STATUS OF THE DOUBLE-CRESTED CORMORANT 

(PHALACROCORAX AURITUS) IN NORTH AMERICA

PREPARED BY:

LINDA R. WIRES

FRANCESCA J. CUTHBERT

DALE R. TREXEL

ANUP R. JOSHI

UNIVERSITY OF MINNESOTA

DEPARTMENT OF FISHERIES AND WILDLIFE

1980 FOLWELL AVE.

ST. PAUL, MN 55108

USA

MAY 2001

PREPARED UNDER CONTRACT WITH *U.S. FISH AND WILDLIFE 

SERVICE

*CONTENT MATERIAL OF THIS REPORT DOES NOT NECESSARILY 

REPRESENT THE OPINIONS OF USFWS

Recommended citation: Wires, L.R., F.J. Cuthbert, D.R. Trexel and A.R. Joshi. 2001. Status of 

the Double-crested Cormorant (Phalacrocorax auritus) in North America. Final Report to 

USFWS.
FINAL DRAFT Executive Summary i

EXECUTIVE SUMMARY

Introduction: Since the late-1970s, numbers of Double-crested Cormorants (Phalacrocorax 

auritus) (DCCO) have increased significantly in many regions of North America. A variety of 

problems, both real and perceived, have been associated with these increases, including impacts 

to aquaculture, sport and commercial fisheries, natural habitats, and other avian species. Concern 

is especially strong over impacts to sport and commercial fishes and aquaculture. Because of 

increasing public pressure on U.S. government agencies to reduce DCCO conflicts, the USFWS 

is preparing an Environmental Impact Statement (EIS), and in conjunction with the U.S. 

Department of Agriculture/Wildlife Services (USDA/WS) and state resource management 

agencies, will develop a national management plan for the DCCO. This assessment will be used 

to prepare the EIS and management plan.

Populations and trends: The DCCO breeding range in North America is divided into five 

geographic areas. Since at least 1980, numbers have clearly increased in three of the breeding 

areas: Canadian and U.S. interior, Northeast Atlantic Coast and Southern U.S. In these 

populations, much of the growth occurred between the late 1970s – early 1990s; from the early 

1990s – 2000 growth rates have slowed or appeared to stabilize in many states and provinces. 

For the Pacific Coast and Alaskan breeding populations it was not possible to summarize trends 

overall because recent data for birds breeding in significant portions of these regions (e.g., 

Alaska, Mexico) are not available, or have not been collected in a coordinated and timely fashion 

for the populations as a whole. Along some parts of the Pacific Coast, breeding numbers 

declined in the 1990s (e.g., British Columbia, species is listed as Vulnerable and is being 

considered for Threatened status). In other areas significant increases occurred. Concurrently, 

numbers also increased on the wintering grounds, particularly in the Mississippi River Delta 

region, an area of high human-cormorant conflict over catfish resources.

Many historical records from across the continent indicate that the species was or may have been 

more abundant and widespread than is currently presumed. While most of these early accounts 

are largely qualitative, many report huge numbers of cormorants, suggesting that recent 

population increases may represent recovery towards historical (presettlement) levels in certain 

regions. In some areas where the DCCO has been documented as a recent breeder, the species is 

actually re-colonizing after an absence of 50 – 300 years.

Reasons for population increases: There appear to be five major factors that led to dramatic 

increases in DCCOs in North America since about 1970. These include:

1. Ban on DDT (1972) and other pesticide reduction regulation. Prior to this time (but post 

WWII) widespread use of DDT occurred. Cormorants accumulated high levels of DDT 

through their food supply, which interfered with reproduction. Depressed populations began 

to increase after DDT was banned.

2. In 1972 the DCCO was added to the Migratory Bird Treaty Act protected bird list. Before 

1972, federal legislation did not prevent killing or harassment of cormorants during their 

annual cycle. Some states also provided special protection for DCCOs around this time.
FINAL DRAFT Executive Summary ii

3. Human induced changes (e.g. accidental and intentional introduction of exotics; over fishing; 

changes in water quality) in aquatic communities in the breeding range.

4. Development of aquaculture (e.g. catfish farms) in the south (especially Mississippi Delta 

region) that provided a new food source.

5. Creation of additional breeding and foraging habitat (e.g. reservoirs; dredge spoil islands).

Diet and native fish populations: DCCO diet is characterized by great temporal and spatial 

variation. The DCCO is known to feed on > 250 species of fresh and saltwater fishes. 

Cormorants are generalists and eat abundant fish in the size range 3 – 40 cm; < 15 cm is 

preferred. Review of diet studies (> 40) indicates most sport and commercially valuable fish 

species do not contribute substantially to DCCO diet. Though there are exceptions, most studies 

conclude that sport and commercially valuable fish species are not negatively impacted by 

DCCOs, and that DCCOs have minor effects on fish populations relative to human harvest and 

other mortality factors. The most common claim against DCCOs is that they reduce sport or 

commercial catches, but the actual relationship between cormorant predation, fish population 

size and human harvest is poorly understood. This lack of information contributes to the 

complexity of cormorant-fish-fishery interactions.

Rigorous quantification of cormorant predation on fish populations or on subsequent sport or 

commercial catches requires more precise estimates of several key parameters, including: prey 

fish population sizes; prey fish mortality sources and rates; age class distribution of fish 

consumed. Additionally, a better understanding of compensatory processes within prey fish 

populations is essential (e.g., predation may reduce competition so that remaining fish survive 

longer or younger fish grow faster). However, no study conducted so far has obtained robust 

estimates for all of these parameters. Therefore, while DCCOs may cause fish populations to 

decline, none of the studies reviewed provided data rigorous enough to demonstrate that they do 

so. The effect of cormorant predation can be either compensatory (if the cormorants do not eat 

them, the same proportion may be removed by other factors) or additive (mortality due to 

cormorant predation is not replaced by another factor). However, investigators have rarely 

examined cormorant predation in the context of other mortality or limiting factors.

Because of great spatial variation in DCCO diet and unique complexities of individual aquatic 

ecosystems, DCCO predation impacts need to be assessed locally. To do this biologists need a 

more comprehensive understanding of local fish population dynamics and standardized methods 

for assessing cormorant diet.

Diet and aquaculture facilities: Studies show DCCOs may eat large numbers of catfish locally 

and temporally. However, no study has quantified the economic impact on net harvest. Only one 

study has examined the issue of additive and compensatory mortality and concluded that 

mortality due to DCCO predation impacts were additive under certain circumstances, but 

insignificant in others.

Impacts on vegetation: Most colonial waterbirds destroy vegetation at breeding and / or 

roosting sites to some extent, and cormorants cause some of the most dramatic change. 

Cormorants impact vegetation through deposition of guano (excrement) that kills underlying 

vegetation and eventually trees, and through nest building behavior when they strip leaves and 
FINAL DRAFT Executive Summary iii

small branches. In the short term these changes are of greatest concern if they affect rare plant 

communities or private property. From a long-term perspective these changes may be 

insignificant on an ecosystem scale. Few studies have been conducted to characterize and 

quantify vegetation change due to cormorant nesting and roosting habits.

Impacts on other bird species: DCCOs are hypothesized to have two potential effects on other 

colonial waterbird species: competition for nest sites and habitat degradation. Direct interspecific 

competition for nests and nest sites may occur but has not been documented through careful 

study. Most impacts appear to occur indirectly through habitat degradation (e.g. defoliation, tree 

die-off). While there is some evidence that DCCOs may displace other species, no studies have 

clearly established DCCO impact on other birds at even a colony level scale.

Management options: Humans have attempted to manage cormorant numbers in the western 

hemisphere for at least 400 years. Currently in the U.S. all lethal take requires permits from the 

USFWS, except at aquaculture facilities in those states under the 1998 Federal Depredation 

Order. Depredation permits can be obtained to prevent economic impacts or impacts to 

endangered, threatened or species of conservation concern. Non-lethal harassment of birds 

depredating or about to depredate does not require permits. To reduce cormorant impacts 

primarily to fisheries, aquaculture, vegetation and other colonial waterbirds, a large number of 

techniques has been developed or proposed. These techniques utilize lethal and non-lethal 

measures and may be used at local, regional or population levels. The effectiveness of these 

measures is difficult to assess because in many cases impacts have been poorly quantified.

Most techniques used at the local level are non-lethal. Lethal control may help reinforce local 

non-lethal control techniques. However, because cormorants are highly mobile, lethal control at 

the local level may be ineffective at decreasing local populations. Although economic 

effectiveness cannot be assessed for individual control techniques, some appear more effective 

than others; future research should focus on reducing the costs of the most promising techniques. 

Many techniques have been poorly investigated; therefore conclusions about their economic and 

numerical effectiveness may be premature. Because aquaculture ponds are high quality foraging 

sites (high fish densities; lack of escape cover), control of cormorants on the breeding grounds is 

unlikely to eliminate the need to practice local control. To make aquaculture ponds less desirable 

foraging sites, some form of control at the local level (e.g. exclosures, harassment) will likely 

still be needed. Previous efforts indicate that population control in general must be large scale 

and will require sizable human and capital inputs to be effective. Additionally, potential density 

dependent effects that compensate for control related mortality are poorly understood. 

Addressing these and numerous other uncertainties will enhance the development of a 

scientifically based, large-scale population control effort.

Finally, no control is a management option that is economically justified if the costs of control 

are greater than the losses associated with cormorant impacts.

Population Models: Models have identified data gaps critical for understanding population 

dynamics and predicting control effectiveness; modeling is potentially a very strong tool for 

gaining insights into cormorant management. Prediction of future DCCO population trends and 

analysis of control methods is hampered by lack of age-specific data for this species. More effort 
FINAL DRAFT Executive Summary iv

needs to be put into obtaining data needed to strengthen model predictions, and increased effort 

should focus on predicting management outcomes and follow progress. Until better data are 

available, however, such modeling efforts should include rigorous sensitivity analyses to 

investigate uncertainties in parameters used and assumptions made in the model.

Current research and monitoring efforts: Of 33 U.S. states and nine Canadian provinces to 

which surveys were sent, nine reported research in progress and 19 have monitoring programs. 

Research addresses: cormorant diet, bioenergetics, impacts to aquaculture, sport and commercial 

fisheries, foraging range and foraging behavior. Additional studies are attempting to determine 

effectiveness of harassment at day and night roosts, effectiveness of barriers at aquaculture 

ponds, and nutrient enrichment in aquatic and terrestrial habitats. A satellite telemetry study will 

determine migration patterns, breeding locations and winter movements of cormorants at catfish 

farms. All monitoring efforts are used to determine population distribution and trends.

Future research priorities: The assessment identified many research needs. Highest priority 

studies on DCCOs fall within the following broad topics: (1) demography, (2) impacts on 

fisheries and aquaculture, (3) management techniques, (4) impacts on flora and fauna and (5) 

distribution.
FINAL DRAFT List of Figures v

LIST OF FIGURES

Figure 1. Distribution of Double-crested Cormorant (Phalacrocorax auritus) breeding colonies in 

North America 1970 – 2000. .............................................................................................15

Figure 2. DCCO Christmas Bird Count sites (1984 – 1998) where ≥ 100 birds were counted at 

least once. .........................................................................................................................16

Figure 3. December estimates of DCCOs in Alaska based on CBCs, 1984 – 1998.....................18

Figure 4. Distribution and size of active colonies in Alaska at time of last surveys (1970 – 1999).

..........................................................................................................................................19

Figure 5. December estimates of DCCOs on the Pacific Coast, Zone 2*, based on CBCs, 1984 –

1998..................................................................................................................................23

Figure 6. Distribution of Pacific Coast DCCO breeding population (Zone 2), 1970 – 2000. ......25

Figure 7. Distribution and size of active colonies on north Pacific Coast at time of last surveys 

(1990s)..............................................................................................................................26

Figure 8. Distribution and size of active colonies on south Pacific Coast at time of last surveys 

(CA = 1989 – 1998; MX = 1973 – 1992)...........................................................................27

Figure 9. Distribution and size of active colonies in the Pacific Coast southwestern states (1993 –

2000).................................................................................................................................28

Figure 10. December estimates of DCCOs in Arizona based on CBCs, 1984 – 1998.................30

Figure 11. Changes in number of pairs at Great Chain, Five Finger and Mandarti Islands, Gulf 

Islands, B.C., 1987 – 2000.................................................................................................33

Figure 12. December estimates of DCCOs in British Columbia based on CBCs, 1984 – 1998...34

Figure 13. December estimates of DCCOs in California based on CBCs, 1984 – 1998..............38

Figure 14. December estimates of DCCOs at the Salton Sea based on CBCs, 1984 – 1999........38

Figure 15. Changes in number of breeding pairs at Anaho Island, Pyramid Lake, Nevada, 1986 –

1998..................................................................................................................................42

Figure 16. December estimates of DCCOs in New Mexico based on CBCs, 1984 – 1998. ........44

Figure 17. Changes in number of breeding pairs at East Sand and Rice Islands, Columbia River 

Estuary, Oregon, 1991 – 2000. ..........................................................................................47

Figure 18. Changes in numbers of breeding pairs at Malheur and Upper Klamath Lakes, 1986 –

1999..................................................................................................................................47

Figure 19. December estimates of DCCOs in Oregon based on CBCs, 1984 – 1998..................48

Figure 20. Changes in breeding numbers at colonies located along the Washington outer coast, 

1979-1999.........................................................................................................................50

Figure 21. Changes in breeding numbers at colonies located in theWashington inner coastal 

waters, 1983-2000.............................................................................................................51

Figure 22. Changes in breeding numbers in interior Washington, N. Potholes Reservoir, 1978 –

1997..................................................................................................................................51

Figure 23. December estimates of DCCOs in Washington based on CBCs, 1984 – 1998...........52

Figure 24. Changes in numbers of breeding pairs on the Great Lakes, 1970 – 1997...................56

Figure 25. Distribution of Interior U.S. and Canadian DCCO breeding population (Zone 3), 1979 

– 2000...............................................................................................................................58

Figure 26. Distribution and size of active colonies in Saskatchewan during last complete survey 

(1991). ..............................................................................................................................59

Figure 27. Distribution and size of active colonies on Lake Winnipegosis during last complete 

survey (1999). ...................................................................................................................60
FINAL DRAFT List of Figures vi

Figure 28. Distribution and size of active colonies in the Interior west-central states during last 

complete surveys (1986 – 1999)........................................................................................61

Figure 29. Distribution and sizes of active colonies in Interior mid-central states at time of last 

complete surveys...............................................................................................................62

Figure 30. Distribution of active colonies (size data not available) in South Dakota and 

Minnesota (MN = 1981 – 1995; SD = 1988 – 1992)..........................................................63

Figure 31. Distribution and size of active colonies in Wisconsin during last complete survey 

(1997). ..............................................................................................................................64

Figure 32. Distribution and size of active colonies in Michigan during last complete survey 

(1997). ..............................................................................................................................65

Figure 33. Distribution and size of active colonies in Ontario during last complete survey (1997).

..........................................................................................................................................66

Figure 34. Detail of Lake Huron showing size and distribution of active colonies (1997)..........67

Figure 35. Distribution and size of active colonies in Ohio, New York, and Vermont................68

Figure 36. Changes in numbers of breeding pairs in Illinois, 1986 – 1997.................................73

Figure 37. Changes in breeding numbers on Lake Winnipegosis, 1979 – 1999..........................79

Figure 38. Changes in numbers of breeding pairs in Michigan, 1977 – 1997. ............................81

Figure 39. Changes in number of breeding pairs at Goose Lake, Crescent Lake NWR, Nebraska, 

1973 – 1996. .....................................................................................................................86

Figure 40. Changes in number of breeding pairs at Marsh Lakes, Valentine NWR, Nebraska, 

1977 – 2000. .....................................................................................................................86

Figure 41. Changes in number of breeding pairs on Little Galloo Island, 1977 – 1998...............88

Figure 42. Changes in numbers of breeding pairs in Ohio, 1992 – 1998. ...................................92

Figure 43. Changes in breeding numbers in Ontario, 1973 – 1997.............................................94

Figure 44. Changes in breeding numbers in Saskatchewan, 1968 – 1991...................................95

Figure 45. Changes in breeding numbers at Bear River Refuge, Utah, 1971 – 1996...................98

Figure 46. Changes in numbers of breeding pairs in Vermont, 1982 – 1999. ...........................100

Figure 47. Changes in breeding numbers in Wisconsin, 1973 – 1997. .....................................102

Figure 48. December estimates of DCCOs in the Southeastern U.S., Zone 4,* based on CBCs, 

1984 – 1997. ...................................................................................................................107

Figure 49. Distribution of South and Southeastern U.S. DCCO breeding population (Zone 4) 

1980 – 1999. ...................................................................................................................109

Figure 50. Distribution and size of active colonies in South Carolina during last comnplete 

survey (1996). .................................................................................................................110

Figure 51. Comparison of Christmas Bird Count sites (with ≥ 100 birds) with roost sites located 

by USDA/WS in Arkansas, Mississippi and Alabama. ....................................................111

Figure 52. December estimates of DCCOs in Alabama based on CBCs, 1984 – 1997. ............113

Figure 53. December estimates of DCCOs in Arkansas based on CBCs, 1984 – 1997. ............115

Figure 54. December estimates of DCCOs in Florida based on CBCs, 1984 – 1997 ................118

Figure 55. December estimates of DCCOs in Georgia based on CBCs, 1984 – 1997...............119

Figure 56. December estimates of DCCOs in Louisiana based on CBCs, 1984 – 1997. ...........123

Figure 57. December estimates of DCCOs in Mississippi based on CBCs, 1984 – 1997..........125

Figure 58. Double-crested Cormorant from Brickell (1737), possibly the first published 

representation of this species. ..........................................................................................127

Figure 59. December estimates of DCCOs in North Carolina based on CBCs, 1984 – 1997. ...127

Figure 60. December estimates of DCCOs in Oklahoma based on CBCs, 1984 – 1997. ..........129
FINAL DRAFT List of Figures vii

Figure 61. Changes in number of breeding pairs in South Carolina, 1989 – 1996. ...................131

Figure 62. December estimates of DCCOs in South Carolina based on CBCs, 1984 – 1997. ...132

Figure 63. Changes in number of breeding pairs in Tennessee, 1992 – 1999. ..........................134

Figure 64. December estimates of DCCOs in Tennessee based on CBCs, 1984 – 1997. ..........134

Figure 65. December estimates of DCCOs in Texas based on CBCs, 1984 – 1997. .................137

Figure 66. Changes in numbers of breeding pairs in New England, 1977 – 1994/95. ...............141

Figure 67. December estimates of DCCOs in the Northeast Atlantic, Zone 5*, based on CBCs, 

1984 – 1998. ...................................................................................................................142

Figure 68. Distribution of Northeast Atlantic Coast DCCO breeding population (Zone 5), 1980 –

1999................................................................................................................................145

Figure 69. Distribution and size of active colonies in the mid-Atlantic states at time of last 

complete surveys.............................................................................................................146

Figure 70. Distribution and size of active colonies in New England at time of last surveys......147

Figure 71. Distribution and size active colonies in Maine during last complete survey (1994 –

1995)...............................................................................................................................148

Figure 72. Distribution and size of active colonies in New Brunswick, Novia Scotia and Prince 

Edward Island during last complete surveys. ...................................................................149

Figure 73. Distribution of active colonies in Quebec at time of last surveys (1970 – 1998)......150

Figure 74. Distribution and size of active colonies in Quebec at time of last surveys (1970 –

1998)...............................................................................................................................151

Figure 75. Detail of the St. Lawrence River Estuary and Gaspe Peninsula showing distribution 

and size of active colonies at time of last surveys (1970 – 1998)......................................152

Figure 76. Distribution and size of active colonies along the coast of Newfoundland and on 

offshore islands during last complete survey (1996). .......................................................153

Figure 77. Changes in numbers of breeding pairs in Connecticut, 1986 – 1998. ......................155

Figure 78. Changes in numbers of breeding pairs in Maine, 1977 – 1994/95. ..........................158

Figure 79. Changes in number of breeding pairs in Maryland, 1990 – 1999.............................159

Figure 80. Changes in numbers of breeding pairs in Massachusetts, 1977 – 1994/5.................161

Figure 81. December estimates of DCCOs in New Jersey based on CBCs, 1984 – 1998..........166

Figure 82. Changes in breeding numbers in New York (coastal) 1985 – 1998. ........................168

Figure 83. Changes in breeding numbers in Nova Scotia, 1972 – 1992....................................170

Figure 84. Changes in numbers of breeding pairs on Prince Edward Island, 1976 – 1999. .......172

Figure 85. Changes in breeding numbers in Québec, 1970 – 1990...........................................175

Figure 86. Changes in breeding numbers in Rhode Island, 1981 – 1998. .................................177

Figure 87. Annual totals of DCCOs in Virginia based on CBCs, 1984 – 1998.........................179

Figure 88 ................................................................................................................................241

Figure 89. The relationship between annual percent increase and r..........................................286

Figure 90. Numbers of Apparently Occupied Nests (AON) on Pigeon Island, Ontario with 

population growth estimates. ...........................................................................................286

Figure 91. Annual percent change in number of apparently occupied nests (AON) in Pigeon 

Island, Lake Ontario........................................................................................................287

Figure 92. Nesting population change on Gull Island, Lake Superior and logistic model based on 

1978 – 1985 nest counts. .................................................................................................294
FINAL DRAFT List of Tables ix

LIST OF TABLES

Table 1. Zone 2, Pacific Coast. Summary of Population data and trends. (+ = increasing; - = 

decreasing; 0 = no change). ...............................................................................................24

Table 2. Numbers of chicks fledged at DCCO colonies along the east coast of Vancouver Island 

(from Moul 2000)..............................................................................................................33

Table 3. Summary of most recent census data for California breeding regions...........................37

Table 4. Zone 3, Canadian and U.S. Interior. Summary of Population Data and Trends. (+ = 

increasing; - = decreasing; 0 = no change).........................................................................57

Table 5. Zone 4, Southern U.S. Summary of Population Data and Trends (+ = increasing; - = 

decreasing; 0 = no change). .............................................................................................108

Table 6. Zone 5, Northeast Atlantic Coast. Summary of Population Data and Trends (+ = 

increasing; - = decreasing; 0 = no change).......................................................................143

Table 7. Pacific Coast: Occurrence of Salmonids in the diet of Double–crested Cormorants....208

Table 8. Pacific Coast: Occurrence of littoral or estuarine fish in the diet of Double–crested 

Cormorants. ....................................................................................................................209

Table 9. Pacific Coast: Occurrence of littoral-benthic fish in the diet of Double–crested 

Cormorants. ....................................................................................................................211

Table 10. Pacific Coast: Occurrence of pelagic fish in the diet of Double–crested Cormorants.

........................................................................................................................................212

Table 11. Great Lakes: Occurrence of open-fresh water fish in the diet of Double–crested 

Cormorants. ....................................................................................................................213

Table 12. Great Lakes: Occurrence of littoral-freshwater fish in the diet of Double–crested 

Cormorants. ....................................................................................................................215

Table 13. Great Lakes: Occurrence of bottom dwelling-fresh water fish in the diet of Double–

crested Cormorants..........................................................................................................219

Table 14. Great Lakes: Occurrence of Salmonids in the diet of Double–crested Cormorants. ..221

Table 15. Great Lakes: Locally important fish species in the diet of Double–crested Cormorants.

........................................................................................................................................222

Table 16. Southern U.S.: Occurrence of Shad in the diet of Double–crested Cormorants.........223

Table 17. Southern U.S.: Occurrence of Catfish in the diet of Double–crested Cormorants. ....224

Table 18. Southern U.S.: Occurrence of Centrarchids in the diet of Double–crested Cormorants.

........................................................................................................................................225

Table 19. Southern U.S.: Locally important fish species in the diet of Double–crested 

Cormorants. ....................................................................................................................226

Table 20. Atlantic Region: Occurrence of Marine bottom fish in the diet of Double–crested 

Cormorants. ....................................................................................................................226

Table 21. Atlantic Region: Occurrence of Estuarine / Diadromous fishes in the diet of Double–

crested Cormorants..........................................................................................................229

Table 22. Atlantic Region: Occurrence of Freshwater fishes in the diet of Double–crested 

Cormorants. ....................................................................................................................232

Table 23. Atlantic Region: Occurrence of Salmonids in the diet of Double–crested Cormorants.

........................................................................................................................................234

Table 24. Atlantic Region: Occurrence of Pelagic fishes in the diet of Double–crested 

Cormorants. ....................................................................................................................235
FINAL DRAFT List of Tables x

Table 25. Atlantic Region: Occurrence of Crustaceans in the diet of Double–crested Cormorants.

........................................................................................................................................236

Table 26. Sources of information on DCCO control at aquaculture facilities. ..........................243

Table 27. Percent of winter band recoveries (1923-1988) that occurred in the Lower Mississippi 

Valley for birds originally banded during the nesting season in various geographic regions 

(Dolbeer 1991)................................................................................................................260

Table 28. States and provinces practicing Double-crested Cormorant control. .........................282

Table 29. Impacts in Zone 3 (U.S. and Canadian Interior).......................................................333

Table 30. Impacts in Zone 4 (Southern U.S.)...........................................................................333

Table 31. Impacts in Zone 5 (North Atlantic)..........................................................................334

Table 32. Summary of fisheries survey data: states / provinces reporting declines linked to 

DCCOs. ..........................................................................................................................335

Table 33. Summary of Fisheries Survey Data: Fish species reported to be in decline and factors 

thought responsible. ........................................................................................................336

Table 34. States and provinces engaged in current DCCO or DCCO related research or 

monitoring programs, and or with identified research priorities. ......................................337
FINAL DRAFT Acknowledgements xi

ACKNOWLEDGEMENTS

Of the hundreds of individuals who contributed to this review, five provided significant 

intellectual insight, guidance and friendship: (1) Jeremy Hatch carefully reviewed multiple drafts 

of this and related manuscripts, and never failed to provide copious (constructive) criticisms. His 

suggestions on organization of the population data were exceptionally helpful and shaped the 

entire document. (2) D.V. (Chip) Weseloh, through review of the manuscript and numerous 

conversations, provided important ideas on the biology and management of cormorants. His 

consistent support, cooperation and encouragement were invaluable. (3) David Carss contributed 

in ways too numerous to mention. His publications, review of this assessment, and his 

enthusiasm for discussing transatlantic cormorant issues revolutionized our thoughts on 

cormorant diet assessment and impacts to fisheries. (4) Thomas Bregnballe provided and helped 

us obtain the extensive literature on the Great Cormorant in Europe. He contributed information 

that helped broaden our understanding of the complexity of population-level management, and 

his work and insights were inspirational. (5) Thomas Keller provided stimulating ideas about 

trans-continental cormorant conflicts, particularly on diet, human perceptions and management 

issues.

Over fifty people reviewed earlier drafts of this assessment. We are grateful for all of their 

comments and suggestions. We especially thank Connie Adams, Robert Ross and Jim Glahn for 

providing careful, detailed reviews that challenged our ideas and interpretations, and made us 

consider complex issues from multiple perspectives.

Other persons who contributed to our thoughts on cormorants, development of the status 

assessment, and problems with piscivorous birds in general include Bill Montevecchi, Bill 

Koonz, Walt Lysack and John Trapp. Also, we are grateful to Steve Lewis and Shauna Hanisch 

for their support, encouragement and patience; Bruce Vondracek helped organize ecological 

groupings for fish species and answered many questions related to fish population dynamics.

Finally, we cannot thank enough the numerous persons identified in Appendix V who 

contributed information on biology, distribution, current numbers and recent changes of the 

Double-crested Cormorant in North America. Their dedication enabled us to compile an accurate 

and detailed status assessment of this species.

The U.S. Fish and Wildlife Service funded preparation of this assessment.
FINAL DRAFT Table of Contents xiii

TABLE OF CONTENTS

EXECUTIVE SUMMARY..........................................................................................................i

LIST OF FIGURES ....................................................................................................................v

LIST OF TABLES.....................................................................................................................ix

ACKNOWLEDGEMENTS .......................................................................................................xi

TABLE OF CONTENTS......................................................................................................... xiii

INTRODUCTION ......................................................................................................................1

Background Information .........................................................................................................1

The Status Assessment............................................................................................................2

PART I: NATURAL HISTORY OF THE DOUBLE-CRESTED CORMORANT, Phalacrocorax 

auritus ........................................................................................................................................3

GENERAL INFORMATION..................................................................................................3

Taxonomy...........................................................................................................................3

Physical Description............................................................................................................3

Range..................................................................................................................................4

Breeding Range...............................................................................................................4

Winter Range. .................................................................................................................4

Habitat ................................................................................................................................5

Breeding Season Habitat Requirements. ..........................................................................5

Winter Habitat Requirements. .........................................................................................5

BIOLOGY ..............................................................................................................................5

Migration ............................................................................................................................5

Reproduction.......................................................................................................................6

Foraging Ecology................................................................................................................7

Diet.....................................................................................................................................8

Longevity, Survival and Mortality.......................................................................................8

PART II. POPULATION DATA AND TRENDS.......................................................................9

INTRODUCTION ..................................................................................................................9

Breeding Birds....................................................................................................................9

Biologically Relevant Units and Organization of Data.....................................................9

Colony Locations ..........................................................................................................10

Trends...........................................................................................................................11

Wintering Birds.................................................................................................................11

Locations and Winter Distribution.................................................................................11

Use of Christmas Bird Count Data.................................................................................11

Wintering Numbers and Trends.....................................................................................12

Migrant Birds....................................................................................................................12

Migration / Stop-over Sites............................................................................................12

Trends...........................................................................................................................12

Non-breeding Birds and Total Population Size ..................................................................13

ZONE 1: SUMMARY OF POPULATION DATA FOR ALASKA.......................................17

Introduction.......................................................................................................................17

Alaska...............................................................................................................................17

ZONE 2: SUMMARY OF POPULATION DATA FOR THE PACIFIC COAST..................21
FINAL DRAFT Table of Contents xiv

Introduction.......................................................................................................................21

Breeding Information ....................................................................................................21

Winter and Migrant Birds..............................................................................................22

Historical Information ...................................................................................................22

Arizona .............................................................................................................................29

British Columbia ...............................................................................................................30

California ..........................................................................................................................34

Mexico..............................................................................................................................39

Nevada..............................................................................................................................41

New Mexico......................................................................................................................42

Oregon..............................................................................................................................44

Washington.......................................................................................................................48

ZONE 3: SUMMARY OF POPULATION DATA FOR CANADIAN AND U.S. INTERIOR 

POPULATION .....................................................................................................................53

Introduction.......................................................................................................................53

Breeding Information ....................................................................................................53

Winter and Migrant Birds..............................................................................................54

Historical Information ...................................................................................................54

Alberta..............................................................................................................................69

Colorado ...........................................................................................................................69

Idaho.................................................................................................................................70

Illinois...............................................................................................................................72

Indiana ..............................................................................................................................73

Iowa..................................................................................................................................74

Kansas ..............................................................................................................................76

Manitoba...........................................................................................................................77

Michigan...........................................................................................................................79

Minnesota .........................................................................................................................81

Missouri............................................................................................................................83

Montana ............................................................................................................................84

Nebraska ...........................................................................................................................85

New York (Inland)............................................................................................................87

North Dakota.....................................................................................................................89

Ohio ..................................................................................................................................90

Ontario..............................................................................................................................92

Saskatchewan....................................................................................................................94

South Dakota ....................................................................................................................96

Utah..................................................................................................................................97

Vermont............................................................................................................................99

Wisconsin .......................................................................................................................100

Wyoming ........................................................................................................................102

ZONE 4: SUMMARY OF POPULATION DATA FOR SOUTHEASTERN U.S. ..............105

Introduction.....................................................................................................................105

Zone Boundaries. ........................................................................................................105

Breeding Information ..................................................................................................105

Winter and Migrant Birds............................................................................................105
FINAL DRAFT Table of Contents xv

Historical Information .................................................................................................106

Alabama..........................................................................................................................112

Arkansas .........................................................................................................................113

Florida ............................................................................................................................116

Georgia ...........................................................................................................................118

Kentucky.........................................................................................................................120

Louisiana ........................................................................................................................121

Mississippi......................................................................................................................123

North Carolina ................................................................................................................125

Oklahoma........................................................................................................................128

Puerto Rico / Virgin Islands ............................................................................................130

South Carolina ................................................................................................................130

Tennessee........................................................................................................................132

Texas ..............................................................................................................................135

ZONE 5: SUMMARY OF POPULATION DATA FOR THE NORTHEAST ATLANTIC 

COAST...............................................................................................................................139

Introduction.....................................................................................................................139

Breeding Information ..................................................................................................139

Winter and Migrant Birds............................................................................................140

Historical Information .................................................................................................140

Summary of Population Data and Trends.....................................................................154

Historical Information .................................................................................................154

Land Ownership ..........................................................................................................154

Productivity.................................................................................................................154

Delaware.........................................................................................................................155

Maine..............................................................................................................................156

Maryland.........................................................................................................................158

Massachusetts.................................................................................................................160

New Brunswick...............................................................................................................162

Newfoundland.................................................................................................................163

New Hampshire ..............................................................................................................164

New Jersey......................................................................................................................165

New York (Coastal) ........................................................................................................166

Nova Scotia.....................................................................................................................168

Pennsylvania ...................................................................................................................170

Prince Edward Island ......................................................................................................171

Québec............................................................................................................................173

Rhode Island ...................................................................................................................176

Virginia...........................................................................................................................177

West Virginia ..................................................................................................................179

PART III: DIET......................................................................................................................181

Introduction ........................................................................................................................181

Diet Studies Reviewed ....................................................................................................181

Diet Assessment Methods: Advantages and Shortcomings ..............................................181

Pellets .........................................................................................................................182

Regurgitations.............................................................................................................183
FINAL DRAFT Table of Contents xvi

Stomach Contents........................................................................................................184

Recovered Tags From Stocked Fish.............................................................................185

Conclusions: Diet Assessment Methods ......................................................................185

Measurements Used to Report Diet .................................................................................185

Results of Diet Review........................................................................................................186

Regional Diet Descriptions..............................................................................................186

Diet on the Pacific Coast .............................................................................................186

Diet in the Great Lakes................................................................................................187

Diet in the Southeastern U.S........................................................................................187

Diet in the Atlantic Region..........................................................................................188

Conclusions: Variation in the Diet...............................................................................188

Fish species declines associated with DCCO predation (perceived or real) ......................189

Studies assessing impacts to sport and / or commercial fish populations..........................190

Great Lakes.................................................................................................................190

Southeastern U.S.........................................................................................................197

Conclusions: Impacts to Sport and / or Commercial Fisheries......................................199

Studies assessing impacts to aquaculture .........................................................................200

Surveys .......................................................................................................................201

Diet Studies and Observations at Ponds.......................................................................202

Bioenergetics Modeling...............................................................................................205

Conclusions: Impacts to Aquaculture...........................................................................207

Studies on food preferences.............................................................................................207

PART IV. IMPACTS TO VEGETATION AND COLONIAL WATERBIRDS ......................237

Impacts to Vegetation .........................................................................................................237

Impacts to avian species......................................................................................................238

Newcastle Disease ..............................................................................................................239

PART V. MANAGEMENT OPTIONS...................................................................................241

Introduction ........................................................................................................................241

Managing Impacts to Aquaculture.......................................................................................244

1) Exclusion Devices: Physical and Functional Barriers ..................................................244

Netting ........................................................................................................................244

Wires...........................................................................................................................245

Floating Ropes ............................................................................................................246

Other Flight Inhibitors.................................................................................................247

Underwater Exclusion Devices....................................................................................247

2) Non-lethal Harassment at Aquaculture Facilities.........................................................247

Human Harassment .....................................................................................................247

Simulated Human Harassment.....................................................................................249

Other Harassment........................................................................................................250

3) Lethal Control at Aquaculture Facilities......................................................................251

Reinforcing Non-lethal Harassment Techniques..........................................................251

Reducing Local Populations ........................................................................................252

Accidental Killing of Other Species.............................................................................253

4) Non-lethal Harassment at Night Roosts.......................................................................254

5) Provision of Alternative Prey and Foraging Sites ........................................................255

Provision of Alternative Prey.......................................................................................255
FINAL DRAFT Table of Contents xvii

Alternative Foraging Sites...........................................................................................256

6) Alteration of Aquaculture Practices.............................................................................256

Decrease Fish Densities...............................................................................................257

Pond Dyes...................................................................................................................257

Water Turbulence........................................................................................................257

Aquaculture Facility Location .....................................................................................258

Improved Facility Design ............................................................................................258

Alter Stocking Practices ..............................................................................................258

Passive Harassment .....................................................................................................259

Alteration of Fish Behavior .........................................................................................259

Chemical Avian Repellants .........................................................................................259

Conditioned Taste Aversion ........................................................................................259

Manage Other Avian Species.......................................................................................259

7) Population Control......................................................................................................259

8) No Control..................................................................................................................262

9) Conclusions: Managing Impacts to Aquaculture..........................................................262

Managing Impacts to Vegetation and Habitat ......................................................................263

Techniques......................................................................................................................264

Harassment of Adults..................................................................................................264

Nest / Tree Removal....................................................................................................265

Egg and Nestling Destruction......................................................................................266

Egg Oiling / Spraying..................................................................................................268

Avian Contraceptives..................................................................................................270

Killing Adults..............................................................................................................270

No Control ..................................................................................................................272

Case Studies: Multiple Techniques..................................................................................272

Lake Winnipegosis, Manitoba .....................................................................................273

St. Lawrence Estuary, Québec .....................................................................................274

Lake Champlain, Vermont (USFWS 1999a)................................................................275

Lake Ontario, New York (USFWS 1999b) ..................................................................277

Conclusions: Managing Impacts to Vegetation and Habitat .............................................278

Managing Impacts To Fisheries...........................................................................................278

Hatchery-raised Trout Releases.......................................................................................278

Salmon Smolt Releases / Runs ........................................................................................279

Nets / Weirs ....................................................................................................................279

Equipment Modification..............................................................................................280

Harassment..................................................................................................................280

Alternative Perches and Prey.......................................................................................281

Reduce Time Fish Spend in Nets / Weirs.....................................................................281

4) Conclusions: Managing Impacts To Fisheries..............................................................281

Survey Results....................................................................................................................281

PART VI. POPULATION MODELING.................................................................................283

INTRODUCTION ..............................................................................................................283

DETERMINISTIC EXPONENTIAL MODELS .................................................................284

Basic Exponential Growth Models ..................................................................................284

Age-Structured Exponential Growth Models...................................................................288
FINAL DRAFT Table of Contents xviii

Lake Ontario ...............................................................................................................288

St. Lawrence Estuary, Québec .....................................................................................289

Lake Champlain, Vermont...........................................................................................291

DETERMINISTIC LOGISTIC MODELS...........................................................................293

Basic Logistic Growth Models ........................................................................................293

Age-Structured Logistic Growth Models.........................................................................294

Great Cormorants in Europe................................................................................................295

CONCLUSIONS ................................................................................................................296

PART VII. RESEARCH NEEDS............................................................................................297

DISTRIBUTION INFORMATION.....................................................................................297

DEMOGRAPHIC INFORMATION ...................................................................................297

DIET...................................................................................................................................298

IMPACTS TO FISHERIES AND AQUACULTURE..........................................................298

IMPACTS TO FLORA AND FAUNA................................................................................299

MANAGEMENT TECHNIQUES.......................................................................................299

ECONOMIC IMPACTS .....................................................................................................300

LITERATURE CITED...........................................................................................................301

APPENDIX I. PERCEIVED IMPACTS ASSOCIATED WITH DOUBLE-CRESTED 

CORMORANTS.....................................................................................................................325

Survey Response.................................................................................................................325

Impacts to commercial / sport fish (reported by wildlife biologists, etc.)..........................325

Impacts to commercial / sport fish (reported by fisheries biologists)................................326

Impacts to Aquaculture ...................................................................................................327

Impacts to Vegetation......................................................................................................328

Impacts to avian species..................................................................................................329

Impacts to other fauna .....................................................................................................330

Damage to Private Property.............................................................................................330

Disease Transmission......................................................................................................331

Impacts to Water Quality.................................................................................................331

Impacts to Public Health .................................................................................................332

APPENDIX II. CURRENT RESEARCH................................................................................337

APPENDIX III. LIST OF COMMON AND SCIENTIFIC NAMES .......................................343

Birds...................................................................................................................................343

Fishes..................................................................................................................................344

Mammals............................................................................................................................346

Arthropods..........................................................................................................................347

Other Invertebrates..............................................................................................................347

Plants..................................................................................................................................347

APPENDIX IV. LIST OF ABBREVIATIONS .......................................................................349

APPENDIX V. LIST OF PERSONAL COMMUNICATIONS ...............................................351
FINAL DRAFT Introduction 1

INTRODUCTION

Background Information

Since the 1970s, numbers of Double-crested Cormorants (Phalacrocorax auritus) 

(DCCO) have increased significantly in many regions of North America. A variety of problems, 

both real and perceived, have been associated with these increases, including impacts to 

aquaculture, sport and commercial fisheries, natural habitats, and other avian species. Concern is 

greatest over impacts to sport and commercial fishes, and to aquaculture. Impacts to the latter 

appear more significant, but studies thus far have not determined the magnitude of these impacts.

The DCCO is protected in the United States by the Migratory Bird Treaty Act (MBTA) 

of 1918, as amended (16 U.S.C. 703-712). DCCOs were added to the list of protected species (50 

Stat. 1311, T.S. No. 12) in 1972 following amendment of the U.S. Convention with Mexico (23 

U.S.T. 260, T.I.A.S. 7302). DCCOs are not included in the U.S. Convention with Great Britain, 

acting on behalf of Canada for the Protection of Migratory Birds (39 Stat. 1702 T.S. No. 628 ), 

and are therefore not protected by the Canadian Federal Government, although they receive 

protection by provincial governments. In addition to U.S. federal protection, DCCOs may also 

receive some protection at the state level.

Due to various concerns, legal actions have been taken to control DCCO numbers in the 

U.S. In March, 1998, USFWS established a depredation order (Trapp 1998) in response to 

complaints from catfish farmers and baitfish dealers. This order allows those engaged in 

commercial aquaculture to shoot cormorants without a federal permit at freshwater aquaculture 

premises or state-operated hatcheries in Minnesota and 12 southeastern states. Cormorant control 

programs also exist in individual states to control numbers to reduce their impacts to island 

vegetation and other colonial waterbirds, but these can only be carried out under the terms of a 

federal permit.

Illegal actions to control cormorants have also occurred. In the summer of 1998, 

frustrated fishermen who believed cormorants were responsible for game fish declines in the 

eastern basin of Lake Ontario illegally shot an estimated 1500 – 2000 Double-crested 

Cormorants on Little Galloo Island, Lake Ontario. In 1999, the State of New York requested a 

depredation permit to reduce cormorants on eastern Lake Ontario primarily for the purpose of 

controlling their predation on smallmouth bass, a popular sport fish. The State’s secondary 

concern was to limit cormorant competition with other bird species. The USFWS issued a permit 

to prevent all reproduction through egg oiling on Little Galloo Island, and to continue nest 

destruction efforts on other islands on Lake Ontario to benefit other bird species and their 

habitats. In Canada, legal and illegal control for similar reasons has also occurred.

A resolution was signed by George W. Bush, then the governor of Texas, asking USFWS 

to evaluate the economic impact of the DCCO on sport fishing and to “consider removal of the 

Double-crested Cormorant from the protection of the Migratory Bird Treaty Act if national 

economic losses warrant severe control methods to keep the cormorant population at a 

manageable level within a given region.” Because of these strong concerns over potential 

impacts associated with increases in DCCOs, the USFWS is preparing an Environmental Impact 

Statement (EIS), and in conjunction with the U.S. Department of Agriculture / Wildlife Services 

(USDA/WS) and state resource management agencies, plan to develop a national management 

plan for the DCCO. This status assessment will serve as a primary resource to aid in the EIS and 

development of the management plan.
FINAL DRAFT Introduction 2

The Status Assessment

To assess the status of the Double-crested Cormorant, we reviewed natural history, 

population data and trends, diet studies, cormorant impacts to vegetation and colonial waterbirds, 

cormorant control, population modeling, current research needs and future research priorities. 

Much of this information was obtained through a review of published literature, literature in 

press, and unpublished reports. In addition to summarizing information, we evaluated some 

studies (mainly those which attempted to determine impacts of DCCO predation) for accuracy 

and thoroughness. Similar increases in numbers of the Great Cormorant (Phalacrocorax carbo) 

(GRCO) have occurred in Europe, and similar conflicts with human interests have arisen. 

Because the DCCO and the European GRCO are ecological counterparts, and much significant 

research has been conducted on the latter species, we incorporated relevant insights gained 

through study of the GRCO.

We also conducted two surveys to obtain information. We first conducted a DCCO 

survey to obtain data on breeding, wintering and migration numbers and distributions, population 

trends, and current research, monitoring and management / research priorities; and to obtain 

information on the perceptions of impacts to natural resources and damage problems associated 

with DCCOs in each state / province. We sent this survey to agency (both government and nongovernment) wildlife biologists, university and museum biologists, and to USDA/WS personnel 

in areas where problems associated with cormorants are high. Survey recipients were asked to 

identify a fisheries biologist to whom we could address specific questions regarding impacts to 

sport and commercial fish. We then conducted a second survey with fisheries biologists to obtain 

more information on perceptions of impacts to fish and to gain a wider perspective on this 

problem. Where questions arose regarding information provided in the surveys, we made followup calls and tried to resolve unclear issues.

FINAL DRAFT Part I. Natural History 3

PART I: NATURAL HISTORY OF THE DOUBLE-CRESTED 

CORMORANT, PHALACROCORAX AURITUS

[This section is a brief summary based largely on the Birds of North America species 

account of the Double-crested Cormorant (Hatch and Weseloh 1999). Parts of some sections 

(breeding and winter range) come directly from this account; for a full treatment of the natural 

history of this species and for specific data sources the reader should see account. Other 

literature and information sources were also occasionally included.]

GENERAL INFORMATION

Taxonomy

The Double-crested Cormorant (DCCO) is in the order Pelecaniformes, family 

Phalacrocoracidae. Worldwide, between 30 – 40 species of cormorants are recognized. In earlier 

taxonomies most species were placed in the single genus Phalacrocorax; recent accounts favor 

dividing the family into two main groups, the typical cormorants and the strictly maritime ones, 

the “shags.” In North America, there are six species of cormorants, including the Double-crested; 

these include Great Cormorant (P. carbo), Neotropic Cormorant (P. brasilianus) [formerly called 

Olivaceous Cormorant (P. olivaceus)], Brandt’s Cormorant (P. penicillatus), Pelagic Cormorant 

(P. pelagicus), and Red-faced Cormorant (P. urile). The range of the Double-crested overlaps 

those of Brandt’s and Pelagic Cormorants on the Pacific Coast, from southern Alaska to the Baja 

Peninsula; the Red-faced Cormorant mostly along the coast of southern Alaska; the Neotropic in 

Texas, Louisiana, and w. Mexico, mainly Sinaloa; and the Great Cormorant along the east coast, 

from Newfoundland to Florida. The Double-crested is most closely related to the Neotropic 

Cormorant and is in the same subfamily (genus) as the Great (carbo) and Brandt’s (penicillatus) 

cormorants. The other subfamily includes species provisionally named as shags.

Five subspecies of the Double-crested Cormorant have been described, based on size and 

crest characters: 1) P.a. auritus (Northern Double-crested Cormorant), most numerous, breeds 

widely in the interior and on the northeast coast; 2) P.a. cincinatus (formerly White-crested or 

White-tufted Cormorant) occurs in Alaska; 3) P.a. albociliatus (formerly Farallon Cormorant) 

breeds on Pacific Coast and inland, possibly to New Mexico, Utah and Montana; 4) P.a. 

floridanus (formerly Florida Cormorant) is resident in Florida and Caribbean; 5) P.a. heuretus

(no common name) is resident on San Salvador I., and possibly other islands of the Bahamas (see 

figures in Palmer 1962, p. 331; Johnsgard 1993, Fig. 51, p. 200). Characteristics of birds 

breeding in Mexico have not been established. Banding recoveries suggest that little mixing 

occurs across the Rocky Mountains.

Physical Description

The DCCO is a medium to large (typical length 70 – 90 cm, body mass 1.2 – 2.5 kg) dark 

waterbird. Cormorants have elongated bodies, moderately long necks and bills, long wedgeshaped tails, and totipalmate feet. Resting birds often hold wings in a spread-wing posture, 

thought to aid in drying wet feathers. Distinctive characteristics of the DCCO include brilliant 

turquoise eyes, a golden throat pouch, elegantly patterned back feathers, and for a short time 

prior to breeding season, a cobalt-blue mouth. Plumage is black or dark brown with a dull 
FINAL DRAFT Part I. Natural History 4

greenish or bronze gloss. There is considerable variation in body size, and in color and size of 

crest. Colors of eye-lid may vary geographically or individually from blue to orange. The genus 

name, Phalacrocorax, is Greek for “bald-headed raven”; the species name, auritus, is Latin for 

“eared” and refers to the crests above the eyes, feathers which are fully developed only for a 

short time early in the breeding season.

Range

Widely distributed in North America, but not elsewhere. Vagrants have been reported 

from England 1989, and the Azores in 1991.

Breeding Range.

Five major breeding zones have been defined:

1) Alaska

2) Pacific Coast

3) Canadian and U.S. Interior

4) Atlantic Coast 

5) Florida and the western Caribbean

The five breeding zones are partly reflected in subspecific designations. Populations of 

P.a. auritus have expanded and contracted from two areas: freshwater breeders in the Prairie 

provinces, and maritime birds in the northwest Atlantic. The other four subspecies are 

concentrated in Alaska (P.a. cincinatus); along the Pacific Coast (P.a. albociliatus); in Florida 

and the Caribbean (P.a. floridanus); and in the Bahamas and Cuba (P.a. heuretus). The Alaskan 

population breeds at Nunivak Island., and se. Bering Sea from e. Aleutian Islands. to the 

southeast coast, including Kodiak Island. The Pacific Coast population breeds between s. British 

Columbia and Sinaloa, Mexico; most breeding is coastal, though some occurs inland. The 

Canadian and U.S. interior population breeds from n. Alberta through central Ontario; James Bay 

and sw. Québec, south to central Utah, central Colorado, w. central Nebraska, se. South Dakota, 

w. central Minnesota, ne. Iowa, central Wisconsin, and n. Lower Peninsula of Michigan; range 

extends west to sw. Idaho and east along GL to lower St. Lawrence River. Also breeds locally in 

other areas within the interior (see Hatch and Weseloh 1999). On the Atlantic Coast, breeds from 

Newfoundland to New York, and small numbers elsewhere (see Hatch and Weseloh 1999). Most 

breeding is coastal. In the southeast, resident in Florida, and occurring locally in small numbers 

along Gulf Coast to Texas. Scattered residents in Cuba, and uncommon resident in Bahamas. 

Also nests on coast of Yucatan Peninsula and n. Belize. Recent expansion has led to blurred 

boundaries for Zones 3, 4 and 5. For further discussion and delineation of the five breeding 

zones, see Part II, Biologically Relevant Units and Organization of Data.

Winter Range.

On the Pacific Coast and Alaska, chiefly resident, though some dispersal occurs. Birds 

breeding in the interior and on the Atlantic Coast are strongly migratory, and most winter 

coastally from North Carolina to the western Gulf of Mexico. Also winters along the Gulf of 

Mexico from Tamaulipas south to the Yucatan Peninsula and Belize. Inland, significant numbers 

occur at lakes, rivers and impoundments; regularly winters inland from Atlantic Coast along 

major rivers to se. Pennsylvania, central Maryland, and throughout lower coastal plains of 
FINAL DRAFT Part I. Natural History 5

Virginia and the Carolinas. Also winters inland from Gulf Coast (especially along the 

Mississippi and other rivers) north to central Georgia, n. Tennessee, sw. Indiana, s. Illinois, n. 

Arkansas, e. Kansas, eastern half of Oklahoma, Texas (except for Panhandle). Small numbers are 

increasingly observed in coastal New England, the Maritimes, the Great Lakes, and the prairies. 

Other populations breeding in Florida, Mexico, Cuba, and the Bahamas are resident.

Habitat

Breeding Season Habitat Requirements.

Colonies require sites safe from ground predators and close to feeding areas (usually < 10 

km). Ponds, lakes, slow-moving rivers, lagoons, estuaries and open coastlines are utilized. Where 

available, selects small rocky or sandy islands. May also use artificial sites such as bridges, 

wrecks, abandoned docks or purpose-built towers. Though nests on ground or in trees, ground 

nesting may be the ancestral and preferred habit; tree nesting may be response to predators. Trees 

used for nesting are usually standing in or near water, on islands, in swamps, or at tree-lined 

lakes. Where predators are present, depends on flooded snags or live riparian trees. Also nests on 

emergent vegetation in marshes. In all seasons requires suitable places for nighttime roosts and 

daytime resting or loafing. Roosts and resting places are often on exposed sites such as rocks or 

sandbars, pilings, wrecks, high-tension wires or trees near favored fishing sites.

Winter Habitat Requirements.

Largest numbers of wintering birds occur along the southern coasts, and require similar 

characteristics in feeding, loafing and roosting sites as when breeding. Cormorants wintering 

along the North Carolina coast roost in or near the inlets on sandbars in dense flocks of up to 

5,000 – 10,000 birds, or more, so that the sandbars are often blackened. They roost so close they 

are nearly touching each other. Sandbars or shoals chosen are high enough that they are not 

underwater at high tides. During the day, birds feed in large flocks, often numbering several 

thousands, in the inlet, sounds, and inshore ocean. Very little roosting is in trees, but some roost 

on channel markers, pilings, etc. Birds also rest on sandbars and markers during the day. They 

are often seen flying to sandbars, and then flying out to sea in the afternoon; at dusk they return 

to the sandbars (H. LeGrand, pers. comm.). Around aquaculture sites winter numbers are 

increasing, and in Mississippi many that winter near catfish farms roost in isolated cypress 

swamps. There may be differential selection between sexes in winter habitat use; in Mississippi, 

males predominated in samples shot at inland roosts (Glahn et al. 1995), and in Texas females 

predominated in a sample collected in Houston Ship Channel (King et al. 1987).

BIOLOGY

Migration

Atlantic Coast and interior nesting birds are highly migratory; all age groups migrate. In 

other areas, the species is mostly resident within the breeding range and migratory habits are 

poorly understood. More northerly populations breeding in Alaska may migrate south to s. 

British Columbia and Washington.

Migrants wintering in the Gulf of Mexico begin heading northward as early as February. 

Along the eastern seaboard they follow the Atlantic coastline; in the interior they follow river 

systems and fly overland. Earliest individuals reach Oklahoma and Virginia by 4 – 5 March; 
FINAL DRAFT Part I. Natural History 6

Massachusetts, the southern Great Lakes, Minnesota, S. Dakota, and s. Idaho by late Mar – early 

April; upper Great Lakes by early to mid April; more northerly destinations, e.g., the prairie 

provinces, coastal Maine, are not reached until mid-late April. In British Columbia, migration 

occurs mid-Mar to late May, peaking late April – early May. Analysis of banding recoveries 

indicates that first-year birds return north about a month later than older birds. Recently, some 

birds have been remaining year-round and breeding along the Gulf Coast.

Autumn migration is essentially the reverse of spring. Earliest autumn migrants appear to 

reach wintering grounds faster than they reached breeding grounds in spring. Migration 

underway on the Canadian prairies by mid-late August, at which time first migrants are arriving 

on the Upper Mississippi River, Ohio, Massachusetts, Virginia and Texas. By September, 

migration well underway throughout range. Massachusetts, Rhode Island and Minnesota report 

peak numbers of transients in October. Most banded migrants recovered in Oct and Nov. Along 

coastal New England, bulk of migration observed 25 September – 17 October. Individuals arrive 

on winter grounds September to November. Some fly across open ocean.

Inland, migrates in small flocks, < 50 – 100; along coasts, near shore flocks may consist 

of 1000s of birds. Migratory flight begins soon after dawn and continues all day, with flocks 

observed flying late in evening.

Reproduction

Usually forms dense breeding colonies (100s – 1000s); solitary nests very rare. Often 

nests in mixed-species colonies. DCCOs are believed to have extreme fidelity to colony sites. 

Many young first breed at natal colonies, and philopatry to proximity of natal colony is probable; 

in banded birds recovered in June that were at least 3 years old, the median distance to natal 

colony was only 25 km.

In British Columbia, most DCCOs began breeding as 3 year olds (in their 4th summer). 

However, a small portion of color banded birds first bred as 1 year olds (4.7 %) and 2 year olds 

(16.5 %). Immatures (1 and 2 yr olds) are present on breeding colonies by June.

Ground nesting cormorants typically nest on low-lying rock islands or reefs, away from 

or with sparse vegetation. Arboreal nesters use evergreen and deciduous trees, initially alive but 

killed within 3 – 10 years due to guano deposition over time. Nesting substrates include 

limestone and granite bedrock, large boulders or fallen trees, gravel, cobbles, beach ridges, soil 

or standing trees. Possibly usurps attended or unoccupied nests of some herons and egrets. Nest 

characteristically includes finger-sized sticks and other bulky items collected from diverse 

locations, some several kilometers away. Along seacoasts seaweed forms the usual nest-base; 

flotsam and jetsam often incorporated, as are parts of dead birds. Lining is usually of grass, 

rootlets and similar materials. As breeding season progresses, the nest receives pebbles and 

bones from pellets, and a coating of guano on the outside that seals the nest together. Many nests 

are used repeatedly and built upon each year, becoming tall turret-like structures; some reach 

heights of 2 m or higher. Tree nests are usually much smaller but have a deeper inner cavity than 

ground nests. Ground nesters defend small area around nest within beak range (0.38 m). 

Reported inter-nest distances vary widely, from 3.3 – 220 cm.

Double-crested cormorants lay between 1 – 7 eggs. Average clutch size ranges from 2.7 –

4.1; modal clutch size is four. First eggs are laid 2 – 4 weeks after arrival to breeding colonies. In 

Ontario, egg laying occurs late Apr to Aug; in the St. Lawrence River estuary from about 10 

May to Jul. Eggs are cylindrical ovate in shape. Shells are pale blue and unmarked, but 

pigmented layer often obscured by variable outer calcite cover that is initially white and porous. 
FINAL DRAFT Part I. Natural History 7

Calcite layer has chalky texture and gives irregular surface. Egg mass is small compared to other 

seabirds; reported averages range from 44.9 – 46.5 g, 2.7 % of adult mass. Eggs are laid daily or 

on alternate days. Following loss of complete clutch, relaying is frequent, occurring within 19 

days. However, successful second broods are rare. Eggs are extremely cold tolerant, but 

vulnerable to heat stress.

Incubation begins gradually, reaching a maximum intensity when third egg is laid. Eggs 

are incubated by lying on top of warm webs of feet; abdomen and breast are lowered onto them. 

Duration of incubation varies from 25 – 28 days. Both sexes incubate, but the female does more 

during the first half of the incubation period. Changeovers occur at intervals of 1 – 3 hours. 

Hatching occurs asynchronously, but intervals between eggs typically less than 1 day.

Chicks are altricial, and barely able to move at hatching. Eyes open at 3 – 4 days; egg 

tooth drops off between 4 – 7 days. Down appears within about one week. Thermoregulatory 

ability not complete until 2 weeks. Young are cared for equally by both parents. Brooding is 

nearly continuous for the first 12 days, but ceases with the appearance of down and effective 

endothermy. Young will remain in ground nests for about 28 days if undisturbed (though have 

walked from ground nests as early as 21 days when approached by humans or predators). Young 

in tree nests or in nests on cliffs may remain there until they are able to fly, which occurs 

between 6 – 8 weeks of age. Diving ability develops and plumage is complete at about the same 

time. Young are completely independent of parents at 10 weeks of age.

Annual reproductive success varies within and among colonies, but hatching success 

usually 50 – 75 %; fledging success 1.2 – 2.4 young / nest or 74 – 95 %. Chick loss from 

hatching to fledging is often low, e.g. 5 % in coastal B.C. All figures much lower for DDEcontaminated populations. In St. Lawrence estuary, reproductive success parameters lower for 

late nesting cormorants (June) compared to early nesters (May). Average lifetime production 

(lifetime reproductive success) calculated for birds breeding on Mandarte I., B.C. was 3.28 

young (van der Veen 1973).

Foraging Ecology

The DCCO is an opportunistic, generalist feeder, preying mainly upon abundant, easy-tocatch fish species. Usually slow-moving or schooling fish, ranging in size from 3 – 40 cm but 

commonly < 15 cm, are taken. The DCCO appears to be strictly diurnal in its feeding habits, and

usually forages in shallow water (< 8 m) within 5 km of the shore. The prey of Atlantic birds 

suggests that they are more likely to feed at the bottom of the water column, while that of Pacific 

and inland birds suggests that they feed in mid-water. Cormorants respond rapidly to high 

concentrations of fish and often congregate where fish are easily caught, such as put and take 

lakes, stocking release sites, aquaculture ponds, dams, and other areas where fish are 

concentrated.

To capture food, cormorants dive from the surface and pursue prey underwater. Prey is 

grasped in the bill and may be swallowed underwater. If the prey is large or difficult to handle, 

such as eels, flounders or spiny fish, it may be swallowed at the surface after first being shaken 

and hammered on the water. Prey is sometimes thrown in the air, caught and swallowed headfirst. Frequently forages individually, but readily gathers to form feeding flocks (tens to 

hundreds). When feeding on schooling prey, sometimes loosely-coordinated foraging flocks are 

formed. This behavior has been observed most often in the breeding season and in late summer / 

early fall. Bottom-feeding is usually solitary.
FINAL DRAFT Part I. Natural History 8

Diet

Information on the diverse diet of this species has been recorded from as early as 1835, 

when Audubon (1835) observed that the food of Double-crested Cormorants consisted of 

“shrimps, lents, capelings, codlings, and other fishes”. In general, primary prey are forage 

species of little or no commercial value, but cormorants will take advantage of abundant species 

in the right size range. Over 250 fish species from more than 60 families have been reported as 

prey items. Occasionally, other aquatic animals, such as insects, crustaceans and amphibians are 

also taken. Diet is discussed specifically by region in PART III: DIET.

Longevity, Survival and Mortality

Oldest banded bird reported was 17 years, 9 months of age; however, wear and loss of 

aluminum bands is likely to lead to underestimates of survival based on recoveries. For birds 

banded as fledglings on Mandarte I., B.C., first year survival was estimated at 0.48, second-year 

at 0.74, and subsequent annual survival of 0.85; mean adult life expectancy 6.1 year (van der 

Veen 1973). Mortality factors for young birds include disturbance at breeding colonies, which 

can result in large mortality of hatchlings from exposure, and of eggs and young by predation; 

Newcastle disease can also cause significant mortality among young birds. Adult and large chick 

mortality factors include predation by Bald Eagles (Haliaeetus leucocephalus), entanglement in 

fishing gear, and shooting. Rates of mortality due to predation were not available. Fishing gear is 

reported as a major cause of death. Of birds banded in the Great Lakes between 1928 – 1995, 9 

% of those recovered were shot; of band recoveries in Texas, 17 % were shot.
FINAL DRAFT Part II. Population Data and Trends, Introduction 9

PART II. POPULATION DATA AND TRENDS

INTRODUCTION

Most population and trend data were obtained through the survey sent to wildlife 

biologists. We also utilized published literature, breeding bird atlases, unpublished reports and / 

or papers in press regarding trends and / or the history of cormorants in specific states and 

provinces. Survey recipients were asked to provide data on breeding, wintering, and migration 

concentrations. For breeding birds we asked recipients if cormorants were recent (1972 or later) 

breeders in their state / province, and to provide number of pairs, ownership of site (if colony >

300 pairs), and any productivity estimates. For wintering populations, we asked for number of 

birds and site ownership information (if concentrations > 1000 birds). For migration 

concentrations, we asked for peak migration times and numbers and land ownership for 

significant stopover sites (concentrations of > 500 birds). We also telephoned and e-mailed many 

individuals who provided us with data to get more precise information regarding numbers and 

trends in their states and provinces. Individuals completing surveys sometimes provided possible 

reasons for population increases; these were incorporated into the state and provincial profiles.

Breeding Birds

Biologically Relevant Units and Organization of Data

Five main zones for breeding Double-crested Cormorants in North America were defined 

by Hatch and Weseloh (1999) and these largely correspond to subspecies ranges (see Part I: 

Breeding Range.). In an effort to present biologically relevant information on biological units, or 

“populations”, we organized and presented population data based largely on these population 

zones, with a few modifications. In North America, populations of breeding cormorants occupy 

five main areas: Alaska (Zone 1); the Pacific Coast, from British Columbia to Mexico (Zone 2); 

the Canadian and U.S. interior (Zone 3); the Southeastern U.S (Zone 4), and the Atlantic Coast, 

from Newfoundland to New York (Zone 5) (Figure 1). For the most part, these zones are 

geographically or biologically (in terms of subspecies) distinct. Data were organized in this 

manner to aid and encourage individuals to view the biological unit, rather than political or 

organizational boundaries, when developing management plans.

These zones are thought to reflect fairly distinct breeding populations, but recent DCCO 

expansion and re-colonization has blurred the boundaries between these zones (Hatch and 

Weseloh 1999). Border states between interior and southeastern populations, interior and western 

populations, and southeastern and northeast Atlantic populations (e.g., Texas, New Mexico, 

Idaho, inland southern states, the Carolinas), qualify as “gray areas.” In these areas, it is not 

possible to determine precisely which zone breeding cormorants belong to; therefore, it was not 

possible to delineate absolute boundaries for each zone. However, because we were interested in 

presenting as much biologically relevant information as possible, and birds breeding in “gray 

areas” were in relatively small numbers, we determined the most likely population zone for each 

colony documented between 1970-2000 (Figure 1). These determinations were based on the best 

information available on subspecies range (Palmer 1962; Johnsgard 1993; Hatch and Weseloh 

1999), and consideration of logical geographic units.

Additionally, to help with decisions for birds breeding in inland southern states (AR, OK, 

TN), southeastern coastal states (NC, SC), and southwestern states (TX, NM), we also 
FINAL DRAFT Part II. Population Data and Trends, Introduction 10

considered winter distribution, and the fact that birds breeding in many of the southern states are 

likely year-round residents, while birds from neighboring Zones 3 and 5 are migratory. The 

winter range in North America is quite distinct and occupies two main areas: in the eastern and 

central portion of the continent, the species winters mostly from Texas to Florida along the Gulf 

Coast, and along the Atlantic Coast through the Carolinas and Virginia; in western North 

America, the species winters mostly along the Pacific Coast, from southern British Columbia to 

Baja California Sur and the Gulf of California. Large numbers also winter inland in Texas, 

Oklahoma, and Arkansas. The large number of birds wintering in the eastern and central portion 

is a fairly easily defined, succinct, and very important unit in terms of human-cormorant 

conflicts. Therefore, it made the most sense to keep the southern states together and acknowledge 

the possibility that the small number of birds that breed in the boundary states of this zone may 

in fact be part of the interior or northeast Atlantic populations. For birds breeding in Idaho, 

where some have been found to possess characters intermediate between subspecies P.a.

albociliatus and P.a. auritus (Burleigh 1972), suggesting that mixing occurs at the “boundaries” 

of Zones 3 and 5, we decided to rely on Burleigh’s (1972) conclusion that Idaho birds examined 

came closer to P.a. auritus than to P.a. albociliatus; thus we grouped Idaho birds with the 

interior population.

Therefore, when viewing these population zones, we suggest that special attention be 

paid to boundaries, especially “gray areas”. Extent and areas of overlap need to be more closely 

defined, especially if management actions are considered at a regional level or potential impacts 

to a population are under review. More detail is presented on population organization in the 

individual summaries for the zones; the problem is also addressed in PART VII. RESEARCH 

NEEDS.

Colony Locations

Survey recipients were asked to provide latitude-longitude coordinates for each colony 

location known since 1970. If these data were not available, we made follow-up calls and tried to 

determine approximate locations. We were able to obtain relatively good location data for most 

colonies and have included maps showing the distribution of colonies in each population zone 

during the last 30 years (Figure 1). In Zone 1 we show distribution of all known colony sites. In 

Zone 2 we show distribution of all known colony sites with the exception of interior California. 

D Shuford at Point Reyes Bird Observatory will supply a map for this area (D. Shuford, pers. 

comm.). In Zone 3, we were unable to obtain complete data for the following states and 

provinces: for Iowa, we were unable to obtain coordinates for 4 of 10 known colonies; for 

Minnesota, we were unable to obtain coordinates for 7 of 75 known colonies; for Alberta, 

Montana, North Dakota and Colorado we were unable to obtain any precise information. In Zone 

4, we obtained coordinates for all known breeding colonies, with the exception of Florida, where 

coordinates were provided for “most” cormorant colonies in the state. In Zone 5, we obtained 

coordinates for all known colonies except for 5 of 12 on Prince Edward Island, and 2 of 9 in 

Newfoundland.

Maps showing distribution of active colonies by size at time of last survey are included 

for states and provinces for which we have the appropriate data in the five breeding zones. In 

Zone 4, the only state for which we produced this type of map was South Carolina. With the 

exception of Florida, for which the necessary data were not available, most states in this zone 

have marginal breeding numbers with few and relatively small colonies.
FINAL DRAFT Part II. Population Data and Trends, Introduction 11

Trends

To determine trends in breeding numbers, we asked survey recipients to supply data on 

all known breeding colonies over 4 time periods: 1970, 1980, 1990, and 1998. Not surprisingly, 

many states and provinces did not have data for these specific years, so we asked for data from 

censuses conducted around these years. If that was not possible, we asked for whatever data were 

available. If complete counts were made for at least two years we generated population trends 

(Table 1, Table 4,Table 5,Table 6; see also figures in state and province summaries). To estimate 

percent average annual rate of change in number of pairs, we calculated er-1, where 

r = ln (recent count) – ln (earlier count) and e

r 

= λ

years between counts

(Smith 1992). For figures we used all complete data available. For eastern and central North 

America (Zones 3, 4, 5), when no complete counts were available, we examined colonies within 

a state or province that had > 2 years of data and compared the number of colonies that had 

increased at last count with the number that had decreased.

We also provided estimates of size of regional breeding populations for Zones 3 – 5 

(Table 4, Table 5, Table 6), based on latest totals of breeding pairs in each state and province that 

made up the regional population. We caution that estimates for each state and province may 

utilize different survey methodologies, and may not all be obtained in the same year, thus 

regional estimates should be considered rough approximations at best. For Zones 1 and 2, 

comprehensive data were not available for an estimate of the entire region.

Because data were incomplete for many areas and / or time periods we did not attempt to 

calculate a rate of change for the continental or even regional populations. However, Tyson et al. 

(1999) reported that the number of DCCOs nesting in the U.S. and Canada increased about 2.6 % 

annually from 1990 through 1994. This trend was based on data that ranged from recent 

complete counts to conjectures based on old or incomplete information. While the mean percent 

annual change in the number of nesting pairs in those states and provinces that had recent 

complete counts was 16.2 %, the majority of states and provinces did not have recent complete 

counts, and thus Tyson et al. (1999) suggest that these rates of change be used with caution.

Wintering Birds

Locations and Winter Distribution

Survey recipients were asked to provide latitude-longitude coordinates for all known 

wintering locations, including feeding and roosting sites where major concentrations (> 1000 

birds) occurred. However, the majority of U.S. and Mexican states, and Canadian provinces do 

not conduct surveys for winter birds, and could not supply latitude-longitude information for 

DCCO wintering areas. Thus it was not possible to create a precise winter distribution map. We 

suggest that Hatch and Weseloh (1999) be consulted for winter distribution across the continent. 

To provide general information on wintering cormorants, we utilized data obtained through 

annual Christmas Bird Counts. A few states (AL, AR, and MS) provided locations for winter 

roosts (see below). 

Use of Christmas Bird Count Data

The most comprehensive data for wintering DCCOs in the U.S. and Canada were 

obtained from annual Christmas Bird Counts (CBCs). Annual CBCs have been the only method 
FINAL DRAFT Part II. Population Data and Trends, Introduction 12

to consistently monitor winter birds on a broad scale. While CBCs are not undertaken to produce 

precise population estimates, they provide a useful index of population trends and valuable 

information on winter occurrence. For some states, CBC data provided the only information 

available about wintering cormorants. Therefore, for states with major wintering numbers (> 100 

birds overwinter) we obtained CBC data (locations and numbers) collected between 1984 – 1997 

for Zone 4 and 1984 – 1998 for Zones 1, 2 and 5. We also obtained data collected between 

1984-1998 for states in Zone 3 that border Zones 2,4 and 5. CBC data were accessible for 1984-

1998 at the time of our assessment (Sauer et al. 1996). For each state and province, we included 

all CBC sites where > 100 birds were counted at least once during the period under consideration 

(see figures in state summaries). These sites are shown in Figure 2.

Wintering Numbers and Trends

In general, data on numbers of over-wintering birds were limited and it was not possible 

to determine trends. The information we provide on wintering trends is based mainly on 

“qualitative” assessment of apparent changes in numbers counted during CBCs. For some states 

(AL, BC, CA, FL, GA, KS, LA, ME, MA, MI, NJ, NY, NC, OK, OR, TN, TX, VA, WA) Sauer 

et al. (1996) analyzed CBC data (1959 – 1988) for trends, included in this assessment. CBC data 

(1989 – 1999) have not been analyzed, but are included to provide a broad picture of recent 

changes in winter numbers. CBC data should be interpreted cautiously, for a number of reasons 

(e.g., number of counts is not always consistent within states and provinces; observer skill level 

varies).

Limited survey data from Mississippi were included to provide a broader picture of 

recent changes in wintering numbers in this state, mainly in the Mississippi Delta region. 

Additionally, limited survey data from Alabama, Arkansas and Mississippi data were 

incorporated to compare CBC site distribution with known night roosts located by Wildlife 

Services surveys (Figure 51). Additionally, comments on perceived trends from biologists and 

other individuals or sources providing us with information on winter numbers were incorporated. 

At this time, it is not possible to provide an estimate for winter population size because data are 

incomplete. However, this may be an important estimate for management actions geared toward 

reducing impacts on the breeding and wintering grounds (see Non-breeding Birds and Total 

Population Size, below, and PART VII. RESEARCH NEEDS).

Migrant Birds

Migration / Stop-over Sites

Survey recipients were asked to provide latitude-longitude coordinates for all major (>

500 birds) migration stop-over sites. For most states, migrant numbers were very difficult to

obtain because often no information was available, or migrants could not be distinguished from 

wintering birds. Some states provided general information about where concentrations of migrant 

birds were regularly seen, but because data were so limited we did not map them.

Trends

We provide limited information for a few states and provinces on trends in numbers of 

migrants in the specific state/provincial accounts.
FINAL DRAFT Part II. Population Data and Trends, Introduction 13

Non-breeding Birds and Total Population Size

We did not obtain information on non-breeders, which include sub-adult and adult nonbreeding birds. Currently there is very little information about this segment of the population, 

and without this information, it is not possible to estimate the continental population size with 

any degree of precision. Life-tables have not been established for any of the breeding 

populations, age of first breeding can vary, and year-to-year differences in non-breeding by 

adults can be significant (Hatch 1995; Hatch and Weseloh 1999). At various times, different 

investigators (e.g. Lewis 1929; McLeod and Bondar 1953; Price and Weseloh 1986; Watson et 

al. 1991) have estimated that a range of 1.0 – 4.0 non-breeding cormorants per breeding pair 

determines the size of the non-breeding segment of North American cormorant populations. In 

recent estimates of population size, these estimates have been applied to breeding numbers. 

Hatch (1995) estimated the continental breeding population at > 360,000 pairs from data 

obtained mainly in the 1980s and early 1990s; applying the range of estimates for non-breeders, 

he estimated a continental population size of 1 – 2 million birds. Tyson et al. (1999) estimated a 

minimum of 372,410 breeding pairs in the U.S. and Canada, based on data collected between 

1975 – 1997 (most data were collected 1994 – 1997). Applying the range of estimates above for 

non-breeders, Tyson and colleagues conservatively estimated that > 1 million individual DCCOs 

currently occur in the U.S. and Canada. Hatch (1995) noted that totals are imprecise because 

large regions have been poorly studied and because some of the largest populations are the least 

well known (e.g., Manitoba, Mexico). Additionally, review of the estimates by Lewis (1929), 

McLeod and Bondar (1953) and Price and Weseloh (1986) indicated estimates for non-breeders 

were not based on detailed scientific study and rigorous data. Before estimates for this segment 

can be made with any confidence, careful study and observations at specific colonies (e.g. older, 

stable colonies vs. newer, rapidly expanding ones), as well as at locations without colonies where 

non-breeders may aggregate, need to be undertaken (D.V. Weseloh, pers. comm.).

In addition to providing important information for total population size, an estimate of 

non-breeders is also important for predicting population growth, and effectiveness and impacts 

of potential management strategies. For example, at the large Oostvaardersplassen Great 

Cormorant (GRCO) colony in The Netherlands, Van Eerden and Van Rijn (1997) estimated that 

at least 40 % of birds present did not breed. Based on this estimate, they concluded that any 

intervention in colonies aimed at reducing number of breeders is likely to allow non-breeders to 

take over the empty territories, or disperse and colonize new territories.

For predicting appropriateness of management strategy, an estimate equally important to 

total population size (e.g., regional, continental) is the proportion of birds utilizing various 

habitats and resources. For example, Van Eerden and Van Rijn (1997) reported that fish farms 

provide food for only about 0.5 – 1 % of the total population of European GRCOs in winter, and 

thus managing on the local level was considered more appropriate. We do not have good 

estimates for the proportion of the DCCO population utilizing aquaculture farms. For example, 

Nisbet (1995) estimated that the number of DCCOs wintering in the Mississippi Valley is 

probably equivalent to no more than five percent of the total interior population. However, this 

estimate represents the number present at a given point in time rather than the total number of 

birds moving through the region, which may be a larger proportion of the total population. 

Dolbeer (1991) estimated from banding records that 120,000 birds might move through this 

region during winter months. J. Glahn (pers. comm.) suggested that the number currently passing 

though has probably more than doubled since Dolbeer’s (1991) estimate. There are two levels of 

uncertainty surrounding these estimates. First, we cannot determine the proportion of the 
FINAL DRAFT Part II. Population Data and Trends, Introduction 14

population the migrants represent because total population size is unknown. Second, because it is 

not clear how many of these birds are overwintering and how many are simply passing through, 

it is difficult to determine level of impact this portion of the population has on aquaculture. 

Therefore, better estimates on the proportion of the population utilizing fish ponds and the 

variation of residence time among birds present in the Mississippi Delta region are important for 

decisions about whether to manage cormorants on a population or local level (see PART V. 

MANAGEMENT OPTIONS).
FINAL DRAFT Part II. Population Data and Trends, Introduction 15

Figure 1. Distribution of Double-crested Cormorant (Phalacrocorax auritus) breeding 

colonies in North America 1970 – 2000.
FINAL DRAFT Part II. Population Data and Trends, Introduction 16

Figure 2. DCCO Christmas Bird Count sites (1984 – 1998) where 3 100 birds were counted 

at least once.
FINAL DRAFT Part II. Population Data and Trends, Zone 1 17

ZONE 1: SUMMARY OF POPULATION DATA FOR ALASKA

Introduction

In Zone 1, the breeding population occurs entirely in Alaska, at Nunivak Island and the 

southeast Bering Sea, and from the Aleutian Islands to the southeast coast, including Kodiak 

Island. Inland it occurs to Lake Louise; the first nest in the Yukon Territory was confirmed at 

Lake Laberge in 1998 (Hatch and Weseloh 1999). The westernmost limit occurs in the eastern 

Aleutian Islands at Chuginadak Island (52°51’01” N, 169°49’41” W) (Carter et al. 1995). 

Because DCCOs breeding in Alaska represent the entire P.a. cincinatus subspecies (Carter et al. 

1995), the largest of the five subspecies (Hatch and Weseloh 1999), the summary of this 

population zone is fully treated in the Alaska state summary, below.

Alaska

Summary of Population Data and trends

(Surveys completed by Don Dragoo, Alaska Maritime NWR, USFWS, Homer; and Shawn 

Stephenson, Migratory Bird Management, USFWS, Anchorage.)

Breeding.

While we did not determine when DCCOs were first documented as a nesting species in 

Alaska, it is clear that the species has been a long time breeder in the state (see Historical 

Information, below). Between 1970 and 2000, the species was confirmed breeding at 126 

colonies. Most breeding occurs along the southern coast and on the Aleutian Islands (Figure 4). 

Because Alaska has not conducted statewide censuses of all colonies in the same year, the 

number of breeding birds is not known. Additionally, most colonies have only been censused 

once or not since the 1970s (Carter et al. 1992); therefore population trends are not available. 

However, of 126 known colonies, 106 were counted at least once between 1970 and 2000; the 

vast majority (93 %) were small, with < 100 pairs on average. Carter et al. (1995) estimated a 

total of 2,811 pairs bred at 90 coastal colonies between 1970 and 1992, at which time comprised 

12 % of the Pacific Coast population, defined by Carter et al. (1995) to encompass P.a. 

cincinatus in Alaska, and P.a. albociliatus, breeding from southern British Columbia to Sinaloa, 

Mexico. In the interior, colonies have not been adequately surveyed.

Numbers are thought to have declined since historical times, especially after introduction 

of predators (see Historical Information, below). At present, oil spills threaten cormorants in 

several areas, and human disturbance at breeding colonies remains a problem (Carter et al. 

1995). On coastal islands, introduced predators [e.g., red and arctic fox, Norway rat, ground 

squirrel (Spermophilus undulates), rabbit are being removed], which may enable former breeding 

sites to be re-colonized by cormorants (Bailey 1993; Carter et al. 1995).

Winter.

In winter DCCOs occur, but no information was provided on wintering locations and 

winter censuses are not conducted. Gabrielson and Lincoln (1959) reported the species was 

resident in its breeding range, except possibly in the more northern areas, and that it was a fairly 

common winter bird in southeastern Alaska. During CBCs conducted between 1984 – 1998, only 

3 sites had > 100 DCCOs during at least one count, and fairly small numbers were observed 
FINAL DRAFT Part II. Population Data and Trends, Zone 1 18

overall (Figure 3). The CBC data have not been analyzed by the Patuxent Wildlife Research 

Center for trends during this period, and no trends are readily apparent from Figure 3.

Migration.

No information was available on migrants or migration in Alaska.

Historical Information

The DCCO is a long time resident and breeding species in Alaska. Remains of DCCOs 

found in middens on Amchitka Island date back 2,650 years (Siegel-Causey et al. 1991). Midden 

remains indicate DCCOs were formerly more abundant in the central Aleutian Islands, and 

climate changes are thought to have probably reduced numbers in Alaska over time (Carter et al. 

1995). While it has been suggested that the introduction of arctic foxes onto nesting islands 

beginning in 1750 (Bailey 1993) probably greatly reduced breeding numbers (Carter et al. 1995), 

it is not entirely clear how severely these introductions impacted DCCOs. In the late 1800s, 

DCCOs were reported as abundant residents and breeders in the Near Islands in the western 

Aleutians (Turner 1885), where foxes at this time were already present; thus whatever damage 

foxes had done would already have been evident (V. Byrd, pers. comm.).

Land Ownership

No information provided.

Productivity

In 1998, productivity was monitored at Aiktak Island, and 1.8 chicks / nest fledged (Byrd 

et al. 1999). On Duck Island, productivity was monitored in both 1998 and 1999, and was 0.0 

and 0.13, respectively (Byrd et al. 1999; D. Dragoo, pers. comm.).

Figure 3. December estimates of DCCOs in Alaska based on CBCs, 1984 – 1998.

0

50

100

150

200

250

300

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals
FINAL DRAFT Part II. Population Data and Trends, Zone 1 19

Figure 4. Distribution and size of active colonies in Alaska at time of last surveys (1970 –

1999).

FINAL DRAFT Part II. Population Data and Trends, Zone 2 21

ZONE 2: SUMMARY OF POPULATION DATA FOR THE PACIFIC COAST

Introduction

Breeding Information

In this zone, the breeding population occurs mostly along the coast from southern British 

Columbia south to at least Bird Island, Sinaloa, Mexico; some birds may nest farther south 

(Carter et al. 1995; Hatch and Weseloh 1999). Birds in this zone comprise the P.a. albociliatus 

subspecies (Baird et al. 1884; Palmer 1962). Though most nesting occurs along the coast, 

significant colonies also occur or have occurred inland in Washington, Oregon, California and 

Mexico. Additionally, most birds breeding in Nevada, Arizona and New Mexico are presumed to 

be members of this subspecies. However, Johnsgard (1993) suggested that birds breeding in 

western Nevada at Pyramid Lake would seem to be more geographically affiliated with auritus, 

but we have chosen to group the Nevada birds in population Zone 2 based on geographic 

proximity to California and Arizona birds. In New Mexico, most of the breeding birds are 

probably P.a. albociliatus, but a small number likely represent auritus (S. Williams, pers. 

comm.). At this time we decided to include New Mexico birds in the Pacific Coast population 

because auritus numbers appear to be marginal and data are still being analyzed (see New 

Mexico profile for further detail). Birds breeding in Idaho and Utah have characters intermediate 

between P.a. albociliatus and auritus, but these birds have been grouped in Population Zone 3 

(see summary for Zone 3: Summary of Population Data for Canadian and U.S. Interior 

Population for further information). Table 1 shows the status (Breeding, Wintering, Migrant) of 

the Double-crested Cormorant throughout Zone 2.

No survey for this assessment was completed by several states in this region because 

breeding data for much of the Pacific Coast have been compiled into one database by the 

USFWS, Office of Migratory Birds and Habitat Programs-Pacific Region. We used this 

database to acquire information on cormorant distribution and abundance. 

For many states and provinces in this zone and Alaska, we cite percent of the Pacific 

Coast population that breeding pairs in a particular area comprised, based on data from Carter et 

al. (1995). They define the Pacific Coast population to encompass P.a. cincinatus in Alaska, and 

P.a. albociliatus, breeding from southern British Columbia to Sinaloa, Mexico. Therefore, when 

we give estimates of percent of the Pacific Coast population that pairs from a particular area 

comprised, we refer to the Pacific Coast population as defined by Carter et al. (1995). Though 

their definition of the Pacific Coast breeding population differs from ours, we cite Carter et al.’s 

(1995) percent estimates to provide additional information on the number of birds occurring 

within this region.

Along the coast, from British Columbia to Mexico, Carter et al. (1995) estimated that 

approximately 21,849 pairs nested between 1968-1992. More recent estimates for the entire 

region are not available, but significant changes occurred during the 1990s. Large increases were 

documented along the coast in Oregon at the mouth of the Columbia River, and inland in 

California at the Salton Sea; increases in these two areas account for most of the growth in the 

region. Sharp declines were observed along coastal British Columbia and Washington colonies.

Because data are not available for birds breeding in significant portions of this region 

(e.g., Mexico, some interior areas) and data collection has not been coordinated at a regional 

scale, it is not possible to summarize trends for the population as a whole. Additionally, trends in 

this region are strongly affected by apparent movements of birds during El Nino oceanographic 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 22

conditions, habitat loss at interior colonies, and use of artificial habitat in some areas (Carter et 

al. 1995). With these habitat changes, distributional changes appear to be occurring; large 

numbers of immigrants may account for much of the growth seen in particular areas (see 

California and Mexico profiles). How these distributional changes affect overall population 

growth for Zone 2 is not clear.

Winter and Migrant Birds

Though portions of the Pacific Coast and the interior provide wintering habitat for 

DCCOs breeding in this region, no state- or province-wide counts of wintering DCCOs have 

been conducted by state or provincial agencies monitoring cormorant numbers. Therefore, with 

the exception of British Columbia, which reported that all breeding DCCOs are residential, no 

precise estimates are available for winter numbers in this region. Additionally, while many of the 

birds that breed in this zone are year round residents, some migration does occur, and it is 

difficult to distinguish migrants from residents. Overall estimates of migrants in the region have 

not been obtained.

Analysis of recent (1989 – 1998) CBCs conducted in this region has not been undertaken,

and trends are not readily apparent from data collected during counts conducted between 1984 –

1998 (Sauer et al. 1996) (Figure 5). However, CBCs indicate both coastal and interior areas of 

California provide major winter habitat; numbers from California counts are much larger than 

those reported in any of the other states (but see California profile for limitations of this data). 

CBC data also suggest that coastal areas in British Columbia, Washington and Oregon provide 

winter habitat for significant numbers of birds. In the interior, significant numbers are reported in 

Arizona on larger reservoirs along the Colorado River, especially near Yuma, and in New 

Mexico in the Elephant / Butte Caballo and Carlsbad areas (Figure 2).

Several areas in Mexico also provide significant winter habitat. The DCCO is commonly 

found along both coasts of Baja Sur, and along the Gulf Coast of Tamaulipas and Campeche. It 

is also a fairly common to common resident from the northwest Yucatan to northern Belize 

(Howell and Webb 1995). However, no information is available on winter numbers in Mexico.

Historical Information

Carter et al. (1995) reviewed the historical background of the DCCO on the Pacific 

Coast, and that information is summarized here. As in many parts of its range, the DCCO 

experienced substantial decline and loss of breeding colonies along several portions of the 

Pacific Coast in the 1800s and early 1900s. Human activities resulted in habitat disturbance and 

occupation, and breeding birds were directly persecuted by humans at their nesting colonies, 

where they were shot and their nests were destroyed. In the interior, agricultural and water 

developments replaced much important habitat. The decline in numbers is thought to have been 

more pronounced than the available sparse literature indicates, and probably resulted in the 

complete loss of DCCO colonies in regions first settled by Europeans (e.g., southern British 

Columbia, Puget Sound, Columbia River, San Francisco Bay). While small populations did 

survive, reproductive success (through DDT and other pesticide-caused eggshell thinning, see 

Carter et al. 1995) and population growth was suppressed by environmental contaminants. 

However, after several decades of reduced human persecution and disturbance, legislation 

introduced in the 1970s to ban DDT and regulate pesticide use, and addition of the DCCO to the 

Migratory Bird Treaty Act Protected List (1972), the DCCO began to recover and expand in this 

region. The source for this expansion was probably the remnant breeding groups that had 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 23

persisted in several areas (e.g., Olympic Peninsula, South Farallon Islands, Channel Islands, 

Mexico). Nevertheless, habitat in this region has been extensively changed by humans, and as a 

consequence DCCOs still show local declines in certain areas. Additionally, records from the 

late 1800s and early 1900s indicate that DCCOs existed in much larger numbers than they do 

today in many areas of this region (e.g., see California and Mexico profiles).

Figure 5. December estimates of DCCOs on the Pacific Coast, Zone 2*, based on CBCs, 

1984 – 1998.

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals

* States and provinces included: AZ, BC, CA, NV, NM, OR, WA; annual totals based on sites 

with ≥ 100 individuals.
FINAL DRAFT Part II. Population Data and Trends, Zone 2 24

Table 1. Zone 2, Pacific Coast. Summary of Population data and trends.

(+ = increasing; - = decreasing; 0 = no change).

State/

Province

Status 

(B,M,W,

w)1

Number 

colony 

sites/time 

period

Active 

colonies/last 

year 

surveyed

Number of 

pairs/last 

year 

surveyed

Breeding 

trend

Winter 

number 

(birds)

Migrant 

number 

(birds)

AZ B, M,W 12/1968-99 10/1993-99 NA NA 1000s? NA

BC B, W 18/1070-99 14/1983-91 800/1999 - 1600 -

CA B, M(?), 

W

45 coastal;

<55 interior 

1970-99

39/1989-91 

coastal;

32/1999 

interior

5,092/1989-

91 coastal;

6,900/1999 

interior

+ 1000s, X = 

20,945 

(10,688-

35,657)

NA

MX B, M(?), 

W

27/1968-92 27/1968-92 7,269/1968-

92

NA NA NA

NM1 B, M(?), 

W

2/199622/19962600/1996 -/? 2,000+3 NA

NV B, M, W 5/1986-

2000

5/1986-1999 NA -/? 10-100s? 1000s

OR B, M, W 40/1979-

2000

30/1988-98 6,249/1988-

92

+ 1000s, X = 

1,548 (658-

2,746)

NA

WA B, M(?), 

W

43/1970-99 22/1992 

coastal;

1/1997 

interior

1,618/1992 

coastal; 

652/1997

interior

-/? 1000s, 

X=4228

(2250-

6821) 

NA

1 Lower Rio Grande colonies; excludes birds in Zone 3 (Table 4)

2 Multiple sites at Elephant Butte and Caballo Reservoirs

3 Did not distinguish P.a. albocilatus from auritus.
FINAL DRAFT Part II. Population Data and Trends, Zone 2 25

Figure 6. Distribution of Pacific Coast DCCO breeding population (Zone 2), 1970 – 2000.

*

* Interior California colony 

locations not available
FINAL DRAFT Part II. Population Data and Trends, Zone 2 26

Figure 7. Distribution and size of active colonies on north Pacific Coast at time of last 

surveys (1990s)1

 

1

 87% counted in 1990s with one colony last counted in 1970s.
FINAL DRAFT Part II. Population Data and Trends, Zone 2 27

Figure 8. Distribution and size of active colonies on south Pacific Coast at time of last 

surveys (CA = 1989 – 1998; MX = 1973 – 1992).
FINAL DRAFT Part II. Population Data and Trends, Zone 2 28

Figure 9. Distribution and size of active colonies in the Pacific Coast southwestern states 

(1993 – 2000).
FINAL DRAFT Part II. Population Data and Trends, Zone 2 29

Arizona

Summary of Population Data and Trends

(Survey completed by Troy Corman, Arizona Game and Fish Department.)

Breeding.

We did not determine when the DCCO was first documented as a nesting species in 

Arizona (see Historical Information, below); currently there are 12 known colony sites. Of these, 

10 were active during the Arizona Breeding Bird Atlas Survey, conducted between 1993 – 1999 

(Arizona Breeding Bird Atlas, unpubl. data). The other two colony sites were documented prior 

to the Atlas survey, one at Imperial NWR, where the species was reported breeding between 

1975 – 1984 (Rosenberg et al. 1991), and the other at the San Carlos Reservoir, where breeding 

was documented in 1968 (Monson and Phillips 1981). Because DCCOs are not monitored in 

Arizona and regular surveys are not conducted, information on trends is not available. DCCOs in 

Arizona are presumed to be P.a. albociliatus (Baird et al. 1884; Monson and Phillips 1981).

Winter.

In winter, the DCCO occurs throughout southern and western Arizona on all lakes, rivers 

and canals. However, estimates of the number of overwintering birds are not available. 

Concentrations > 500 are probable only at larger reservoirs along the Colorado River, especially 

near Yuma. During Christmas Bird Counts conducted between 1984 – 1998, only 3 sites had >

100 DCCOs during at least one count (Figure 10 ). The majority of birds were counted at 

Martinez Lake-Yuma, where a high count of 3,092 birds was recorded in 1992. While the CBC 

data have not been analyzed by the Patuxent Wildlife Research Center for this period, Figure 10

suggests that numbers have declined since 1992. In the last 3 counts included here, conducted 

between 1995 – 1998, numbers observed were the lowest recorded during the15 year period 

examined.

Migration.

During spring migration, peak numbers occur late March – April; in fall, peak numbers 

are observed October – November. No information was available on peak numbers, and 

locations of major stopover sites were not provided.

Historical Information

Little historical information was obtained. The species was described as “rather common” 

around Tucson from March 29 to June 20, 1897 (Phillips et al. 1964). Elliot Coues (1866) 

mentions that it occurred on the lower Colorado River. Date of first documented breeding was 

likely recorded between 1910 and 1964. In 1910 the California Museum of Vertebrate Zoology 

conducted an expedition down the Colorado River from April to May, and documented the 

species at various points along the river between the Laguna Dam and Yuma. The Birds of 

Arizona (Phillips et al. 1964), states that the DCCO breeds locally along the Colorado River (T.

Corman, pers. comm.).

Land Ownership

No information provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 2 30

Productivity

No estimates available.

Figure 10. December estimates of DCCOs in Arizona based on CBCs, 1984 – 1998.

0

500

1000

1500

2000

2500

3000

3500

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals

*

* No data available for 1989.

British Columbia

Summary of Population Data and Trends

(Survey completed by M. J.Chutter, B.C. Min. Env., Lands and Parks, Wildlife Branch; I. 

Moul, Foul Bay Ecological Research Ltd., B.C., contributed breeding and trend data.)

Breeding.

The DCCO has been documented as a breeding species in British Columbia since 1927; 

however, archaeological evidence indicates the species occurred and may have been breeding in 

the area much earlier (see Historical Information, below). British Columbia represents the 

northernmost extension of the subspecies P.a. albociliatus, breeding as far north as Mitlenatch 

Island (49°57’ N, 125° W). Between 1970 and 2000, a total of 19 breeding sites was recorded, 18 

along the coast and 1 in the interior. (Figure 7). Along the coast most colonies occur in two main 

regions, the Northern Strait of Georgia and the Gulf Islands; most breeding occurs in the latter 

region. Carter et al. (1995) reported that 1,751 pairs nested at 14 active colonies in southern 

British Columbia between 1983 – 1991, comprising 7 % of the Pacific coast marine population. 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 31

Currently, 602 pairs are estimated to breed along the coast (I. Moul, pers. comm.). In the interior, 

breeding was first documented in the mid-1990s in the central portion of the province at Stum 

Lake, where small numbers (6 – 12 pairs) have been reported (M. Chutter, pers. comm.).

Between 1935 – 1987, numbers of breeding birds increased in the province fairly steadily 

(Carter et al. 1995). However, in the 1990s, many colonies were abandoned and most had 

population declines. In recent years these declines became more dramatic. Between 1987 and 

2000, provincial breeding numbers declined from 1,981 pairs to 602 pairs. In the Gulf Islands,

counts from 13 colonies conducted in 2000 indicate a highly significant drop (70 %) in estimated 

number of nesting pairs since 1987. Numbers at the three largest Gulf Islands colonies (Great 

Chain, Mandarti and Five Finger Islands) have declined fairly steadily since the late 1980s. In 

1987, these three colonies comprised about 82 % of the total pairs estimated in British Columbia, 

and about 94 % of the total pairs breeding in the Gulf Islands. Declines were observed beginning 

in 1990, with the sharpest declines occurring in the late 1990s (Figure 11). The province’s largest 

colony, located at Mandarti Island, had nearly 1500 nests in 1981, but in 2000, only 215 nests 

were counted. Additionally, there appeared to be no successful reproduction at any of these 

colonies in 1998 and 1999 (data not available for 2000).

Because of these declines, the DCCO is designated “at risk” in the province and is on the 

provincial Blue List of species considered to be Vulnerable. Recent re-evaluation of the DCCO 

suggests that it may warrant upgrading to the province’s Red List for consideration of 

Threatened Status (M. Chutter, pers. comm.).

It is not clear if declines in numbers are in response to an imbalance in the ecosystem, 

caused by human activities, or reflect some long term natural cycle. Moul (2000) reported 

repeated observations of Bald Eagles flying into the vicinity of colonies located along the 

southeast coast of Vancouver Island. In response to eagle arrival, the entire colonies would flush 

from nests. Gulls, and occasionally crows, often returned to nest sites quicker than cormorants 

and depredation of unattended cormorant eggs and chicks was observed. [Moul also observed 

similar responses to human caused disturbances, i.e. marine wildlife viewing expeditions getting 

close enough to flush birds (M. Chutter, pers. comm.).] This scenario happened at least once / 

day and sometimes several times per day until late June / early July (Moul 2000). Colony failure 

appeared linked to this phenomenon (Moul 2000). Human activities such as fishing, marine 

wildlife viewing or picnicking in close proximity to nesting cormorants can also contribute to 

colony declines and abandonment (Campbell et al. 1990; Carter et al. 1995; Moul pers. comm.; 

M. Chutter, pers. comm.). Presently, there is concern that the DCCO could soon be extirpated as 

a breeding species (Moul 2000).

Winter.

In winter, DCCOs are widely scattered in protected waters along the outer coast. They are 

most abundant in the vicinity of the Strait of Georgia and Juan de Fuca Strait, where the species 

is resident (Campbell et al. 1990; M. Chutter, pers. comm.). Based on the number of coastal 

breeders in this area, at least 1200 individuals overwintered in 2000. However, this is likely an 

underestimate. Campbell et al. (1990) notes the occurrence of seasonal movements of birds, most 

likely involving Alaskan breeders in spring and fall. Presumably some of these birds overwinter 

in BC, which would add to the province’s overwintering resident population (M. Chutter, pers. 

comm.). There is also no information available on the number of nonbreeding and young birds 

that overwinter.

Campbell et al. (1990) reported wintering numbers counted on CBCs at Vancouver, 

Ladner and Victoria noticeably increased between 1958 and 1984; between 1960 and 1984, the 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 32

average wintering population at these three sites nearly tripled. During counts conducted at these 

three sites between 1984 – 1998, numbers fluctuated; in some years increases occurred but were 

then followed by declines (1987, 1991, 1992, 1996). This appears to be the general pattern for 

the province as a whole during this time period (Figure 12). During earlier counts conducted 

between 1959 – 1988 an increase of 3.6 % per year was observed (Sauer et al. 1996). More 

recent data have not been analyzed by the Patuxent Wildlife Research Center and information on 

trends is not available. CBCs conducted in 1998 estimated 2,390 birds in the province. Based on 

known breeding survey data and the assumption that all breeders are resident, M. Chutter (pers. 

comm.) reported that the winter population probably peaked in the late 1980s at a minimum of 

4,000 – 5,000 birds. The high CBC for the province, 3,726 birds, was recorded in 1993 (about 45 

% of these birds were counted at the three sites mentioned above).

Migration.

Interior breeding birds at Stum Lake are migratory. There are no known important 

migratory stopover sites (M. Chutter, pers. comm.).

Historical Information

Though the first nesting record was not obtained until 1927 at Mandarti Island, 

subsequent observations suggest that the colony at Ballingal Islets was probably established 

earlier than this (see Carter et al. 1995 for records). Furthermore, DCCO bones are abundant at 

archaeological sites throughout the Strait of Georgia and indicate DCCOs occupied this area for 

the past 5,000 years (Hobson and Driver 1989). Thus, fairly recent colonization of the province 

probably represents re-colonization following a previous extirpation (Carter et al. 1995; 

Campbell et al. 1990).

After the initial discovery of breeding, numbers grew slowly, and have varied 

dramatically between colonies. In 1946, only two colonies were known, with a total of 50 pairs. 

By 1960, four colonies were active, with a total of 150 pairs. In 1975, there were six known 

colonies, with a total of 671 pairs (see Campbell et al. 1990 for records). Most rapid growth 

occurred sometime between 1975 – 1988. A high of 2,032 pairs breeding at 15 coastal sites was 

recorded in 1988 (Rodway in press, cited in Campbell et al. 1990). These increases were then 

followed by significant overall declines in the 1990s (see above).

Land Ownership

In British Columbia > 90 % of the land base is owned by the Crown. With the possible 

exception of Bare Point (which could be on land owned by the local pulp mill), all known 

breeding sites are presumed to be on Crown Land (land owned by the province). Several of these 

sites have some form of protected status (M. Chutter, pers. comm.). Marine wintering sites are 

under the jurisdiction of the federal government. Fresh water areas are technically on provincial 

lands, though surrounding lands could be privately owned.

Productivity

Some productivity data, collected in the 1990s (Moul 2000), were available for select 

colonies. These data are summarized below (Table 2).
FINAL DRAFT Part II. Population Data and Trends, Zone 2 33

Table 2. Numbers of chicks fledged at DCCO colonies along the east coast of Vancouver 

Island (from Moul 2000).

Colony Year Active Nests Sample size Successful 

Nests

Chicks 

fledged

1995 432 432 0 0

1998 300 300 0 0

Chain 

Islands

1999 100 100 0 0

1992 280 101 56 127

1994 403 403 2 6

1995 288 288 80 ?

1998 178 178 0 0

Mandarti

1999 43 43 0 0

1991 78 78 38 91

1992 74 74 71 181

Crofton

1999 83 83 58 165

1991 67 54 54 131

1992 30 30 0 0

Hudson 

Rocks

1995 15 015 0 0

Five Finger 1991 118 47 45 115

1992 191 26 24 72

Figure 11. Changes in number of pairs at Great Chain, Five Finger and Mandarti Islands, 

Gulf Islands, B.C., 1987 – 2000.

0

200

400

600

800

1000

1200

1400

1600

1800

1986 1988 1990 1992 1994 1996 1998 2000

Year

Number of Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 2 34

Figure 12. December estimates of DCCOs in British Columbia based on CBCs, 1984 –

1998.

0

500

1000

1500

2000

2500

3000

3500

4000

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals

California

Summary of Population Data and Trends

(No survey completed; breeding data collated by M. Naughton, Migratory Birds and 

Habitat Programs–USFWS, Pacific Region; and D. Shuford, Point Reyes Bird Observatory, CA.)

Breeding.

The DCCO has been a long time resident and breeding species in California (see 

Historical Information, below). Between 1970 and 1999 a total of 96 colonies was documented. 

Because interior colonies were not completely surveyed until 1997 – 1999, the total number of 

birds breeding in the state was not precisely known prior to 1997. Carter et al. (1995) reported 

results of surveys conducted by the USFWS and Humboldt State University between 1989 –

1991 for six breeding regions along the coast, where an estimated 5,092 pairs nested at 39 active 

colonies during this period, and comprised 21 % of the Pacific Coast marine population. Using 

aerial photography, complete surveys of coastal colonies have occurred annually from 1993 –

2000 by USFWS, U.S. Geological Survey, and Humboldt State University but data have not yet 

been obtained from photographs for all areas and years (Carter et al. 1996, 2000, unpubl. data). 

Complete interior surveys were first conducted between 1997 – 1999 by Point Reyes Bird 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 35

Observatory, and a total of about 6,900 pairs was estimated at 32 active colonies (D. Shuford, 

unpubl. data). The most recent data available for all regions are reported in Table 3.

For the coastal and interior regions, Carter et al. (1995) summarized trends up to 1992. In 

northern and central California, increases in coastal colonies were documented between the late 

1960s and early 1970s up to late 1980s and early 1990s. In northern California, a new population 

emerged in the 1960s and has continued to expand. During 1969 – 1974, < 600 breeding birds 

were recorded between the Oregon border and Cape Mendocino; by 1989, numbers had 

increased to 3,252 birds, and several new colonies had formed. Along the south section of the 

northern coast and along parts of the central coast, declines occurred in the mid 1990s (Table 3). 

The colony at the South Farallon Islands is the only well documented breeding colony along the 

central coast. After being sharply reduced for nearly a century, increases were observed between 

the 1970s and 1980s, with colony peaks in 1982 and 1989. Though declines linked to major El 

Nino events occurred in 1983 and 1992, overall numbers of breeding cormorants increased from 

1972 – 1995. However, data collected between 1990 and 2000 indicate this colony has been 

slowly declining over the last decade (Ainley and Boekelheide 1990; Carter et al. 1995, 1996, 

2000; Abraham et al. 2000). In San Francisco Bay, DCCOs were common in the late 1800s and 

early 1900s, but were not recorded nesting in the Bay area until 1978 when they nested at Russ 

Island. By 1990, nine colonies were found on artificial structures or human-altered areas. More 

recent data for this area are available (M. Naughton, pers. comm.) but were not included in this 

assessment. 

In southern California, numbers were very low in the early 1970s due to impacts from 

pollutants, oil spills (especially the 1969 Santa Barbara oil spill), and human disturbance (Gress 

et al. 1973; Gress 1994; Carter et al. 1995). DDT was banned around this time, and DCCOs 

began increasing shortly after 1975. Numbers in this area went from 208 pairs in 1975 – 1977 to 

1,264 pairs in 1991, a growth rate of 12 – 14 %. This growth rate could not be explained by 

productivity alone and probably reflected immigration of birds from Mexico or interior 

California (Carter et al. 1995). Between 1991 – 2000, H. Carter (pers. comm.) reported numbers 

in southern California to have declined. Possible causes likely include: major El Nino events in 

1992 – 1993 and 1997 – 1998; continuing pollutant issues (DDT and PCB); high levels of 

hooking by recreational fishermen in certain years; some deaths in gill nets and oil spills, 

especially the 1990 American Trader oil spill; and perhaps Newcastle's disease (H. Carter, pers. 

comm.).

In interior regions, Carter et al. (1995) reported only 1,403 pairs were known to nest at 12 

active colonies by 1992. DCCOs had been decimated in these areas in the 1800s due to habitat 

destruction, loss of large lakes, and colony disturbances due to water and agricultural 

developments. However, a major increase in nesting occurred in the 1990s. By 1999, DCCOs 

nested at multiple locations. In 1999, about 80 % of all pairs occurred at the Salton Sea, where 

dramatic growth began occurring in the mid – late 1990s. Prior to this time period, DCCOs 

nested at the Salton Sea irregularly or in much smaller numbers. Between 1981 – 1995 at least 

partial counts / surveys were conducted each year. No more than 75 pairs were estimated in any 

count, and in many years no DCCOs were recorded (Salton Sea NWR files). In 1996, explosive 

population growth began coincident with colonization of Mullet Island at the south end of the 

Sea, and the population on Mullet Island reached 5,425 pairs in 1999 (Shuford et al. 2000). 

During this year, PRBO conducted a year-long reconnaissance survey of birds at the Salton Sea. 

As part of this work, four aerial photographic surveys of the nesting colony at Mullet Island (1 

Feb, 19 Feb, 25 Mar, 16 April) were conducted. Cormorants on the sea or roosting along the 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 36

shoreline were counted by airplane, and during the breeding season the counts of cormorants at 

Mullet Island were added to the aerial surveys to provide grand totals of cormorants. These totals 

were 18,504 (early Feb), 11,160 (mid-Apr), 3023 (mid-Aug), and 15,179 (mid-Nov). Seven other 

colonies were active at the Salton Sea (largest 106 pairs) in 1999, but many of these may 

represent birds that failed at first attempts to nest at Mullet Island (Shuford et al. 2000).

The 1999 estimate for the Mullet Island Salton Sea breeding colony alone is essentially 

equivalent to the 1989 – 1991 estimate reported by Carter et al. (1995) for the entire coast. 

Similar to the growth observed in southern California between the mid-1970s to 1991, growth at 

the Salton Sea likely can not be explained by productivity rates alone; most of these birds are 

believed to be immigrants from unknown locations, possibly Mexico (D. Shuford, pers. comm.). 

Correspondingly large increases in winter numbers have also been observed here in the late 

1990s (see below).

Winter.

The DCCO is mostly resident in California, and in winter large numbers occur over much 

of the breeding range. The species is observed throughout the near coastal counties and the 

Central Valley, along the lower Colorado River (Hatch and Weseloh 1999), and is fairly 

common at the Salton Sea year round (Garrett and Dunn 1981). CBCs conducted between 1959 –

1988 suggested an increase of 4.5 % per year (Sauer et al., 1996). In CBCs conducted between

1984 – 1998, significant numbers of DCCOs (> 100 birds in at least one year) were recorded at 

65 sites, and a high of 35,657 birds was counted for the state in 1997. Though numbers appear to 

have increased in the late 1990s (Figure 13), the CBC data may be misleading and require 

cautious interpretation for a number of reasons. The recent data have not been analyzed for 

trends by the Patuxent Wildlife Research Center, and the number of CBCs conducted in 

California has increased during this period. Additionally dramatic increases have been observed 

at the Salton Sea during CBCs conducted in the mid-late 1990s (Figure 14). However, DCCOs 

start nest building at this site in December; therefore, CBCs at this site are tracking some 

breeding numbers. The recent large increases at the Salton Sea may mask any trend for the state 

as a whole (D. Shuford, pers. comm.). Large numbers also winter around Oakland, and an 

unusually high CBC of 6,781 birds was recorded in 1997.

Migration.

No information was obtained.

Historical Information

Breeding records obtained in the late 1800s and early 1900s are available for coastal and 

interior locations (Grinnell and Miller 1944; Carter et al. 1995). However, remains of cormorant 

chicks found in middens suggest breeding occurred prior to the arrival of Spanish explorers in 

the 1770s (Sher 1994 in Carter et al. 1995). The fact that bones were retrieved from middens 

indicates that native harvest of birds was occurring at the colonies. The colony at the South 

Farallon Islands is well documented, and in the mid 1800s was probably one of the largest 

colonies on the coast, as it was reported to have thousands of nesting DCCOs. Numbers here 

declined in the late 1800s due to human disturbance and remained low until the early 1970s, 

when increases began to be recorded, but numbers have not reached their former size (Ainley 

and Lewis 1974; Ainley and Boekelheide 1990; Carter et al. 1995).
FINAL DRAFT Part II. Population Data and Trends, Zone 2 37

Overall increases that began in the 1970s are probably best described as recovery rather 

than expansion or growth, because they occurred as DDT and other pesticides became less 

widely used and reproductive success improved (M. Naughton, pers. comm.).

Land Ownership

Coastal colonies are owned by: National Park Service (Channel Islands and Redwood 

National Parks), U.S. Bureau of Land Management (California Islands Wildlife Sanctuary), 

USFWS (Farallon Islands), Tolowa tribe (Prince Island), and others. Many colonies in the San 

Francisco Bay area occur on artificial structures (e.g., bridges) managed by various state and 

federal agencies.

Productivity

Stenzel et al. (1995) reported fledging rates for the colony at the Richmond-San Rafael 

Bridge in the San Francisco Bay estuary of 0.98, 1.78 and 1.70 chicks per nest between 1988 –

1990, respectively; in the South Farallon Islands colony, 35 km west of the mouth of San 

Francisco Bay, they reported fledging rates of 1.29, 1.13 and 0.76 during the same years. (Also 

see Ayers 1975; Gress et al. 1973, 1995; Lewis and Gress 1988; Ainley and Boekelheide 1990; 

Ingram 1992; Gress 1994; Ingram and Carter 1997; and Martin and Sydeman 1998 for 

productivity at coastal colonies in California.)

Table 3. Summary of most recent census data for California breeding regions.

N = number of colonies for which data were available;

n = number of colonies active

Region 

(N = known number 

of colonies, 1970-99) 1989 1990 1991 1995 1999

Northern Coast –

North Section

 (N = 17)

1,408 pairs1

(N = 15;

 n = 14) 

Northern Coast –

South Section

(N = 4)

218 pairs1

(N = 4;

 n=2)

75 pairs2

(N = 3

 n >3)

Central Coast –

Outer coast North

(N = 3)

570 pairs1

(N = 3;

 n = 1)

413 pairs2

(N = 3;

 n = 1)

Central Coast –

San Francisco Bay

(N = 9)

1,429 pairs1

(N = 9;

 n = 9)

Central Coast –

Outer Coast South

(N = 6)

198 pairs1

(N = 6;

n = 6)

Southern Coast

(N = 6)

1,264 pairs1

(N = 6;

n = 6)

Interior

(N < 55)

6,900 pairs3

(N = 37;

n = 32)

1 = Carter et al. 1995
FINAL DRAFT Part II. Population Data and Trends, Zone 2 38

2 = Carter et al. 1996, 2000

3 = Point Reyes Bird Observatory, unpublished data.

Figure 13. December estimates of DCCOs in California based on CBCs, 1984 – 1998.

0

5000

10000

15000

20000

25000

30000

35000

40000

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals

Figure 14. December estimates of DCCOs at the Salton Sea based on CBCs, 1984 – 1999.

0

2000

4000

6000

8000

10000

12000

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

Year

Individuals
FINAL DRAFT Part II. Population Data and Trends, Zone 2 39

Mexico

Summary of Population Data and Trends

(No survey completed for Mexico; population data from Carter et al. 1995)

Breeding.

The DCCO has been documented as a nesting species in Mexico since at least the 

beginning of the 20th century (see Historical Information, below). The southern limit of the 

known breeding range for P.a. albociliatus is at Bird Island, Sinaloa, but some birds may nest 

farther south (Carter et al. 1995). This subspecies is currently known to nest in Baja California 

(Norte and Sur), Sonora and Sinaloa, but no data were available post 1992. Birds nesting farther 

inland in adjoining Durango probably belong to populations in eastern North America (Carter et 

al. 1995). The following data were summarized by Carter et al. (1995).

In Baja California, 3,394 pairs bred at 20 colonies between 1968 – 1992, comprising 14 

% of the Pacific Coast marine population. Most of these birds nested on the west coast of the 

Baja California peninsula. Some areas in this region have not yet been completely surveyed, 

especially Piedra near Guerro Negro and the Islas San Benito. Formerly, the largest known 

DCCO colony ever documented occurred on this peninsula, located at Isla San Martin, but was 

abandoned in the late 1970s (see Historical Information, below). Thus, this colony was not 

included in Carter et al.’s (1995) Pacific Coast total. The large colony at Isla Santa Margarita 

(Las Tijeras Mangrove) was one of the largest (currently active) on the Pacific Coast at the time 

of last count (1992).

In Sonora and Sinaloa, 3,575 pairs bred at seven known colonies from 1973 – 1991, and 

comprised 15 % of the Pacific Coast population. Large colonies at Isla Alcatraz, Sonora, and 

North Altamura, Sinaloa, were among the four largest currently active colonies on the west coast 

based on their totals (1500 pairs each) when they were last surveyed in 1975. At least 300 pairs 

were estimated to nest inland in Sonora at Rodriguez and El Molinito Reservoirs. Russell and 

Monson (1998) reported that the principal nesting in inland Sonora occurs along the Rio Yaaqui, 

wherever there are dead trees or brush to support nests.

Precise information is not available on population trends. However, in 1980, breeding 

numbers in the Gulf of California were roughly estimated at 10,000 pairs (Anderson 1983). The 

more recent estimates by Carter et al. (1995) are in that order of magnitude, but of more detail. 

Exact DCCO status in this region is basically unknown, but probably stable. Factors that might 

threaten these birds include: aquaculture operations which are increasing in DCCO habitat along 

the west coast of Mexico (threaten because cormorants would be expected to be controlled as 

they depredate aquaculture; the opposite might just as well be true, that DCCO are increasing 

due to this); the other effect of aquaculture is habitat destruction, where nesting sites have been 

disturbed or destroyed by increasing human activities in the mangrove and other swamps of 

western Mexico. It is also possible that DCCO have declined on some of the islands in the 

northern Gulf by disturbances and possibly even Newcastle disease, where it is likely in DCCOs 

in the northern Gulf (D.Anderson, pers. comm.).

Winter.

Howell and Webb (1995) reported that the DCCO is a fairly common year round resident 

along both coasts of Baja Sur. It occurs locally on the coast of northern Sonora, and is rare in 

winter south to Nayarit. In Durango it is an uncommon and local resident. From northwest 

Yucatan to northern Belize it is a fairly common resident. Along the Gulf Coast of Tamaulipas 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 40

and Campeche, it is a fairly common winter visitor, and is uncommon to rare to Veracruz and 

Tabasco. Occurs locally inland in northern Mexico to Sonora, Nuevo Leon and San Luis Potosi.

No CBCs are conducted in Mexico.

Migration.

No information obtained.

Historical Information

Early in the 20th century, colonies were reported on just about every island on the west 

coast of Baja California (Grinnell 1928). The earliest nesting record we obtained is from Wright 

(1913) who estimated nearly 350,000 DCCO nests at San Martin Island in Baja California, the 

largest colony ever reported in North America. Wright (1913) reports seeing

“a steady stream of cormorants, flying about eight or ten abreast. This stream poured from these 

hills continuously and reached as far as we could see, toward the bay of San Quentin. The stream 

was like a great black ribbon that waved in the breeze and reached to the horizon...the birds kept 

coming as though there were no limit to their numbers...the flow of birds was continuous during 

the daylight hours of each day we were there. The flow was unbroken—simply one steady stream 

going, all day, and a steady stream returning.”

While Carter et al. (1995) state that Wright’s estimate was likely an overestimate due to 

incorrect extrapolation, it is clear that this colony, even if it was only 1/10 the size of Wright’s 

estimate, was enormous compared to currently existing DCCO colonies, or other cormorant 

colonies anywhere in the world. In the 1970s the colony still existed, but numbers were greatly 

reduced; counts obtained in 1969 and 1975 estimated 2,500 and 10,000 -12,000 breeding pairs, 

respectively. However, in 1977 there were no nests but many people were observed on the 

island. This historic colony is thought to have disappeared due to human disturbance, the 

introduction of alien predators, and to a smaller degree, contaminants (Carter et al. 1995).

Other early colonies were also reported to be fairly large. At Islas Los Coronados, about 

1,000 pairs bred in the early 1900s, but by the mid 1920s only a few pairs remained due to 

human disturbance from tourism. At last count in 1991, 174 nests were counted at this site.

Overall, continued human disturbance, predations by feral cats, and persistent 

contaminants have resulted in low breeding success, colony abandonments, and shifts in nesting 

locations between years (Carter et al. 1995). While greater effort is needed to monitor Mexico’s 

nesting population, it is clear that it has been much reduced since the early 20th century.

Land Ownership

No information available.

Productivity

No information obtained.
FINAL DRAFT Part II. Population Data and Trends, Zone 2 41

Nevada

Summary of Population Data and Trends

(Survey completed by L. Neel, Nevada Division of Wildlife; numbers for Anaho Island 

colony provided by D. Withers, Stillwater NWR.)

Breeding.

The DCCO has been documented as a breeding species in Nevada since at least 1950 

(Alcorn 1988). Breeding colonies occur in the southern and western portions of the state; since 

1986 a total of five known colonies has been reported. The largest and most persistent of these is 

at Anaho Island NWR in Pyramid Lake; the number of nests reported between 1950 – 1998 

ranged from a low of 200 to a high of 2500. Between 1986 – 1998, the average number of nests 

was 957; however, numbers appear to have declined since 1986 (Figure 15). Because regular 

statewide surveys are not conducted, overall population trends are not available.

Winter.

In winter, the DCCO occurs in southern Nevada, and was recorded as a permanent 

resident on the lakes in the Pahranagat Valley and on the Colorado River in the 1950s (Alcorn 

1988). No information was provided on current numbers or locations. Christmas Bird Counts 

conducted between 1984 – 1998 (Sauer et al. 1996) recorded only one site, Henderson, with 

counts of > 100 birds in at least one year. During this time, numbers fluctuated between 15 – 110 

birds. With the exception of 110 birds counted in 1996, numbers were < 100 birds in all years 

during this period.

Migration.

During migration DCCOs are transient over most of the state (Alcorn 1988). Significant 

concentrations are reported at Walker Lake (500 – 1000 in fall, 100 – 350 in spring) and Pyramid 

Lake (2,300 in fall).

Historical Information

No information obtained.

Land Ownership

Information provided for one colony at Humboldt Wildlife Management Area (WMA), 

which is federal land being leased by the state.

Productivity

No information provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 2 42

Figure 15. Changes in number of breeding pairs at Anaho Island, Pyramid Lake, Nevada, 

1986 – 1998.

0

500

1000

1500

2000

2500

3000

1985 1987 1989 1991 1993 1995 1997 1999

Year

Number of nests

New Mexico

Summary of Population Data and Trends

(Survey completed by S. Williams, New Mexico Game and Fish.)

Breeding.

The DCCO has been documented as a nesting species in New Mexico since 1937, but 

possibly occurred earlier (see Historical Information, below). Both P.a. albociliatus and auritus

probably occur in New Mexico, a fact that may provide important information for delineation of 

breeding populations. In the Lower Rio Grande drainage birds tend to have the white nuptial 

plumes of albociliatus, while birds in the Upper Rio Grande drainage (Abiquiu Reservoir) and 

possibly elsewhere in the north tend to have the black nuptial plumes of auritus (S. Williams, 

pers. comm.). We included breeding birds occurring in the Lower Rio Grande drainage area (at 

least 82 % of breeders), in Zone 2; birds breeding in the Upper Rio Grande and Middle Pecos 

River were included in the summary of Zone 3. Data are still being gathered and analyzed to 

determine more specific information on subspecies delineation (S. Williams, pers. comm.). 

The last statewide estimate for numbers of breeding pairs and colonies was made in 1996. 

At that time 730 pairs were estimated at about five major colony sites. DCCOs breed in three 

regions, all on reservoirs: 1) the Lower Rio Grande Valley, primarily at Elephant Butte and 

Caballo reservoirs and vicinity, about 600 pairs (there are multiple sites in this area, and birds 

shift locations from year to year); 2) the Upper Rio Grande, at Jemez and Abiquiu reservoirs, 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 43

about 100 pairs; and 3) the Middle Pecos Valley at Santa Rosa reservoir, about 30 pairs. In 1992 

the same numbers of birds were estimated at the same colonies.

Precise information on population trends is not available due to scarcity of early data and 

lack of systematic nest searches. However, available data suggest the following trends: from the 

late 1930s to the mid 1970s no real change occurred in the Elephant Butte Reservoir area. 

Establishment of nesting sites in the Caballo Reservoir in the late 1970s combined with generally 

larger numbers noted in the Elephant Butte – Caballo region by the mid-1980s suggests that the 

nesting population may have increased in the lower Rio Grande Valley during that time. Two 

new colonies in the Upper Rio Grande Valley and one new colony in the Middle Pecos River 

Valley in the early to mid-1980s also suggests an increasing trend in New Mexico during that 

period. More recent reports suggest breeding numbers at Elephant / Caballo reservoirs have 

decreased in recent years, as snags fell and were not replaced. In the Upper Rio Grande Valley, 

Jemez Reservoir is scheduled to be drained and abandoned, so this colony may disappear (S. 

Williams, pers. comm.).

Winter.

Significant winter concentrations of DCCOs occur in the Elephant Butte / Caballo 

(1000+ birds) and Brantley – Avalon (1000+ birds) areas (Carlsbad areas). The maximum 

number of birds estimated to winter in the state is 2000+ individuals, about equally distributed in 

the two areas. However, winter numbers vary from year to year, depending on water levels, food 

availability, and other factors (S. Williams, pers. comm.). CBCs conducted between 1984 – 1998 

recorded three sites where > 100 DCCOs were counted in at least one year (the two sites 

mentioned above and at Loving). At the latter site, an unusually high count of 805 birds was 

recorded in 1996; in most years counts at this site total < 10 birds. Large annual variation in 

winter numbers is apparent in the CBCs (Figure 16), and no trend information is available. In 

recent years, numbers in the Elephant / Caballo area may be lower, while in the Brantley –

Avalon area numbers may be higher (S. Williams, pers. comm.). Data from CBCs at these sites 

conducted between 1996 – 1998 support this statement.

Migration.

Information on migrants in New Mexico is very limited. Migrants may occur, but extent 

of occurrence and number of migrants is not well understood (S. Williams, pers. comm.).

Historical Information

The DCCO was first recorded in the state in 1913 when one of three individuals was 

collected in December near present day Truth or Consequences, Sierra County. First breeding 

record was obtained in 1937 at Elephant Butte Reservoir area, when two colonies totaling 250 

“nesting” were reported. (It is not clear if number reported referred to individuals or pairs.) 

However, DCCOs were possibly breeding in the same vicinity in 1932 (S. Williams, pers. 

comm.). From the late 1930s on it was presumed to be resident in this area. Between 1950 –

1963 up to 24 pairs nested at Bosque del Apache NWR in Socorro County, but this area was 

abandoned due to habitat changes. In 1975, 260 nests were estimated at Elephant Butte Marsh, 

and in 1979, 300 nests were estimated at Caballo Reservoir. In 1985 surveys estimated about 450 

nests at these two reservoirs. Colonies in the Upper Rio Grande Drainage were discovered in the 

1980s and have been regular since then. The colony at Santa Rosa Reservoir in the Middle Pecos 

River drainage was discovered in 1985 and has also been regular.
FINAL DRAFT Part II. Population Data and Trends, Zone 2 44

Land Ownership

Reservoirs where breeding colonies occur are assumed to be on public lands, managed by 

the U.S. Bureau of Reclamation and the U.S. Army Corps of Engineers (USACE) (S. Williams., 

pers. comm.).

Productivity

No information provided.

Figure 16. December estimates of DCCOs in New Mexico based on CBCs, 1984 – 1998.

0

200

400

600

800

1000

1200

1400

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals

Oregon

Summary of Population Data and Trends

(Survey completed by R. Lowe, USFWS; most data on breeding birds from R. Lowe, 

USFWS, M. Naughton, USFWS, and Carter et al. (1995); additional data supplied by M. Lawes, 

USFWS, J. Hainline, USFWS, and A. Clark, USFWS.)

Breeding.

The DCCO has been documented as a nesting species in Oregon since at least the late 

1800s (see Historical Information, below). Between 1979 and 2000, approximately 40 known 

colony sites were active. While annual statewide censuses for breeding DCCOs are not regularly 

conducted, Carter et al. (1995) summarized trends for the breeding regions in Oregon through 

1992, and these data are summarized below.

Carter et al. (1995) divided the coast into four regions: the Columbia River estuary, the 

north coast, the central coast, and the southern coast. Between 1988 – 1992 an estimated 6,249 

pairs nested along this coastal region, comprising 26 % of the Pacific Coast marine population. 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 45

The largest numbers occur in the Columbia River estuary, where large increases have occurred in 

the last 20 years. Though DCCOs were not documented nesting in the estuary until 1980, by 

1992, more than half of Oregon’s DCCOs nested here. Up until 1998, the greatest concentrations 

have occurred in two colonies in the estuary: East Sand Island, the largest (currently active) 

colony on the Pacific Coast, and Rice Island. In 1998, however, numbers declined on Rice Island 

and increased on East Sand Island to 6147 breeding pairs (Collis et al. 1999). In 1999, no 

DCCOs nested on Rice Island, and numbers on East Sand increased to an estimated 7,242 pairs 

(D. Roby and K. Collis, pers. comm.). In this year management was initiated to discourage 

Caspian Terns from nesting on Rice Island, and DCCOs from Rice Island are thought to have 

immigrated to East Sand Island, probably as a result of these efforts. In 2000, East Sand Island 

supported an estimated 6390 breeding pairs (D. Roby pers. comm.). Double-crested Cormorant 

numbers in the Estuary declined for unknown reasons (Figure 17), and again no nesting occurred 

on Rice Island (D. Roby, pers. comm.).

Along other regions of the coast increases also occurred. On the north coast, 983 pairs 

were estimated between 1988 – 1992, primarily at four large colonies on islands and mainland 

cliffs. For this region, no data were provided post-1992. On the central coast, 599 pairs were 

estimated at four active colonies on islands and the mainland; increases occurred at three of the 

four colonies. With the exception of a small new colony (4 pairs) found at Yaquina Bay in 2000, 

no data are available for this area post-1992. On the southern coast, 1,357 pairs were estimated at 

11 active colonies; increases occurred at the majority of these colonies. With the exception of 

two of these colonies, plus a new colony found at the McCullough Tower, no data are available 

for this area post-1992..

In the interior, the most significant interior colonies are at Malheur and Upper Klamath 

Lakes; between 1986 – 1999 numbers at both of these colonies underwent significant declines 

(Figure 18). In 1986, the combined total of breeding pairs at these lakes was 1,882, while in 1999

the combined total was 759 pairs. The interior lake region of eastern Oregon is a very dynamaic 

ecosystem and the availability of nesting habitat near water fluctuates in response to wet and dry 

climate cycles. For example, Malheur Lake increased from 40,000 acres to > 175,000 acres 

between 1982 and 1986. Drought conditions since the late 1980s caused the lake to decrease in 

size to about 300 acres in 1992 (Carter et al. 1995). DCCO populations respond to these changes 

in habitat and consequently breeding populations fluctuate (M. Naughton, pers. comm.). The 

only other interior colony that appeared to be censused regularly was at Summer Lake, where the 

highest number of pairs estimated during counts between 1988 – 2000 was 60 pairs.

Winter.

In winter DCCOs are reported to occur over much of their breeding range, except where 

water freezes over (Gabrielson and Jewett 1940). Along the coast DCCOs commonly occur in 

virtually every estuary, some open ocean locations and most freshwater lakes. Coastal locations 

that may support 500 or more wintering DCCOs include the lower Columbia River, Coos Bay, 

and Siltcoos Lake (R. Lowe, pers. comm.). No information is available on maximum numbers, 

and no current information was provided or obtained on wintering birds in the interior. Other 

coastal estuaries (e.g., Nestucca, Tillamook) could potentially support larger numbers of DCCOs 

and other cormorant species, however, a state supported hazing program operates to harass 

cormorants away from these areas (M. Naughton, pers. comm.). CBCs conducted between 1959 

– 1988 suggest an increase of 1.9 % per year (Sauer et al. 1996). Counts conducted between 

1984 – 1998 recorded DCCOs at 14 sites, with a high of 2,746 birds for the state in 1993 (Figure 

19). During these 15 counts large numbers (> 500 birds) were recorded at Sauvie Island and in 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 46

the Columbia Estuary. These more recent data have not been analyzed for trends by the Patuxent 

Wildlife Research Center, and no trends are readily apparent from Figure 19. 

Migration.

During migration, flocks moving along the Oregon coast are routinely observed in spring 

and fall. However, information on the number of migrants and peak periods is not available. 

While some DCCOs in this area are undoubtedly year-round residents, it is believed that 

significant numbers do migrate (R. Lowe, pers. comm.). Limited data from chicks banded at East 

Sand Island suggest that birds migrate to Puget Sound and also south of their natal colony (A. 

Clark, pers. comm.).

Historical Information

The first definite record of the DCCO in the Columbia River is from Townsend (1839), 

though Lewis and Clark (1814) reported “cormorants” at the mouth of the Columbia River as 

early as 1805. Early breeding records from 1875 – 1888 describe “huge” colonies in Oregon’s 

“lake counties” (Gabrielson and Jewett 1940). On the north coast, Finley (1902, 1905) reported 

DCCOs as the second most abundant cormorant nesting at middle and west rocks of Three Arch 

Rocks around 1900, but nesting in this area has greatly declined (Carter et al. 1995). Declines 

also occurred at Haystack Rock between 1930 – 1957, and remained low until 1988, after which 

time numbers began to increase again. In the Columbia River Estuary the dramatic increases 

since 1980 probably reflected movement from interior nesting areas, and some birds may have 

immigrated from Grays Harbor, Washington. In the interior portion of the state at Malheur Lake, 

breeding numbers averaged 200 pairs in the early 1980s, but increased dramatically to 1120 pairs 

by 1987. This increase was linked to creation of new feeding and nesting habitats from 1982 –

1986. However, severe drought conditions since the late 1980s caused the lake area to drop and 

in 1992, only 50 pairs attempted to nest. Changes in DCCO numbers at this lake are thought to 

be representative of changes at other lake colonies in Oregon, California and Utah, while 

increases on the outer coast of the state may be related to immigration from interior populations 

(both within and outside Oregon). Other factors that contributed to recent increases in DCCO 

numbers may include reduced human persecution and protection of nesting habitat, though some 

fishermen have continued to harass cormorant species in Oregon (Carter et al. 1992).

Land Ownership

Ownership of coastal colonies is primarily public, however, seven colonies are on private 

land. The breakdown of public ownership: USFWS/NWR (19), USACE (1), US Coast Guard 

(1), State of Oregon (4), unknown public (3). The three unknown sites are Miller Sands 

Navigational Aids, Trestle Bay and Desdemona Sands Pilings. East sand Island is owned by 

USACE; Rice Island is owned by the State of Oregon (M. Naughton, pers. comm.). The three 

wintering sites are all managed by the State of Oregon.

Productivity

In 1997, Double-crested Cormorant nests located on channel markers in the vicinity of 

Rice Island (n = 64) fledged an average of 1.55 young per initiated nest. Active nests on East 

Sand Island (n = 50) that were checked late in the chick-rearing period contained an average of 

1.61 young, while the comparable number on the successful channel marker nests near Rice 

Island was 2.11 young (Roby et al. 1998). Cormorant nesting productivity within the estuary in 

1998 was sampled by monitoring nests (n = 70) in each of three different locations: the channel 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 47

markers in the vicinity of Rice Island, on Rice Island and on East Sand Island; nests in each 

location fledged an average of 1.59, 0.55, and 1.23 young per initiated nest, respectively (Collis 

et al. 1999). In 2000, the channel markers in the vicinity of Rice Island experienced a sharp 

decline in productivity (0.43 young per initiated nest; n = 75; Anderson et al., unpubl. data). The 

East Sand Island cormorant colony fledged an average of 1.20 young per initiated nest during 

2000 (n = 40) (C. Anderson pers. comm.). This was similar reproductive success as on East Sand 

Island in 1998, although it was significantly lower than the channel marker nests in 1997 and 

1998 and significantly higher than on Rice Island in 1998.

Figure 17. Changes in number of breeding pairs at East Sand and Rice Islands, Columbia 

River Estuary, Oregon, 1991 – 2000.

0

1000

2000

3000

4000

5000

6000

7000

8000

1990 1992 1994 1996 1998 2000

Year

Number of pairs

Figure 18. Changes in numbers of breeding pairs at Malheur and Upper Klamath Lakes, 

1986 – 1999.

0

200

400

600

800

1000

1200

1984 1986 1988 1990 1992 1994 1996 1998 2000

Year 

Number of pairs

Malheur Lake

Upper Klamath Lake
FINAL DRAFT Part II. Population Data and Trends, Zone 2 48

Figure 19. December estimates of DCCOs in Oregon based on CBCs, 1984 – 1998.

0

500

1000

1500

2000

2500

3000

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals

Washington

Summary of Population Data and Trends

(No survey completed; population data from U. Wilson (USFWS) and summarized from 

Carter et al. 1995.).

Breeding.

The DCCO has been documented as a nesting species in Washington since at least 1907 

(see Historical Information, below). Since 1970, a total of 43 breeding colonies has been 

documented. Nearly all are on offshore islands, and nests are mainly on the ground or on cliffs. 

Carter et al. (1995) reported results of surveys conducted in 1992 for five breeding regions along 

the coast: the San Juan Islands, Juan de Fuca Strait East, Olympic Peninsula Outer coast, Grays 

Harbor Bay, and the Columbia River mouth. An estimated 1,618 pairs nested at 22 colonies, and 

comprised about 7 % of the Pacific Coast marine population. Fairly large colonies (50 – 350 

pairs) were observed in the San Juan Islands, the Juan de Fuca Strait East, on the Olympic 

Peninsula, and in the Grays Harbor Estuary at Goose and Unnamed Sand Islands; about 54 pairs 

were estimated in the Columbia River mouth. The largest numbers were reported for Juan de 

Fuca Strait East (33 %) and the Olympic Peninsula Outer Coast (35 %). Between 1975 – 1992, 

numbers of breeding DCCOs increased on both the inner and outer coastal waters, though 

increases were more pronounced on inner waters. Carter et al. (1995) note that Wilson (1991) 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 49

showed few birds nested on the outer coast during strong El Nino years in the 1980s, and 

numbers there declined during and after El Nino years. Carter et al. (1995) suggest that birds 

may move to inner waters during El Nino years, and back to outer coast waters afterwards. 

Therefore, El Nino conditions that occurred off the Pacific Coast in 1992 may have affected the 

distribution and numbers of breeding DCCOs recorded for different parts of the Washington 

coast (Carter et al. 1995).

Although an increasing trend was observed for the Washington coastal colonies from 

1975 - 1992 (Carter et al. 1995), overall there was no significant trend from 1979 - 2000 (U. 

Wilson, pers. comm.). Along the Washington outer coast of the Olympic Peninsula, DCCO 

breeding populations peaked in 1990 (Figure 20). Between 1990 - 2000 a significant decline of 

about 72% occurred in this area (rs = -0.7 18, n = 11, p < 0.02) (U. Wilson, unpubl. data). More 

inland, in the eastern part of Juan de Fuca Strait and the San Juan Islands, colonies showed no 

trends from 1983 - 2000 (rs = -0.110, n = 18, p > 0.5) (Figure 21, U. Wilson, unpubl. data). 

Along the south central coast, the Grays Harbor colonies have almost disappeared since 1992 

when 440 pairs nested here (Carter et al. 1995). Goose Island, a former breeding site, has 

entirely washed away. Sand Island supports few nesting DCCOs; approximately 10 pairs nested 

in 2000 (D. Roby, pers. comm.). Some birds continue to nest on the pilings and navigational 

aids in Grays Harbor. DCCOs in Washington frequently shift colony locations. The Lower 

Columbia River colonies (East Sand and Rice Island) increased in size significantly during the 

period 1990 - 2000. These colonies are located in Oregon, at the Oregon/Washington border, 

and it is likely that birds moved from Washington to the Lower Columbia River colonies (M. 

Naughton, pers. comm.).

In the interior, numbers were available for only one colony, located at the Northern 

Potholes reservoir. This colony was last counted in 1997, at which time 652 pairs were estimated 

(J. Stofel and J. Tabor, pers. comm.). At this site increases have occurred steadily since 1978 

(Figure 22).

Winter.

In winter, thousands of DCCOs occur on Puget Sound. Two series of surveys in this area 

found a 62% decrease in numbers between 1978/1980 and 1992/1999 (D. Nysewander, pers. 

comm.). The species also occurs east to Seattle, south on Puget Sound to Tacoma and 

Steilacoom, and on the open coast to Cape Disappointment (Jewett et al. 1953). Larrison and 

Sonnenberg (1968) describe it as a fall, winter and spring visitor to Puget Sound and to inland 

lakes (Sept. to late May). CBCs conducted between 1959 – 1988 suggest an increase of 10.4 % 

per year (Sauer et al. 1996). More recent counts, conducted between 1984 – 1998, have not been 

analyzed for trends by the Patuxent Wildlife Research Center, but suggest increases continue to 

occur; a high of 6,821 birds was recorded in 1995 (Figure 23). During these more recent counts, 

DCCOs have been recorded at 21 sites, with high numbers (> 500 birds) counted in Tacoma, 

Seattle, Kitsap County, Padilla Bay, San Juan Islands Archipelago and Bellingham.

Migration.

No information obtained.

Historical Information

Carter et al. (1995) reported Historical Information for DCCOs in Washington, and this 

information is summarized below. The earliest nesting records we obtained were collected 

between 1907 – 1915, when 12 colonies (about 400 pairs) on the outer coast of Washington were 
FINAL DRAFT Part II. Population Data and Trends, Zone 2 50

reported. Nesting was documented at White Rock and Bird Rocks when breeding expanded to 

the San Juan Islands in 1937. Between the 1940s and 1970s, 12 new colonies appeared in this 

area. But beginning in the 1960s and 1970s, many of these colonies were abandoned; by 1980 

only four colonies were still active. Between 1983 – 1986, three disappeared and only Bird 

Rocks remained active. Increasing human disturbance since the 1950s is thought to be the cause 

of these declines. In the Juan de Fuca Strait colonies on Smith and Protection Islands have 

experienced severe human disturbance and nest predation by immature Bald Eagles, which 

resulted in no productivity from 1990 – 1992. Despite these events, numbers increased in these 

areas up to 1992. Between 1997 – 1999, numbers at Smith and Protection Islands underwent 

significant declines (see above). On the outer coast, numbers reported in the late 1970s were 

similar to those reported for the 1907 – 1915 period, though colony location changed.

In eastern Washington, DCCOs have been present since at least 1932 in the south-central 

Columbia Basin (Smith et al. 1997), but we did not determine status at that time.

In 1992, eggshell thinning was detected in eggs at Goose Island at the Grays Harbor 

estuary (Kiff 1994). 

Land Ownership

No information obtained or provided.

Productivity

Henny et al. (1989) made observations on nest success and productivity on Colville 

Island in the San Juan Islands in 1984. They did not follow individual nests, but recorded that 66 

young from 230 nests (0.29 young / occupied nest) were reared to two-thirds of adult size.

Figure 20. Changes in breeding numbers at colonies located along the Washington outer 

coast, 1979-1999.

0

100

200

300

400

500

600

700

800

900

1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999

Number of Breeding Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 2 51

Figure 21. Changes in breeding numbers at colonies located in theWashington inner coastal 

waters, 1983-2000

0

100

200

300

400

500

600

700

1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Number Breeding Pairs

Figure 22. Changes in breeding numbers in interior Washington, N. Potholes Reservoir, 

1978 – 1997.

0

100

200

300

400

500

600

700

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998

Year

Number of Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 2 52

Figure 23. December estimates of DCCOs in Washington based on CBCs, 1984 – 1998.

0

1000

2000

3000

4000

5000

6000

7000

8000

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals
FINAL DRAFT Part II. Population Data and Trends, Zone 3 53

ZONE 3: SUMMARY OF POPULATION DATA FOR CANADIAN AND U.S. INTERIOR 

POPULATION

Introduction

Breeding Information

In Zone 3, the breeding population spans across the Prairie Provinces of Canada (Alberta, 

Saskatchewan and Manitoba), the Canadian and U.S. Great Lakes and southwestern Québec, and 

extends west of Minnesota to sw. Idaho; range extends as far south as central Utah, and central 

Colorado. Also breeds locally in central Kansas and possibly northern New Mexico. Table 4

shows the status (Breeding, Wintering, Migrant) of the Double-crested Cormorant in this zone. 

Some of the birds included in this region, those from Idaho and Utah, show characters 

intermediate between P.a. albociliatus and auritus. In Idaho, 11 specimens were examined, and 

found to be intermediate in their characters between these two races, but closer to auritus

(Burleigh 1972). In Utah, Behle (1941) reached a similar conclusion for the birds of Great Salt 

Lake. Thus we included these birds as part of Zone 3. However, in these areas some mixing 

obviously occurs between races. In New Mexico, the majority of birds breed in the Lower Rio 

Grande drainage and are believed to be P.a. albociliatus, while the small number that breed in 

the Upper Rio Grande drainage and possibly elsewhere in the north are thought to represent P.a. 

auritus. Because the majority of birds breeding in New Mexico are thought to be representative 

of P.a. albociliatus, New Mexico birds were grouped in the Pacific Coast population (see 

summaries for Zone 2: Summary of Population Data for the Pacific Coast and New Mexico).

Over the last two decades the number of breeding Double-crested Cormorants has greatly 

increased in the interior portion of the continent. Counts in the 1990s (mostly 1997) estimated a 

minimum of over 150,000 pairs in the region. This is a substantial underestimate, since numbers 

of breeding birds were not available for several areas, including most of Manitoba, where we 

know large numbers of DCCOs breed. Of 24 states and provinces for which we obtained data on 

breeding cormorants, 10 had increasing cormorant numbers, one had declining numbers, one had 

possibly stable or increasing numbers, and one had possibly stable / declining numbers. Data 

were not available for the other 11 locations to determine trends. Individual summaries for each 

state and province in this zone are included at the end of this summary and are in alphabetical 

order.

Prairie Provinces.

Very large numbers breed at Lake Winnipegosis, and Manitoba has the largest number of 

breeding cormorants in North America. However, complete province-wide surveys have not 

been regularly conducted in this province, so trends cannot be clearly demonstrated. Currently 

numbers appear to be fairly stable on Lake Winnipegosis in Manitoba, and are increasing in 

Saskatchewan and Alberta. Current numbers for the Prairie Provinces as a whole are not 

available; see provincial accounts for specific population data.

Great Lakes.

In the Great Lakes, increases have been dramatic since the 1980s (Figure 24), especially 

in Ontario, Michigan and Wisconsin waters, where the largest concentrations of breeding birds 

occur. Many of the Great Lakes states and Ontario have conducted regular state and provincewide surveys over several years. From 1970 to 1991, the population in the Great Lakes climbed 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 54

from 89 to 38,115 pairs, an average annual increase of 33 %. Growth in the Great Lakes has 

substantially declined in the last decade, even though numbers of cormorants are still increasing. 

From 1991 to 1997, the population went from 38,115 to 88,000 pairs, an average annual increase 

of 15 %. In 1997, about 75 % of these birds were in provincial Ontario and state of Michigan 

waters.

The most dramatic increases have occurred on Ontario, Michigan and Wisconsin waters. 

In some Ontario locations, growth appears to be slowing, which may indicate that numbers are 

approaching asymptotic values (Korfanty et al. 1997). While growth in Michigan and Wisconsin 

has also slowed from its very rapid initial increase during the first 10 years of the cormorant’s 

recovery, numbers of cormorants rapidly increased in these states through 1997. Based on 

surveys of breeding colonies in 2000 for D. Trexel’s M.S. thesis, growth rates on US waters of 

lakes Huron and Michigan between 1997 and 2000 appear to have declined even further.

Winter and Migrant Birds

No substantial numbers winter in this zone. We reviewed CBC data collected between 

1984 – 1998 (Sauer et al. 1996) for all states that bordered Zones 2, 4 and 5 (Idaho, Utah, 

Colorado, Kansas, Missouri, Illinois, Indiana, and Ohio). Sites with ≥ 100 individuals during at 

least one count were identified in Idaho, Kansas, Missouri and Illinois, but there were not enough 

consistent data to create figures for these states. Overall, small numbers were occasionally 

observed during CBCs in the states reviewed, with a few exceptions (see state profiles). 

Substantial numbers are reported during migration, with as many as 25,000 – 50,000 birds 

reported in some of the Great Lakes states.

Historical Information

The earliest breeding record we have for this zone comes from Lake of the Woods, 

Ontario, where the species was first recorded breeding in 1798. Early breeding records (19th

century) are also available for Manitoba and Saskatchewan, and for many of the Great Lakes 

states.

Great Lakes.

It is not exactly clear when the species began breeding on the Great Lakes themselves. 

Weseloh et al. (1995) note that breeding was not suspected to occur on the Lakes before 1913, 

when breeding was first documented on the far western end of Lake Superior. Colonies then 

spread slowly eastward. However, the second edition of the American Ornithologists’ Union 

Checklist of North American Birds (1895) states the breeding range of the Double-crested 

Cormorant as “the Bay of Fundy, the Great Lakes, Minnesota and Dakota northward.” While it is 

not clear what records were used to support the claim of breeding on the Great Lakes at this time, 

individuals involved on the AOU committee to revise matters relating to distribution included 

Elliot Coues, J.A. Allen, William Brewster, C. Hart Merriam, and Robert Ridgway, prominent 

and careful ornithologists. Additionally, breeding was well documented in many of the states 

surrounding the Great Lakes, and the species appears to have been abundant throughout the 

region during the 1800s (Lewis 1929). Therefore, it seems likely that the Double-crested 

Cormorant may have nested on the Great Lakes earlier than 1913, but may have been extirpated 

from the area before good documentation was obtained. A review of literature by early Great 

Lakes explorers may shed light on this question.

By the turn of the century, the Double-crested Cormorant had been greatly reduced or 

extirpated as a breeding species from the Great Lakes states (MN, OH, WI, IN, IL), Missouri, 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 55

Iowa and probably several of the prairie states. In some areas, migrants were also severely 

reduced. These declines appeared due to human persecution and exploitation. However, the few 

cormorants that remained in the region soon began a population expansion into or a recolonization of the Great Lakes. Nesting was reported in Lake Superior between 1913 – 1920, 

reached lakes Ontario and Erie by the late 1930s, and the Upper St. Lawrence River in 1945 

(Ludwig 1989; Weseloh et al. 1995). Numbers increased steadily during the 1930s and 1940s, 

and by the early 1950s cormorants had become so abundant in some areas of Ontario that control 

measures were authorized to reduce perceived competition with sport and commercial fisheries 

(Weseloh and Collier 1995). Aggressive illegal control measures also began around this time. 

Persecution from commercial fishermen and predation during nesting probably kept the numbers 

of birds breeding in Michigan and Canadian waters of Lake Huron at low levels (Ludwig and 

Summer 1995). Controls largely ended by around 1960, and probably only slowed the growth of 

the population in this region.

By 1960, even after control measures stopped, the cormorant population declined and 

continued to do so through the early 1970s. Cormorants disappeared from Lakes Michigan and 

Superior as a breeding species and declined rapidly in Lake Huron. In 1970, the population in the 

Great Lakes was estimated at only 89 pairs, nesting at eight sites (Weseloh et al. 1995). These 

dramatic declines are mostly attributed to high levels of toxic contaminants, such as DDE and 

PCBs, that were present in the Great Lakes and elsewhere throughout the annual range. These 

contaminants resulted in eggshell thinning and reproductive failure (Weseloh et al. 1983; 

Weseloh and Collier 1995; Ludwig and Summer 1995).

In the mid 1970s, cormorant numbers began to recover, due to a number of factors. 

Lower contaminant levels in the environment resulted from legislation implemented around this 

time to restrict the use of DDT and related pesticides (Weseloh and Collier 1995). Additionally, 

the DCCO was added to the list of birds protected by the Migratory Bird Treaty Act in 1972. 

Human induced changes in fish communities in the breeding range, development of aquaculture 

on the winter range, and creation of additional habitat also likely contributed to the recovery.

Prairie Provinces.

In the Prairie Provinces of Canada, certain colonies also appeared to decline around the 

turn of the century, but numbers did not drop as drastically as in the U.S. interior. In many areas 

cormorants remained relatively abundant. As in the Great Lakes, cormorant numbers reached 

high levels in the mid part of this century; from this point on cormorants have a history similar to 

that of the Great Lakes. In 1943 the Manitoba government employed cormorant control on Lake 

Winnipegosis, as the species was perceived to be responsible for losses to commercial fisheries. 

Numbers were greatly reduced by the early 1950s, when control stopped, but cormorant numbers 

continued to decline through the 1960s. Cormorants also declined in Saskatchewan and Alberta. 

Vermeer (1973) estimated a total of 6500 nesting birds in the prairie provinces for the 1967 –

1972 period, which is only about 2/3 the number counted on Lake Winnipegosis alone in 1945. 

Contaminants, illegal control and habitat loss were thought to be the main factors contributing to 

these declines (Vermeer and Rankin 1984; Koonz and Rakowski 1985; Korfanty et al. 1997). 

Cormorants received protection in all of these provinces during the 1970s and early 1980s, and 

numbers began increasing again. The greatest changes in numbers occurred in Saskatchewan and 

Manitoba, where populations increased from 5,850 to 32,558 pairs during the 1970s and early 

1980s (Vermeer and Rankin 1984).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 56

Figure 24. Changes in numbers of breeding pairs on the Great Lakes, 1970 – 1997.

a b

c

d

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

1970 1975 1980 1985 1990 1995 2000

Year

No. Pairs

a = 89 pairs (Weseloh et al. 1995)

b = 1798 pairs (Ludwig 1984)

c = 38,115 pairs (Weseloh et al. 1995)

d = 88,000 pairs (this report)
FINAL DRAFT Part II. Population Data and Trends, Zone 3 57

Table 4. Zone 3, Canadian and U.S. Interior. Summary of Population Data and Trends.

(+ = increasing; - = decreasing; 0 = no change).

State/ 

Province

Status 

(B, M, 

W, w)1

Number 

colony sites/

time period

No. active 

colonies/ last 

yr. surveyed

No. pairs/

last year 

surveyed

Breed. 

trend

Winter 

number 

(birds)

Migrant 

number 

(birds)

AB2 B NA NA NA + NA NA

CO B, M, w 21/1987-95 11/1992 1,000 0/+? < 100 1000s

IL B, M, w 14/1994-97 6/1997 > 754 - 100s (?) 1000s

IN M, w - - - - 200-300 100s-1000s

IA B 10/1989-98 5/199838443+ -

ID B, m, w 11/1993 11/1993 1175-1401 + 50-150 NA

KS B, M, w 6/1951-2000 3/1992-97 NA NA 100s 10,000-

100,000

MB B, M (?) 156/1979-91 

(Manitoba)

33/19994

(L. Win. only)

36,1804 NA - -

MI B, M, w 48/1998 45/1997 31,079 0? 200 50,000

MN B, M 75/1981-97 NA NA5- /? - 100s-1000s

MO M, w - - - - 100s 1000s

MT B, m 50-100/? NA NA NA - NA

NE B, M 14/1970-2000 14/1970-2000 NA +/? - 1000s

NM6 B, M(?), 

W

3/1996 3/1996 130/1996 -/? NA NA

NY(inland) B, M 12/1980s-97 12/1997 9,0727+ - 1000s

ND B, M NA NA NA +/? - NA

OH B, M, w 2/1992-98 2/1998 1510 + 50 birds 25,000

ON B 116/1980-97 74/1997 35,159 + - -

QC (sw) B 5/1990-97 5/1990-97 212 + - -

SK B 14/1980-91 10/1991 19,547 + - -

SD B, M 36/1988-92 36/1988-92 > 2,962 -/? - NA

UT B, m, w 8/1971-1999 8/1971-99 NA NA < 100 100s

VT B 5/1982-99 2/1999 2886 + - -

WI B 45/1980-97 23/1997 10,546 + - 1000s

WY B, M 25/1981-99 < 17/1994 NA -/? - NA

Totals 490 217 > 153,056 1,000s > 81,000

1 = B, Breeding; M, Migrant; W, Wintering; w, small numbers wintering

2 = For Alberta, we did not receive a completed survey. Only partial data available at this time.

3 = Number may be an underestimate, as not all known colonies were counted 

4 = Number is for Lake Winnipegosis only; last province-wide survey was done in 1979, at 

which time 60 colonies were identified with 22,642 nests.

5 = No complete statewide census has ever been completed for Minnesota.

6 = Upper Rio Grande and Middle Pecos River Drainage; excludes colonies in Zone 2 (Table 1).

7 = Estimate for inland colonies only; est. pairs for entire state in 1997 = 12,675 (Miller 1997).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 58

Figure 25. Distribution of Interior U.S. and Canadian DCCO breeding population (Zone 

3), 1979 – 2000.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 59

Figure 26. Distribution and size of active colonies in Saskatchewan during last complete 

survey (1991).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 60

Figure 27. Distribution and size of active colonies on Lake Winnipegosis during last 

complete survey (1999).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 61

Figure 28. Distribution and size of active colonies in the Interior west-central states during 

last complete surveys (1986 – 1999)1.

 

1

 Majority of colonies visited in 1990s; only 2 colonies with last visits in 1980s.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 62

Figure 29. Distribution and sizes of active colonies in Interior mid-central states at time of 

last complete surveys.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 63

Figure 30. Distribution of active colonies (size data not available) in South Dakota and 

Minnesota (MN = 1981 – 1995; SD = 1988 – 1992).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 64

Figure 31. Distribution and size of active colonies in Wisconsin during last complete survey 

(1997).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 65

Figure 32. Distribution and size of active colonies in Michigan during last complete survey 

(1997).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 66

Figure 33. Distribution and size of active colonies in Ontario during last complete survey 

(1997).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 67

Figure 34. Detail of Lake Huron showing size and distribution of active colonies (1997).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 68

Figure 35. Distribution and size of active colonies in Ohio, New York, and Vermont.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 69

Alberta

Summary of Population Data and Trends

(We did not receive a completed survey for Alberta.)

Korfanty et al. (1997) did not report breeding numbers for this province, but did state that 

the Double-crested Cormorant was added to the non-game list and removed from the endangered 

list in 1987, because numbers were increasing due to habitat protection.

Historical Information

In 1977, the species was designated as an endangered animal in Alberta due to population 

declines and habitat risks at nesting colonies.

No other information available.

Land Ownership

No information available.

Productivity

No estimates available.

Colorado

Summary of Population Data and Trends

(Surveys completed by K. Giesen, Colorado Division of Wildlife, T. Leukering and R. 

Levad, Colorado Bird Observatory.)

Breeding.

The DCCO has been documented as a nesting species in Colorado since 1931 (Kingery 

1998). Beginning in 1987 and continuing through 1995, the Colorado Breeding Bird Atlas 

survey was conducted. Since the Atlas survey’s inception, nesting has been documented at a total 

of 21 sites (R. Levad, pers. comm.), with about 1000 pairs estimated in the big mountain parks, 

along the Arkansas and Platte rivers, and on the northeastern plains (Kingery 1998). In 2000, a 

total of 28 nesting sites was recorded in the database. Of these, 19 were visited and 11 were 

active, with a total of 710 nests, but this was not a complete count (R. Levad, pers. comm.). 

Presently, precise locations and information on trends are not available. However, numbers of 

DCCO colonies and breeding pairs are known to have increased between the late 1930s and 

1980s (Kingery 1998). In the 1980s the state population was estimated at 1000 pairs (Ryder 

1996), and in 1992, 11 nesting colonies were reported (Andrews and Righter 1992). While 

numbers of colonies appeared to increase during the Atlas survey, numbers of breeding birds 

remained similar to those estimated in the 1980s (Kingery 1998). Construction of reservoirs is 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 70

presumed to be the main reason for increases and expansion (Andrews and Righter 1992). 

Location information may become available in 2001. Visits to all known colonies are planned for 

2001, and annual counts will likely get underway (R. Levad, pers. comm.).

Winter.

In winter the DCCO is a very rare resident on eastern plains near foothills, and has also 

been recorded in western valleys in Mesa County (Andrews and Righter 1992). T. Leukering 

(pers. comm.) reported a few winter birds (< 10) at reservoirs in Boulder and Pueblo West, and 

K. Giesen (pers. comm.) reported < 20 birds overwinter in the state. Review of CBC data 

collected between 1984 – 1998 (Sauer et al. 1996) identified no sites where ≥ 100 individuals 

were estimated during any count, but small numbers were observed at several sites.

Migration.

The species is reported as a common to abundant spring and fall migrant on eastern 

plains, with a high count recorded on Oct. 17, 1989, in Otero County (Andrews and Righter 

1992). Peak numbers are observed in mid-October.

Historical Information

The earliest records we found of DCCOs in Colorado were those by Sclater (1912), who 

reported that only four records were verified up to 1912. The first breeding record was not 

obtained until 1931, when eight nests were found at Barr Lake.

By 1939, this colony had increased to 30 pairs. In 1995, 248 nests were reported at this 

site (Kingery 1998).

Land Ownership

No information provided.

Productivity

No information provided.

Idaho

Summary of Population Data and Trends

(Survey completed by Chuck Trost, Idaho State University.)

Breeding.

The DCCO has been documented as a nesting species in Idaho since at least 1941 (see 

Historical Information, below). Burleigh (1972) reported that 11 specimens collected from 

different locations in Idaho were examined to determine subspecies, and were found to be 

intermediate in their characters between P. a. auritus and P. a. albociliatus, but closer to auritus
FINAL DRAFT Part II. Population Data and Trends, Zone 3 71

(Burleigh 1972). The most recent survey for colonially nesting waterbirds in the state, conducted 

in 1993, documented 11 active colonies and estimated 1,175 – 1,401 pairs (Trost 1994). Prior to 

this, a survey for colonially nesting waterbirds conducted in 1984 found five colonies and 

estimated 805 – 880 pairs (Trost 1985). Peterson (1977) reviewed the distribution of colonial 

waterbirds in Idaho and reported six known DCCO colonies in the state, but did not report 

number of pairs. While numbers of breeding pairs and colonies appear to have increased since 

1984, increases may be due, in part, to a better response to the questionnaire developed for the 

1993 survey (Trost 1994).

Winter.

In winter, small numbers (40 – 50 birds) can be found along the lower Snake River (C. 

Trost, pers. comm.). Review of CBC data collected between 1984 – 1998 (Sauer et al. 1996) 

identified one site where ≥ 100 individuals were estimated: Hagerman Valley (129 in 1995). 

Data collected at this site in the 1990s indicate that numbers are slowly increasing, but analysis 

of recent data has not yet been conducted. Multiple sites with smaller numbers were also 

identified.

Migration.

No important stopover sites or significant concentrations of migrants were reported, 

though it does occur as a spring and fall migrant (C. Trost, pers. comm.).

Historical Information

The earliest documented nesting record appears to be from Bear Lake in southeastern 

Idaho, and was obtained in 1941 (Behle 1941). However, the bird was documented in the state 

much earlier than this. It was included in the first list of birds for the state. Lewis, in his account 

of the Lewis and Clarke expedition, recorded the species on the Clearwater River, October 8, 

1805. Other early records of birds apparently in migration were obtained in the late 1800s in both 

the northern and southern portions of the state (Burleigh 1972).

Land Ownership

The following land ownership information was provided for breeding colony sites: 

Blackfoot Reservoir is probably owned by the Bureau of Reclamation (BREC), but the water 

belongs to the Bureau of Indian Affairs (BIA); Mud Lake WMA is state land managed by Idaho 

Fish and Game; Bear Lake, Minidoka and Deer Flats NWRs are federal land, managed by 

USFWS; the Mormorn Reservoir may be owned by BREC; Island Park Reservoir maybe owned 

by Henry’s Fork Canal Co.; and American Falls Reservoir is managed by the BREC but the land 

is on BIA.

Productivity

No information provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 72

Illinois

Summary of Population Data and Trends

(Survey completed by J. Herkert, Illinois Endangered Species Protection Board, 

Springfield, IL.)

Breeding.

The Double-crested Cormorant was first documented as a breeding species in Illinois by 

Ridgway (1874). Currently, the state has a relatively small number of nesting birds. In the 1990s, 

there was a total of 14 known colony sites, all at inland lakes, rivers and marshes. Eleven 

complete censuses of breeding colonies have been conducted between 1979 and 1997. Since 

1979, significant increases have been reported, and in 1995, numbers peaked at 1077 pairs. 

Thereafter, numbers declined; in 1997, the last year for which survey data were available, > 754 

pairs were estimated at 6 colonies. Complete survey data were only available for 7 years between 

1986 – 1997, when numbers increased at an average annual rate of 11.6 % (Figure 36).

Winter.

A small number of birds were reported to winter in the state (J. Herkert, pers. comm.). 

Review of CBC data collected between 1984 – 1998 (Sauer et al. 1996) identified three sites 

where ≥ 100 individuals were estimated: Horseshoe Lake (100 in 1993); Rend Lake (> 100 have 

been recorded on multiple counts in the 1990s, with a high of 1,597 individuals in 1998); and 

Pere Marquette Park (100 in 1998). Data were not consistent enough at the three sites combined 

to create a figure showing any potential trends. At the Rend Lake site large increases occurred in 

1990, 1991, 1994 and 1998, and may indicate that this site is becoming a relatively significant 

wintering area for this region.

Migration.

Thousands are reported during the peak of spring migration, which occurs during March 

– April (J. Herkert, pers. comm.).

Historical Information

In the late 1800s, the Double-crested Cormorant nested near Mt. Carmel (Ridgway 1874), 

Lacon (Barnes 1890) and Peoria (Loucks 1893). Nesting was also recorded near Havana and 

Clear Lake in 1910, but these birds were severely persecuted (Lewis 1929). At this point, nesting 

appears to have ceased in the state or to have become very sporadic; Lewis (1929) was unaware 

of any nesting after this time. Bohlen and Zimmerman (1989) reported that the Havana colony, 

which had “several nests”, was shot out by fishermen in 1910, but that there were probably 

colonies elsewhere in the state. In 1960, the species was added to the Illinois endangered species 

list; at this time there was only one known nest site at Thomson, IL, on the Mississippi River 

with just seven nests. Artificial nesting platforms were later installed, and the colony at Thomson 

began to recover; 16 nests were recorded in 1978, and >110 were recorded in 1986. Bohlen and 

Zimmerman (1989) reported that very small colonies had also “recently” been found at Rend 

Lake, Lake Renwick, Putnam Co., and Bartonville; we assume these colonies were discovered in 

the late 1970s and early 1980s, because the IL DNR reported only 25 nests for 1979, the earliest 

year for which the DNR had state totals (though number excludes Thomson colony; J. Herkert, 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 73

pers. comm.). More small colonies continued to be discovered, and in 1997 there was a total of 

14 known nesting sites.

Land Ownership

Land ownership was provided for two colony and migration stopover sites, Carlyle Lake 

and Rend Lake. Both of these were reported as public lands. Carlyle Lake is federal land, owned 

by the Army Corps of Engineers; no additional information was provided for Rend Lake.

Productivity

No estimates available.

Figure 36. Changes in numbers of breeding pairs in Illinois, 1986 – 1997.

0

200

400

600

800

1000

1200

1984 1986 1988 1990 1992 1994 1996 1998

Year

No. Pairs

Indiana

Summary of Population Data and Trends

(Survey completed by J.S. Castrale, Indiana Dept Natural Resources, Mitchell, IN.)

Breeding.

Does not breed in the state. Although not reported by atlas workers or Breeding Bird 

Survey participants in Indiana, this species was as frequent as Common Loons on Summer Bird 

Counts, and recorded most commonly in northwestern Indiana (Castrale et al. 1998). Some 

recent observations of birds carrying nesting materials have been reported, but nesting has not 

been documented (J. Castrale, pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 74

Winter.

In most years a small number of birds, 200 – 300, linger on into winter months (J. 

Castrale, pers. comm.). Review of CBC data collected between 1984 – 1998 (Sauer et al. 1996) 

identified no sites where ≥ 100 individuals were estimated during any count, but small numbers 

were observed at some sites.

Migration.

During the peak of fall migration, September – October, up to 9000 birds have been 

reported. During the peak of spring migration, mid-April – early-May, smaller numbers ranging 

from 100 – 300 birds reported. Recently, records in all seasons have become more numerous and 

birds have been observed throughout the state (J. Castrale, pers. comm.).

Historical Information

The earliest record of the Double-crested Cormorant in Indiana dates from 1858 (West 

1958), and Robert Ridgway thought the cormorant nested in Gibson and Knox counties prior to 

1900 (Butler 1898). However, the first circumstantial evidence of nesting was not obtained until 

September 1929, when cormorants were present in “considerable numbers” at Hovey Lake and a 

nest was observed. Cormorants nested at this site until at least 1953 in small numbers (< 10 

pairs). Nest building was also noted at Willow Slough Fish and Wildlife Area in 1953 (Mumford 

and Keller 1984). The last breeding record was obtained in the same year.

Land Ownership

Land ownership was provided for the 1 major stopover site reported, Hovey Lake FWA. 

This is state land, owned by the Indiana Division of Fish and Wildlife.

Productivity

Not applicable.

Iowa

Summary of Population Data and Trends

(Survey completed by K. Bogenschutz, Iowa Dept Natural Resources, Boone, IA.)

Breeding.

The Double-crested Cormorant returned as a fairly regular breeder in Iowa in 1984 (see 

Historical Information, below, for breeding history), with nesting documented at Sabula in 

Jackson County, on the Mississippi River, and at the Coralville Reservoir in eastern-central 

Iowa. Over the last decade, nesting has been documented at a minimum of 10 sites. The Iowa 

DNR reported 8 colony sites on islands or lakes along the Mississippi River that were active 

between 1989 – 1998. Five of these colony sites were counted in 1998, and 844 pairs were 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 75

estimated. However, this may be an underestimate, as not all colony sites on the Mississippi were 

counted. Additionally, the Iowa DNR did not report the Coralville Reservoir site as one of the 

known breeding sites, though nesting was observed there in 1990. They also did not report 

nesting in north-central Iowa, which was documented in 1991 at the Union Slough National 

Wildlife Refuge in Kossuth County (Ehresman 1996). We were unable to obtain current 

information on the status of these colonies. Because Iowa has not conducted systematic statewide 

censuses of all known cormorant colonies, we estimated a population trend by examining 

changes in colonies that had been counted at least twice. Of the 10 colonies, five fit this 

requirement; three showed increases, while two showed decreases. Based on changes in these 

colonies, cormorants appear to be increasing in the state, which supports the opinion of the Iowa 

DNR (K. Bogenschutz, pers. comm.).

Winter.

Cormorants are not known to winter in the state (K. Bogenschutz, pers. comm.).

Migration.

No significant numbers of migrants were reported (K. Bogenschutz, pers. comm.).

Historical Information

The Double-crested Cormorant once bred abundantly in northern Iowa, but by the turn of 

the century, it occurred mainly as a migrant throughout the state (Cooke 1888; Anderson 1907; 

Lewis 1929 ). It was not recorded as a breeding species again until 1934 and 1936, when up to 

12 nests were documented along the Mississippi River in Lee County. Between this time and 

1984, scattered breeding records were reported along the Mississippi and Missouri Rivers, and at 

the Coralville Reservoir. The breeding colonies documented in 1984 were the first colonies 

reported since 1966 (Ehresman 1996).

Land Ownership

All colonies in Iowa are on federal lands (K. Bogenschutz, pers. comm.).

Productivity

At one of the colonies, Butler Lake, fledging rates (young / nest) were provided, and 

ranged from 1.25 the first year of breeding to 2.7 in 1994. In 1996, when the colony was 

estimated at 364 pairs, the fledging rate was 2.3.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 76

Kansas

Summary of Population Data and Trends

(Survey completed by W. Busby, Kansas Biological Survey.)

Breeding.

DCCOs have been documented as a nesting species in Kansas since at least 1951 

(Thompson and Ely 1989). A total of six known colonies was reported between that time and 

2000. The Kansas Breeding Bird Atlas, conducted 1992 – 1997, confirmed breeding at only three 

sites: Cheyenne Bottoms, Quivira National Wildlife Refuge and Wolf Creek Reservoir. Kansas 

does not conduct regular statewide surveys for DCCOs, and trend information is not available. 

However, breeding numbers are thought to be small (all colonies are always < 300 pairs) and use 

of individual breeding sites from year to year is erratic (W. Busby, pers. comm.). The largest 

colony reported was that at Glen Elder Reservoir in Mitchell County, which had as many as 220 

pairs since 1974, but by 1989 there were many fewer (Thompson and Ely 1989).

Winter.

In winter DCCOs are not usually present, though a few sometimes remain until 

midwinter when there is no more open water (Thompson and Ely 1989). A few have also been 

observed in mild years. CBCs conduced 1959 – 1988 suggested an increase of 3.2 % per year 

(Sauer et al. 1996). Review of CBC data collected between 1984 – 1998 identified three sites 

where ≥ 100 DCCOs were estimated: Lawrence (150 in 1990, 216 in 1994); Waconda Lake (190 

in 1998); and Webster Reservoir (110 in 1998). However, at the latter two sites CBCs were not 

initiated until 1992, and there were not enough consistent data for the three sites combined to 

create a figure showing any potential trends.

Migration.

Large numbers of migrants, 10,000 – 100,000, are estimated to occur, with most found at 

large Army Corps and Bureau of Reclamation reservoirs in the eastern 2/3 of the state, but this 

information has not been formally collected (W. Busby, pers. comm.). Migration peaks occur in 

mid-April and early October (Thompson and Ely 1989).

Historical Information

No information obtained.

Land Ownership

Land ownership information was provided for four breeding sites: the Kirwin and Glen 

Elder Reservoirs are both owned by the Army Corps of Engineers, the Cheyenne Bottoms colony 

is on state land, and the Norton Reservoir colony is on Bureau of Reclamation land. Kirwin, 

Glen Elder, and Norton are all federal reservoirs owned and operated by the Bureau of 

Reclamation.

Productivity

No information provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 77

Manitoba

Summary of Population Data and Trends

(Survey completed by W. Koonz, Manitoba Dept. of Nat Res, Winnipeg, Manitoba.)

Breeding.

Manitoba has the largest number of breeding Double-crested Cormorants in North 

America. Birds nest on shallow lakes throughout the southern half of the province and on lakes 

Manitoba, Winnipeg, Winnipegosis and in the central portion of the province. Because Manitoba 

is so large and has so many colonial waterbird breeding sites, province-wide surveys are very 

difficult to accomplish in one season. The last province-wide survey was conducted in 1979 over 

a four day period with a fixed wing aircraft; numbers counted in this survey probably 

underestimated the breeding population (B. Koonz, pers. comm.). Because complete provincewide surveys have not been regularly conducted, province-wide population estimates and trends 

are not available. However, data collected from colonies over the last 20 years provide some 

information on population changes. Between 1979 – 1991, breeding was documented at 

approximately 156 sites; of these, 26 sites were counted at least twice, and 19 showed increases. 

The province-wide survey conducted in 1979 counted 22,642 active nests at 60 colonies; 9053 

nests were at 17 colonies on Lake Winnipegosis (Koonz and Rakowski 1985). In 1987, 35,181 

nests were reported at 37 colonies on Lake Winnipegosis alone, suggesting a dramatic increase 

over previous Lake Winnipegosis and province-wide numbers (Hobson et al. 1987). The 

population at Lake Winnipegosis continued to increase until at least 1989, when a high of 51,788 

pairs on 40 islands was estimated (B. Koonz, pers. comm). Lake Winnipegosis was censused 

again in 1999, at which time 36,180 nests were counted on 33 islands. (Figure 37).

Most of the growth on Lake Winnipegosis appears to have occurred during the 1980s, 

when the average annual growth rate was 19 %. We did not calculate an average annual increase 

rate based on the 1999 survey data for the following reasons. While breeding numbers appear to 

have declined since the late 1980s, this decline is probably not significant. During 1999 surveys, 

at least 1000 dead birds that had been shot were observed at several colonies, and many nonbreeding birds were also observed. Active shooting and harassment of birds occurred at one 

colony even while surveyors were trying to obtain nest counts. Because human disturbance was 

so prevalent, many birds were probably not breeding, possibly explaining why fewer nests were 

counted. The number of birds breeding on Lake Winnipegosis, overall, has probably remained 

fairly stable over the last decade (B. Koonz, pers. comm.).

One important observation from surveys done in the late 1980s and in 1999 at Lake 

Winnipegosis was that a large proportion of cormorant nests was found in trees on islands not 

traditionally used for nesting by cormorants. Historic records from Bent (1922) and McLeod 

(1943) consider cormorants to nest exclusively on the ground. Vermeer (1973) also noted that 

cormorants nested in trees in the prairie provinces only when there were no treeless islands in the 

vicinity. Establishment of new colonies on forested islands may be due to increases in cormorant 

numbers and / or disturbance of traditional cormorant nesting islands (Lewis 1929; Vermeer 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 78

1969, 1973; B. Koonz, pers. comm.); the forested islands colonized are usually distant from 

major travel routes where unofficial cormorant control frequently occurs (B. Koonz, pers. 

comm.).

Winter.

No birds are known to winter in the province (B. Koonz, pers. comm.).

Migration.

The peak period of migration occurs in late August – early September, when birds leave 

for southern wintering grounds (B. Koonz, pers. comm.).

Historical Information

The Double-crested Cormorant has been an established breeder in Manitoba for well over 

a century. Lake Winnipegosis was recognized for its cormorant colonies just prior to 1900 

(Koonz and Rakowski 1985), and Bent (1922) remarked that he had not seen greater densities of 

Double-crested Cormorants than on this lake. Lewis (1929) noted early records (1891 – 1909) of 

cormorants nesting at Shoal Lake, lakes north of Touchwood Hills, Ossowa, and Selkirk 

Settlement, but cormorants appeared to have ceased nesting at these places by the time Lewis 

was writing. Mendall (1936) recorded a total of 9,320 nests from at least 27 islands on seven 

Manitoba lakes.

Koonz and Rakowski (1985) and Hobson et al. (1987) described the history of the 

Double-crested Cormorant in Manitoba during the twentieth century, and that information is 

summarized here, unless otherwise noted. As in many areas of the continent, commercial 

fishermen blamed cormorants for losses in Manitoba’s commercial fishery, and from 1943 –

1951, the Manitoba government conducted a cormorant control program on Lake Winnipegosis. 

Human disturbance reduced numbers on this lake from 9,862 nests in 1945 to 4,656 nests in 

1951. However, even after government control stopped, declines continued on Lake 

Winnipegosis through the 1960s, and by 1969, there was only a total of 1,403 nests counted. 

These declines were attributed to large scale destruction of eggs and young birds by people 

(Vermeer 1973).

In 1963, the species became officially protected under the provincial Wildlife Act. In 

1969 Vermeer (1969) recorded 4772 nests counted from 37 islands on 13 of Manitoba’s lakes. 

Unofficial destruction of eggs, nestlings and adults, combined with the effects of DDT on nest 

success, probably contributed to the post-government control decline. During the 1970s and early 

1980s, numbers began increasing again, likely in response to the ban on DDT. Persecution by 

humans continues, apparently with little substantial impact to overall numbers, but substantial 

changes in Manitoba’s population distribution may result (B. Koonz, pers. comm.; see Impacts to 

Avian Species section).

Land Ownership

Most if not all Manitoba islands used by cormorants for breeding are Crown owned (B. 

Koonz, pers. comm.).

Productivity

Hobson et al. (1987) provided productivity estimates for 32 colonies on Lake 

Winnipegosis in 1987. Productivity was defined as number of mobile chicks per initiated nest in 

July and August. Thirteen colonies showed evidence of human disturbance; the mean 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 79

productivity at these colonies was approximately 0.4 chicks per pair (range 0 – 1.2). Nineteen 

colonies were undisturbed, and mean productivity at these colonies was 1.8 chicks per pair 

(range was 0 – 4.0; the 0 was at a colony that may have been disturbed by humans or a storm).

Figure 37. Changes in breeding numbers on Lake Winnipegosis, 1979 – 1999.

0

10000

20000

30000

40000

50000

60000

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

Year

No. Pairs

Michigan

Summary of Population Data and Trends

(Survey completed by J.P. Ludwig, The Sere Group, Kingsville, Onatrio; additional 

information provided by Glenn Belyea, Michigan Dept Nat. Resources, E. Lansing, MI.)

Breeding.

Cormorants disappeared from Michigan as a breeding species in the 1960s; at the end of 

the 20th century, however, Michigan has by far the largest number of breeding cormorants in the 

U.S. Great Lakes, and constitutes one of the major breeding sites in North America. The species 

resumed nesting in the state in 1977, when nine nests were counted at Gravelly Island. Since that 

time, two more complete censuses of the Great Lakes have been conducted (1989, Scharf and 

Shugart 1998; 1997, Cuthbert, unpublished data), and 45 breeding sites were documented in 

1997. In 1998, three more breeding sites were discovered, bringing the total number of known 

breeding sites in Michigan to 48. Ludwig (1991) reported 1 inland colony site, but status of this 

site was not determined for this assessment. Between 1977 and 1997 numbers increased at an 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 80

average annual rate of 50 % (Figure 38). However, the rate of increase has slowed; between 1989 

– 1997, the average annual rate of increase was 27.8 %, compared to 67.4 % during 1977 – 1989. 

The current rate of increase is not known, but in 2000 D. Trexel censused Michigan colonies 

(with the exception of three sites) and found that overall growth rate for these colonies, while 

still positive, had declined significantly. In addition, only one new colony site was found. A few 

concurrent years of data would help determine by how much growth has slowed and whether 

cormorants are approaching carrying capacity in this state.

Winter.

A small number of birds winter in the state (J. Ludwig, pers. comm.).

Migration.

During peak migration periods, April and August, estimates of 50,000 birds are reported 

(J. Ludwig, pers. comm.).

Historical Information

Changes in cormorant historical distribution and numbers within Michigan have been 

described by Ludwig and Summer (1995) (unless otherwise noted), and are summarized here. 

Small numbers of migrants (<20) were reported in the late 1800s during spring and fall 

migrations (Barrows 1912; Wood 1950). Given distribution known in the state in the late 1800s 

and early 1900s, Barrows (1912) speculated that cormorants might nest about the Great Lakes, 

but no records for Michigan were obtained until the 1930s. F.E. Ludwig (unpublished data) 

accumulated occasional breeding records at islands near Alpena (Lake Huron) between 1932 and 

1941, and at Pismire, Hat and Bellows islands (Lake Michigan) in the 1930s; the population in 

the 1940s is estimated to have been between 200 – 500 pairs. Persecution by commercial 

fishermen and predation are thought to be responsible for regulating the population at these low 

levels. By the outbreak of WWII, creation of artificial habitat, such as flooded inland 

impoundments, provided breeding refugia for cormorants. However, DDT came into widespread 

use in the late 1940s, and by 1960, Great Lakes cormorant populations were in rapid decline due 

to DDE-mediated eggshell thinning. The species vanished from all Michigan Great Lakes sites 

by 1960, and from inland impoundments by 1964. Double-crested Cormorants did not breed in 

Michigan again until 1977. Since that time, the species has undergone a phenomenal increase in 

this state.

Land Ownership

Of the 51 known colonies, ownership was identified at only 14. Of these, three (about 21 

%) were on private lands, and the rest were on federal lands. One of the colonies was on private 

land, a site soon to be under federal ownership.

Productivity

No estimates of productivity were available.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 81

Figure 38. Changes in numbers of breeding pairs in Michigan, 1977 – 1997.

0

5000

10000

15000

20000

25000

30000

35000

1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998

Year

No. Pairs

Minnesota

Summary of Population Data and Trends

(We did not receive a completed survey for Minnesota; information on breeding numbers 

and locations was provided by T. Roden, MN Dept Natural Resources, Natural Heritage 

Program, St. Paul, MN and by J. McKearnon, Univ of Minnesota, Dept. Fisheries and Wildlife, 

St. Paul, MN.)

Breeding.

The Double-crested Cormorant has been documented as a nesting species in Minnesota 

since 1875, when full-grown young were observed in treetops at Lake Minnetonka (Roberts 

1932). A total of 75 nesting sites were recorded between 1981 – 1997. Only three of these are on 

the Great Lakes (Lake Superior); the rest are all on inland lakes and marshes. Between 1977 –

1997, the Minnesota Department of Natural Resources conducted annual counts of colonial 

waterbird colonies, but because it has never conducted complete statewide censuses, total 

numbers for the state during any one year are not available. However, efforts have been made to 

consistently count the large or major colonies. McKearnon (1997) estimated population trends of 

cormorants in Minnesota by using a Breeding Bird Survey analysis method, in which each 

colony was treated as a route. Based on this analysis, McKearnon reported that cormorants 

increased in the state from 1981 – 1990 at an estimated rate of 20 % per year, and then declined 

at a rate of – 8 % per year until 1995. A few more years of data will help determine whether 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 82

cormorants are past their peak and are declining or fluctuating around an asymptote. In 1997, 13 

colonies were counted and there was a total of 3943 pairs, compared to a total of 5059 pairs 

counted at 22 colonies in 1995.

Because Minnesota is one of the states where the Depredation Order is in effect, a higher 

priority should be placed on monitoring numbers in this state. Complete state-wide surveys of 

breeding birds should be conducted so that numbers of colonies and breeding pairs can be 

accurately assessed. Without this type of information, impacts of and to the birds and effect of 

the depredation order in terms of buffering DCCO impacts cannot be determined.

Winter.

No birds are known to winter in the state.

Migration.

During migration, peak flock size is usually 200 – 500 birds, though 1000 were reported 

in Oct, 1983. Peak spring migration usually occurs in the second half of April; peak fall 

migration occurs during the third week of October (Janssen 1987).

Historical Information

Lewis (1929) notes that Minnesota once undoubtedly contained a large number of 

Double-crested Cormorant breeding colonies, with nesting documented at Lake Minnetonka, 

Kawishiwi Lake, Lake of the Woods, Elbow Lake, Lake Shetack, Dead Lake, Heron Lake, Loon 

Lake and Lake Andrew, near Lanesboro and Faribault, and probably in many other places (Lewis 

1929). Many of these colonies were described as “large”; at Loon Lake, hundreds of birds nested 

on the ground and in trees, and when the island was visited in the late 1800s, it was “covered” 

with nests (Roberts 1932). Huge numbers of birds were also observed during the migration 

around the turn of the century; Sennett (1891, cited in Lewis 1929) reported a flock of 

cormorants passing through the state that was “4 miles long and 11⁄2 miles wide”. However, 

congregations of large numbers in the spring provoked professional fisherman and anglers to 

shoot adult DCCOs and destroy nest sites. In April, 1918, the Game and Fish commissioner 

received numerous complaints that > 10,000 cormorants were in the bottom-lands of the 

Minnesota River and were going out daily to the neighboring lakes to feed upon the fish; 

requests were made to drive away or destroy the birds. Roberts (1932) noted that the number of 

birds that remained in the spring to nest was growing steadily less each year, chiefly because of 

human persecution. By the mid 1920s, cormorants were known to breed at only Lake 

Minnetonka, Kawishiwi Lake, and Lake of the Woods, and they were rapidly disappearing from 

these sites (Roberts 1932). Additionally, Lewis (1929) reported the huge flocks observed by 

Sennett (1891) during migration were no longer seen. However, large numbers were still at least 

occasionally observed in the region through the mid 1920s (see Wisconsin summary).

Janssen (1987) states that migrants were “abundant” in the state until the 1950s; however, 

he reports flocks of only 1,000 – 5,000 birds, nowhere near the historic highs reported around the 

early part of this century. Numbers of migrants declined between the 1950s – 1980s, and then 

began to increase again; however, peak flocks reported in the 1980s consisted of only 200 – 500 

birds (Janssen 1987). While current numbers of breeding pairs in the state appear to be abundant, 

it is not possible to compare these numbers with earlier ones reported around the turn of the 

century, because we do not have nest counts for earlier colonies.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 83

Land Ownership

Land ownership was available for only two Great Lakes sites. Both of these colonies 

were on federal lands owned by the BLM.

Productivity

No estimates available.

Missouri

Summary of Population Data and Trends

(Survey completed by J.D. Wilson, Missouri Department of Conservation.)

Breeding.

The status of the Double-crested Cormorant in Missouri is not clear. J. Wilson (pers. 

comm.) reported that it does not breed in the state, but Robbins and Easterla (1992) reported that 

some breeding may occur along the Mississippi River.

Winter.

Generally, cormorants do not winter in Missouri, except for small numbers that may 

remain on the larger southernmost lakes in mild winters (J. Wilson, pers. comm. ). Review of 

CBC data collected between 1984 – 1998 (Sauer et al. 1996) identified three sites where ≥ 100 

individuals were estimated: Springfield (100 in 1991); Montrose Lake Wildlife Area (104 in 

1997); and Horton-Four Rivers (988 in 1998). At the first two sites DCCOs have been observed 

during the 1984 – 1998 time frame fairly consistently. At the Horton-Four Rivers site, counts 

were not conducted until 1995; DCCOs were recorded in 1996, when one bird was observed. 

Data were not consistent enough at the three sites combined to create a figure showing any 

potential trends.

Migration.

Several thousand pass through during spring and fall migration; peak numbers occur in 

April and October (J. Wilson, pers. comm.). The species is more common throughout the state 

during the fall migration, and high counts of 2000 + birds have been recorded (Robbins and 

Easterla 1992).

Historical Information

Early in the century, Widmann (1907) reported that the Double-crested Cormorant bred 

in “considerable numbers” in the Mississippi Lowlands. However, the species apparently 

declined as a breeder, and Bennitt (1932) listed it as an “uncommon breeder”. Robbins and 

Easterla (1992) note that it is not clear if Bennitt’s statement was based on current information at 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 84

the time or whether it was a reiteration of Widmann’s observations. The last documented nesting 

in the state occurred in 1956 (Robbins and Easterla 1992).

Land Ownership

Five major stopover sites (roosting and feeding) were reported, and land ownership was 

provided for all of them. Four of the sites were on federal land, owned by the Army Corps of 

Engineers and USFWS; the other site was on state land, owned by the Missouri Department of 

Conservation (J. Wilson, pers. comm. ).

Productivity

No estimates of productivity were available.

Montana

Summary of Population Data and Trends

(Survey completed by D. Flath, Montana Fish, Wildlife and Parks.)

Breeding.

We were not able to determine when the DCCO was first documented as a breeding 

species in Montana. Currently 50 – 100 colonies are estimated to occur statewide, but colonies 

are not monitored. Colonies probably range in size from 15 – 120 pairs; average colony size may 

be around 40 pairs (D. Flath, pers. comm.). No information is available on population trends. 

However, based on aerial observations, the species appears to be increasing along the 

Yellowstone River where heron colonies occur (D.Flath, pers. comm.).

Winter.

In winter, the species rarely occurs; there are only two known winter records (D. Flath, 

pers. comm.).

Migration.

No significant concentrations of migrants or stopover sites were reported (D. Flath, pers. 

comm.).

Historical Information

Very little information obtained. The species was recorded as a rare migrant along the 

large rivers in the eastern part of the state by Saunders (1921). This observation was based on 

records of migrants collected in the late 1800s and early 1900s.

Land Ownership

No information provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 85

Productivity

No information provided.

Nebraska

Summary of Population Data and Trends

(Survey completed by J. Dinan, Nebraska Game and Parks Commission.)

Breeding.

The DCCO has been documented as a nesting species in Nebraska since 1930 (Farrar 

1997), but likely occurred as a nester earlier than this (see Historical Information, below). 

Between 1970 and 2000, a total of 14 colonies were known to be active. Regular statewide 

surveys for DCCOs are not conducted, so information on population trends is not available. 

However, in recent years, the number of colonies in the state appears to have increased. Most of 

these are small (< 100 pairs); only a few have persisted for many years. The apparent increase in 

number of colonies is likely a re-colonization of old breeding grounds (Farrar 1997). The two 

largest and most persistent colonies in recent years have been on islands in the Crescent Lake 

and Valentine national wildlife refuges. During the 1950s and 1960s, Canada Goose nesting 

structures were placed at both these refuges in an effort to restore Canada Geese as breeding 

birds to the Sandhills region. DCCOs took advantage of these structures and fairly large colonies 

developed at Goose Lake in the Crescent Lake NWR and at Marsh Lakes in the Valentine NWR. 

In 1991 the island at Goose Lake was inundated, and the number of nests declined (Figure 39). 

At the Marsh Lakes colony the number of nests fluctuated from a low of 240 to a high of 922 

with lake levels; in 2000 this colony was estimated at 840 pairs (Figure 40) (J. Dinan, pers. 

comm.; Farrar 1997).

Winter.

In winter, DCCOs are usually not present, though during very mild winters when some 

lakes remain open individuals can be observed occasionally (Sharpe et al. 2001).

Migration.

In spring and fall, the DCCO is an abundant and regular migrant statewide. Peak 

concentrations are observed in April and October. During migration DCCOs utilize medium to 

large sized lakes and reservoirs, and less frequently rivers such as the Platte and Missouri 

(Sharpe et al. 2001). High counts ranging from 350 – 1500 individuals were made between 1994 

and 1998 at Medicine Creek and Harlan Reservoirs, the North Platte NWR, Branched Oak Lake, 

Nebraska City and Holmes Lake, Lincoln. In 1999, a high of 5000 + individuals was observed at 

Harlan County Reservoir.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 86

Historical Information

The first nesting record was not obtained until 1930 (Thorp Lake), but breeding was 

suspected in the early 19th century. Ducey (1988, 2000) described the species as a likely historic 

breeder in western Nebraska, and noted that it was a potential breeder as early as 1820 at the 

Engineer Cantonment area, Washington County. At this time it was a regular migrant along the 

Missouri River, and Coues (1874) listed it as a breeder in this area.

Land Ownership

Land ownership was provided only for the two large colonies at the Crescent Lake and 

Valentine NWRs; both are public lands administered by the USFWS (J. Dinan, pers. comm.)

Productivity

Data were provided on the number of fledged chicks at Valentine NWR between 1985 –

1991. Fledging rates ranged from 0.04 to 2.1 chicks / nest (J. Dinan, pers. comm.).

Figure 39. Changes in number of breeding pairs at Goose Lake, Crescent Lake NWR, 

Nebraska, 1973 – 1996.

0

100

200

300

400

500

600

1970 1975 1980 1985 1990 1995 2000

Year

Number of pairs

Figure 40. Changes in number of breeding pairs at Marsh Lakes, Valentine NWR, 

Nebraska, 1977 – 2000.

0

200

400

600

800

1000

1975 1980 1985 1990 1995 2000

Year

Number of pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 3 87

New York (Inland)

Summary of Population Data and Trends

(Survey completed by R. Miller, N.Y. State Dept. of Env. Cons., Delmar, NY; some 

breeding data from Weseloh and Ewins 1994.)

Breeding.

The Double-crested Cormorant was first reported breeding in inland New York in 1945. 

Over the last two and a half decades breeding numbers and locations have increased dramatically 

in this region. Nest counts at Little Galloo Island (the largest colony in the state, and one of the 

largest in North America) have been made yearly since 1974 (Weseloh and Ewins 1994; R. 

Miller, pers. comm.). This colony increased fairly slowly until the mid-1980s, then began to 

increase rapidly until 1996, when numbers peaked at 8,410 pairs. In 1998, numbers declined to 

5,839 pairs (Figure 41). Between 1974 – 1992 numbers increased at an average annual rate of 

35.8 %. However, the rate of increase has greatly slowed in recent years; between 1992 – 1998 

the average annual rate of increase was only 1.2 %. As of 1997, there was a total of 12 known 

colony sites in upstate New York, with an estimated 9,072 pairs; most (84 %) were on Little 

Galloo Island [for entire state, upstate and Atlantic Coast, 12,346 pairs estimated at 21 colonies 

in 1997 – 1998, R. Miller pers. comm.; see New York (Coastal) profile].

Winter.

Cormorants do not winter in this part of the state (R. Miller, pers. comm.).

Migration.

During spring and fall migration, large flocks are observed along the eastern shore of 

Lake Ontario. Up to 2000 individuals have been reported at Oneida Lake in the fall (R. Miller, 

pers. comm.; Miller 1998). Fall migration peaks in late September; spring migration peaks late 

March – early April (R. Miller, pers. comm.).

Historical Information

The history of the Double-crested Cormorant and its distribution in New York was 

described by Miller (1997, 1998), and is summarized for upstate New York here, unless 

otherwise noted. In 1945, Kutz and Allen (1947) found a colony with about 14 nests on Gull 

Island in eastern Lake Ontario, about four miles from Henderson Harbor, Jefferson County. A 

long time resident reported that this was the first nesting of cormorants in the area in 75 years of 

his memory. This is believed to be the first authentic record of cormorants nesting in New York. 

The Gull Island colony supported about 20 pairs of cormorants and was presumed to be the only 

colony in the state for about two decades. As the Great Lakes cormorant population declined 

throughout the 1960s and early 1970s, the Gull Island colony probably died out in the 1960s. 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 88

Following the reduction of contaminants due to government anti-pollution programs and laws, 

cormorants made a remarkable recovery in the Great Lakes and elsewhere (see Zone 3: Summary 

of Population Data for Canadian and U.S. Interior Population Summary). In 1974, Little Galloo 

Island, which is near the former Gull Island colony, became occupied and began to increase 

steadily. New nesting sites at other upstate New York locations were later colonized, probably as 

a result of the expanding Great Lakes population, and colonies were established at Lake 

Champlain, Oneida Lake, and Lake Erie. In 1996, 8,410 pairs were counted on Little Galloo 

Island alone. In the summer of 1998, frustrated commercial fishing guides, believing cormorants 

responsible for lowered fishing success, illegally slaughtered 1500 – 2000 cormorants on Little 

Galloo. Ten individuals plead guilty to charges stemming from the slayings. Five who plead 

guilty to the most serious charges were sentenced to home confinement and fined. In spring of 

1999, the USFWS issued a permit to New York state to oil eggs in up to 7,500 nests on Little 

Galloo Island to reduce recruitment into the Eastern Basin and to slow expansion of cormorants 

to new nesting sites. Control efforts are also undertaken on Oneida Lake, in the form of nest 

destruction during the breeding season, and harassment of migrant cormorants during the fall 

(VanDeValk et al. 1999; R. Miller, pers. comm.). See summary of New York (Coastal) for 

additional information on DCCOs in New York.

Land Ownership

Ownership information was provided for all 12 inland colonies. Seven were on privately 

owned lands, which were owned by NGOs such as The Nature Conservancy, private 

corporations, organizations and individuals. The other five were on public lands managed by 

federal and state agencies (R. Miller, pers. comm.).

Productivity

Weseloh and Ewins (1994) conducted undisturbed counts of medium-large young at the 

Little Galloo Island colony in July during 1981 – 1991, and estimated an average productivity of 

2.14 young per active nest. On Wantry Island, Oneida Lake, 1.6 chicks per nest were allowed to 

complete incubation (VanDeValk et al. 1999).

Figure 41. Changes in number of breeding pairs on Little Galloo Island, 1977 – 1998.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

Year

No. Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 3 89

North Dakota

Summary of Population Data and Trends

(Survey completed by C. Grondahl, North Dakota Game and Fish Dept.)

Breeding.

The DCCO has been documented as a nesting species in North Dakota since the late 

1800s (see Lewis 1929 and Stewart 1975 for records). Currently, the DCCO nests in several 

areas across the state, but breeding surveys have not been conducted and precise locations are not 

known. Stewart (1975) reported “during recent years, the largest segment of the breeding 

population in the state has become established as scattered colonies along the margin of Lake 

Sakakawea or Garrison Reservoir. Other recent colonies have been found on the Oahe Reservoir 

about 10 miles below Bismarck and on scattered lakes and river impoundments located within 

the Prairie Pothole Region and in the Turtle Mountains.” He also described two distinct nesting 

habitats: all colonies along the Missouri River reservoirs nested in the tops of dead trees, mainly 

cottonwoods; colonies located on natural lakes were mostly on the ground on isolated islands. C. 

Grondahl (pers. comm.) suggested that, presently, all appropriate habitat east of the Missouri 

River system may have DCCOs. Some prime nesting locations include Devils Lake in the 

northeastern portion of the state, the back bays of upper Lake Sakakawea, several areas on Lake 

Oahe, and wetlands throughout central North Dakota. The southeastern and northeastern portions 

of the state also have plenty of good habitat. While no estimates of breeding numbers are 

available, personal observations of several North Dakota Game and Fish Department employees 

suggest that DCCOs have increased since 1975, when state status and distribution was reviewed 

by Stewart (1975) (C. Grondahl, pers. comm.).

Winter.

In winter, the DCCO does not occur (C. Grondahl, pers. comm.).

Migration.

No information was provided on migrants.

Historical Information

It is not clear when the DCCO was first recorded breeding in North Dakota, but some 

early observations of nesting colonies were made in 1897 at Sweetwater Lake, and in 1898 at 

Devils and Stump Lakes. The Devils Lake colony was active through 1920, and the Stump Lake 

colony was active as late as 1925. It is not clear when the Devils Lake colony was re-colonized. 

Some high counts prior to 1975 include 1,260 breeding pairs on all of Lake Sakakawea, 1958; 

500 nests at Chase Lake, Stutsman County, 1967; 200 – 300 nests in one colony on Lake 

Sakakawea or Garrison Reservoir, 1970; 225 nests on the Upper Souris NWR, Renville County, 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 90

1965; 170 nests on J. Clark Salyer NWR, McHenry County 1946 (see Stewart 1975 for these and 

additional records).

Land Ownership

No information provided.

Productivity

No information provided.

Ohio

Summary of Population Data and Trends

(Survey completed by M. Shieldcastle, Crane Creek Wildlife Research Station, Oak 

Harbor, OH.)

Breeding.

In 1987, the Double-crested Cormorant returned to Ohio as a nesting species, and though 

no chicks were fledged from this attempt, this nesting provided the first nesting record for the 

state in this century (see Historical Information, below). The first successful nesting was 

documented in 1992 at West Sister Island, where numbers increased steadily through 1995. A 

very large increase occurred between 1994 – 1995, then for the next 3 years remained fairly 

stable (Figure 42). In 1998, 1500 pairs were estimated at West Sister Island. Overall, this colony 

increased at an average annual rate of 41.6 % since discovery. A second small colony of 10 pairs 

was found at Mercer Wildlife Area in 1998, bringing the state total to 1510 pairs (M. 

Shieldcastle, pers. comm.).

Winter.

A small number of birds (50) are estimated to winter in the state (M. Shieldcastle, pers. 

comm.). Review of CBC data collected between 1984 – 1998 (Sauer et al. 1996) identified no 

sites where ≥ 100 individuals were estimated during any count, but small numbers were observed 

at some sites.

Migration.

Large numbers (25,000) are reported during peak migration, which occurs in the spring 

approximately April – May, and in the fall August – October. Migrants are observed roosting on 

islands around and feeding on western Lake Erie, including Sandusky Bay (M. Shieldcastle, 

pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 91

Historical Information

In the early 1800s cormorants were regular migrants along Lake Erie. The creation of 

canal reservoirs in the state provided new habitat, breeding populations formed in the 1860s and 

1870s at Buckeye Lake and Lake St. Mary’s (Peterjohn and Zimmerman 1989). However, 

cormorants may have been nesting at Buckeye Lake earlier than this, since Wheaton (1882) 

writes that the species “was said to have nested years ago at the Licking Reservoir [Buckeye 

Lake].” There is also some confusion over how many colony sites were actually documented. 

Lewis (1929) reported that prior to the 1900s, cormorants nested at Grand Reservoir, St. Mary’s 

Reservoir and possibly at Licking County. However, Grand Reservoir and St. Mary’s Reservoir 

may be the same site, since they both occur in the same counties (Mercer and Auglaize), and 

today Lake St. Mary’s is called Grand Lake St. Mary’s. “Large numbers” of birds were reported 

to nest at St. Mary’s colony, while only 10 – 15 pairs were recorded at the Buckeye Lake site. 

However, these colonies were subject to indiscriminant hunting and egg collecting (Peterjohn 

and Zimmerman 1989). Numbers decreased rapidly at St. Mary’s Reservoir after 1867 (Langdon 

1878), and by the early 1880s, this colony and the one at Buckeye Lake had disappeared 

(Henninger 1904; Peterjohn and Zimmerman 1989). By the turn of the century (or earlier) the 

cormorant no longer nested in the state (Dawson 1903; Jones 1903).

Between 1900 and 1950 migrants were still observed during the spring and fall along 

Lake Erie, where they were uncommon to fairly common, though flocks were seldom larger than 

10 – 20 individuals. In the late 1950s, reduced numbers became evident, and by 1965 they were 

rare throughout Ohio with < 10 sightings annually of 1 – 5 individuals. Declines in the state were 

thought to have resulted from widespread DDT use and other contaminants throughout the 

region (Peterjohn and Zimmerman 1989).

In the 1980s, the cormorant population began to recover in Ohio and hundreds of birds 

were recorded during migration. Large concentrations ( 500 – 2000) along western Lake Erie, a 

major staging area for the Great Lakes nesting population, have become a regular fall 

occurrence. In 1987, six pairs nested at the Ottawa Wildlife Refuge on w. Lake Erie, and though 

these nests ultimately failed, this provided Ohio’s first nesting record in this century.

Land Ownership

Land ownership was provided for both breeding sites. The West Sister Island colony is 

on federal land, owned by the USFWS; the Mercer WA is on state land, owned by the Ohio 

Division of Wildlife (M. Shieldcastle, pers. comm.).

Productivity

No estimates available.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 92

Figure 42. Changes in numbers of breeding pairs in Ohio, 1992 – 1998.

0

200

400

600

800

1000

1200

1400

1600

1991 1992 1993 1994 1995 1996 1997 1998 1999

Year

No. Pairs

Ontario

Summary of Population Data and Trends

(Survey completed by J. Harcus, Ministry of Natural Resources, Petersborough, Ontario; 

breeding data provided by D.V. Weseloh, Canadian Wildlife Service, Downsview, Ontario.)

Breeding.

Ontario has the largest number of breeding pairs of Double-crested Cormorants in the 

Great Lakes region, and perhaps the second largest on the continent. Between 1980 – 1997, there 

was a total of 116 known colony sites; in 1997, 74 colonies were active, and there was an 

estimated 35,159 pairs nesting in the St. Lawrence River and on Lakes Ontario, Erie, Huron and 

Superior. Large increases have occurred over the last 25 years (Figure 43); these have been 

attributed to decreased contaminant loads in the Great Lakes, and to significant increases in 

alewife and rainbow smelt (Weseloh and Collier, 1995; Weseloh et al. 1995). However, growth 

appears to be slowing in several locations, e.g. the North Channel, Lake Huron; Lake Ontario; 

and Lake Superior. Between 1981 – 1990, the average annual rate of increase was approximately 

38 %; but D.V. Weseloh reported that between 1993 / 1994 – 1997, the annual rate of increase 

was only 3.7 % (in Korfanty et al 1997). Numbers in the upper Great Lakes are expected to level 

off in the next decade (Ludwig and Summer 1997).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 93

Winter.

Birds are not known to winter in the province (J. Harcus, pers. comm.).

Migration.

Migrants do not pass through, as birds found in the province originate there (J. Harcus, 

pers. comm.).

Historical Information

The history of the Double-crested Cormorant in Ontario was described by Korfanty et al. 

(1997) and is summarized here, unless otherwise noted. Cormorants have nested in Ontario for 

hundreds of years; the first documented breeding dates back to 1758, when cormorants were 

recorded breeding at Lake of the Woods, where they continue to nest today. Between 1900 –

1920 they moved eastward to lakes Superior and Nipigon; the first documented nesting in the 

Great Lakes occurred on the far western end of Lake Superior in 1913. Lewis (1929) reported 

active colonies in eastern L. Superior, Lake Nipigon and Lake of the Woods between 1927 –

1928 with an estimated 80 pairs. In 1931 they began nesting in the North Channel of Lake 

Huron, and by 1938 and 1939, breeding colonies were established on lakes Ontario and Erie, 

respectively. By the 1950s, the cormorant population on the Canadian Great Lakes peaked at 

about 900 nests, an abundance that led sport and commercial fishermen to call for a cormorant 

control program. In response, a cormorant control program was introduced in Ontario, primarily 

on Georgian Bay. Efforts included destruction and spraying of eggs. Illegal control also 

occurred, with fish harvesters shooting adults, and destroying eggs, nests and young. However, 

cormorant control only slowed the growth of the cormorant population in the 1940s and 1950s, 

control was continued on the Canadian Great Lakes until 1966.

Between the 1950s and the 1970s, the number of nesting pairs on the Canadian Great 

Lakes was reduced by 86 %. This reduction is thought to have occurred mainly as a result of 

toxic chemical concentrations (e.g. DDT, DDE, and PCBs) in the Great Lakes and their impact 

on cormorant reproduction. DDT and other pesticides were banned in the 1970s, and as levels of 

contaminants declined in the Great Lakes, cormorant reproduction began to improve and 

cormorant numbers began to dramatically increase.

Land Ownership

No land ownership information was provided.

Productivity

No productivity information was provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 94

Figure 43. Changes in breeding numbers in Ontario, 1973 – 1997.

0

5000

10000

15000

20000

25000

30000

35000

40000

1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

Year

No. Pairs

Saskatchewan

Summary of Population data and Trends

(Survey completed by K. Roney, Royal Saskatchewan Museum, Regina, Saskatchewan.)

Breeding.

In Saskatchewan, there has been a total of 19 known colonies since 1968, but some have 

become inactive. In 1980, there was a total of 14 known colonies; in 1991, the last year Doublecrested Cormorants were censused, there were 10 active colony sites with an estimated 19,547 

pairs (K. Roney, pers. comm.). Complete censuses have been conducted since 1968, at which 

time 1078 pairs were estimated (Vermeer 1970). Numbers appear to have steadily increased 

(Figure 44); the average annual growth rate between 1968 – 1991 was 13.4 %. The greatest 

growth occurred during the 1970s; between 1985 – 1991, growth slowed to 2.7 %. The species is 

currently protected under the province’s Wildlife Act (K. Roney, pers. comm.), but we were 

unable to determine the exact date when the species received province-wide protection (see 

Historical Information, below).

Winter.

No information provided.

Migration.

No information provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 95

Historical Information

The earliest mention that we found of the Double-crested Cormorant in the Saskatchewan 

area was by Nuttall (1834, cited in Mendall 1936), who noted that a DCCO (identified as P. 

dilophus) was collected on the Saskatchewan River in the month of May. Audubon (1843) also 

noted that the species bred on the Saskatchewan River. The species has been nesting in the 

province for over a century. W. Spreadborough found a colony with 27 nests at Crane Lake in 

1894, and J. Macoun reported breeding at Old Wives Lake in 1895; around this time, the species 

was reported as “abundant and breeding westward to Old Wives Lake and Crane Lake,” and to 

breed in all suitable places around Prince Albert and at Big Stick Lake (Macoun and Macoun 

1909, cited in Lewis 1929). Seton (1908) reported a large colony at Lake Isle a la Crosse. By the 

time Lewis was writing, breeding appears to have ceased at Old Wives, Crane and Big Stick 

lakes, but was documented at 9 other locations with roughly 1600 pairs.

During 1950 – 1965, large scale destruction of nests by fishermen and ranchers occurred 

in the province because cormorants were perceived as a threat to fishing interests. The first 

thorough survey conducted in 1968 found nine colonies, though one or two may have been 

missed. Several colonies had disappeared by this time due to declining lake levels and human 

disturbance, and the breeding population had declined since the late 1920s (Vermeer 1970). In 

1971, five colonies were given protection, and numbers have increased dramatically in the 

province since that time (Vermeer and Rankin 1984; K. Roney, pers. comm.).

Land Ownership

Ownership information was provided for eight of the most recently active colonies. Most 

were on public lands in either wildlife refuges, bird sanctuaries, or National Park lands; one was 

on land that is part of a National Defense Area (K. Roney, pers. comm.).

Productivity

No estimates provided.

Figure 44. Changes in breeding numbers in Saskatchewan, 1968 – 1991.

0

5000

10000

15000

20000

25000

1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994

Year

No. Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 3 96

South Dakota

Summary of Population Data and Trends

(Survey completed by E. Dowd-Stukel and D. Backlund, South Dakota Dept. of Game, 

Fish and Parks; breeding data supplied by R. Peterson, South Dakota Ornithologist’s Union.)

Breeding.

The DCCO has been documented as a nesting species in South Dakota since at least the 

late 1800s (see Historical Information, below). Between 1988 – 1992, surveys were conducted 

during the South Dakota Breeding Bird Atlas, and 36 DCCO colonies were documented 

(Peterson 1995). Of these, 10 were on human influenced habitats (e.g., flooded timber of the 

Missouri River reservoirs and other impoundments, E. Dowd-Stukel, pers. comm.), while 25 

were on natural habitat; habitat type for one colony was not reported. The majority of colonies 

are found in the eastern portion of the state, and nesting habitat includes islands, sandbars and 

trees near lakes (Whitney et al. 1978). Colonies also occur along the Missouri River. In the Sand 

Lake NWR all nests were in cattail marshes on floating mats. Large numbers occurred in 

Kingsbury County, where a total of 2,962 pairs nested in 11 colonies in 1991 (Petersen 1995). 

No data post-1992 were available. Because regular statewide censuses are not conducted, 

information on breeding trends and state breeding total is not available. Colonies along the 

Missouri River may be in decline as flooded timber disappears. Formerly large colonies at Crow 

Creek and Grand River are much smaller, and this is probably true on most reservoirs (E. DowdStukel and D. Backlund, pers. comm.).

Winter.

In winter, not reported to occur (E. Dowd-Stukel and D. Backlund, pers. comm.). The

South Dakota Ornithologists’ Union (1991) notes that early winter records are probably late or 

crippled migrants.

Migration.

No areas known to provide important stopover sites for migrants (E. Dowd-Stukel and D. 

Backlund, pers. comm.). In spring, migrants are normally observed the first two weeks of April; 

in fall, mid-September to early October (Whitney et al. 1978; South Dakota Ornithologists’ 

Union, 1991).

Historical Information

The earliest record we obtained was that of Agersborg (1885, cited in Lewis 1929), who 

reported that “DCCOs once nested commonly in trees along streams in southeastern South 

Dakota, but that with the cutting of the timber they abandoned the region.” Lewis (1929) 

reported “present colonies” only in the northeastern portion of the state. No additional 

information was obtained.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 97

Land Ownership

No information provided.

Productivity

No information provided.

Utah

Summary of Population Data and Trends

(Survey completed by F. Howe, Utah Division of Wildlife Resources.)

Breeding.

The DCCO has been documented as a nesting species in Utah for the last 150 years (see 

Historical Information, below). The subspecies occurring here is believed to be P.a. auritus

(Hayward et al. 1976). Between 1971 and 1999, a total of eight colonies was known to be active. 

Consistent data are not collected on DCCOs in this state, so statewide totals and precise 

population trends are not available. However, up until 2000, Utah’s largest colony and most of 

Utah’s breeding population occurred at Bear River Refuge, where data have been collected 

regularly. Numbers at Bear River have fluctuated since 1971, but increased overall up to 1997. In 

1998 and 1999, declines occurred, and in 2000 the unit was dry due to construction and no 

nesting occurred (Figure 45). With the exception of Great Salt Lake (GSL), all other colonies 

have been small (< 55 pairs). At GSL, no information has been obtained on numbers of breeding 

pairs or nests, though numbers of birds were counted each July at five state Waterfowl 

Management Areas on the east shore of GSL between 1980 – 1992. During these counts, the 

number of individual birds ranged from 29 to 429 (F. Howe, pers. comm.).

Winter.

In winter, the DCCO is not reported to occur (F. Howe, pers. comm.). Review of CBC 

data collected between 1984 – 1998 (Sauer et al. 1996) identified no sites where ≥ 100 

individuals were estimated during any count, but small numbers were observed at a few sites.

Migration.

Peak fall migration occurs mid-to-late September; no information was provided for 

spring migration. The Bear River Refuge was reported as a significant stopover site for migrant 

birds, with numbers ranging from 2 – 500 birds (F. Howe, pers comm.).

Historical Information

Stansbury (1852) gave the first description of DCCO colonies on Egg Island, Great Salt 

Lake, when he visited this site in 1850. Ridgway (1877) also recorded the species around Great 

Salt Lake. There are numerous early nesting records from this area, and early breeding colonies 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 98

reported in the literature all occurred in northern and western Utah. Records obtained between 

1926 – 1942 include nesting at Egg Island, Great Salt Lake, in Salt Lake County, and in Cache 

County, five miles west of Logan. The latter colony nested in dead trees along the banks of Little 

Bear River (Bee and Hutchings 1942). Breeding colonies have also been reported in Rich, 

Millard, Box Elder, Davis and Utah counties. Between the 1920s and 1970s numbers of nesting 

DCCOs decreased in Utah. In 1974, a total of 13 colonies was known to have existed in the state, 

but only five were still in use. During the 1930s and 1940s the larger colonies may have 

supported a combined total of > 1000 birds, but by 1973 a total of 386 birds (number of birds, 

not pairs, reported) was estimated to nest in five colonies (Mitchell 1977).

Land Ownership

Information was provided only for the Bear River Refuge, which is federal land managed 

by USFWS (F. Howe, pers. comm.).

Productivity

No information provided.

Figure 45. Changes in breeding numbers at Bear River Refuge, Utah, 1971 – 1996.

0

50

100

150

200

250

300

350

400

1970 1975 1980 1985 1990 1995 2000

Year

Number of Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 3 99

Vermont

Summary of Population Data and Trends

(Survey completed by L. Garland, Vermont Fish and Wildlife, Essex Junction, VT.)

Breeding.

The Double-crested Cormorant appears to be a fairly recent breeder in Vermont, with the 

first known nesting documented in 1982 (Laughlin and Kibbe 1985). In 1999, there were five 

known colony sites; two were active with a total of 2805 pairs. After initial discovery numbers 

slowly increased for the first seven years. By 1995, numbers had nearly quadrupled since 1989, 

and have continued to steadily increase (Figure 46). Over the last decade numbers increased at an 

average annual rate of 22 %. Nearly all of the growth has occurred on Young Island; in 1999, > 

90 % of pairs were on this island (L. Garland, pers. comm.). 

Winter.

No significant numbers winter in the state (L. Garland, pers. comm.).

Migration.

No significant concentrations are reported during migration (L. Garland, pers. comm.).

Historical Information

The earliest records we were able to obtain were from the 1930s, when three specimens 

and one sight record were listed for the state. During the Vermont Atlas Project Survey period 

(1976 – 1981) observers documented increasingly frequent summer occurrences of Doublecrested Cormorants on Lake Champlain, especially around Young Island. The first nesting for 

the state was documented on Young Island the year after the atlas survey, 1982; one nest was 

observed in a tree. Breeding was not recorded at other sites until 1994, when cormorants 

expanded nesting to Mud Island. In 1996 they expanded further to Bixby, Shad and Popasquash 

Islands. Between 1994 – 1997, nests were removed from Mud , Bixby and Popasquash Islands. 

In 1998, cormorants did not return to nest at Mud or Popasquash Islands, but did return to Bixby 

Island, where harassment techniques prevented successful nesting and eventually drove the birds 

from the island (L. Garland, pers. comm.).

Land Ownership

Both Young and Mud Islands are on state land; Shad Island is on federal land; 

Popasquash is on private land, owned by the Green Mountain Audubon Society; and Bixby 

Island is owned by a private individual (L. Garland, pers. comm.).

Productivity

Fowle et al. (1997) estimated productivity on Young Island on Lake Champlain in 1995 

and 1996. Multiplying the mean number of young per nest by the number of nests for each 

season, they estimated that 2.54 young fledged per nest in 1995 and 1996.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 100

Figure 46. Changes in numbers of breeding pairs in Vermont, 1982 – 1999.

0

500

1000

1500

2000

2500

3000

3500

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

Year

No. Pairs

Wisconsin

Summary of Population Data and Trends

(Survey completed by S. Matteson, Wisconsin DNR, Bureau of Endangered Resources, 

Madison, WI.)

Breeding.

Wisconsin has the second largest number of breeding pairs in the U.S. Great Lakes. In 

1997, there was a total of 46 known colony sites. Of these 23 were active, with a total of 10,546 

pairs. Seventeen colonies are on the Great Lakes (Apostle Islands, Lake Superior; Green Bay / 

Lake Michigan Islands); the rest are on inland lakes, marshes and the Upper Mississippi River. 

However, the Great Lakes colonies, particularly those on the Green Bay / Lake Michigan 

Islands, support a far greater number of nesting pairs than do the inland colonies. By 1990, the 

number breeding on the Great Lakes was about five times larger than that utilizing inland habitat. 

Increases in breeding numbers have occurred steadily since 1973 (Figure 47). Matteson et al. 

(1999) used a regression model to calculate trends in the state during 1973 – 1997, and reported 

an average annual increase of 24.7 %; they also noted that the rate of increase significantly 

slowed in recent years, 13.2 % during 1986 – 1997, compared to 36.9 % during 1973 – 1985. 

During the more recent period, increases were significantly greater on Great Lakes sites than on 

inland sites.
FINAL DRAFT Part II. Population Data and Trends, Zone 3 101

Winter.

Cormorants are not known to winter in the state in any significant numbers (S. Matteson, 

pers. comm.).

Migrants.

Spring migration occurs late March – late April; fall migration, mid-to-late August –

November. During fall migration several thousand (5,000 – 7,000 in 1991 and 1992) are seen on 

the Upper Mississippi River, and large numbers congregate around LaCrosse and Trempeleau 

(Kirsch 1995).

Historical Information

Around 1875 DCCOs were common migrants in suitable waters throughout Wisconsin. 

But by the turn of the century, numbers of migrants appear to have declined; Kumlien and 

Hollister (1903) note that “even when it was more common, comparatively few were noticed in 

the interior during the fall.” However, large numbers were still at least occasionally observed 

into the 1920s; on April 24, 1926, a huge number of birds was observed migrating up the 

Mississippi River past LaCrosse, estimated at 100,000 to 1,000,000 individuals, a flock size 

which by this time was considered “unusual” for the region. The flock was so large “that at times 

it was impossible to see the sunset sky through the mass” (Grassett 1926). USFWS unpublished 

reports note that 20,000 to 40,000 cormorants used the Upper Mississippi River during spring 

and fall migration between 1939 – 1950 (Kirsch 1995). Anderson and Hamerstrom (1967) also 

report that “large” numbers of migrants, 2,000 – 5,000 individuals, were still reported in the 

1940s and 1950s, but these are nothing like the huge numbers reported by Grassett (1926). By 

the late 1950s, migrants appeared to decline and continued to do so through the 1960s, with none 

to very few reported during the migration (Anderson and Hamerstrom 1967). Numbers of 

migrants on the Upper Mississippi River have apparently increased some since the late 1960s, 

but have remained much lower than those reported in the 1940s (Kirsch 1995).

The breeding history of the cormorant and changes in its numbers and distribution in 

Wisconsin were reviewed thoroughly by Matteson et al. (1999), and are summarized here. The 

first published reports of nesting did not occur until 1919 and 1921, when cormorants nested on 

Lake Wisconsin (formerly Okee Flowage) in south-central Wisconsin. However, Kumlien and 

Hollister (1903) report communications from C.F. Carr that cormorants nested earlier than this 

on “some of the larger, isolated lakes in the northern and central part of the state.” Additionally, 

Anderson and Hamerstrom (1967) report that cormorants probably bred in most of the state in 

the early 1900s. From 1921 to the mid-1950s breeding records were regularly obtained; during 

this time cormorants occupied 17 known colony sites in 16 counties. The total number of nesting 

pairs reached at least several hundred in peak years. However, from the 1950s to the early 1970s, 

numbers of breeding cormorants and colony sites sharply declined due to combined effects from 

bioaccumulation of DDT and its metabolites, human persecution and habitat loss. By 1966 only 

three active colony sites (24 pairs total) were identified; in 1972 the Double-crested Cormorant 

was officially listed by the state as endangered.

The installation of artificial nesting platforms, declines in DDE levels and protection as 

an endangered species led to a marked recovery. In 1973 a statewide survey revealed that the 

small population in 1966 had more than doubled, with a total of 66 pairs counted. By 1982 the 

state population had increased to 1028 pairs in 16 colonies, and due to its comeback, its official 

status was changed from “endangered” to “threatened.”
FINAL DRAFT Part II. Population Data and Trends, Zone 3 102

Land Ownership

No information provided.

Productivity

Statewide, mean annual production per nesting pair during 1973 – 1997 was 1.58, with a 

range of 0.00 – 3.36 young per nesting pair (Matteson et al.1999).

Figure 47. Changes in breeding numbers in Wisconsin, 1973 – 1997.

a

0

2000

4000

6000

8000

10000

12000

1970 1975 1980 1985 1990 1995 2000

Year

No. Pairs

a = conservative estimate, as not all colonies were censused in this year (Matteson et al. 1999)

Wyoming

Summary of Population Data and Trends

(Survey completed by A. Cerovski, Wyoming Game and Fish Dept.)

Breeding.

The DCCO has been documented as a breeding species in Wyoming since 1928 (see 

Historical Information, below). Since 1981, a total of 25 colonies have been documented, and 

occur throughout the state, with the exception of the southwest (Figure 28). All known colonies 

are not censused yearly, and precise information on trends is not available, nor is an estimate of 

the total number of birds breeding in the state. However, Findholdt (1985) reported a dramatic 

increase in numbers of nesting DCCOs between historical times and the mid-1980s. The main 
FINAL DRAFT Part II. Population Data and Trends, Zone 3 103

reason for this increase is thought to be construction of reservoirs with isolated islands. In 

addition to supplying breeding habitat, reservoirs likely increased prey abundance and 

availability. Also, decline in contaminants may also have enabled numbers to increase.

Between 1986 – 1994, numbers appear to have declined. In 1986, 21 colonies were 

counted; of these, 16 were active and a total of 1,457 nests was estimated. Large numbers (113 –

303 pairs) occurred at Cooper and Ocean Lakes, Pathfinder Reservoir Bird Island, Soda Lake 

West Island and Soda Lake Manmade Island. In 1994, 15 colonies were censused and an 

additional two were known to be active. At most colonies declines occurred. At the censused 

colonies, a total of 364 nests was estimated, and only one large (> 100 pairs), located at 

Yellowstone Lake Molly Island (125 pairs), was reported. Eight colonies were known to be 

inactive. Changes in water levels were responsible for lack of breeding at least one colony, 

Cooper Lake (A. Cervoski, pers. comm.).

Winter.

In winter the DCCO is not known to occur in any significant numbers (A. Cervoski, pers. 

comm.).

Migration.

There are no known important migrant stopover sites, and no estimates of migrants were 

provided (A. Cervoski, pers. comm.).

Historical Information

The earliest records of DCCOs in Wyoming that we obtained were reported in Grave and 

Walker (1913) who note there are “but three Wyoming records, two from the Laramie Plains and 

one from near Buffalo.” The first confirmed breeding record was obtained on Molly Islands, 

Yellowstone Lake, at Yellowstone National Park in July 1928 (see Findholdt 1985 for records). 

Observations of small nesting numbers (< 25 pairs) were occasionally obtained at this location 

until the mid 1980s, at which time numbers began to increase. In the late 1980s – mid 1990s, 85 

– 148 pairs nested in any one year. Additional colonies were documented on the North Fork Arm 

of the Shoshone Reservoir in 1938, at Ocean Lake in 1942, and at Bamforth Lake in 1972 (see 

Findholdt 1985 for records). In 1984, Findholdt (1985) documented 15 active and five inactive 

colonies.

Land Ownership

Land ownership information was supplied for 20 colony sites. Of these, nine were on 

public lands, while the other 11 were owned by private individuals or corporations (A. Cervoski, 

pers. comm.).

Productivity

Productivity information (number of chicks fledged / nest) was provided for the colony 

located at Yellowstone Lake Molly Island. Between 1985 – 1996, productivity ranged from 1.4 

to 2.2. Productivity rates at this colony showed nearly steady increases during this time period, 

and the colony has grown fairly steadily from 25 nests in 1981 to126 nests in 1996 (A. Cervoski, 

pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 3 104
FINAL DRAFT Part II. Population Data and Trends, Zone 4 105

ZONE 4: SUMMARY OF POPULATION DATA FOR SOUTHEASTERN U.S.

Introduction

Zone Boundaries.

In addition to Florida, Hatch and Weseloh (1999) included Georgia, Alabama, 

Mississippi and Louisiana in the southeastern breeding zone, but noted that birds are generally 

rare or absent as breeders from these states. However, as numbers grow in the southeast, this 

description may no longer hold true. With the exception of Alabama, breeding is currently 

occurring in small numbers in all of these states. Additionally, breeding is occurring in other 

southern states, such as North and South Carolina, Tennessee, Arkansas, Oklahoma and Texas. 

Some of these nestings are recent, while some represent re-colorizations. Based mainly on winter 

distribution of cormorants and possible floridanus subspecies designations (birds in TX, NC and 

SC, see below), we have expanded the boundaries of this population zone to include all of these 

southern states.

As noted in the introduction to this section, the winter range is a succinct geographic unit 

and it would not make biological sense to split it up. Additionally, birds breeding in some 

southern states may be year-round residents, while birds from the interior and northeast Atlantic 

are migratory. Another factor that influenced our decision to include additional states in this zone 

is that birds breeding in Texas may be the floridanus subspecies (Palmer 1962; Johnsgard 1993), 

though it is possible that birds breeding in northwest Texas may be the auritus or albociliatus

subspecies, and be part of the interior or Pacific Coast populations, respectively. Similarly birds 

breeding in Oklahoma could be part of the interior Zone 3 population (Palmer 1962). Hatch and 

Weseloh (1999) included birds breeding in the Carolinas with the northeast Atlantic population, 

but the possible floridanus subspecies designation of these birds (Palmer 1962; Clapp and 

Buckley 1984; Post and Post 1988), along with winter distribution, prompted us to include them 

with the southeast population.

Breeding Information

With the exception of Florida, numbers of Double-crested Cormorants breeding in this 

zone are marginal; compared to Zones 3 and 5, numbers have remained small over the last two 

decades. The species has just recently returned as a breeder to Arkansas, Mississippi, and 

Tennessee, and is thought to have just recently colonized South Carolina and Georgia. These 

latter states were the only states in this zone that had enough consistent data to estimate trends in 

breeding numbers; in both of these states numbers are increasing, though total numbers are 

insignificant compared to increases in Zones 3 and 5. If the estimate of breeding pairs in Florida 

is fairly accurate (see Florida summary), total number of breeding pairs in this zone is probably < 

10,000 (Table 5).

Winter and Migrant Birds

The southern and southeastern coastal U.S. provides much of the wintering habitat for 

Double-crested Cormorants breeding in the interior and along the northeast Atlantic Coast of the 

continent (Zones 3 and 5). Of the zones where DCCOs winter, this one provides habitat for the 

greatest number of birds (however, numbers wintering in Mexico are unknown). Most of the 

birds that breed in this zone are year round residents, based on the Florida population alone. Over 

the last few decades, numbers of wintering cormorants appear to have increased dramatically in 
FINAL DRAFT Part II. Population Data and Trends, Zone 4 106

this zone [based on surveys conducted by Wildlife Services and CBC data (Sauer et al. 1996)]. 

Data from CBCs conducted between 1959 – 1988 have been analyzed for several states in this 

zone, and show increases ranging from 3.5 – 18.7 %. The largest increases occurred in 

Louisiana, Mississippi, Oklahoma and Texas. Data collected during the last 10 years suggest that 

numbers are still increasing in these states and others, but these data have not yet been analyzed.

Based on CBCs conducted between 1984 – 1997 (Sauer et al. 1996), the number of birds 

occurring in December in southeastern U.S. ranged between 105,000 – 313,000 (Figure 48). This 

is an obvious underestimate, given that our incomplete estimate of breeding individuals from the 

interior population alone is > 300,000 birds, and does not include young of the year, nonbreeders or breeders from several states and provinces. Because CBC sites are selected to 

maximize variety of species and numbers counted, they do not sample all areas where 

cormorants are likely to occur, such as important roost sites around aquaculture facilities. 

Therefore, they tend to underestimate numbers (see data for Mississippi and Georgia, Table 5, 

and state summaries); however, they do provide useful information on trends and are used here 

as a general index of population change (see Intro for this section). Figure 51 compares 

distribution of DCCOs during winter months based on CBC data and winter roost locations 

obtained during surveys conducted by USDA/WS in Alabama, Arkansas, and Mississippi.

Historical Information

Prior to the twentieth century, the Double-crested Cormorant appears to have been an 

abundant year round resident, both as a breeding and wintering bird, along the Gulf Coast from 

Texas to Florida, and along the southern Atlantic Coast up through the Carolinas. Audubon 

(1843) described the floridanus subspecies as “constantly resident in the Floridas and their Keys, 

and along the coast to Texas.” He noted that the auritus subspecies wintered on the eastern coast, 

but rarely farther south than the Capes of North Carolina, where it encountered floridanus. 

Breeding was documented in the Carolinas, at least North Carolina, as early as the 18th century. 

Though breeding was not documented in most of the southern states until the 20th century, based 

on Audubon’s (1843) and other early observers’ comments, breeding may have occurred earlier 

in states such as Arkansas, Kentucky, Tennessee, Mississippi and Texas (see state summaries).

Information on status around the turn of the century is very limited; however, declines in 

breeding numbers in Arkansas and North Carolina were apparent by this time. In other states, 

such as Tennessee and Louisiana, declines in breeding numbers occurred later, between the 

1940s and 1970s; also declines occurred again in North Carolina during this time. In Mississippi, 

there is little information on breeding, but numbers reported in the 1950s appeared to be in 

decline. Based on Christmas Bird Count data, significant declines in wintering numbers in the 

mid-south states also occurred during this period. Declines have been attributed to human 

persecution, habitat degradation and pesticides (Howell 1911; Jackson and Jackson 1995).

During this low point in cormorant numbers, the catfish industry in the mid-South 

underwent its major growth. Commercial catfish farming got underway in Kansas and Arkansas 

in the 1950s. By 1965, the first commercial catfish pond was established in Mississippi, and over 

7,000 acres were developed for aquaculture in Arkansas. Texas and Louisiana also had 

significant acreage. However, Mississippi quickly became the industry leader, and by 1985 had 

over 85,000 acres of commercial catfish ponds (Stickley and Andrews 1989). As breeding 

populations in the interior and along the Atlantic Coast began to recover in the mid-to-late 1970s, 

numbers of wintering cormorants began increasing in the South. Increases in the Mississippi 

Delta and Texas have been especially dramatic.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 107

Jackson and Jackson (1995) point out that the timing of the mid-south breeding 

population decline and recent increase coincides with the DDT era, the subsequent banning of 

DDT and the decline of pesticide levels in Mississippi Delta ecosystems, which suggests that the 

mid-South at least shared the cause of the problem leading to the decline. Additionally, with the 

advent of aquaculture in the region, the development and re-colonization of southern breeding 

populations may be enhanced. Wintering birds have certainly been able to regularly take 

advantage of catfish ponds for foraging and loafing. This steady resource has likely contributed 

to greater overwinter survival, enabling more birds to return to northern breeding grounds, and to 

remain in the south as year-round residents.

Figure 48. December estimates of DCCOs in the Southeastern U.S., Zone 4,* based on 

CBCs, 1984 – 1997.

0

50000

100000

150000

200000

250000

300000

350000

84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals

* Includes AL, AR, FL, GA, KY, LA, MS, NC, OK, SC, TN, and TX; annual totals based on 

sites with ≥ 100 individuals
FINAL DRAFT Part II. Population Data and Trends, Zone 4 108

Table 5. Zone 4, Southern U.S. Summary of Population Data and Trends

(+ = increasing; - = decreasing; 0 = no change).

State Status 

(B, M, 

W, w)1

Known 

colony sites/

time period

No. active 

col./last yr 

surveyed

No pairs/ 

last year 

surveyed

Trend 

(winter 

birds)2

Winter number Migrant 

number

AL M, W - - - + 30,000-40,0003

;

X = 19,000 (7,000-

33,000)4

50,000

AR B,M, W 1/1999 1/1999 72-100 + X = 6200 (1,000-

16,000)4

1000s

FL B, M, W > 110/1989 84/1999 6,7455+/? X = 44,000 

(28,000-67,000)4

NA

GA B, M, W 7/1990s NA NA + 15,000-30,0003

;

X = 3176

(1500-6000)4

NA

KY M, w - - - + 500-10004

100s1000s

LA B, M(?), 

W

2/1994-99 2/1994-99 427 + X=19,000

(11,000-33,000)4

NA

MS B, M, W 2/1998 2/1998 33 + 65,0003

; X=12,000 

(1,000-23,000)4

NA

NC B, M, W 3/1980s-90s NA NA +/? 25,000-50,0003

;

X = 50,000 

(10,000-105,000)4

NA

OK B, M, W 2/1985-94 1/1996 30 + X = 6600

(2,000-14,000)4

NA

PR w - - - 0 Very few -

SC B, M(?), 

W

17/1985-96 10/1996 895 + 1000s3

; X=7,000 

(2,000-17,000)4

NA

TN B, M, w 3/1992-99 1/1999 29 +/? 100s3

; X=235

(21-791)4

2000

TX B, M, W > 5/1990s NA NA + X = 66,000 

(50,000-90,000)4

NA

Total 152 > 101 > 8231 NA NA

1 = B, Breeding; M, Migrant; W, Wintering; w, small numbers winter

2 = Winter numbers reported for most states based on CBC data, which have not yet been 

analyzed, but precursory examination suggests increases over the last 10-15 years; for some 

states other data also support increasing trend (see summaries).

3 = Estimate from winter surveys conducted or reported by state, federal or non-governmental 

agencies

4 = Mean and Range based on number of individuals counted on Christmas Bird Counts 

conducted over the last decade.

5 = Numbers of individuals were estimated, but very wide range of possible numbers reported, 

due to inaccuracy of aerial survey method. See Florida summary for detail.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 109

Figure 49. Distribution of South and Southeastern U.S. DCCO breeding population

(Zone 4) 1980 – 1999.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 110

Figure 50. Distribution and size of active colonies in South Carolina during last comnplete 

survey (1996).
FINAL DRAFT Part II. Population Data and Trends, Zone 4 111

Figure 51. Comparison of Christmas Bird Count sites (with 3 100 birds) with roost sites 

located by USDA/WS in Arkansas, Mississippi and Alabama.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 112

Alabama

Summary of Population Data and Trends

(Survey completed by F. Boyd, USDA Wildlife Services, Auburn AL.)

Breeding.

The Double-crested Cormorant occurs throughout the year in Alabama, and is common 

October to May (Imhof 1976). However, it has not been documented as a breeder in the state (F. 

Boyd, pers. comm.).

Winter.

In winter, the number of birds fluctuates, and it is difficult to provide estimates of 

wintering vs. migrating birds, but large numbers occur. Roughly 30,000 – 40,000 individuals are 

thought to overwinter (F. Boyd, pers. comm.). There are 16 known sites where birds concentrate 

during both migration and winter (Figure 51); 15 of these sites are used for both feeding and 

roosting. Only one site in Gainesville was reported as strictly a feeding site (F. Boyd, pers. 

comm.). Numbers of wintering and migrant birds are abundant around the coast, and have been 

gradually increasing inland since the 1980s (see Historical Information, below); however, 

numbers may now be leveling off (F. Boyd, pers. comm.). Christmas Bird Counts conducted 

between 1959 – 1988 suggested an increase of 11.6 % per year. More recent Christmas Bird 

Counts (1989 – 1998) estimated that approximately 7,000 – 33,000 birds wintered in the state 

during the last decade. Though these data have not yet been analyzed, numbers appear to still be 

increasing, with high counts between 23,000 – 33,000 reported during 1995-1998 (Figure 52) 

(Sauer et al. 1996).

Migration.

While it is difficult to differentiate migrant from overwintering birds, in late winter large 

numbers of migrants begin to pass through. In March – April, peak numbers of 50,000 birds 

(combined migrants and wintering birds) are reported (F. Boyd, pers. comm.).

Historical Information

Little information available. Imhof (1976) reported that the species was abundant on the 

Gulf Coast in winter and during migration; it also occasionally summered on the coast. However, 

the species had begun to decline inland around 1945, and in 1961 was considered rare 

everywhere inland in the state (Imhof 1976). Numbers remained scarce inland until about 1980, 

at which time the species started showing up again in western Alabama. In the late 1980s, 

cormorants were observed in other areas of the state and along river systems. These were mostly 

migrants in northeastern Alabama and on the Eufala Reservoir in the southeast, where high 

concentrations are currently reported (F. Boyd, pers. comm.).

Land Ownership

Land ownership was provided for 11 of the 16 known roost / feeding sites. Six of these 

sites are on federal land, owned by the Army Corps of Engineers. The other five are on private 

lands, one of which is owned by a hunting club (F. Boyd, pers. comm.). No other information 

was provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 113

Productivity

No documented breeding (F. Boyd, pers. comm.).

Figure 52. December estimates of DCCOs in Alabama based on CBCs, 1984 – 1997.

0

5000

10000

15000

20000

25000

30000

35000

84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals 

Arkansas

Summary of Population Data and Trends

(Surveys completed by M. Hoy, USDA/WS, Stuttgart, AR, and K. Rowe, Arkansas Game 

and Fish Commission, Humphrey, AR; winter data provided by S. Werner, USDA/WS, 

Mississippi State University, MS.)

Breeding.

In 1999, the Double-crested Cormorant was documented as a nesting species in Arkansas 

(Anonymous, 1999) for the first time since 1951 (James and Neal 1986), though unreliable 

reports of breeding have been made since that time (K. Rowe, pers. comm.). On May 19, 1999, 

more than 100 nests were discovered on Millwood Lake in southwestern Arkansas between 

Yarborough Landing and the dam. The birds were using only live Cypress trees. During this 

discovery, 11 cormorants were collected for DNA testing, food habits and other studies. On June 

30, 72 active nests were documented and 137 adult and young cormorants were counted on the 

lake; 106 of these were collected for the previously mentioned studies, for a total of 117 birds 

collected (Hutchinson 1999). The state director for Wildlife Services stated that his agency 
FINAL DRAFT Part II. Population Data and Trends, Zone 4 114

hoped to “keep this population down and maybe eliminate it,” as the catfish industry and sports 

fishermen do not want cormorants to become established year-round in Arkansas (Anonymous 

1999).

Winter.

James and Neal (1986) reported that, in winter, the species is present locally in small 

numbers in the southern areas of the state, but that most birds leave after the arrival of severe 

temperatures. However, sometimes large numbers are still present in the latter half of December, 

especially in recent years. While no estimates of wintering numbers were available from state 

agencies, USDA/WS conducts winter cormorant surveys which are not state wide, but focus on 

catfish production areas. In 1991, 36 cormorant night roosts were located in southeastern 

Arkansas during February and March (Figure 51); numbers of cormorants observed at these 

roosts ranged from 1 to > 1000 individuals (S. Werner, pers. comm.). Additionally, during 

Christmas Bird Counts, large numbers have been tallied at reservoirs around the state. Counts 

conducted between 1985 – 1998 ranged between approximately 100 – 16,000 birds (Figure 53) 

(Sauer et al. 1996). Large increases appear to have occurred over the last decade, but percent 

change is not available, as these data have not yet been analyzed. In 1996, a high count of 16,000 

birds was made; high numbers at Holla Bend NWR, Pine Bluff and Magnolia-Lake Columbia 

contributed most to this count. Jackson and Jackson (1995) examined Christmas Bird Count data 

from 1946 – 1991 and noted that counts in western Arkansas that included major reservoirs had 

increases paralleling those along the Mississippi River. During the last two Christmas Bird 

Counts (1997, 1998) numbers appear to have dropped considerably (Figure 53).

Migration.

Large numbers are recorded during spring and fall migration along the Arkansas River 

valley, Mississippi River bottoms, Bayou Meto bottoms, Millwood Lake, and at fish farming 

regions in the ne, se, and central portions of the state (M. Hoy, pers. comm.). In the fall, peak 

numbers are observed from mid-September to early-December. In the spring, peak numbers are 

recorded in February and March.

Historical Information

Audubon (1843) noted that in the early 1800s, shortly after young had fledged, many 

cormorants could be found as far north as the Arkansas River. In the late 1800s, the species was 

recorded as fairly abundant in parts of the state, and was apparently a year round resident. In the 

early part of 1889, cormorants were considered “fairly common in Poinsett County” (James and 

Neal 1986), suggesting that the species wintered in the state in fair numbers, at least during some 

years. Howell (1911) reported large numbers of migrants observed in the late 1800s and early 

1900s at Helena, Menasha Lake and along the White River at Crocketts Bluff, but by the early 

20th century, cormorants had apparently declined. By the time Howell was writing, the species 

could only be found in the wilder and more remote parts of the state, but was noted as being 

“formerly abundant in the rivers and swamps of eastern Arkansas.” Howell (1911) attributed the 

cormorant’s decline to human modification of the landscape. Though the first Arkansas nesting 

was not documented until 1910, the species apparently had nested in the state previously and in

greater numbers; Howell (1911) reported that the colony of 100 – 200 nests in cypress trees that 

he observed in 1910 at Walker Lake, Mississippi County, was probably the only large colony 

that remained in the state at that time. By 1924, Walker Lake had been drained and the one large 
FINAL DRAFT Part II. Population Data and Trends, Zone 4 115

nesting colony had disappeared. Prior to 1999, the last known nesting in the state was in 1951 at 

Grassy Lake (James and Neal 1986).

The species remained in the state mostly as a migrant, and large numbers were reported 

during spring migration as early as 1939, when 2000 were seen at Horseshoe Lake in February; 

4000 were observed at Big Lake Refuge in March, 1946. Wintering numbers have recently 

increased; during Christmas Bird Counts between 1948 and 1961 few were reported. James and 

Neal (1986) suggested that the large numbers remaining in recent years may be attributable to 

the impoundment of large amounts of water and to recoveries from population declines 

noticeable in Arkansas by the 1960s.

Land Ownership

Land ownership information was provided by K. Rowe and M. Hoy. K. Rowe provided 

information on nine sites: one wintering / migration site, Lake Millwood, and eight additional 

sites used during migration. Of these nine sites, three were on public lands owned by the Army 

Corps of Engineers; two were on state lands, and four were on private lands. M. Hoy provided 

information on wintering and migration sites at a broader, more general scale (e.g., Mississippi 

River bottoms, etc.). He noted that for the Arkansas and Mississippi River bottoms and Bayou 

Meto bottoms, where birds concentrate in winter and during migration, ownership was both 

public and private. Other wintering and migration sites were on private lands.

Productivity

No information provided.

Figure 53. December estimates of DCCOs in Arkansas based on CBCs, 1984 – 1997.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals
FINAL DRAFT Part II. Population Data and Trends, Zone 4 116

Florida

Summary of Population Data and Trends

(Survey completed by J. Rodgers, Jr., Florida Game and Fresh Water Fish Commission, 

Gainesville, FL.)

Breeding.

The Double-crested Cormorant is a year round resident in Florida. P.a. floridanus is the 

breeding race and is present year round; P.a. auritus migrates into the state for the winter. In 

1999, the Florida Fish and Wildlife Conservation Commission conducted a statewide colonial 

waterbird survey and estimated there were 6,745 pairs (approximately 13,490 individuals) of 

Double-crested Cormorants at 84 colonies. Number of pairs at each specific colony was not 

available, thus we could not map colonies by size. However, the following data were provided: 

average colony size was 80.3 nests; 19 % of colonies were < 50 nests; 6 % of colonies ranged 

from 51 – 100 nests; 7.1 % ranged from 101 – 150 nests; only about 7 % of colonies had greater 

than 150 nests. Though estimates from statewide colonial waterbird surveys conducted in 1989 

(8,898 – 23,980 individuals at 110 colonies) and 1977 (28,000 + individuals at 77 colonies) were 

also provided, at this time it is not possible to determine if cormorants in Florida are stable, 

increasing or decreasing (J. Rodgers, pers. comm.) for a number of reasons. The survey method 

consisted of flying over a colony and estimating the size class of the colony. At the end of the 

survey, size classes were tallied and numbers of birds summed up. The large range in the 1989 

estimate is indicative of the difficulty of estimating the number of birds through aerial counts; 

dark plumaged species such as cormorants, it was noted, generate especially crude estimates that 

bias current and future population analyses. Additionally, the actual number of birds in 1989 was 

probably underestimated, because coverage of coastal sites was incomplete during this survey 

year. Finally, survey methods were modified in 1999 for better species detection and the data 

have not yet been analyzed. However, in 1989 only 84 colonies were located statewide, while in 

1989, 110 colonies were counted (J. Rodgers, pers. comm).

Winter.

Large numbers winter throughout the state, but no information was provided on winter 

numbers. Christmas Bird Counts conducted from 1959 – 1988 suggested an increase in wintering 

birds of 3.5 % per year. More recent Christmas Bird Counts (1989 – 1998) estimated that 

approximately 28,000 – 67,000 birds wintered in the state during the last decade (Sauer et al. 

1996). These data have not yet been analyzed, and trends are not readily apparent; however, high 

counts of approximately 67,000 and 54,000 birds were observed in 1992 and 1993, respectively 

(Figure 54).
FINAL DRAFT Part II. Population Data and Trends, Zone 4 117

Migration.

Cormorants migrate through the state; November – March was reported as the peak 

migration period, but no information was available on migration numbers (J. Rodgers, pers. 

comm.).

Historical Information

The Double-crested Cormorant has been a long time, year round and abundant resident in 

Florida. Audubon (1843) reported that the Florida Cormorant (P. a. floridanus) was a constant 

resident in Florida, the Florida Keys, and along the coast to Texas. The Double-crested 

Cormorant (P.a. auritus), he noted, rarely went farther south than the Capes of North Carolina. 

However, Howell (1932) noted that the Double-crested (P.a. auritus) occurred in the winter “not 

uncommonly,” south to Okeechobee Lake, Florida. He noted that “cormorants are moderately 

common in winter in Choctawhatchee Bay and all along the Gulf Coast; probably some of them 

are of this northern race.”

The earliest nesting record for Florida was obtained in April of 1832, when Audubon 

visited several of Florida’s small keys. Audubon (1943) reported large colonies of cormorants, 

with “many thousands of these birds,” and collected large numbers of birds and eggs. Nesting 

was also documented in the late 1800s, when egg sets were collected at Gainesville and at the 

Wakulla River (Stevenson and Anderson 1994). Around the first third of the twentieth century, 

Howell (1932) mapped the breeding distribution of the Florida Cormorant from the s. tip of the 

Peninsula north to the Chassahowitzka River and Merritt Island. He also showed a colony on the 

Wacissa River. Though he did not show colonies on several of the Florida Keys, he reported that 

breeding occurred there. Howell (1932) stated that the Florida Cormorant was “very abundant on 

the Gulf Coast, and less numerous on the east coast and on many lakes in the interior.” He 

reported that cormorants “may be seen in small or large flocks in all the bays and lagoons along 

the coasts, in most of the larger rivers and lakes, and in the ocean and Gulf. The abundance of the 

bird has led to the erection of a series of racks for many miles along the Gulf Coast to catch the 

excrement of the birds, which is collected to use for fertilizer.”

Howell (1932) also reported numerous observations of very large winter concentrations. 

On December 4, 1910, about 10,000 cormorants were seen at Passage Key. On January 21, 1919, 

a flock of 5,000 – 6,000 was observed at Terra Ceia Bay. Great flocks of cormorants were also 

observed in March 1925, along with ibises and herons, flying from the mainland at Cape Sable to 

outlying keys to night roosts. Additionally, Pangburn (1919) estimated 12,000 cormorants were 

at Pass-a-Grille on February 11, 1918.

Sprunt (1954) revised Howell (1932) and noted that for both subspecies, “no change of 

any consequence has taken place ... since Howell wrote in 1932.”

As discussed above, increases in breeding numbers may have occurred, at least in the last 

few decades. Additionally, very large winter concentrations have been recorded in the 1980s and 

1990s; Christmas Bird Count maxima include 15,100 at North Pinellas county, December 22 

1984, and about 25,000 off Anclote Key, January 15, 1991 (Stevenson and Anderson 1994).

Land Ownership

No information provided.

Productivity

No information provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 118

Figure 54. December estimates of DCCOs in Florida based on CBCs, 1984 – 1997

0

10000

20000

30000

40000

50000

60000

70000

80000

84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals

Georgia

Summary of Population Data and Trends

(Survey completed by B. Winn, Georgia Dept of Nat Resources,Coastal Nongame Office, 

Brunswick GA.)

Breeding.

The Double-crested Cormorant is a recent breeder in Georgia; first nesting was 

documented in May, 1990 at Blalock Lake (Brisse 1990). Since that time a total of seven 

colonies has been discovered, but no statewide surveys for cormorants have been conducted, so 

the total number nesting in the state is not known (B. Winn, pers. comm.). Recently, cormorants 

have been found in increasing numbers inland, especially in the Piedmont and along the “fall 

line,” mostly in the Macon and Augusta areas (Brisse 1990).

Wintering.

Large numbers of birds, estimated between 15,000 – 30,000 winter in the state, roosting 

and feeding at sites along the Lower Altamaha River and Delta, at lakes and reservoirs, and 

along the coast (B. Winn, pers. comm.). Christmas Bird Counts conducted between 1959 – 1988 

suggested an increase of 5.9 % per year. More recent Christmas Bird Counts (1989 – 1998) 

estimated that approximately 1,000 – 6,000 birds wintered in the state during the last decade 

(Figure 55) (Sauer et al. 1996). This is likely an underestimate; however, the CBC data appear to 
FINAL DRAFT Part II. Population Data and Trends, Zone 4 119

support an increasing trend, especially since the mid 1980s, though these data have not yet been 

analyzed and percent change is not available.

Migration.

During migration, consistent large flocks are observed along the entire coast, from Tybee 

Island to Cumberland Island, but no estimate of numbers is available. In spring, peak numbers 

occur in March and April; in fall, peak numbers are reported in November – December (B. 

Winn, pers. comm.).

Historical Information

The only Historical Information we were able to obtain for Georgia was from records 

published in Burleigh (1958).The earliest record in the state was of a specimen taken at 

Summerville, Chattooga County, on March 1, 1886; this individual was identified as P. a. 

floridanus. In the early part of the twentieth century, the species was fairly abundant in the 

winter at various locations within the state. Non-breeding summering birds were documented in 

1939. At the time of Burleigh’s (1958) writing, the cormorant was characterized as “resident on 

the coast, occurring as a common winter visitant, and in smaller numbers throughout the summer 

months.” In the interior, it was a “fairly common” winter resident in the southern portion of the 

state. Since that time, the population, particularly in the winter, has obviously increased.

Land Ownership

Land ownership was provided for wintering sites along the Lower Atlanta River and 

Delta, where cormorants roost and feed on state, federal and private land; and for migration areas 

along the coast, which is mostly state and federal land (B. Winn, pers. comm.).

Productivity

No information provided.

Figure 55. December estimates of DCCOs in Georgia based on CBCs, 1984 – 1997.

0

1000

2000

3000

4000

5000

6000

7000

84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals
FINAL DRAFT Part II. Population Data and Trends, Zone 4 120

Kentucky

Summary of Population Data and Trends

(Survey completed by B. Palmer-Ball, Jr., Kentucky State Nature Preserve’s Commission, 

Frankfort, KY.)

Breeding.

The Double-crested Cormorant has not been documented to breed in Kentucky since 

1951 (Palmer-Ball 1996).

Winter.

Depending on the severity of winter weather, up to 1000 birds will winter in the state. 

Numbers are believed to have increased recently, since the late 1980s. In 1998, 919 birds were 

counted during the CBC conducted at Lake Barkley (Sauer et al. 1996), the only regular 

wintering area in the state, where birds both roost and feed.

Migration.

During migration, birds are observed regularly at Lake Barkley and at Reelfoot Lake, 

though birds at this latter site are probably flying from roost sites at Reelfoot Lake in Tennessee. 

Migrants also sometimes occur on the Ohio River at Markland Dam. In the spring, peak numbers 

are observed in April – May; in the fall, peak numbers occur October-November. The highest 

count recorded was 2400 at Markland Dam, Nov 6, 1997, but birds are usually present only in 

the hundreds at any one location (B. Palmer-Ball, pers. comm.).

Historical Information

Palmer-Ball (1996) notes that the Double-crested Cormorant was not documented to have 

nested in Kentucky prior to the twentieth century. The earliest records we were able to obtain for 

the species in the state were those reported by Pindar (1925), who made observations of the birds 

of Fulton County, Kentucky, between 1890 – 1893. Based on his observations, Pindar (1925) 

noted that the Double-crested Cormorant (P. a. auritus) was “a common migrant, a fairly 

common summer habitant, and rare in winter” in Fulton county. He considered the Florida 

Cormorant (P.a. floridanus) “a rare summer visitant,” but noted that it probably bred at Reelfoot 

Lake, and possibly at other suitable locations.” Mengel (1965) reported three colonies in the late 

1930s in the western portion of the state. In 1949, Mengel visited one of these colonies (Fulton 

County) and observed about 50 nests. Mengel (1965) described the state breeding distribution as 

“the Ohio River bottom lands from Henderson westward, through much of the low country of the 

Purchase, especially in Fulton, Hickman, Carlisle, and Ballard counties, the species occurs 

locally but in numbers near colonies [the three mentioned above] and in localities where 

unknown colonies probably exist.” Mengel (1965) also mentions that he saw cormorants in June 

and July on each of many visits he made to Swan and Clear Lakes in Ballard County.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 121

Mengel (1965) reported that in winter cormorants were “rare and irregular” except near 

Reelfoot Lake, where they were common year round. He noted that a few birds probably 

wintered around Kentucky Lake and on the lower Ohio river.

Land Ownership

Land ownership was provided for Lake Barkley, which is federal land owned by the 

Army Corps of Engineers; and for the Reelfoot Lake area, where birds roost and feed on 

privately owned land (B. Palmer-Ball, pers. comm.).

Productivity

No information provided.

Louisiana

Summary of Population Data and Trends

(Survey completed by B. Vermillion, Louisiana Dept of Wildlife and Fisheries, Baton 

Rouge, LA.)

Breeding.

The number of Double-crested Cormorants breeding in Louisiana appears to be 

increasing, but it is not known by how much. Prior to the late 1980s – early 1990s, the species 

was only known to nest in small numbers in the state, but since that time larger colonies have 

been noted. Presently, only two breeding sites are known. One is at the mouth of the Mississippi 

River; in 1994 this colony had about 50 pairs, and in 1999 about 30 pairs nested there . The other 

colony is on the Toledo Bend, a large reservoir on the Louisiana / Texas border; numbers were 

counted in 1995 only, at which time 397 nests were found scattered in dead trees throughout the 

Bend (B. Vermillion, pers. comm.).

Winter.

Large numbers winter in the state. Christmas Bird Counts conducted between 1959 –

1988 suggested that numbers increased by 17.4 % per year. More recent counts (1989 – 1998) 

estimated that approximately 11,000 – 33,000 birds wintered in the state during the last decade 

(Figure 56) (Sauer et al. 1996). Though these data have not yet been analyzed, numbers appear to 

still be increasing, with high counts of approximately 27,000 and 33,000 birds observed in 1996 

and 1998, respectively. In the latter year, an unusually high count of nearly 19,000 birds was 

made at Shreveport; the previous high count for Shreveport occurred in 1996 with 9,264 birds

observed.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 122

Migration.

It is not known if cormorants pass through or stop over in Louisiana during migration; it 

is assumed that birds observed in the state are wintering, but possibly migrants from Texas going 

back to northern breeding grounds may pass through Louisiana (B. Vermillion, pers. comm).

Related Species.

Louisiana is one of only two states (the other is Texas) where substantial numbers of 

Neotropic Cormorants nest and overwinter (Telfair and Morrison 1995). Because the 

Depredation Order is in effect in these states, special efforts should be made to distinguish the 

Neotropic from the Double-crested Cormorant; in the U.S. numbers of Neotropics, while 

increasing, are much lower than those of Double-crested Cormorants. Neotropic cormorants 

likely feed in aquacultural areas in southwestern Louisiana (C. Hunter, pers. comm.) and are shot 

because of perceived impacts to fish, but extent to which shooting has or may affect this species 

is unknown (Telfair and Morrison 1995).

Historical Information

Audubon (1843) recorded P. a. floridanus as “constantly resident in the Floridas and their 

Keys, and along the coast to Texas”, thus we assume the species was present along the Louisiana 

coast and probably nested there at the time of Audubon’s visit. Audubon (1843) reports seeing 

cormorants “offered for sale in the New Orleans market, the poorer people there making gumbo 

soup of them.” However, it is not clear if these cormorants were obtained in Louisiana or if they 

were shipped into the state from Florida. Beyer et al. (1907) described the Double-crested 

Cormorant (P.a. auritus) as an uncommon winter resident in Louisiana, found mostly along the 

Gulf Coast as late as April. The Florida Cormorant (P.a. floridanus) occupied the same localities, 

but was much more common and bred everywhere within the state that it occurred; its 

distribution and abundance remained uniform throughout the year. Oberholser (1938) 

documented nesting in southwest Louisiana in Cameron Parish, in West Feliciana Parish and in 

Devil’s Swamp north of Baton Rouge, East Baton Rouge Parish. However, by the time Lowery 

(1974) revised his “Louisiana Birds”, there was no longer any evidence that the species still 

nested in the state. Though Lowery (1974) noted that cormorants were most common in 

Louisiana in the winter and early spring, he remarked that cormorants appeared less abundant in 

all seasons than they had been during the 1930s – 1950s. He also noted that, while the cormorant 

“used to be a winter fixture at City Park and University lakes in Baton Rouge, now its 

appearance on these lakes is unusual.” Apparently, numbers began increasing sometime after this 

period.

Land Ownership

No information provided.

Productivity

No information provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 123

Figure 56. December estimates of DCCOs in Louisiana based on CBCs, 1984 – 1997.

0

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84 85 86 87 88 89 90 91 92 93 94 95 96 97

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Individuals

Mississippi

Summary of Population Data and Trends

(Survey completed by J. Glahn, USDA Aphis/Wildlife Services, Mississippi State, MS.)

Breeding.

There are only three breeding records for the Double-crested Cormorant in Mississippi. 

The first was reported from a cypress brake near Clayton in May, 1952; 12 nests were observed 

(Coffey 1952). The latter two records were both obtained in 1998, with two nests observed at 

Lewis Swamp, and 31 nests observed at Jones Lake (Reinhold et al. 1998).

Winter.

Very large numbers winter in the Delta Region of Mississippi. USDA/WS conducts 

yearly winter cormorant surveys which are not state wide, but focus on catfish production areas. 

In 1999, there was a total of 76 known night roosting sites (Figure 51), and 64,169 birds were 

counted. A high count of 68,179 birds was recorded in 1998. Results from annual censuses 

conducted in February, 1990 – 1999, show dramatic increases, from 28,584 birds in 1990 to the 

present numbers. A low of 16,405 birds was counted in 1994 due to icing over of the majority of 

fish ponds in the Delta (J. Glahn, pers. comm.). Christmas Bird Counts conducted between 1959 

– 1988 suggested an increase of 18.7 % per year, the largest increase reported for any of the 

wintering states. More recent Christmas Bird Counts (1989 – 1998) estimated that approximately 

1,000 – 23,000 birds wintered in the state during the last decade (Figure 57) (Sauer et al. 1996).
FINAL DRAFT Part II. Population Data and Trends, Zone 4 124

While these counts are obvious underestimates (see Summary for Zone 4: Summary of 

Population Data for Southeastern U.S.), and these data have not yet been analyzed for percent 

change, they appear to support an increasing trend since the mid-1980s.

Migration.

Peak migration numbers occur in March, but no estimates are available (J. Glahn, pers. 

comm.).

Historical Information

Jackson and Jackson (1995) described the history of the Double-crested Cormorant in 

Mississippi, which we have summarized here, unless otherwise noted. In the mid 1800s, 

cormorants were abundant in late summer and early fall along the Yazoo and Mississippi rivers. 

Lewis (1929) reported that interior nesting birds wintered along the coast from western Florida to 

eastern Texas, and in the first half of the 20th century, Burleigh (1944) reported it as one of the 

most common winter birds of the Mississippi Sound, late September to early May. In freshwater 

areas, such as the lower Mississippi River, Lewis (1929) reported that winter numbers were very 

small.

At the time of the first breeding in the early 1950s, tenant farmers in the Clayton area 

reported that there had been many more birds, but that much cypress had been cut. A pattern of 

decline from the 1940s – 1970s occurred throughout the south central U.S., and is thought to be 

linked to DDT usage and persecution on both the breeding and wintering range. It was during 

this declining point that the catfish industry underwent its major growth in the mid-South, with 

the heart of the catfish industry located in the Mississippi Delta at the confluence of the Arkansas 

and Mississippi rivers, both major migratory pathways for many avian species. Cormorants 

began recovering in the south in the late 1970s, and since that time, the number of wintering 

cormorants has increased substantially, with larger numbers appearing to arrive earlier and stay 

later each year. The species is now regularly found on the Mississippi coast year round, although 

substantial numbers are reported only in winter.

Land Ownership

The two breeding sites documented in 1998 were both on private lands; Jones Lake is 

owned by a private individual, and Lewis Swamp is owned by a private corporation. Nineteen of 

the 76 night roosting sites used by wintering birds are on public agency lands; the other 57 are on 

lands owned by private individuals (J. Glahn, pers. comm.).

Productivity

Of the 31 nests on Jones Lake, 13 fledged one or more young (Reinhold et al. 1998).
FINAL DRAFT Part II. Population Data and Trends, Zone 4 125

Figure 57. December estimates of DCCOs in Mississippi based on CBCs, 1984 – 1997.

0

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30000

84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals

North Carolina

Summary of Population Data and Trends

(Survey completed by D.H. Allen, North Carolina Wildlife Resources Commission, 

Trenton, NC.)

Breeding.

The Double-crested Cormorant has been nesting in North Carolina for at least two and a 

half centuries, though not consistently (see Historical Information, below). In the mid 1980s and 

1990s, breeding was documented at three sites within the state: Great Lake / Lake Ellis Simon 

near the coast; along the Chowan River; and in the Piedmont at Jordan Lake, near Raleigh. In 

1996, only the Great Lake / Lake Ellis Simon site was active, and 135 pairs were counted (D. 

Allen, pers. comm.); at this time, cormorants had shifted from Great Lake to nearby Lake Ellis 

Simon [which is 0.8 km southeast of Great Lake (Doig et al. 1989)]. Cormorants are no longer 

breeding at the Chowan River site. No data are available on the number of pairs from 1996 –

1998 at the Piedmont site, but in 1999, 50 breeding pairs were estimated (W. Golder, pers. 

comm.). No trend information is available, as most colonial waterbird survey efforts in North 

Carolina have focused on coastal nesters (D. Allen, pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 4 126

Winter.

The wintering population is thought to be very large, estimated at 25,000 – 50,000 birds, 

possibly more, depending on the severity of the winter. Most birds winter on the inlets of 

Pamlico Sound, with the largest numbers (10,000 – 30,000) at Hatteras Inlet and Oregon Inlet 

(H. LeGrand and W. Golder, pers. comm.). Substantial numbers also winter at Beaufort Inlet and 

at Bald Head Island, Cape Fear; smaller numbers also winter at some inland sites (H. Legrand, 

pers. comm.). Christmas Bird Count data from 1959 – 1988 suggest an increase of 9.9 % per 

year. More recent Christmas Bird Counts (1989 – 1998) estimated that approximately 10,000 –

105,000 birds wintered in the state during the last decade (Figure 59) (Sauer et al. 1996), but 

these data have not yet been analyzed, and recent trend information is not available.

Migration.

No information available on migration numbers or peak periods; since large numbers 

winter in the state, it is difficult to distinguish migrants (H. LeGrand, pers. comm.).

Historical Information

The Double-crested Cormorant appears to have been a very abundant breeding species in 

North Carolina, and possibly the surrounding region, during the colonial era. The earliest record 

we obtained for the state was from Brickell (1737), cited in Lewis (1929); Lewis noted that this 

record “doubtless included P.a. floridanus and P.a. auritus.” Brickell reports:

The Cormorants are the same as in Europe, only those of this Province are larger. They are as 

numerous all over these Parts of America, as in any part of the World, especially at the run of the 

Herrings, which is in March and April; at which time they are seen sitting upon the Sand Banks, or 

Logs of Wood in the Rivers, and catch vast quantities of Fish, which is their only Food, and 

whereof they are very ravenous and greedy. They lay their Eggs in the beginning of the Spring, in 

the Islands, in the Sound and near the Sea Shoar in the Banks, and sometimes on high trees, as the 

Shags do...”

Lewis (1929) also noted that Brickell (1737) included a representation of the Cormorant 

of North Carolina, which is the earliest representation of P. auritus he had seen (Figure 58). The 

next record we obtained for the state was from Audubon (1843), who reported only that, after 

young fledge, many proceed as far south as the Capes of North Carolina. He did not mention 

nesting, and numbers apparently declined; nesting was not documented in the state again until 

1898, when 150 nests were counted on the coastal plain at Great Lake, Craven county (Pearson 

et al. 1959). This colony was visited again in 1909 and 1911, at which times 123 and 159 nests, 

respectively, were counted. Pearson et al. (1959) noted that the colony shifted its nesting site 

several times as the trees that it occupied were killed by the birds’ excrement. The colony 

appears to have slowly declined after this time, and in 1939, only 41 nests were counted. At the 

time of the first printing of Pearson et al. (1942), the authors noted that this was “the only known 

colony of breeding cormorants between Florida and Maine.” [However, breeding was 

documented in Massachusetts in 1940, and was reported to occur as early as 1937, and perhaps 

earlier (see Massachusetts summary)]. In 1948, 35 nests were counted at Great Lake. In 1953, 

nesting was recorded at Lake Ellis. In 1956 there was no nesting at Great Lake and only nine 

nests recorded at Lake Ellis (Pearson et al. 1959). In 1975, 12 nests were reported at Great Lake / 

Lake Ellis. Breeding numbers began to slowly increase in the 1980s, when a total of 74 pairs 

were counted at three sites (D. Allen, pers. comm.). 
FINAL DRAFT Part II. Population Data and Trends, Zone 4 127

Pearson et al. (1959) note that most, if not all, cormorants seen in the state in winter were 

the P.a. auritus subspecies. These authors also provide records of birds taken in the fall between 

1923 – 1938.

Figure 58. Double-crested Cormorant from Brickell (1737), 

possibly the first published representation of this species.

Land Ownership

The nesting site on the Chowan River is owned by a private timber company. The nesting 

site on Great Lake is federal land, part of the Croatan National Forest. The nearby Lake Ellis site 

is on privately owned land. The Jordan Lake site is owned by the federal government and 

administered by the Army Corps of Engineers. Wintering areas along the coast are primarily 

owned by the federal and state government, though there is also some private ownership (W. 

Golder, H. LeGrand, and D. Allen, pers. comm.).

Productivity

No information provided.

Figure 59. December estimates of DCCOs in North Carolina based on CBCs, 1984 – 1997.

a

0

20000

40000

60000

80000

100000

120000

84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals

a = In 1991, counts for Portsmouth and Ocracoke Islands gave very large numbers that appeared 

incorrect, thus data for these islands in this year were thrown out and replaced with the yearly 

average for the last 10 years (G. LeBaron, pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 4 128

Oklahoma

Summary of Population Data and Trends

(Survey completed by M. Howery, Oklahoma Dept of Wildlife Conservation, Oklahoma. 

City, OK.)

Breeding.

The Double-crested Cormorant currently nests at the Salt Plains Reservoir, Alfalfa 

County; it is the only known nesting site for the species in the state of Oklahoma. The discovery 

of breeding birds at this site in 1994 was the first breeding record for the state since 1985. The 

Salt Plains Reservoir colony was last estimated at about 30 pairs in 1996 (M. Howery, pers. 

comm.).

Winter.

The species overwinters on reservoirs in the state, but no winter surveys have been 

conducted. However, USDA/WS conducted a year-long wildlife assessment on Lake Overholser 

in Oklahoma City, for Will Rogers World Airport, from February 1997 to January 1998. During 

that time cormorant numbers fluctuated with seasonal migrations, but were most noticeable from 

February to April. Average counts for February, March, and April were 118, 428, and 355, 

respectively. Although the counts were averaged, during one count in April 1350 cormorants 

were observed foraging for fish. Additionally, during the same time period, 3,000 – 4,000 

cormorants were observed roosting at Lake Arcadia, approximately 10 miles east of Lake 

Overholser. This could indicate a much larger number of cormorants wintering in Oklahoma than 

was previously known (P. Robinson, pers. comm.). Anglers believe cormorant numbers and the 

extent of their residency in the state have increased (Simmonds et al. 1997). Christmas Bird 

Counts conducted between 1959 – 1988 suggest an increase of 16.5 % per year during this 

period. More recent Christmas Bird Counts (1989 – 1998) estimate that approximately 2,000 –

14,400 birds wintered in the state during the last decade, with high counts occurring 1991 – 1993 

(Figure 60) (Sauer et al. 1996). Though these data have not yet been analyzed, they suggest that 

numbers may still be increasing.

Migration.

During migration, large numbers occur, with peak fall numbers recorded October – early 

November; peak spring numbers are recorded March – April (M. Howery, pers. comm.). At most 

reservoirs where birds are observed, densities are greatest in fall migration, followed by spring 

migration and winter residency (Simmonds et al. 1997).
FINAL DRAFT Part II. Population Data and Trends, Zone 4 129

Historical Information

The earliest record we obtained of the species in the state was of a flock reported in fall 

1916, apparently during migration (Nice 1931). The earliest nesting record we found was for a 

small colony (3 – 7 nests) at the north end of the reservoir in the Salt Plains National Wildlife 

Refuge between 1945 – 1950 (Sutton 1967; Baumgartner and Baumgartner 1992). Later, the 

species bred at the Robert S. Kerr Reservoir at the Sequoyah National Wildlife Refuge from 

1973 – 1980 (M. Howery, pers. comm.) and again at the Sequoyah NWR in 1985 (Baumgartner 

and Baumgartner 1992). It was not recorded breeding again until 1994 when it returned to the 

Salt Plains Reservoir (M. Howery, pers. comm.).

Sutton (1967) recorded the species as transient throughout the state September to 

December, and from March to May, and noted that there were only three January records, and no 

February record. However, during the 1984 CBC, 2,100 birds were counted at the Sequoyah 

National Wildlife Refuge (Baumgartner and Baumgartner 1992). We are not sure when winter 

numbers began to increase, or whether these data actually represent an increase, since we found 

very little information on winter numbers and distribution in the state.

Land Ownership

Both breeding sites are on federal lands that are part of National Wildlife Refuges (M. 

Howery, pers. comm.).

Productivity

In 1974, 203 young were produced in 86 nests at the Robert S. Kerr Reservoir, giving a 

rate of 2.4 chicks per nest (M. Howery, pers. comm.).

Figure 60. December estimates of DCCOs in Oklahoma based on CBCs, 1984 – 1997.

0

2000

4000

6000

8000

10000

12000

14000

16000

84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals
FINAL DRAFT Part II. Population Data and Trends, Zone 4 130

Puerto Rico / Virgin Islands

Summary of Population Data and Trends

(No survey completed.)

In the eastern Caribbean, the Double-crested Cormorant occurs only as a winter vagrant. 

Cuba is the only Caribbean location that may have larger numbers of cormorants (J. Saliva, pers. 

comm.).

South Carolina

Summary of Population Data and Trends

(Survey completed by T.M. Murphy, South Carolina Dept of Natural Resources, Green 

Pond, SC.)

Breeding.

The Double-crested Cormorant was documented as a breeding species relatively recently 

in South Carolina, with the first nesting recorded in 1985. There are currently17 known colony 

sites, all inland; during the last statewide count, 10 colonies were active. Complete ground 

counts for all known colonies were conducted in 1995 and 1996, and 717 and 895 pairs, 

respectively, were estimated. Complete ground counts were also conducted in 1989 – 1990, 

when 186 and 126 pairs, respectively, were counted. The number of breeding pairs is definitely 

increasing (Figure 61), but it is not known at what rate, because complete censuses have not been 

regularly conducted; additionally, some colonies were not known at the time complete censuses 

were conducted, so number of pairs may be underestimates. In 1998, 725 pairs were counted, but 

data were available for only five colonies (T. Murphy, pers comm.).

Winter.

Large numbers winter in the state, feeding and roosting at inland lakes and reservoirs, and 

along the entire coast. In the last decade the population has increased along the coast. Christmas 

Bird Counts conducted between 1989 – 1998 estimate that approximately 2,000 – 17,000 birds 

wintered in the state during the last decade (Figure 62). While these data have not yet been 

analyzed for trends, numbers appear to be increasing (T. Murphy pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 4 131

Migration.

It is not known if areas in the state provide important stopover sites for migrants, or how 

many migrants pass through, probably because large numbers winter in the state and it is difficult 

to distinguish migrant from wintering birds (T. Murphy pers. comm.).

Historical Information

The earliest record we obtained in the state was that of Hoxie (1892, cited in Lewis 1929) 

who observed cormorants foraging off the coast of South Carolina in winter. Sprunt and 

Chamberlain (1949) described P.a. auritus as an abundant winter resident, mostly coastwise. 

Wayne (1910) reported that the Florida Cormorant (P.a. floridanus) bred in the state, but he was 

never able to locate a colony. Sprunt and Chamberlain (1949) reported that the Florida 

Cormorant was a fairly common permanent resident, chiefly along the coast and that it was 

presumed to breed there. However, W. Post (pers. comm.) noted that cormorants probably did 

not breed in the state until the large hydroelectric lakes were built over the vast Santee Swamp. 

The first nest was finally found on L. Marion, about 55 mi from the coast, in 1985.

Land Ownership

No information provided.

Productivity

Post and Seals (1991) reported a rate of 2.20 fledglings per nest at Hog Swamp, a newly 

established colony, in 1989.

Figure 61. Changes in number of breeding pairs in South Carolina, 1989 – 1996.

0

100

200

300

400

500

600

700

800

900

1000

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997

Year

No. Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 4 132

Figure 62. December estimates of DCCOs in South Carolina based on CBCs, 1984 – 1997.

0

2000

4000

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8000

10000

12000

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16000

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84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals

Tennessee

Summary of Population Data and Trends

(Survey completed by R.M. Hatcher, Tennessee Wildlife Resources Agency, Nashville, TN

Breeding.

The Double-crested Cormorant has recently returned to Tennessee as a nesting species, 

with the first nesting since 1955 documented in 1992 on an island in the Holston River inside a 

U.S. Army ammunition plant near Kingsport (Alsop, in press; Robinson 1990). Currently there 

are three known breeding sites, two in the eastern portion of the state, and one in the western 

portion in Humphreys county. Only the Holston colony near Kingsport in the northeast was 

active in 1999; at this time, 29 pairs were counted. This is the largest of the colonies and has 

increased since initial nests were discovered in the mid 1990s (Figure 63) (B. Hatcher, pers. 

comm.).

Winter.

A small number of cormorants winter at Reelfoot Lake and a few other sites within the 

state. Robinson (1990) reported that in recent years, winter flocks have generally numbered 

fewer than 100 birds. However, Jackson and Jackson (1995) show increases to about 250 birds 

between 1990 – 1992 at Reelfoot Lake, and note that Christmas Bird Counts from the late 1970s 
FINAL DRAFT Part II. Population Data and Trends, Zone 4 133

to the present indicate numbers at Reelfoot Lake exceeded those of the 1940s. During the 1992 

Christmas Bird Count an unusually high count of 661 birds was recorded at this location 

(LeBaron 1992). Nevertheless, winter numbers at Reelfoot Lake have not come anywhere near 

the high count of 4,500 reported for this location on Dec 20, 1952 (Jackson and Jackson 1995), 

and since 1992 have ranged between only 3 – 260 birds. Between 1985 – 1998 numbers counted 

for the entire state have remained under 1000 during CBCs (Sauer et al. 1996), but numbers do 

appear to recently be increasing, though CBC data have not yet been analyzed for this period 

(Figure 64). After mid-winter, fewer birds are found within the state (Robinson 1990).

Migration.

Peak spring migrant numbers are recorded in April – May; in fall, peak numbers are 

observed in October (Robinson 1990). Numbers during migration are estimated to be around 

2000 birds (B. Hatcher, pers. comm.). Flocks of 1000 – 1500 are reported on Mud Lake, Shelby 

County, while smaller flocks of approximately 100 birds have been observed on the Mississippi 

River near Reelfoot Lake (Alsop, in press). Migrants are also observed on the Mississippi River 

near Memphis (B. Hatcher, pers. comm.).

Historical Information

Alsop (in press) reported that the Double-crested Cormorant was abundant at Reelfoot 

Lake in the late 1800s, and existed there in the thousands into the late 1930s. The species was a 

permanent year-round resident at this site, and numbers “swelled” with the arrival of northern 

migrants in the fall (Jackson and Jackson 1995). Breeding was first documented in the state at 

this site in 1919; about 75 nests were counted. In 1932, about 200 nests were counted (Jackson 

and Jackson 1995); in the late 1930s, between 250 – 400 nests were estimated (Robinson 1990; 

Alsop in press). Breeding was documented at Reelfoot Lake through the early 1940s. However, 

in the late 1930s and early 1940s, several visitors to this colony reported numbers of cormorants 

shot; at this time, cormorants were shot regularly by locals for fish bait and for sport (Jackson 

and Jackson 1995; Alsop in press). Other colonies documented in the state include one at the 

Duck River Unit of the Tennessee National Wildlife Refuge, Humphreys County, which in April, 

1949, had about 100 nests; and one at Kentucky Lake, Benton County, with about 10 nests in 

1955. Robinson (1990) reports this latter nesting record as the last for the state, prior to that 

recorded in 1992. In 1963, the Refuge Manage at Reelfoot Lake reported that the cormorant 

population there was declining yearly; only one pair was seen in that year. Therefore, it is not 

clear when breeding at Reelfoot Lake ceased (Jackson and Jackson 1995).

The species formerly wintered in large numbers at Reelfoot Lake, and also at the Duck 

River Unit. At the latter site, 3000 were reported on 3 January 1954 (Robinson 1990). Alsop (in

press) noted that indiscriminate shooting and probably the sudden increase of pesticides ended 

the days of the cormorant as a permanent resident in w. Tennessee. As numbers did elsewhere, 

the cormorant population in Tennessee plummeted in the 1960s and 1970s, and the species was 

initially listed as “In Need of Management” in 1976. In the 1980s, however, the species began to 

recover, and in the 1990s, breeding was documented in both west and east Tennessee. 

Additionally, more cormorants are being recorded annually on bird counts throughout the year.

Land Ownership

No information available.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 134

Productivity

No estimates available.

Figure 63. Changes in number of breeding pairs in Tennessee, 1992 – 1999.

0

5

10

15

20

25

30

35

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Year

No. Pairs

Figure 64. December estimates of DCCOs in Tennessee based on CBCs, 1984 – 1997.

0

100

200

300

400

500

600

700

800

900

84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals
FINAL DRAFT Part II. Population Data and Trends, Zone 4 135

Texas

Summary of Population Data and Trends

(Survey completed by J. Herron, Texas Parks and Wildlife, Program Director Wildlife 

Diversity; breeding data provided by B. Howe, USFWS, Albuquerque, NM.)

Breeding.

The Double-crested Cormorant was first documented nesting in Texas in 1926 

(Oberholser 1974). In the 1990s, the species was found nesting in small numbers (< 50 nests per 

site) in the western and eastern portions of the state in Hansford, Hudspeth, Randall, Reeves, 

Sabine and Wood counties (B. Howe and R. Telfair II, pers. comm.). The species may also have 

nested in Delta, Harrison and Sabine counties (Texas Ornithological Society, 1995; B. Howe, 

pers. comm.). The quality of these records varies; in some of the northeastern counties (Harrison, 

Delta and Wood), the species may have been confused with the Neotropic Cormorant (R. Telfair 

II, pers. comm.). Better documentation of nesting records is necessary to accurately describe the 

distribution in Texas. At this time, population trends cannot be described; systematic surveys 

have not been conducted and very limited information is available on present numbers and 

distribution of breeding birds (J. Herron, pers. comm.).

Winter.

As in other southern states, cormorants are increasing in Texas; increases there have been 

described as “tremendous” over the last 30 years (J. Herron, pers. comm.). Christmas Bird 

Counts conducted between 1959 – 1988 suggested an increase of 14.5 % per year. More recent 

Christmas Bird Counts (1988 – 1998) estimated approximately 50,000 – 90,000 birds wintering 

in the state, and suggest that numbers are still increasing, but these data have not yet been 

analyzed (Figure 65). High counts between 83,000 – 90,000 were made 1992 – 1995. During 

these years, Christmas Bird Counts were conducted at Calaveras and Lake Livingston (lower), 

but these sites, which had very large numbers of cormorants, were dropped from the count 

beginning in 1995. However, numbers remained high in 1995 partly because an unusually high 

count of nearly 26,000 birds was counted in that year at Texarkana; the high at this site during 

the last decade had previously been around 13,000 (Sauer et al. 1996).

Very large numbers winter at reservoirs east of Dallas / Ft. Worth. Wintering populations 

are found on over 30 reservoirs, and resident populations are found on hundreds of smaller 

reservoirs or impoundments (J. Herron, pers. comm.).

Migration.

It is assumed that areas in the state also provide important stopover sites for migrating 

DCCOs. However, it is not possible to identify peak times or concentrations, as it is very 

difficult to distinguish wintering from migrating birds (J. Herron, pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 4 136

Related Species.

See Related Species. in Louisiana profile (p. 122) regarding importance of distinguishing 

between Neotropic and Double-crested Cormorants in states where Depredation Order is in

effect.

Historical Information

Audubon (1843) reported that the Florida Cormorant (P.a.. floridanus) was “constantly 

resident in the Floridas and their Keys, and along the coast to Texas,” and that the species was 

abundant. Oberholser (1974) notes a record of the species in Tom Green County obtained in the 

fall of 1880. The earliest documented breeding record we obtained for the state was in 1926 at 

Matagorda Peninsula. Nesting was also documented in the 1930s, at which time the Doublecrested Cormorant was a rare and local breeder. After 1939, breeding appeared to cease for more 

than 30 years, with the last nesting record obtained in Wilbarger County. The Cooperative Fisheating Bird Survey, conducted in late May and early June, 1969, along the entire coast found no 

Double-crested Cormorants (Oberholser 1974). Nesting was not documented again until the mid 

1970s, when 14 nests were found on the Toldeo Bend Reservoir at the mouth of Mill Creek Bay 

in April 1974 (Holm et al. 1978); five nests were also reported in Midland County in 1973, but 

the outcome of these nests was not clear (B. Howe, pers. comm.). Breeding has since expanded 

elsewhere within the state.

Obeholser (1974) noted that this species was still numerous and widespread in Texas as a 

migrant and winter resident. He described it in winter (mid-November to early-March) as 

“locally very common to uncommon along entire coast; locally fairly common to uncommon 

inland in eastern half; scarce to rare west of 100th meridian.” In recent years numbers have

increased dramatically; very large numbers winter in reservoirs east of Dallas / Ft. Worth. 

Oberholser noted that the many hundreds of livestock watering lakes constructed in the early 

1940s – early 1970s aided it. Additionally, J. Herron (pers. comm.) stated that, while many 

factors have likely contributed to the species’ growth, the hundreds of artificial lakes and 

reservoirs created new habitat that this species was able to readily occupy.

Land Ownership

No information provided.

Productivity

No estimates provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 4 137

Figure 65. December estimates of DCCOs in Texas based on CBCs, 1984 – 1997.

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

84 85 86 87 88 89 90 91 92 93 94 95 96 97

Year

Individuals

FINAL DRAFT Part II. Population Data and Trends, Zone 5 139

ZONE 5: SUMMARY OF POPULATION DATA FOR THE NORTHEAST ATLANTIC 

COAST

Introduction

Breeding Information

In this zone, the breeding population extends along the Atlantic coast from s. 

Newfoundland, the north shore of the Gulf and estuary of the St. Lawrence River, Anticosti 

Island, Magdalen Island., south to New York City and Long Island. Table 6 shows the status 

(Breeding, Wintering, Migrant) of the Double-crested Cormorant in this region. Over the last two 

decades the number of breeding Double-crested Cormorants has increased along much of the 

northeast Atlantic Coast; currently the breeding population is roughly estimated at about 86,000 

pairs. Of 15 states and provinces for which we had data on breeding cormorants, eight had 

increasing cormorant numbers, one had declining numbers, one had possibly declining or stable 

numbers, one had possibly increasing or stable numbers, and trends in four could not be 

determined.

Eastern Canada.

Very large numbers breed in Québec and the surrounding area, including the St. 

Lawrence River and Estuary, the North Shore, and multiple islands (see Québec summary). 

Colonies in southwestern Québec were grouped with the interior population, based on Hatch and 

Weseloh (1999), and on apparent spatial separation from other Québec locations and proximity 

to interior colonies when viewing colony distribution. Québec has the largest number of breeding 

cormorants in the Northeast Atlantic. Numbers have increased substantially in Québec since 

1970, especially in the St. Lawrence River. These increases may be related to reduction in human 

persecution and changes in prey fish communities (Chapdelaine and Bédard 1995). In 1989, a 

large scale culling program was introduced and reversed population trends in the St. Lawrence 

River Estuary, where rapid growth was occurring.

Very large breeding concentrations are also found on Nova Scotia and Prince Edward 

Island, where rapid increases began occurring in the 1970s. However, during the last decade 

declines have occurred on these two provinces, which may indicate that numbers are beginning 

to stabilize (see summaries). In New Brunswick, cormorants were increasing as of 1997, but the 

current rate of increase has slowed (Korfanty et al. 1997). In Newfoundland, the number of 

breeding birds is comparatively very low, and trend information is not available.

New England and U.S. Coast.

Very large breeding concentrations occur in New England along the coasts of Maine and 

Massachusetts. With the exception of Maine, (where numbers began declining between the mid1980s and early 1990s, and where numbers may have reached carrying capacity), rapid increases 

have occurred since the 1970s. In parts of southern New England (Connecticut, Rhode Island, 

coastal New York) the cormorant has been documented fairly recently as a breeding species; 

mostly steady increases have occurred in these states, and numbers are growing fairly rapidly. 

However, due to declines in Maine, numbers in New England have declined overall between 

1992 (and perhaps earlier) and 1995 at an average annual rate of – 6 % (Figure 66). Further south 

along the Atlantic, the species has expanded to or possibly re-colonized (see Historical 
FINAL DRAFT Part II. Population Data and Trends, Zone 5 140

Information, below) several states, with first breeding records obtained in New Jersey, 

Pennsylvania, Maryland, Delaware and Virginia between the late 1970s and 1990s.

Winter and Migrant Birds 

Small numbers (< 1000 birds) winter in some of the New England states, and several 

thousand likely occur in Virginia and coastal New York. CBC data collected between 1984 –

1998 (Sauer et al. 1996) suggest that numbers may be increasing (Figure 67), particularly in 

Virginia, but data have not yet been analyzed; a few more years of data may be necessary to 

document current trends. Very large numbers of fall migrants are observed along the coast, with 

hundreds of thousands reported from southern New England to Virginia.

Historical Information

Breeding has been documented for this zone for nearly 500 years; Hatch (1982) reported 

that bones of adult and young cormorants found in Indian middens in Boston Harbor, 

Massachusetts, dated back to as early as 1500. Numerous early records also exist for New 

Brunswick, Newfoundland, Nova Scotia, Québec, Maine, Virginia, and possibly Rhode Island. 

Early records indicate that cormorants were very abundant in Atlantic Canada, New England and 

Virginia prior to European settlement. While the full extent of the historic distribution is not 

known, changes in numbers have been mostly within the known historic range (Hatch and 

Weseloh 1999). Sometime during settlement cormorants began declining in most parts of their 

Atlantic range. By the early 19th century they were entirely extirpated as breeders in New 

England. By the late 19th and early 20th centuries, cormorants were basically extirpated from 

Nova Scotia and greatly reduced in the other Maritime Provinces and Québec. Declines across 

the range appear to have been due to human persecution and exploitation. Numerous authors 

have suggested that, given pre-settlement habitat, the known historic distribution of cormorants, 

and other evidence, cormorants may have been present in areas for which we have unclear or no 

early (pre-20th century) records, such as New York, Rhode Island, and Virginia (Mendall 1936; 

Arbib 1998; Ferren and Myers 1998). Thus, some of the areas “colonized” in the last twenty 

years may actually represent a re-colonization rather than a range expansion.

New England

(Summarized from Krohn et al. 1995, unless otherwise noted.) In New England, 

cormorant numbers began to recover around 1925, when they began consistently breeding and 

increasing in Maine. Birds may have immigrated from eastern Canada. The cormorant returned 

as a breeding species to southern New England in the late 1930s and early 1940s, when nesting 

was recorded in Massachusetts, and later in New Hampshire during the 1940s or 1950s (J. 

Kanter, pers. comm.). By the mid-1940s numbers in Maine were perceived to cause problems for 

fishermen and a cormorant control program, utilizing egg spraying, was initiated. The control 

program appeared to coincide with a reduction in population growth (Drury 1973); while growth 

in New England continued, it did so at a very low rate until the 1970s (Hatch 1982; Veit and 

Petersen 1993). Low growth rates may have been due to the combined effects of control efforts, 

pesticides and natural limitations to population size. After the DCCO received protection under 

the Migratory Bird Treaty Act and pesticide regulations were introduced, a second period of 

growth began in the 1970s and has continued through the 1990s, though numbers in Maine have 

begun to decline and are possibly stabilizing (R.B. Allen, pers. comm.). From the late 1970s 

through the 1990s, growth rates have been greater in southern New England than in Maine, 
FINAL DRAFT Part II. Population Data and Trends, Zone 5 141

though Maine still supports the majority of cormorants breeding in New England. During the 

mid-1980s through the early 1990s growth rates in both areas decreased.

Eastern Canada.

We were not able to obtain much detailed historic information on cormorants in 

Newfoundland, New Brunswick, or Prince Edward Island. However, the limited information we 

did obtain suggests that the history of cormorants in eastern Canada was similar to that in New 

England, though numbers in Canada did not appear to drop as low as those in New England. In 

the Maritime Provinces, cormorants were reduced to remnant numbers by 1900 (Erskine 1992). 

Lewis (1929) notes early records of several colonies in the Québec region (including the north 

shore of the Gulf of St. Lawrence, and the Gaspé peninsula), Newfoundland, Nova Scotia and 

New Brunswick, that had all disappeared by the time he was writing or earlier in this century. He 

also provides an estimate of the number of breeding birds in these provinces, roughly calculated 

around the time he was writing, and there appears to have been about 5000 pairs.

Lewis’s estimate (1929) is the only one for all of eastern Canada prior to recent estimates. 

During the 1940s and 1950s, when cormorant numbers were known to be increasing in New 

England, they also increased in the Québec region and in Nova Scotia, while numbers were low 

on Prince Edward Island. No information is available for New Brunswick and Newfoundland at 

this time. In the 1970s, numbers began rapidly increasing in Québec, Nova Scotia and Prince 

Edward Island. 

Figure 66. Changes in numbers of breeding pairs in New England, 1977 – 1994/95.

a

b

c

d

0

5000

10000

15000

20000

25000

30000

35000

40000

1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996

Year

No. Pairs

a = 17,117 pairs, 1977, included ME, NH, MA (Krohn et al. 1995).

b = 34,170 pairs, mid 1980s, includes ME, NH, MA, RI, CT (Krohn et al. 1995).

c = 37,648 pairs, early 1990s, includes ME, NH, MA, RI, CT; numbers for ME based on visits to 

67 colonies, about half the number of known colonies (Krohn et al.1995).

d = 31,391 pairs, 1994-95, includes ME (numbers declined substantially since mid 1980s), MA, 

RI, CT, coastal NY (this report).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 142

Figure 67. December estimates of DCCOs in the Northeast Atlantic, Zone 5*, based on 

CBCs, 1984 – 1998.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals

* States included: CT, DE, MD, NJ, NY, PA, VA; annual totals based on sites with ≥ 100 

individuals.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 143

Table 6. Zone 5, Northeast Atlantic Coast. Summary of Population Data and Trends

(+ = increasing; - = decreasing; 0 = no change).

State/

Province

Status 

(B, M, 

W, w)1

Number 

colony sites/

time period

No. active 

col./ last yr. 

surveyed

No. pairs/ 

last year 

surveyed

Breed. 

Trend

Winter 

number 

(Birds)

Migrant 

number (Birds)

CT B, w 26/1986-98 20/1998 961 + 25-100 -

DE B, M, w 1/1991 1/1991 3 NA NA 1000s (spring); 

100,000s (fall)

ME B, M > 117/1980s90s117/1994-

95

17,100 - - 1000s (spring)

MD B, M, w 4/1990s 4/1999 1046 + - 100,000s (fall)

MA B, M, w 28/1977-95 27/1994-95 7833 + 200 NA

NB3 B, w (?) 15-253/1986-

91

15/1986-91 9,278 + NA -

NF B > 7/1980-96 > 5/1996 343 NA - -

NH B, M 3/1974-98 1/1998 20 NA - NA

NJ B, M, w 5/1993-97 5/1993-97 NA NA 200-

1000

189,768 (fall)

NY 

(coast)

B, M, W 6/1995 6/1995 3,528 + 1000s 1000s

NS 69/1987 15,964 4 B 71-804/1987-

92 ?/1992 12,000

-/0 (?) - -

PA B, M 1/1996-99 1/1999 5 NA - 100s-1000s

PEI B, M (?) 12/1978-99 8/1999 7,695 +/0 (?) - 1000s (?)

QC B 995/1970-98 526/1987-

93

>24,1287 

(1987-93)

+ - -

RI B, M, w 10/1980-98 9/1997 2056 + 200 10,000-

100,000

VA B, M, W 1/1993-97 1/1997 60-70 + 1000s 20,000-

300,000

WV M, w NA NA NA NA <100? 1000s

Totals > 406 > 341 > 86,056 NA NA

1 = B, Breeding; M, Migrant; W, Wintering; w, small numbers winter.

2 = Counts for 5 Long Island colonies not available this year, so 1995 census data used for these 

colonies. Estimate is probably within 10 % of actual population size (B. Miller, pers. comm.).

3 = Numbers from Lock et al. (1994); Korfanty et al. (1997) reported approximately 15,000 

individuals in summer months, but did not provide breeding number or information on how 

estimate was obtained or time frame. Erskine (1992) reported 25 colonies in “recent” surveys 

by provincial and federal officials, but no other information was provided.

4 = Lock et al (1994) provided number of active colonies in 1987; Erskine (1992) reported 80 

colonies in “recent” surveys by provincial and federal officials, but no other information was 

provided. Korfanty et al. (1997) provided estimate of breeding pairs in 1992, but did not 

report information on number of active colonies.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 144

5 = A total of 121 known colonies was reported for Québec, but 22 of them appeared to have 

disappeared during the nearly 30 year period for which data were supplied. We included only 

those colonies that were active in the last year they were visited (J.F. Rail, pers. comm.). 

6 = This is the number of colonies with breeding birds that were visited around 1990 (1987 –

1993). Many colonies were not visited during this time period, but were and are currently 

presumed to be active (J.F. Rail, pers. comm.).

7 = No province-wide surveys have been conducted during a single year in Québec. Number of 

pairs is based on counts conducted around 1990 (all colonies surveyed between 1987 –

1993).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 145

Figure 68. Distribution of Northeast Atlantic Coast DCCO breeding population (Zone 5), 

1980 – 19991

 

1

 A small number of colonies in Quebec have not been surveyed since the 1970s, but are 

presumed active (J.F. Rail, pers. comm).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 146

Figure 69. Distribution and size of active colonies in the mid-Atlantic states at time of last 

complete surveys.*

 

*

Distribution and size of inland New York colonies shown in Figure 35.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 147

Figure 70. Distribution and size of active colonies in New England at time of last surveys.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 148

Figure 71. Distribution and size active colonies in Maine during last complete survey (1994 

– 1995).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 149

Figure 72. Distribution and size of active colonies in New Brunswick, Novia Scotia1 and 

Prince Edward Island during last complete surveys.

 

1

 A complete survey was conducted for Nova Scotia in 1992, but we were not able to obtain this 

information; locations and sizes shown are for 1987.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 150

Figure 73. Distribution of active colonies in Quebec at time of last surveys (1970 – 1998).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 151

Figure 74. Distribution and size of active colonies in Quebec at time of last surveys (1970 –

1998).1

 

1

 All colonies shown are presumed active, though some have not been surveyed since the 1970s 

and 1980s (see Figure 73). Sizes shown are based on last colony visits (J.F. Rail, pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 152

Figure 75. Detail of the St. Lawrence River Estuary and Gaspe Peninsula showing 

distribution and size of active colonies at time of last surveys (1970 – 1998).1

 

1

 All colonies are presumed active, though some have not been surveyed since the 1970s and 

1980s. Sizes shown are based on last colony visits (J.F. Rail, pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 153

Figure 76. Distribution and size of active colonies along the coast of Newfoundland and on 

offshore islands during last complete survey (1996).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 154

Connecticut

Summary of Population Data and Trends

(Survey completed by J. Victoria, Dept. of Environmental Protection, Franklin Wildlife 

Management Area, North Franklin, CT.)

Breeding.

The Double-crested Cormorant appears to be a relatively recent breeder in Connecticut, 

with first nesting documented in 1979 at East White Rock in the Norwalk Islands (Wood 1979). 

Currently there is a total of 26 known colony sites; complete censuses have been conducted 

every three years since at least 1986, when 181 pairs were recorded. Between 1986 and 1989, 

numbers increased by more than 600 %, and reached a peak of 1117 pairs. But by 1992, numbers 

had declined by close to 50 %. Censuses in 1995 and 1998 indicated that numbers were again 

increasing (Figure 77); in 1998, the last survey year, a total of 961 pairs was estimated (J. 

Victoria, pers. comm.). Between 1986 – 1998, the average annual rate of increase was 15 %.

Winter.

Along with the increase in breeding birds, in the 1980s the species became regular in 

winter along the coast and inland during migration (Sibley 1994; Zeranski and Baptist 1990). 

However, only small numbers of birds, 25 – 100 individuals, are estimated to winter in the state 

(J. Victoria, pers. comm.). Review of CBC data collected between 1984 and 1998 identified one 

site, New London, where > 100 birds have been estimated during a count (Sauer et al. 1996).

Migration.

Although observations of migrants are routine, significant concentrations do not stopover 

for any length of time (J. Victoria, pers. comm.).

Historical Information

Sibley (1994) reports that no historical nesting records exist for the Double-crested 

Cormorant in the state of Connecticut, but that it was recorded as a rare migrant through the 

1800s and into the early 1900s.

Land Ownership

No information available.

Productivity

No estimates available.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 155

Figure 77. Changes in numbers of breeding pairs in Connecticut, 1986 – 1998.

0

200

400

600

800

1000

1200

1984 1986 1988 1990 1992 1994 1996 1998 2000

Year

No. Pairs

Delaware

Summary of Population Data and Trends

(Survey completed by C.M. Heckscher, Division of Fish and Wildlife, Smyrna, DE.)

Breeding.

The Double-crested Cormorant appears to be a recent breeder in Delaware. Breeding was 

first documented in 1991 at Little River, where three pairs were counted. However, no other 

information on this site was available at the time this assessment was prepared (C. Heckscher 

pers comm.).

Winter.

Cormorants do winter in the state, but estimates of numbers and wintering locations were 

not provided. Review of CBC data collected between 1984 – 1998 (Sauer et al. 1996) identified 

one site, Rehoboth, where ≥ 100 birds were estimated during at least one count (600 birds in 

1984).

Migration.

During spring migration, significant numbers are observed continuously along the coast, 

but no regular concentration sites were reported. Peak numbers in late April are estimated in the 

thousands; peak numbers migrating down the coast in the fall may be in the hundreds of 

thousands (D. Forsell, pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 156

Historical Information

No information available.

Land Ownership

No information provided.

Productivity

No estimates available.

Maine

Summary of Population Data and Trends

(Survey completed by R.B. Allen, Dept. of Inland Fisheries and Wildlife,Bangor ME.)

Breeding.

Maine, by far, has the largest number of breeding Double-crested Cormorants in New 

England. In 1994 – 1995, at least 17,100 pairs were estimated at 117 known colonies, all of 

which were on coastal islands, except for one inland nesting site (not shown on Figure 71; 

coordinates not provided). Numbers appear to have reached their peak in the mid 1980s, when 

28,760 pairs were counted at 121 colony sites during 1984 – 1985 surveys. While Krohn et al. 

(1995) reported 28,000 pairs in the early 1990s, this estimate was an approximation based on 

visits to only about half of the known colonies. Numbers appear to have declined at an average 

annual rate of – 5 % during the last two complete surveys (1984/85 and 1994/95) (Figure 78). 

This decline may be a result of several factors: 1) in the mid-to-late 1980s, cormorants may have 

exceeded the carrying capacity for their numbers in coastal Maine; current numbers are probably 

more in line with what the habitat and fish resources can actually support; 2) Bald Eagles have 

increased in recent years in Maine and Bald Eagles have been observed to heavily prey on chicks 

and disrupt cormorant colonies; 3) aerial surveys show large variation depending on when birds 

are counted during the nesting cycle (R.B. Allen, pers. comm.).

Winter.

Cormorants are not known to winter in Maine (R.B. Allen, pers. comm.). Review of CBC 

data collected 1984 – 1998 (Sauer et al. 1996) identified no sites where ≥ 100 birds were 

estimated during at least one count, though the species was occasionally found in small numbers.

Migration.

During spring migration, peak numbers (thousands) are seen in April, migrating north to 

the St. Lawrence River and Gulf; no regular stopover sites reported (R.B. Allen, pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 157

Historical Information

Mendall (1936) described the early history of the Double-crested Cormorant in Maine, 

and his work is summarized here, unless otherwise noted. The first mention of Cormorants in 

New England was made by Captain John Smith in 1616. In the summer of 1614, Captain John 

Smith explored the Maine coast in a southwesterly direction from the Penobscot River, and 

Mendall (1936) felt that his reference to “Cormorants” likely applied to Maine. Towards the end 

of the 17th century, reports of how the Indians captured cormorants around Black Point, Maine 

(now the town of Scarborough), were made, but it is not clear if the species in question was 

P.auritus or P. carbo (Mendall 1936). Confusion between these two species and over the status 

of cormorants is apparent in much of the literature that followed. However, by the19th century, 

for reasons that are not clear, “the cormorant” was extirpated as a breeding species along the 

Maine coast (Gross 1944). At the end of the 19th century, Smith (1883, cited in Mendall 1936) 

wrote that no Double-crested Cormorants bred on the coast of Maine.

Though it was observed roosting, summering and migrating along the coast at various 

times, the Double-crested Cormorant was not documented as a breeder in Maine until the late 

1800s. The first Double-crested Cormorant eggs collected in the state were probably taken in 

1892 near Isle au Haut. The following year seven nests were observed, four of them with eggs, 

on Black Horse Ledge. Nesting was again observed at this site in 1895 and 1896. Egg collecting 

at these sites was believed to have contributed to the disappearance of nesting DCCOs in Maine 

in the 1890s (Gross 1944). Though many young and adult cormorants were observed loafing in 

several areas, the species was not recorded nesting in Maine again until 1925. The population 

then increased rapidly through the mid 1940s. In 1931, more than 1700 individual breeding birds 

were observed at various locations. By 1944, Gross (1944) estimated that 10,000 pairs nested in 

32 colonies on the Maine coast. In response to complaints from fishermen that cormorants were 

eating too many fish, a control program was initiated in 1944 and continued until 1953. Between 

the 1940s and 1970s, the population experienced a long period of slow growth, which was 

largely attributed to the control efforts (shooting, oiling eggs, destruction of young and eggs, and 

harassment at nesting colonies) and pesticides (Drury 1973, 1974). The cormorant population 

then underwent a second period of rapid growth from the 1970s through the mid 1980s; in 1977, 

15,333 pairs were reported at 103 colonies. In 1984, the population appears to have reached its 

peak, with 28,760 pairs reported at 121 colonies (Krohn et al. 1995).

Land Ownership

Maine provided land ownership for all colonies that were over 300 pairs (n = 17); 12 

were on private lands owned by individuals (9), or by NGOs such as The Nature Conservancy 

(1) and The National Audubon Society (2); three were on state lands managed by Maine Dept. of 

Inland Fish and Wildlife; one was on federal land managed by USFWS; and one was a 

municipality.

Productivity

No estimates available.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 158

Figure 78. Changes in numbers of breeding pairs in Maine, 1977 – 1994/95.

a

a

b

a

0

5000

10000

15000

20000

25000

30000

35000

1975 1980 1985 1990 1995 2000

Year

No. Pairs

a = Data from B. Blodget, estimate based on complete surveys.

b = Data from Krohn et al. (1995), estimate based on visits to 67 colonies in early 1990s, about 

half the number of known colonies.

Maryland

Summary of Population Data and Trends

(Survey completed by D.F. Brinker, Maryland Department of Natural Resources 

Annapolis, MD.)

Breeding.

The Double-crested Cormorant is a recent breeder in Maryland. First nesting was 

documented on Poplar Island in Talbot County in 1990; 55 nests were counted (Meritt 1996). 

Numbers have increased fairly steadily at an average annual rate of 38.7 % since discovery 

(Figure 79). In 1999, there were four known colony sites, with a total of 1046 pairs. The South 

Point Spoils colony is the only coastal location; the other three are in Chesapeake Bay (D. 

Brinker, S. Smith, pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 159

Winter.

No major concentrations are known to overwinter, but small numbers (< 100) do occur. 

CBC data collected between 1984 – 1998 (Sauer et al. 1996) identified one site, Port Tobacco, 

where ≥ 100 birds were estimated during at least one count (451 birds in 1998).

Migration.

Thousands of migrants occur in Chesapeake Bay. Peak spring numbers are reported April 

5 – 25; peak fall numbers occur October 1 – November 10, and hundreds of thousands are 

estimated migrating down the coast (D. Forsell, pers. comm.).

Historical Information

Audubon (1843) reported that the Double-crested Cormorant was common as far south as 

the coast of Maryland in winter. Baird et al. (1884) stated the species was a common spring and 

fall migrant along the east coast as far south as Maryland. At the turn of the century, the 

cormorant was reported as an uncommon migrant and winter visitor in the state (Meritt 1996). 

Later, it was described as an uncommon to common migrant in tidewater areas and in the 

Allegheny Mountain Section, and rare elsewhere. In recent years it has become more common 

during the summer in areas of the Chesapeake Bay. Field observers for the Maryland Breeding 

Bird Atlas Survey predicted the first Maryland breeding record would be obtained during the 

Atlas Survey years (1983 – 1987); breeding was confirmed in 1990 (Meritt 1996).

Land Ownership

Three of the four colonies were on state land; Spring Island is on federal land, managed 

by the Blackwater National Wildlife Refuge. 

Productivity

No estimates provided.

Figure 79. Changes in number of breeding pairs in Maryland, 1990 – 1999.

0

200

400

600

800

1000

1200

1988 1990 1992 1994 1996 1998 2000

Year

No. Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 5 160

Massachusetts

Summary of Population Data and Trends

(Surveys completed by B.G. Blodget, Massachusetts Division of Fisheries and Wildlife, 

Westborough MA and L. Henze, USDA/WS, Amherst, MA.)

Breeding.

The Double-crested Cormorant returned as a nesting species to Massachusetts around 

1940, possibly earlier (see Historical Information); at that time 53 nests were documented at 

Shag Rocks in Boston Harbor (Hagar 1941). Since 1977, when 1760 pairs were nesting at 11 

known sites, at least three complete surveys have been done. Numbers have increased between 

1977 – 1995 at an average annual rate of 8.6 % (Figure 80). At the time of the last survey, 1994 –

1995, 7833 pairs were counted at 27 colonies; there are a total of 28 known sites (B. Blodget, 

pers. comm.).

Winter.

There were no proven mid-winter records prior to 1976 (Veit and Petersen 1993). With 

the increase in breeders, birds began occurring in the winter with more frequency, and a small 

number, probably < 100 – 200 individuals, now winter in the state (B. Blodget, pers. comm.). 

Review of CBC data collected between 1984 – 1998 (Sauer et al. 1996) identified no sites where 

≥ 100 birds were counted.

Migration.

No significant stopover sites were reported (B. Blodget, pers. comm.), but large numbers 

have been observed, especially in the fall. In spring, migrants peak in late April, and high counts 

of up to 1300 have been reported; fall migration occurs September – October, and high counts of 

5000 + have been reported (Veit and Petersen 1993).

Historical Information

Bones of adult and young Double-crested Cormorants found in Indian middens indicate 

the species was breeding on Calf Island, Boston Harbor, as early as 1500 (Hatch 1982). Mendall 

(1936) notes that in the mid-17th century Roger Williams (1643) stated that “the Indians used 

quantities of Cormorants for food, returning in the morning, after a night’s hunt, with vast 

numbers of the birds.” Mendall (1936) thought that Williams’ observations referred to either 

Massachusetts or Rhode Island, or both. At some point cormorants declined and were entirely

extirpated as breeders in New England by the early 19th century. In the late 1800s, the species 

returned to New England as a nesting species in Maine (Veit and Petersen 1993), but did not nest 

there with regularity until 1925. It began expanding its range apparently in the 1930s. Hagar 

(1941) wrote that the colony he discovered in 1940 around Boston Harbor was "probably of 

several years’ standing;” and noted a communication that cormorants had been breeding at Shag 
FINAL DRAFT Part II. Population Data and Trends, Zone 5 161

Rocks since as early as 1937. In the early 1940s, cormorants were regularly summering on the 

whole of the Massachusetts coast. By the mid 1940s, cormorants were nesting at Shag Rocks, the 

Weepecket Islands in Buzzards Bay, and on at least one island off Salem (Hatch 1984). Numbers 

continued to increase in Massachusetts Bay until 1950, when the number of pairs reached 715 

(Drury 1973). Numbers in the bay then appeared to decline naturally, which coincided with the 

control program that was initiated in Maine in 1944 and continued until 1953 (see Maine

summary). By 1972, the population in Massachusetts Bay had declined to 325 pairs. Cormorants 

began a second period of increase, and the Boston Harbor population more than doubled between 

1977 and 1981, from 1,000 to 2,510 pairs. By 1982, the entire nesting population of 

Massachusetts totaled 5,173 pairs (Veit and Petersen 1993). The population has continued to 

increase, although at a slower rate, and in the latest survey (1994/95) totaled nearly 8000 pairs, 

as noted above.

Land Ownership

Ownership information was provided for nine of the 28 breeding sites. Six of the colonies 

were on public land, while three were on private land. No further information was available.

Productivity

No estimates provided.

Figure 80. Changes in numbers of breeding pairs in Massachusetts, 1977 – 1994/5.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1975 1980 1985 1990 1995 2000

Year

No. Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 5 162

New Brunswick

Summary of Population Data and Trends

(Survey completed by R. Stocek, Maritime Forest Ranger School, Fredericton, New 

Brunswick; breeding locations from Lock et. al. 1994.)

Breeding.

Korfanty et al. (1997) reported that there are about 15,000 Double-crested Cormorants in 

the coastal areas of New Brunswick during summer months. In surveys summarized in Lock et 

al. (1994), 9,278 pairs were estimated between 1986 – 1991 at 15 sites. However, Erskine (1992) 

reported a minimum of 25 colonies in the province, though information was not provided on 

when these colonies were active. Overall, numbers are increasing in this province, but the current 

rate of increase (not available) is less than it was 15 years ago (Korfanty et al. 1997).

Winter.

A small number of Double-crested Cormorants may remain in the province during the 

winter (Korfanty et al. 1997). Review of CBC data collected during 1984 – 1998 (Sauer et al. 

1996) identified no sites where ≥ 100 birds were counted.

Migration

No significant stopover sites were reported for migrants, but birds are observed moving 

along the coastal Bay of Fundy during migration. Peak spring migration occurs mid-late April, 

with birds observed as early as March 28. Peak fall migration occurs mid-October. A special 

volunteer project of the Saint John’s Naturalist Club has been collecting data on migratory 

seabirds at the Point Le Preau Bird Observatory for the last five years. The observatory is about 

25 miles southwest of Saint John’s on the Bay of Fundy. Observations made in spring 1996 –

1998 on heaviest migration days for this species recorded averages of 21 – 56 cormorants per 

hour. However, many cormorants probably bypass this observation point and migrate overland, 

so these numbers may not be representative of numbers coming through the entire province. In 

fall, more cormorants follow the coastline, and averages of 160 – 367 cormorants per hour on 

heaviest migration days have been observed. However, volunteer coverage has been less intense 

than in spring; thus more intense coverage may be necessary to obtain more accurate fall 

estimates (J. Wilson, pers. comm.).

Historical Information

The only early record we obtained for Double-crested Cormorants in New Brunswick 

was from Herrick (1878), who reported that the species probably nested formerly at Grand 

Manan, but had ceased to do so. Lewis (1929) reports a colony present at Saltkill, Ids., Lepreaux, 

estimated at 200 pairs. No other information is available at this time.

Land Ownership

No information available.

Productivity

No estimates available.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 163

Newfoundland

Summary of Population Data and Trends

(No survey was completed for Newfoundland. Distribution and population information 

were provided by W. Montevecchi, Memorial University of Newfoundland, St. Johns, NF, 

Korfanty et al. 1997 and Lock et al. 1994.)

Breeding.

In a survey of coastal Newfoundland and offshore islands in 1996, W. Montevecchi 

reported at least 343 nesting Double-crested Cormorants at a minimum of six locations (in

Korfanty et al. 1997). At many of these sites Double-crested and Great Cormorants occurred 

together; DCCOs may also have nested at Great Barasway, but at this location numbers of 

DCCOs and GRCOs were not distinguished (Korfanty et al.1997; W. Montevecchi, pers. 

comm.). Since 1980, 8 – 9 known breeding colonies have been observed (Lock et al. 1994; 

Korfanty et al. 1997). Numbers are at low density levels in Newfoundland compared to 

population levels in other Maritime provinces, and it is not known if these low density 

population levels are stable (Korfanty et al. 1997).

Winter.

Review of CBC data collected between 1984 – 1998 (Sauer et al. 1996) identified no sites 

where ≥ 100 birds were estimated during a count, though small numbers were occasionally 

observed.

Migration.

No information available.

Historical Information

The earliest mention of cormorants in Newfoundland is in 1594, when Europeans visited 

a small village on the shore of St. George Bay on the west coast of Newfoundland and found 

“foules called Cormorants, which they [the Indians] had pluckt and made ready to have dressed” 

(Hakluyt 1904). Lewis (1929) noted that while the specific identity of these birds is uncertain, 

DCCOs nested here at the time he was writing. Cormorant bones were also abundant at a 17th

century Indian site on this province (Montevecchi and Tuck 1987). Audubon (1843) reported a 

large colony on Cormorant Island as he sailed towards the Straits of Belle Isle. Other early 

records include a colony at Hawk’s Bay (Reeks 1869). Audubon (1843) also documented human 

persecution at this time, noting that while “the fishermen and eggers never gather their 

eggs...they commit great havoc among the young, which they salt for food or bait.” By the time 

Lewis was writing, Double-crested Cormorants were recorded at eight colonies with < 50 pairs 

estimated at most of these; for a few of these colonies, no population estimate was available.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 164

Land Ownership

No information available.

Productivity

No estimates available.

New Hampshire

Summary of Population Data and Trends

(Survey completed by J. Kanter, New Hampshire Fish and Game Dept, Concord NH.)

Breeding.

The Double-crested Cormorant has nested in New Hampshire since the 1940s – 1950s. 

The trend in breeding numbers in this state is not clear. There are only three known colony sites, 

all coastal. Small numbers of breeding pairs (8 – 24) have been reported since the mid-late 

1970s, with small increases in the 1980s to fairly large increases in the early-to-mid 1990s. 

However, these large increases are thought to be from birds dispersing from Maine to New 

Hampshire. Numbers declined again in 1998, after the large newly formed colony in New 

Hampshire at White Island abandoned this site due to harassment employed to preserve the site 

for breeding terns. In 1998, only 20 pairs were reported at one site (J. Kanter, pers. comm.).

Winter.

No substantial numbers winter in the state (J. Kanter, pers comm.).

Migration.

Peak migration numbers are observed in April and October, but no estimates of numbers 

available (J. Kanter, pers. comm.).

Historical Information

No information available.

Land Ownership

Ownership information was provided for the three known breeding sites. Square Rock 

and Lunging Rock, which are next to each other, are both privately owned lands, owned by 

individuals. White Island, where cormorants were harassed and eventually abandoned the site, is 

state owned land (J. Kanter, pers. comm.).

Productivity

No estimates provided.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 165

New Jersey

Summary of Population Data and Trends

(Survey completed by D. Jenkins, New Jersey Division of Fish, Game and Wildlife, 

Trenton,; breeding data provided by J. Walsh, Cape May Bird Observatory, and R. Kane, New 

Jersey Audubon Society.)

Breeding.

In New Jersey, summering patterns of the Double-crested Cormorant throughout the 

1970s and 1980s indicate an increasing population, but breeding was not documented until 1987, 

when colonies were found in lower Newark Bay. Between 1993 – 1997 an atlas survey was 

conducted and cormorants were confirmed nesting on artificial structures at five locations. At all 

five locations cormorants nested on manmade structures. Information on numbers of breeding 

pairs is not available, but colonies are believed to be small, < 100 pairs (J. Walsh, pers.comm; R. 

Kane, pers. comm).

Winter.

The DCCO occurs in winter, but no estimates of numbers were available. Leck (1984) 

reported “small numbers remain into winter.” CBCs conducted between 1959 – 1988 suggested 

an increase of 3.5 % per year. More recent counts (1984 – 1998) suggest that numbers have 

continued to increase (Figure 81), though these data have not yet been analyzed for percent 

change. High statewide counts of > 600 birds were recorded in 1992 and 1998 (Sauer et al. 

1996).

Migration.

In the fall, peak numbers of migrants are seen in early-mid Oct, both along the coast and 

inland. Maximum number recorded was 21,856 on October 17, 1997. Hundreds of thousands 

pass through during migration, with an average number of 189,768; a maximum of 240,676 was 

reported by Avalon Sea Watch in 1996. The spring migration peaks in late April or early May, 

with a maximum count of 3500 birds, Avalon, April 17,1981. Flight is mainly over land, with 

many birds crossing from Delaware and dispersing northward up the Cape May peninsula 

(Walsh et al. 1999).

Historical Information

Little historic information on the Double-crested Cormorant in New Jersey was obtained. 

Stone (1937) summarized records of Double-crested Cormorants observed around Cape May 

between 1890 – 1934. Most observations were of spring and fall migrants. Sightings were also 

reported during the summer, but it was not possible to determine if these birds were early 

migrants or non-breeding birds that had summered in the area. Sibley (1993) notes that the 

Double-crested Cormorant is apparently more numerous around Cape May than formerly, at 
FINAL DRAFT Part II. Population Data and Trends, Zone 5 166

least during the summer, as Stone (1937) only listed five summer records of the species. Most of 

the flocks reported by Stone (1937) were small, < 100 birds, though “an immense flock 

stretching well across the sky” was observed in 1929. Numbers of migrants appear to have 

remained similar since the early 1980s, based on high counts. Leck (1984) reported a high count 

of 14,000 birds at Cape May on Oct 11 – 12, 1981, and Sibley (1993) reported a high count of 

20,000 at Cape May Point on Oct 11, 1983.

Land Ownership

No information provided.

Productivity

No estimates provided.

Figure 81. December estimates of DCCOs in New Jersey based on CBCs, 1984 – 1998.

0

100

200

300

400

500

600

700

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals

New York (Coastal)

Summary of Population Data and Trends

(Survey completed by R. Miller, New York State Dept. of Env. Cons., Delmar, NY.)

Breeding.

The Double-crested Cormorant was first reported breeding in New York in 1945, but was 

not reported as a breeder in coastal New York until 1977 (see Historical Information, below). 

Over the last 15 years, numbers have increased significantly in the Long Island / New York City 
FINAL DRAFT Part II. Population Data and Trends, Zone 5 167

region. In 1985, there were an estimated 585 breeding pairs in this region; in 1998, there were an 

estimated 3,274 pairs (R. Miller, pers. comm.), an average annual rate of increase of 14.2 % 

(Figure 82). In the latter year all known breeding sites were surveyed, with the exception of Plum 

Island and North and South Brother Islands. North and South Brother Islands were surveyed in 

1999, and these counts were figured into the 1998 total (Miller 1998). Excluding Plum Island, 

nine breeding sites were reported during the last survey, but we only received location 

information for five. Based on 1997 estimates for inland New York colonies and 1998 estimates 

for coastal ones, the entire state population (upstate and Atlantic Coast combined) was estimated 

at 12,346 pairs.

Winter.

Up to several thousand birds winter along Long Island and in the New York City region 

(R. Miller, pers. comm.). However, review of CBC data collected between 1984 – 1998 reported 

counts of > 100 birds at only two sites, Queens (103 birds in 1991) and Staten Island (150 birds 

in 1998) (Sauer et al. 1996).

Migration.

During spring and fall migration, large flocks are observed along the shores of Long 

Island. On heavy flight days, daily counts will sometimes top 10,000 in both seasons, especially 

in late September, when fall migration peaks (Miller 1998). Spring migration peaks late March –

early April (R. Miller, pers. comm.).

Historical Information

Though the first authentic record of cormorants nesting in New York was not obtained 

until 1945, and was at an inland location [see summary for New York (Inland)], Arbib (1988) 

notes that in pre-colonial times the cormorant could have nested within the boundaries of the 

future New York state, given its distribution along the East Coast. The first nesting for Atlantic 

New York was not documented until 1977 at Fishers Island (Bull 1981). As increases occurred in 

the Atlantic Coast population, cormorants moved southward from breeding sites in the Maritimes 

to Maine, Massachusetts, and eventually to Long Island, NY. In Long Island / New York City 

region, numbers increased from 585 pairs in 1985 to 3528 pairs in 1995. See summary of New 

York (Inland) for additional information on DCCOs in New York.

Land Ownership

Ownership information was provided for five of the New York-Atlantic breeding 

colonies. Three were on privately owned lands, owned by private corporations, organizations and 

individuals. Two were on public lands that were not identified further (R. Miller, pers. comm.).

Productivity

No information available.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 168

Figure 82. Changes in breeding numbers in New York (coastal) 1985 – 1998.

0

500

1000

1500

2000

2500

3000

3500

1984 1986 1988 1990 1992 1994 1996 1998 2000

Year

Number of Pairs

Nova Scotia

Summary of Population Data and Trends

(Survey completed by R. Stocek, Maritime Forest Ranger School, Fredericton, New 

Brunswick.)

Breeding.

Population information here is summarized from various sources. The last survey was 

conducted in 1992; at that time 12,000 pairs were estimated, and the population appeared to be 

declining (Milton et al. 1995) (Figure 83). A survey was planned for 1997 (Korfanty et al. 1997), 

but we were not able to obtain information as to whether or not this occurred. A review of 

surveys at selected colonies suggests that numbers are no longer growing and may be undergoing 

a small decline (Milton et al. 1995), or may be stabilizing (Korfanty et al. 1997). Lock et al. 

(1994) reported 69 active colonies observed in 1987 (a complete survey year), and a total of 71 

colonies observed between 1987 – 1992. However, Erskine (1992) reported that “recent” surveys 

by provincial and federal officials indicated at least 80 colonies (the Atlas survey work occurred 

between 1986 – 1990). The average annual rate of increase between 1972 – 1992 was 5.4 %; 

however, most growth occurred between 1972 – 1980, when numbers increased at an average 

annual rate of 14.3 %.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 169

Winter.

Review of CBC data collected between 1984 – 1998 (Sauer et al. 1996) identified no sites 

where ≥ 100 birds were estimated during a count.

Migration.

No information available.

Historical Information

There are several early records of cormorants in Nova Scotia from the 17th century. The 

earliest is from 1604, when Europeans visited the Isle of Cormorants, an island west of Cape 

Sable on the southwest coast of Nova Scotia, “so named because of the infinite number of these 

birds, of whose eggs we took a barrel full,” (Champlain 1922). In a footnote to this account, it is 

suggested that this island was probably the “present [1922] Green Island.” Lewis (1929) notes 

that in this same account, cormorants are mentioned among the birds found on the present Mud 

Island group, south of the coast of Yarmouth County. In 1610, cormorants were observed and 

killed in the same region, off southwestern Nova Scotia, and Lewis (1929) notes that in both 

accounts, it is a fair probability that at least some of these cormorants were Double-crested. In 

1634 – 1635, the Indians of Cape Breton were observed to eat cormorants in summer (Lewis 

1929). In 1835, Audubon (1835) reported that “a good number [of DCCOs] breed on Seal Island 

[near Cape Sable] off the Bay of Fundy.” The next record of cormorants appears to be from the 

early twentieth century; Lewis (1929) reports a letter dated 1928 which tells of a cormorant 

rookery that existed “some years ago...on the Bay of Fundy coast, west of the [Digby] ‘Gut’,” 

Digby County. It is not clear which species of cormorant is being referred to.

By the early 1900s, or perhaps earlier, cormorants had greatly declined in this province; 

Korfanty et al. (1997) report that “Double-crested Cormorants were pretty much extirpated from 

Nova Scotia during the early 1900s.” Lewis (1929) reported that ornithologists visiting islands 

off the southwest coast of Nova Scotia during the twentieth century found no nesting cormorants; 

breeding cormorants were reported at only two northeastern locations in the mid-1920s, Bird 

Islands and Antigonish, with a total of 67 pairs. Numbers remained very low or absent for some 

unknown amount of time. However, numbers are believed to have increased to 1,300 – 1,500 

pairs by the mid to late 1950s, and growth during this period is believed to resemble that of 

cormorants along the Maine coast. The first thorough survey of Nova Scotia’s colonies was 

conducted in 1971. During the 10 year period between 1971 and 1980 – 1982, the population 

nearly tripled, growing from 4,150 to more than 12,000 pairs. In 1985 numbers peaked at 15,700 

pairs. Numbers then began to decline.

Land Ownership

No information available.

Productivity

No estimates available.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 170

Figure 83. Changes in breeding numbers in Nova Scotia, 1972 – 1992.

a

a,b

a

c

a

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

1970 1975 1980 1985 1990 1995

Year

No. Pairs

a = Korfanty et al. (1997)

b = Estimate from counts 1980-1982

c = Lock et al. (1994)

Pennsylvania

Summary of Population Data and Trends

(Survey completed by D. Brauning, Pennsylvania Game Commission, Montgomery PA.)

Breeding.

The Double-crested Cormorant appears to be a recent breeder in Pennsylvania; the first 

nesting was not documented until 1996, when one nest with three young was observed at Wade 

Island (McConaughy 1996). Since that time, small numbers of birds (< 5 pairs) have continued 

nesting at this site through 1999 (M. Ross, pers. comm.).

Winter.

No significant numbers are reported to winter in this state. CBC data collected between 

1984 – 1998 (Sauer et al. 1996) identified one site, Lock Haven-Jersey Shore, where ≥ 100 birds 

were estimated during a count (194 birds estimated in 1989, but no birds recorded at this site in 

other years during this period).

Migrants.

Peak spring migrant numbers are recorded in mid-to-late April; fall migration peaks in 

the first half of October. During these times, hundreds to thousands can be seen passing through 

the state. Birds do not remain roosting in large numbers on a regular basis, but four areas were 
FINAL DRAFT Part II. Population Data and Trends, Zone 5 171

identified where migrants concentrate to feed or roost: the Lower Del River in Philadelphia; the 

lower Susquehanna River in the Harrisburg area; at Hammond and Cowanesque Lakes in Tioga 

County, which borders New York; and on Lake Erie, Presque Island State Park. (Ross 1995; D. 

Brauning, pers. comm.).

Historical Information

Little information available at this time. Mendall (1936) reports that Wood (1935) 

observed 19 cormorants at Harrisburg on May 3, 1935, which constituted the first record of more 

than a single individual in this region. Brauning (1992) reported the cormorant as a species

observed during the breeding season, but without sufficient evidence to be considered nesting, 

and remarked that it was likely a candidate for the state’s next “confirmed” breeder. The first 

breeding record was obtained at Wade Island in the Susquehanna River near Harrisburg in 1996 

(McConaughy 1996); non-breeding roosting occurred at this same locale a number of years prior 

to nesting (R. Ross, pers. comm.).

Land Ownership

Land ownership was identified only for three sites: Wade Island is owned by the 

Pennsylvania Bureau of Forestry; Hammond and Cowanesque Lakes are federal dam projects of 

the U.S. Army Corps of Engineers; and Presque Island State Park is under state ownership (D. 

Brauning, pers. comm.).

Productivity

In 1996, three chicks were observed on July 12 in the single nest at Wade Island 

(McConaughy 1996).

Prince Edward Island

Summary of Population Data and Trends

(Survey completed by R. Dibblee, Dept. of Technology & Environment, Fish & Wildlife 

Division, Charlottetown, PEI.)

Breeding.

The Double-crested Cormorant has been breeding on Prince Edward Island since at least 

the 1940s. Yearly surveys for cormorants have been conducted since the 1970s, and all known 

colonies are counted. At this time, there are 12 known colony sites, with eight of them active. 

Data were available for 18 surveys since 1976. Since that time numbers have increased at an 

average annual rate of 13.8 %, though growth has not been steady. Numbers reached their peak 

of 9,769 pairs in 1997, and then declined in 1998 and 1999 (Figure 84). Korfanty et al. (1997) 

suggested that growth may be beginning to level off; an average annual rate of increase of 2.8 % 

over the last decade (1989 – 1999) supports this conclusion.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 172

Winter.

Other than the occasional rare individual, cormorants do not winter in the province (R. 

Dibblee, pers. comm.). Review of CBCs conducted 1984 – 1998 (Sauer et al. 1996) identified no 

sites where ≥ 100 birds were observed in one year.

Migration.

Large numbers (1000s) of migrants have been reported in Malpeque Bay (North Shore –

West), but it is difficult to distinguish between migrants and locally produced birds that are 

flocking together (R. Dibblee, pers. comm.).

Historical Information

First nesting record was obtained in 1941, at which time there were a total of 75 nests at 

one known colony site. No additional information is available about possible earlier occurrences 

in this province (Hatch and Weseloh 1999). Breeding numbers increased very slowly from the 

1940s until the late 1970s, when 423 pairs were counted at two of three known colony sites. In 

the early 1980s numbers began to increase rapidly and new colonies began forming. The most 

rapid growth occurred between 1983 – 1989, when numbers increased by an average annual rate 

of 25 %.

Land Ownership

Land ownership information was provided for six of the breeding colonies. Two were on 

lands owned by the government of P.E.I.; three were on lands owned by private individuals; one 

was on land owned by a private organization, the Island Nature Trust. Additionally, one colony 

was on land that was noted as probably owned by the government of P.E.I. (R. Dibblee, pers. 

comm.).

Productivity

No information available.

Figure 84. Changes in numbers of breeding pairs on Prince Edward Island, 1976 – 1999.

0

2000

4000

6000

8000

10000

12000

1975 1980 1985 1990 1995 2000

Year

No. Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 5 173

Québec

Summary of Population Data and Trends

(Survey completed by J.F. Rail, Canadian Wildlife Service, Ste-Foy, QB.)

Breeding.

The Double-crested Cormorant has been breeding on the St. Lawrence River, Estuary and 

Gulf for hundreds of years (see Historical Information, below). Between 1970 – 1998 a total of 

99 colonies were reported on the St. Lawrence River, the St. Lawrence Estuary, the North Shore, 

the Gaspé Peninsula, Anticosti Island, and the Magdalen Islands. Province-wide surveys 

estimating colony size in the same year have not been conducted, so it is difficult to establish 

size and trends for the entire breeding population in Québec. Data were supplied for colonies 

collected in years around 1970, 1980 and 1990 (e.g., data collected 1967 – 1973 were assumed to 

represent 1970; 1977 – 1982 represented 1980; 1987 – 1992 represented 1990), and data for all 

colonies around each year were totaled. Numbers appear to have increased substantially: in 1970, 

numbers were roughly estimated at 5000 pairs, in 1990, numbers were estimated at about 25,000 

pairs (Figure 85). Obviously, these estimates must be interpreted cautiously because cormorants 

move frequently and size of individual colonies may change dramatically year-to-year (J.F. Rail, 

pers. comm.).

Chapdelaine and Bédard (1995) provided an estimate for 1990 that was calculated in a 

similar manner: data collected at colonies (primarily 1986 – 1990) were combined (a few 

colonies were counted in 1983 and included in this estimate). For the 1990 total, they estimated 

27,320 pairs at 70 active colonies. This number was compared to one calculated for the same 

area in the early 1980s (DesGranges et al. 1984), which estimated 43 active colonies and about 

12,000 breeding pairs. Based on these figures, Chapdelaine and Bédard (1995) determined that 

cormorant numbers increased everywhere in the decade between the late 1970s and late 1980s. 

The composite annual rates of increase were about 11 % for the Estuary, 16.5 % for the North 

Shore, 7 % for the Gaspé Peninsula, and 2 % for the Magdalen Islands. DesGranges et al. (1984) 

concluded that populations had especially increased in the St. Lawrence Estuary between 1965 

and 1979, but that the number of colonies and pairs on the North Shore had declined by more 

than half between 1928 – 1980. Increases may be related to reduction in human persecution and 

illegal population control, and to a possible increase in the stock of small fish species such as 

sand lance and capelin owing to commercial overfishing of large predator fish such as cod 

(Chapdelaine and Bédard 1995). In the last decade population trends in the St. Lawrence River 

have been reversed due to introduction of a 5-year culling program in 1989. In 1991, the estuary 

was estimated at 12,081 pairs, a sizable drop since the culling program began (Bédard et al. 

1995a). The goal for this area was to maintain the population at 10,000 pairs. However, the 

program was halted in 1993 because the population fell below the 10,000 pairs threshold, with 

9,561 pairs estimated in this year (Bédard et al. 1999).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 174

Winter.

Cormorants are not known to winter in Québec (J.F. Rail, pers. comm.). Review of CBCs 

conducted 1984 – 1998 (Sauer et al. 1996) identified no sites where ≥ 100 birds were observed in 

one year.

Migration.

The Canadian Wildlife Service did not have any information on major stopover sites for 

migrating cormorants, nor did they know if such sites existed.

Historical Information

The earliest mention of cormorants in Québec, recorded in 1591 at the Magdalen Islands, 

is perhaps the earliest reported observation in the range of the Double-crested Cormorant (Lewis 

1929). Lewis (1929) notes that “the anonymous author... speaks of a small island called ‘The 

Isle of Cormorants’ (Hakluyt 1904),” which is probably “the one now known as Shag Island, on 

the eastern side of the main group of the Magdalens.” Probably both names were used because it 

was the nesting place of one or more species of cormorant. Other early records (Lewis 1929) 

include nesting colonies of Double-crested Cormorants along the north shore of the Gulf of St. 

Lawrence near the Harbour of Great Mecatina Island observed in 1833 (Audubon 1835; 1843); 

near Sloop Harbor [now (1929) Whale Head] (Coues1861; Packard 1891); Harrington Harbor 

(local accounts); Bird Bay, near the eastern end of Anticosti Island (Schmitt 1902, 1904); 

Bonaventure Island, Gaspé County (Goss 1889); and again Shag Island in the Magdalen Islands 

(Bishop 1889). However, by the time Lewis was writing, DCCOs had disappeared from all of 

these locations, mainly due to human persecution and exploitation. Double-crested Cormorants 

continued nesting at certain other locations in the area, such as those included in bird sanctuary 

areas set aside for the benefit of other species. Lewis (1929) reported 26 active colonies in the 

province between 1915 – 1928, with roughly 3300 pairs. Lewis (1929) noted that in the Gulf of 

St. Lawrence region, “cormorants outside protected sanctuaries supplied fresh meat for Indians, 

fresh eggs for local residents, and a considerable quantity of food for sled-dogs and captive 

foxes...and that some colonies lose practically all their young every year.”

The history of the Double-crested Cormorant in Québec during the last century was 

reviewed by Chapdelaine and Bédard (1995) and DesGranges and Reed (1981), and is 

summarized here. At the beginning of the century, commercial fisheries regarded the cormorant 

as a competitor, which triggered studies on its behavior and feeding habits, and cormorant 

colonies in the St. Lawrence River, Estuary and Gulf were counted and studied. These studies 

showed that it ate fish of little or no commercial value. Nevertheless, the cormorant was targeted 

by fishermen, especially during the first half of the century, and certain colonies were prevented 

from expanding. Legal control was also undertaken in the second half of the century. Island 

landowners felt that cormorants were damaging natural vegetation, and the Canadian Wildlife 

Service collaborated with landowners and Québec provincial authorities to control cormorants on 

Ile aux Pommes in 1954 – 1955, and in 1978 – 1980, but without substantial reduction in 

numbers.

In the second half of the century, illegal cormorant control became less common, and 

cormorant numbers increased. As noted earlier, large scale legal cormorant control was again 

initiated in the last decade in the St. Lawrence Estuary, and has substantially reduced numbers.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 175

Land Ownership

While land ownership information was not provided for all 122 colony sites, sites that are 

on protected areas were identified. A total of 54 colonies was located on lands in National Parks, 

Migratory Bird Sanctuaries, or National Wildlife Areas (J.F. Rail, pers. comm.).

Productivity

Productivity rates ranged from 1.5 – 2.35 fledglings per nest in the Estuary (Cleary 1977; 

Lemire 1985; both cited in Chapdelaine and Bédard 1995); a rate of 2.87 fledglings per nest was 

estimated for one colony on the North Shore (Grenier 1987, cited in Chapdelaine and Bédard 

1995), while at another North Shore colony (located in Refuge des iles Ste.Marie) productivity 

was estimated at 2.68 and 2.18 young (13 – 14 days old) per nest in 1996 and 1997, respectively 

(Rail and Chapdelaine, unpub. data). See Hatch and Weseloh (1999) for additional information.

Figure 85. Changes in breeding numbers in Québec, 1970 – 1990.

0

5000

10000

15000

20000

25000

30000

1965 1970 1975 1980 1985 1990 1995

Year

No. Pairs
FINAL DRAFT Part II. Population Data and Trends, Zone 5 176

Rhode Island

Summary of Population Data and Trends

(Survey completed by C. Raithel, Division of Fish, Wildlife and Estuarine Resources, 

West Kingston, RI.)

Breeding.

The first known nesting of the Double-crested Cormorant in Rhode Island was 

documented in 1981 on the Sakonnet Islands (Ferren and Myers 1998a). In 1997, there was a 

total of 2058 pairs counted at nine colonies. In 1998, a new breeding site was discovered, 

bringing the total number of known colonies to 10. All colonies are located on islands above 

Rhode Island Sound and on the Sakonnet River. Additionally, 1 – 2 pairs have been observed 

breeding on Seekonk River (North) pilings and (South) platform. However, we did not include 

these latter sites in the total number of breeding colonies listed in Table 6 because they were 

single or double pair sites and may not be used regularly (C. Raithel, pers. comm.). Since initial 

discovery in 1981, numbers increased steadily through 1995, then showed small declines in 1996 

and 1997 (Figure 86). Much of the most rapid growth occurred between 1981 – 1990, when the 

average annual rate of increase was 63 %. Between 1991 – 1997, the average annual rate of 

increase slowed substantially to 4.3 %. Rapid growth has occurred at the West Island and Little 

Gould Island colonies (C. Raithel, pers. comm.).

Winter.

A small number of birds, about 200, winter in the state (C. Raithel, pers. comm.). 

Favored overwintering spots are Galilee Harbor and lower Salt Pond (Ferren and Myers 1998a). 

Review of CBCs conducted 1984 – 1998 (Sauer et al. 1996) identified no sites where ≥ 100 birds 

were observed in one year.

Migration.

Spring migration occurs mainly in April, and peaks between April 20 – 30; fall migration 

occurs the last week of September – first week of November, peaking about mid October, when 

as many as 20,000 birds a day are counted (Ferren and Myers 1998a). Up to 100,000 birds are 

estimated to pass through, feeding and roosting mainly along the coast.

Historical Information

Ferren and Myers (1998a) reviewed the historical distribution of the Double-crested 

Cormorant in Rhode Island and their work is summarized here. Prior to the arrival of Europeans, 

the Double-crested Cormorant could have been present in what is today Rhode Island; in preColumbian times the food supply was richer and more dependable than it is today, and nesting 

habitat was probably similar in terms of vegetation and freedom from mammalian predators. 

However, Native Americans could easily reach likely nesting locations in pre-colonial Rhode 

Island, as they did in other parts of New England, and any sizable cormorant colony would not 

have been ignored (see Massachusetts profile, Historical Information). It is fairly certain that 

cormorants were not present in the state in any significant numbers after European settlement, 

and the ensuing period of human exploitation of waterbirds all along the Atlantic seaboard. 

Ferren and Myers (1998a) note that Sturtevant saw only a single cormorant during all his pre1900 summers in the Newport area. Additionally, Hathaway saw only a single bird in August 

1903. Gradually, however, in the first third of the twentieth century, sightings began to increase 
FINAL DRAFT Part II. Population Data and Trends, Zone 5 177

and small numbers of summering individuals and immatures were observed on coastal rocks and 

pilings. Numbers grew slowly; in the 1950s counts rarely exceeded 50, but by 1971, 170 nonbreeders were counted on the Sakonnet islands. By 1980, the state’s non-breeding summering 

population was estimated to have reached a minimum of 1,000 birds. The following year, 

breeding was documented at Sakonnet, where numbers increased rapidly and breeding pairs soon 

spread to other islands.

Land Ownership

Of the 12 known breeding sites, 10 were on privately owned lands; Little Gould Island is 

owned by the Audubon Society of Rhode Island. The other two colonies, Big Gould and Hope 

Island, are on state land (C. Raithel, pers. comm.).

Productivity

No estimates available.

Figure 86. Changes in breeding numbers in Rhode Island, 1981 – 1998.

0

500

1000

1500

2000

2500

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998

Year

No. Pairs

Virginia

Summary of Population Data and Trends

(Survey completed by D. Schwab, Virginia Dept of Game and Inland Fisheries, 

Williamsburg, VA.)
FINAL DRAFT Part II. Population Data and Trends, Zone 5 178

Breeding.

The Double-crested Cormorant may be a fairly recent breeder in Virginia, with first 

known nesting documented in the James River 3 km east of Hopewell in 1978 (Blem et al. 

1980). However, breeding may have occurred historically (see Historical Information). In 1997, 

there was only one known colony on Shank / Cheasman Island in Chesapeake Bay, with an 

estimated 60 – 70 pairs. This colony appears to be slowly increasing; in 1993, the only other year 

for which data were reported, 12 nests were counted (D. Schwab, pers. comm.).

Winter.

In winter, birds can be observed along the James River in the southern end of Chesapeake 

Bay (D. Forsell, pers. comm), but no estimates of wintering numbers were provided. CBCs 

conducted between 1959 – 1988 suggested an increase of 12.6 % / year; more recent counts 

(1989 – 1998) (Sauer et al. 1996) suggest that numbers have continued to increase (Figure 87), 

though these data have not yet been analyzed for percent change.

Migration.

During migration, very large numbers reported; in the spring, estimates at Fisherman 

Island Refuge on the eastern shore of Chesapeake Bay range from 20,000 – 30,000 to as many as 

300,000 (D. Forsell, pers. comm.). Large numbers also observed in fall, September – October, 

with birds roosting and feeding in coastal waters and in the marshes and rivers around 

Chesapeake Bay (D. Schwab, pers. comm.).

Historical Information

Lewis (1929) noted several 17th century references that mention the abundance of 

cormorants in Virginia; one reports cormorants among the birds in Virginia rivers “in such 

abundance as are not in all the world to be equaled.” Lewis (1929) reported that probably most or 

all of these cormorants were Double-crested. The next records of the species we were able to 

obtain were from the 20th century (Mendall 1936). These include a small group (10 birds) of nonbreeding individuals observed in the summer of 1921 at Wallops Island; an account of a 

cormorant found in 1924 in the interior of the state that apparently struck a chimney; and an 

account of a flock of 175 cormorants observed around 1928 during fall migration flying in a 

southerly direction offshore of Cobb’s Island. We found no records of the species in the 18th and 

19th centuries; the species may have been absent or extirpated from the state during this time, as 

it was across New England (see Zone 5: Summary of Population Data for the Northeast Atlantic 

Coast Summary).

Land Ownership

No information provided

Productivity

No estimates available.
FINAL DRAFT Part II. Population Data and Trends, Zone 5 179

Figure 87. Annual totals of DCCOs in Virginia based on CBCs, 1984 – 1998.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Year

Individuals

West Virginia

Summary of Population Data and Trends

(Survey completed by D. Jones, West Virginia Nongame Wildlife & Natural Heritage 

Program, Elkins, WV.)

Breeding.

The DCCO is not known to breed in West Virginia (D. Jones, pers. comm.).

Winter.

Smith (1996) reports the species as fairly common in the state, September through May. 

He notes that the species has increased in recent years, with observations at various locales 

becoming frequent after 1990. In early January of 1995, a count of nearly 50 was made along the 

Ohio river from Nitro to Robertsburg, suggesting that the species is widespread in small numbers 

during the winter months. Review of CBCs conducted 1984 – 1998 (Sauer et al. 1996) identified 

no sites where ≥ 100 birds were observed in one year.

Migration.

During spring and fall migration, thousands are seen along the Ohio River, with peaks 

observed late April and early May, and again in September and early October (P. Morrison, pers. 

comm.). Migrants are also seen along the Potomac River; in spring migration, hundreds are 

reported (K. Leo, pers. comm.). In summer, non-breeding birds are also sometimes observed

along the Ohio river with juveniles, probably 1 year old birds (P. Morrison, pers. comm.).
FINAL DRAFT Part II. Population Data and Trends, Zone 5 180

Historical Information

Hall (1983) reported that the Double-crested Cormorant was a fairly regular migrant in 

small numbers at Cheat Lake, Monongalia County, until the late 1960s. Spring dates were from 

mid-April to mid-May; fall dates were from late October to early December. The species was 

much less common elsewhere in the state, but there are scattered records from other locales. Hall 

(1983) reported that since the mid-1960s very few reports of cormorants were made, which 

paralleled the general decline in the population concomitant with pesticides in the aquatic food 

chain. However, he also noted that declines in reports may be due to the fact that the portion of

the lake where most cormorant sightings were observed became inaccessible by shore. In 

Kanawha Valley, where sightings have increased, there were few records prior to 1990 (Smith 

1996).

Land Ownership

No information available.

Productivity

Not applicable.
FINAL DRAFT Part III. Diet 181

PART III: DIET

INTRODUCTION

In this section, we address several aspects of DCCO diet, and begin by defining relevant 

terms. Diet refers to the proportions of diverse prey consumed by cormorants but provides no 

measure of impact. Daily food intake (DFI) refers to the amounts of prey consumed over a 24-

hour period. Impact is the effect, if any, of consumption of fish by birds on the fishery in 

question. Because there are numerous compensatory effects of prey that may eliminate or reduce 

potential impacts, impact is the most difficult measurement to quantify.

Common names of fishes are not traditionally capitalized in publications (American 

Fisheries Society 1991), whereas standard format for bird common names requires use of capital 

letters (American Ornithologists’ Union 1998). Use of these two different formats respects 

nomenclature decisions followed by these two professional societies.

Diet Studies Reviewed

Trapp et al. (1999) reviewed 25 major studies on the diet of the DCCO conducted in 13 

states and provinces, 1923 – 1994. We identified an additional 16 major studies conducted in 16 

states and provinces; 12 of these were done between 1995 – 1999 (Neuman et al. 1997; Johnson 

et al. 1999; Belyea et al. 1999; Bur et al. 1999; Blackwell et al. 1995, 1997; Hill et al. 1997; 

Milton et al. 1988; Rail and Chapdelaine 1998; Glahn et al. 1995, 1998; Roby et al. 1998), while 

four were conducted prior to 1995 (Robertson 1974; Ainley et al. 1981; Kehoe 1987; and 

Bivings et al. 1989). We also summarized two reviews / compilations of diet studies conducted 

in coastal New England, the Maritime Provinces, and eastern Québec (Cairns 1998), and in 

Wisconsin on Lakes Superior and Michigan, 1983 – 1997 (Matteson et al. 1999). We organized 

these studies and reviews by prey species groups in four regions: the Atlantic Region (Maine, 

New Brunswick, Nova Scotia, Prince Edward Island, and Québec); the Great Lakes (Canadian 

and U.S.); the Southern U.S. (Texas, Mississippi, Alabama and Arkansas); and the Pacific Coast 

(British Columbia to Sonora, Mexico) (Table 7 – Table 25). Because earlier diet studies were 

summarized elsewhere (Hatch and Weseloh 1999; Trapp et al. 1999), and because we are 

interested in current dietary trends for the purpose of this status assessment, we utilized diet 

studies conducted primarily over the last decade. However, to represent each region as fully as 

possible, we also include some of the studies summarized elsewhere (Robertson 1974; Ainley et 

al. 1981; Craven and Lev 1987; Birt et al. 1987; Ludwig et al. 1989; Campo et al. 1993; Neuman 

et al. 1997; Blackwell et al. 1995, 1997).

Diet Assessment Methods: Advantages and Shortcomings

Most information on cormorant diet was obtained from pellets or regurgitations collected 

in nesting colonies (especially fish otoliths within colonies). Stomach contents are reported less 

frequently. Occasionally tags of stocked fish are recovered. All methods have advantages and 

disadvantages. During the 4th European Conference on Cormorants (Bolgna, Italy, 1995), a 

group of 28 cormorant researchers, mostly from Europe, discussed methods of assessing 

cormorant diet and food intake. They also evaluated associated sources of error, attempts to 

quantify them, areas of consensus, and gaps in current knowledge. Carss et al. (1997) 
FINAL DRAFT Part III. Diet 182

summarized the results of these deliberations. We relied heavily on this summary, along with 

remarks from North American investigators studying DCCO diet, to review the advantages and 

disadvantages associated with several dietary assessment methods. We provide this review to 

enhance understanding of conclusions reached in the various studies reviewed.

Pellets

Neuman et al. (1997) state that currently, pellets seem to provide the most cost effective, 

representative, and non-intrusive means of assessing cormorant diet composition. Additionally, 

in a comprehensive review of diet methods, Carss et al. (1997) note that pellets provide large 

samples that can be collected quickly with little or no disturbance to birds, and that analysis is 

fairly easy.

However, Carss et al. (1997) also point out that “pellet analysis is a useful method of 

obtaining a rough index of cormorant diet in qualitative terms but there is serious doubt as to 

whether it can be used to derive quantitative information on, for example, species composition or 

size-range of fish taken.” Carss and colleagues outline several problems and biases associated 

with pellet analysis. One of the most important is that the diagnostic hard parts recovered from 

pellets, namely the sagittal otoliths of teleost fish, are often eroded. Differential loss and 

digestion of otoliths makes identification of some fish species problematic or impossible. For 

example, Craven and Lev (1987) were unable to identify ninespine sticklebacks in the pellets of 

cormorants nesting at Eagle Island, Lake Superior, even though this species was abundant in the 

waters around this site, and most likely consumed by cormorants at the time of the study; this 

species was identified in regurgitations.

Studies on both captive and wild cormorants suggest that estimating consumed prey size 

from otoliths recovered from pellets is highly inaccurate, as otoliths of some fishes are “rapidly 

attacked” and eroded by bird stomach secretions (Duffy and Laurenson 1983; Jobling and Breiby 

1986; Johnstone et al. 1990; Harris and Wanless 1993). Most “lost” or eroded otoliths are from 

the smallest fishes so recovery is size related (Johnstone et al. 1990; Carss et al. 1997), and can 

result in underestimation of smaller fish consumed and impacts to fisheries. Additionally, age of 

prey fishes aged by eroded otoliths is likely underestimated (Harris and Wanless 1993). 

Therefore, attempts to develop an age distribution of fish consumed by cormorants through an 

analysis of otoliths recovered in pellets may underestimate both older and younger fish. To 

minimize this bias and determine the most reliable otolith predictors of fish length, some 

investigators have related otolith erosion levels and size, and developed an otolith classification 

scheme according to degree of erosion (Adams et al. 1999; Ross and Remaley, in review). 

However, some scientists (Johnstone et al. 1990; Harris and Wanless 1993) caution that attempts 

to sort out eroded from uneroded otoliths may be misleading, because erosion sometimes deeply 

etches otolith details making eroded otoliths appear fresh and indistinguishable from uneroded 

ones.

An additional problem associated with otoliths is that of secondary consumption (i.e. 

otoliths in sample represent diet of cormorant prey). Blackwell and Sinclair (1995) caution that a 

species well represented by otoliths can occur solely as a result of secondary consumption (in 

both regurgitations and pellets). Johnson et al. (1997) concluded that consumption of 

invertebrates by DCCOs might be overestimated in cases where primary and secondary 

consumption products cannot be separated. Pellets may also overestimate primary invertebrate 

consumption and underestimate soft-bodied fish (Brugger 1993; Neuman et al. 1997), as hard 
FINAL DRAFT Part III. Diet 183

parts of animals such as crustaceans remain undigested while soft-bodied fishes can be 

completely digested relatively quickly.

A common assumption about pellets is that they integrate meals over an entire day, and 

on average are produced once a day (e.g., Craven and Lev 1987; Orta 1992; Derby and Lovvorn 

1997a; Johnson et al. 1999). However, research indicates that pellets may integrate meals over a 

substantial time period, and a single pellet should not be assumed to represent one day’s food 

intake, as less than or greater than one pellet / day may be produced (Duffy and Laurenson 1983; 

Johnstone et al. 1990; Brugger 1993). More recently, observational and experimental studies 

with European Shags (P. aristotelis) and Great Cormorants in Europe indicate that the 

assumption of one pellet / day overestimates the rate of pellet production for these species. In 

south-east Scotland, Russell et al. (1995) found that pellets were produced by shags during the 

breeding season much less frequently than one / day; on average, a shag produced a pellet once 

every four days. In Germany, Huppop and Frundt (2000) found that Great Cormorants observed 

at winter roost sites regurgitated on average 0.74 pellets / night (range 0.33 – 1.0), about 25 % < 

generally assumed. These results have important consequences for the use of pellets to estimate 

energy requirements of phalacrocoracids, because such models have generally assumed that 

pellet contents give an estimate of daily food intake (e.g., Linn and Campbell 1992; Johnson et 

al. 1999). Less frequent pellet production suggests that models assuming a production rate of 1.0 

pellets / day may overestimate fish consumption and consequently impacts to fisheries.

To enhance the use of pellets as a dietary assessment method and to reduce the biases 

associated with pellet analysis, Carss et al. (1997) suggest that feeding trials be undertaken and 

attempts made to quantify size- (or species-) related differential recovery. Specifically, “such 

trials should involve feeding captive birds on fishes of known species, length, and mass and 

should take into account the possible influences of stress, deliberate disturbance (on pellet 

content), and activity.” Such trials may also produce necessary information on pellet formation 

and the relationship between degree of digestion and quality of fish.

Regurgitations

Regurgitations (boli), compared to pellets, are cheaper and faster to analyze, and the 

results are much less influenced by differential digestibility of prey. Regurgitations often provide 

fresh or relatively undigested specimens, that can be measured for more precise estimates of 

length and age class of fish consumed by cormorants, and for prey biomass or energy consumed. 

Prey in freshly regurgitated boli generally can be identified at a glance and easily numbered; it is 

fairly easy to obtain a good measure of the biomass or volume represented by each taxon (e.g., 

no large inherent bias). Additionally, very small and / or soft-bodied prey that are poorly 

represented or absent in pellets are generally found in regurgitations. (J.F. Rail, pers. comm.).

However, regurgitations contain the contents of recent meals only, and thus may not 

provide information on daily diversity within the diet. For example, Neuman et al. (1997) 

compared prey items found in boli and pellets that were collected on the same day at the same 

colony, and found large differences in the percent frequency of food items indicated by both 

methods (Table 11, Table 13). Additionally, regurgitations do not necessarily provide an 

accurate representation of adult diet, because generally the largest part of the sample is produced 

by young cormorants (J.F. Rail, pers. comm.), and adults may eat low quality food themselves 

and feed higher quality food to chicks (Harris and Wanless 1993). Furthermore, studies that 

utilize otoliths obtained from regurgitations may encounter biases similar to those encountered in 

pellet analysis. For example, because adult birds swallow fish head first, the fish heads will be 
FINAL DRAFT Part III. Diet 184

the most digested parts; some otoliths within them may be eroded or not passed in the 

regurgitation (Carss et al. 1997). Other disadvantages include a greater amount of time required 

to collect an equal sample size of regurgitations as pellets, and greater caution is required to 

minimize disturbance while gathering regurgitations (J.F. Rail, pers. comm.). In addition, 

obtaining complete regurgitations from tree nesting colonies is difficult because whole fish may 

fall from the nest or small parts may become tangled in tree branches and leaves (M. Bur, pers. 

comm.). Overall, however, this method may be very effective during the chick rearing period, 

providing high quality data with possibly fewer biases and problems than pellet / otolith analysis 

(J.F. Rail, pers. comm.). Although collecting pellets or regurgitations may not cause direct 

mortality, it is important to note that entering active colonies flushes adults and leaves eggs and 

chicks vulnerable to predation which may reduce colony-wide reproductive success.

Stomach Contents

Stomach contents (which may be collected from the esophagus, proventriculus and 

gizzard) are most commonly obtained from dead birds but are sometimes also acquired by 

flushing stomachs of live birds. Advantages of studying diet from stomach content analysis 

include: sample is often fresh; it is easy to distinguish recent prey from partially digested fishes 

and prey retained in bottom of stomach. Thus some of the biases associated with pellets can be 

avoided. Stomach contents can also be accompanied by site specific and demographic 

information (Carss et al. 1997).

However, there are several important disadvantages associated with this method. First, 

collecting DCCOs for stomach contents usually requires killing birds. While stomach contents 

can be obtained from live birds by flushing stomachs with stomach pumps and emetics, these 

procedures are stressful for birds (Duffy and Jackson 1986; Harris and Wanless 1993). 

Additionally, both methods are time consuming and / or costly (Hatch and Weseloh 1999). Due 

to these problems, sample sizes are usually small and further reduced if some stomachs are 

empty (Carss et al. 1997).

DFI cannot be determined from stomach content analyses, because it is not possible to 

determine if a bird has finished feeding for the day (Carss et al.1997). Additionally, this method 

is likely to under sample known spatial and temporal variation in prey captured each day (Hatch 

and Weseloh 1999). Foraging behavior of birds may also lead to biased assessments of diet. For 

example, patch foraging may create a non-random distribution of prey among stomachs; birds 

foraging individually (generally thought to take larger fish) are thought to be more vulnerable to 

shooting than birds foraging in groups (presumed to take smaller fish) (Carss et al. 1997). 

However, shooting incoming birds (returning from foraging trips) at random locations from a 

boat-blind 150 – 200 m distant from the island colony reduces variation and randomizes birds 

that may be foraging individually or in groups (M. Bur, pers. comm.).

While it may be possible to obtain more precise information about species and sizes of 

fish eaten with this method (Harris and Wanless 1993), the more digested the stomach contents 

are the greater the potential for bias, at least theoretically, as some items will be more resistant to 

digestion than others. Finally, studies estimating diet from intact items alone will underestimate 

the proportion of small fish species and overestimate the average size for some larger ones 

(Carss et al. 1997).

To enhance use of this method, Carss et al. (1997) suggest that a sample from one 

localized area is more informative than one from a whole “catchment”, and that small samples 

from a restricted time and place are more useful than if spread out over an entire year or 
FINAL DRAFT Part III. Diet 185

watershed. Carss et al. (1997) also propose a standardized technique for analyzing stomach 

contents, and include suggestions for potential key bones to be used in analysis of dried remains, 

which include atlas, thoracic, and caudal vertebra, pharyngeal teeth, pelvic girdles, opercular 

bones, cleithra, lower jaws, and cyclostome teeth. Key bones must be robust and resistant to 

digestion, relatively easy to identify, and diagnostic; the only published examples are salmonid 

atlas and eel thoracic vertebrae. Otoliths were rejected because their presence did not correspond 

well with the occurrence or size of other bones from the same species.

Recovered Tags From Stocked Fish.

At some colonies, tags recovered from stocked salmon smolts were used to determine 

predation. One drawback associated with this method is that converting tag recovery data into 

estimates of diet composition is difficult because tag recovery rates are hard to measure and 

capture of tagged fish may not be representative of untagged fish of the same species (Cairns 

1998). However, Ross and Johnson (1999a) found that previous analyses of lake trout stocking 

losses benefited from the return of coded wire tags, in addition to diagnostic hard parts, 

recovered in pellets.

Conclusions: Diet Assessment Methods

When attempting to determine diet composition, the method of diet assessment needs 

careful consideration, and biases associated with each method should be well understood and 

compensated for. Depending on the circumstances (e.g., colony size, accessibility, reaction of 

birds to intrusion, etc.), and goals of the study, one method may be much more appropriate than 

another, and limitations of the method should be considered in study design (J.F. Rail, pers. 

comm.) and interpretation of results. Many studies are designed to assess impacts to fisheries. To 

assess these impacts, solid information is needed not only on cormorant diet, but also on daily 

energy requirements. Because of associated biases, “good” estimates of daily food intake cannot 

be obtained through pellet analysis, regurgitations or stomach contents of shot birds. Future 

estimates of daily food intake should be based on considerations of the energy requirements of 

wild birds (Carss et al. 1997). Methods to measure these are described in PART VII. 

RESEARCH NEEDS.

Measurements Used to Report Diet

In addition to the method used to assess diet, an important factor in all studies is the 

measurement used to report diet. Measurements, along with assessment methods, vary, and this 

can affect results, conclusions, and comparability among studies. In the diet studies we reviewed, 

five measurements were reported: percent frequency, defined as the proportion of samples 

containing a prey taxon; percent number, defined as the number of specimens of a taxon as a 

percent of all specimens in a sample; percent biomass, defined as the biomass of a taxon as a 

percent of total sample biomass; percent volume, equivalent to percent biomass, except that 

quantities are measured volumetrically; and relative importance, defined as the sum of the 

percent of each taxon divided by the number of samples analyzed, was reported in one study. 

Each measurement reflects different but important aspects of prey utilization.

All of these measurements can be confounded by differential digestibility or variation in 

size, availability, or abundance of prey taxa (Blackwell et al. 1995). Robertson (1974) suggested 

that biomass is probably a more critical indicator of energy flow than number of individual food 

items taken, but this measurement, in isolation, provides no information on the frequency with 
FINAL DRAFT Part III. Diet 186

which the taxon is taken or what proportion of the diet consists of the taxon. Because 

compensatory mechanisms may operate more strongly on different age classes, and because 

number of fish taken does not indicate proportion of each age class available, simply reporting 

number of fish taken does not indicate impact to a fishery. Studies reporting numbers of fishes 

consumed need to include estimates of size of fishes taken, because the removal of even large 

numbers of very small fishes may not significantly affect a fishery. For example, the loss of 

salmon fry or small parr could result in enhanced survival or growth of remaining fish, while 

removal of large parr or smolts could result in a reduction in the numbers of juvenile fish 

reaching the sea and perhaps in the numbers returning as adults (Carss and Marquiss 1997). 

Similarly, in Oneida Lake, consumption of age 0 yellow perch and walleye is buffered by 

compensatory effects, and essentially has no effect on the resulting number of fish reaching the 

fishery, while consumption of age 1 and older fishes of these species is believed to represent 

additive mortality, making fewer fish available to the fishery (C. Adams and L. Rudstam, pers. 

comm.). Therefore a combination of measurements is necessary to accurately reflect the 

importance of and impacts associated with a particular prey species within the diet. Reintjes and 

King (1953) suggest that a prey taxon ranking high in frequency of occurrence, volume, and 

number is important at the time and place sampled.

RESULTS OF DIET REVIEW

Regional Diet Descriptions

An important factor to consider in the following descriptions of cormorant diet in each 

region is that most of the diet studies we reviewed were designed or undertaken to assess the 

impact of cormorant predation on sport or commercial fish species, or to determine the 

importance of sport or commercial fish species within the diet (Belyea et al. 1999; Ross and 

Johnson 1999a,b; Johnson et al. 1999; Blackwell et al. 1995, 1997; Milton et al. 1995; Hill et al. 

1997; Bivings et al. 1989; Campo et al. 1993; Glahn et al. 1995, 1998; Roby et al. 1998). Such 

studies may not generate data representative of DCCO diet in general. For example, studies that 

utilize data from cormorant stomachs that were collected at smolt stocking sites or catfish farms 

may find predictably high frequencies of these species in the diet. Furthermore, these data often 

involve only a few hundred DCCOs while the total population in the particular state or province 

where the samples are collected may consist of many thousands of birds [many of which may not 

participate in movements to these feeding grounds (Cairns 1998)]. Therefore, to avoid overrepresenting the importance of results from preferential sampling, we caution that the results we 

present to describe the diet of DCCOs in each region should be considered in the context of the 

conditions under which the study was conducted, and the size and status (breeding, wintering, 

migrant) of the cormorant population described (J.F. Rail, pers. comm.).

Diet on the Pacific Coast

We incorporated data from three studies that examined diet of DCCOs on the Pacific 

Coast; only one was conducted in the 1990s. Diet in this region consisted almost exclusively of 

fish, though small amounts of shrimp were also consumed. To categorize diet, we developed four 

categories of important prey fish in this region: salmonids, estuarine and or littoral species, 

littoral-benthic species, and pelagic species (Table 7 – Table 10). Though information on diet in 

several areas was included, much of the data we obtained comes from two main locations, the 

Columbia River estuary and Mandarte Island, British Columbia. As on the Atlantic Coast, diet 
FINAL DRAFT Part III. Diet 187

varied regionally to a great extent, and no one species emerged as the most important. However, 

some fish species were very important in certain regions and across the region generally. For 

birds breeding in the Columbia River estuary, salmonids were the most important prey item by 

percent biomass, particularly during the month of May, and were more important overall in the 

diet of birds nesting further upriver than birds nesting closer to the river mouth (Table 7). Littoral 

or estuarine species, such as stickleback and peamouth, were the second and third most important 

species, respectively, by percent biomass. Littoral-benthic species, such as sculpin, were also 

important prey for the estuary birds (Table 8, Table 9). Along the coast and in the Gulf of 

California, Ainley et al. (1981) examined DCCO diet at seven sites, and reported that cormorants 

ate mostly schooling prey that occurred from the surface to near, but not on, bottoms having no 

relief. In general, littoral or estuarine fish, such as shiner perch and other embiotocids, were 

important in the diet of cormorants in many areas of the Pacific Coast (Table 8). Littoral-benthic 

fish, such as sculpin, gunnel, snake prickleback and sucker, also made up a significant portion of 

the diet (Table 9). Locally important species included Pacific sand lance for birds nesting in 

British Columbia (Table 8), and clupeids of the genus Opisthonema (Table 10) for birds breeding 

in Sonora, Mexico.

Diet in the Great Lakes

In the Great Lakes region, the studies we reviewed indicate that diet in this region 

consists almost exclusively of fish, though crustaceans were also consumed in small proportions. 

To describe the principal fish species consumed, we developed five freshwater fish categories 

(B. Vondracek, pers. comm.): open-fresh water fishes, littoral-fresh water fishes, bottom 

dwelling-fresh water fishes, salmonids, and locally important fishes (Table 11 – Table 15). Fish 

species from the open-fresh water category, specifically alewife and gizzard shad, appeared to be 

most important (importance here is defined in terms of region-wide consumption and high 

percentages of frequency occurrence, biomass, and or number consumed), with the exception of 

Lake Superior (Table 11). In Lake Superior fishes from the littoral and bottom-dwelling groups, 

stickleback and sculpin sp., respectively, appeared to be most important (Table 12, Table 13). 

These species were also the second most important species consumed region-wide. Cyprinids, 

especially Notropis sp., appeared to be an important dietary component, in terms of % frequency 

and number, in several locales during specific time periods. Yellow perch had a fairly high 

frequency in the diet in several locales during specific time periods (Table 12); however, none of 

the studies reported significant impacts to the yellow perch fishery. In most locales, salmonids 

did not comprise an important portion of the diet (Table 14). Locally important species include 

burbot, freshwater drum and lake / northern chub (Table 15).

Diet in the Southeastern U.S.

We found four diet studies that were conducted in the 1990s that examined the diet of 

cormorants wintering in southeastern U.S. Most of the diet in this region consists of shad, 

catfish, and sunfish species (Table 16 – Table 19). At inland reservoirs in Texas, lakes in 

Mississippi and Alabama, and at Arkansas aquaculture facilities, shad species, namely gizzard 

and threadfin, appear to be the most important species in the diet, in terms of percent frequency 

and or percent number (Table 16). For birds around catfish farms in the Mississippi Delta, catfish 

were of major importance in the diet during winter months (Table 17), though shad species were 

of great importance in the diet of these birds in the fall (Table 16). At aquaculture facilities in 

central and s.e. Arkansas, catfish occurred in the diet with a surprisingly low % frequency (Table 
FINAL DRAFT Part III. Diet 188

17). However, Bivings et al. (1989) reported an increase in catfish consumption in December, 

and suggested that cormorant diet undergoes a seasonal shift to catfish from fall to winter, which 

is supported by the pattern of catfish consumption reported in the Mississippi delta (Table 17). 

At certain times and in certain locations, centrarchids, mainly sunfish and crappies, appear to be 

moderately important in the diet (Table 18). blue tilapia was locally important in Texas in winter 

months (Table 19).

Diet in the Atlantic Region

In the Atlantic region, the studies we reviewed indicated that diet consisted of fish and 

crustaceans. Six categories were developed (Cairns 1998) for prey species consumed: marine 

bottom fishes, estuarine / diadromous fishes, freshwater fishes, salmonids, pelagic fishes, and 

crustaceans (Table 20 – Table 25). Diet varied regionally to a great extent, and no one species or 

category emerged as the most important on a region-wide basis. However, most colonies are 

located on coastal islands in marine habitats, and several of the marine bottom fish species, e.g., 

cod, sculpin, cunner and gunnel (Table 20), were important in many areas, as were pelagic fishes 

such as sand lance and capelin (Table 24) (J.F. Rail, pers. comm.). In the spring, cormorants are 

also found on lakes, rivers and estuaries, and at this time they feed mainly on estuarine / 

diadromous fishes, such as alewife, rainbow smelt, and stickleback (Table 21); freshwater fishes, 

such as smallmouth bass, yellow perch, pumpkinseed and cyprinids (Table 22); and salmonids, 

mainly Atlantic salmon, especially in May (Table 23). However, this diet may not be 

representative of the diet of migrating DCCOs in general; use of this type of habitat may involve 

only relatively small numbers of individuals compared to large populations migrating / breeding 

in the Atlantic region (J.F. Rail, pers. comm.). Crustaceans, mostly shrimp species (Table 25), 

were important locally; importance of shrimp in the diet was highly variable with month and 

year.

Conclusions: Variation in the Diet

One of the most important features of cormorant diet that emerged from our review of 

diet studies was the great degree of variation that exists in utilization of prey. Table 7 – Table 25

highlight some of the variation that occurs locally, regionally, temporally, seasonally, and interannually. Because of this variation, measurements of diet composition must be interpreted 

cautiously. Neuman et al. (1997) conducted the first extensive study on the diet of DCCOs on the 

lower Great Lakes that explored temporal and spatial variation in detail. They suggested that, as 

a result of the importance of temporal and spatial variation within cormorant diet, the temporal 

and spatial scales at which biologists and fishery managers have previously investigated the 

effect of DCCO predation on fish communities may introduce considerable bias and error. In the 

Atlantic region, Blackwell et al. (1997) did a similarly extensive study exploring temporal and 

spatial variation in cormorant diet. They found that spatial and seasonal variation of prey 

populations had a major influence on cormorant habitat use and diet composition. On the Pacific 

Coast, Roby et al. (1998) studied DCCO diet in two Columbia River estuary colonies about 15 

miles apart, and found that proportions of salmonids and other fish species varied significantly 

between locations. In general, cormorant diets show a high degree of spatial variability and tend 

to reflect the fish species composition for each site, making it necessary to examine cormorant 

diets on a site-by-site basis (Belyea et al. 1999). Additionally, cormorant diet also tends to reflect 

both short term and long term temporal shifts in the abundance of prey species (Blackwell et al. 

1995; Neuman et al. 1997). Therefore, Neuman et al. (1997) recommend that sampling be 
FINAL DRAFT Part III. Diet 189

conducted at a scale appropriate to the question being posed, and noted that extrapolations to 

whole-lake context can only be made meaningful once spatial and temporal variability has been 

quantified.

Fish species declines associated with DCCO predation (perceived or real)

Zone 1, Alaska, is the only portion of the DCCO’s breeding range where no serious 

conflicts between cormorants and fisheries are currently known to exist (V. Byrd, S. Stephensen 

and K. Kuletz, pers. comm.). In Zones 2 – 5, cormorants are associated with negative impacts to 

sport and commercial fish species. In the Great Lakes, fisheries managers and sport and 

commercial fishermen have been concerned that predation pressure from growing populations of 

DCCOs has or will contribute to declines in yellow perch, white perch, smallmouth bass and 

walleye (Craven and Lev 1987; Belyea et al. 1999; Bur et al. 1999; Matteson et al. 1999; Lantry 

et al. 1999; Schneider and Adams 1999). In the Atlantic region, concerns have revolved around 

impacts to Atlantic salmon, brook trout and rainbow smelt (Milton et al. 1995; Blackwell et al. 

1997; Hill et al. 1997). On the Pacific Coast, concerns center on impacts to salmonids, 

particularly in the Columbia River estuary (Roby et al. 1998). In the southeastern U.S. cormorant 

foraging at catfish farms is reported to result in serious economic losses, and impacts to sport 

fishes are also of concern (Glahn et al. 1995; Campo et al. 1993; Simmonds et al. in press). At 

Lake Winnipegosis, Manitoba, commercial fishermen have been concerned that cormorants were 

negatively impacting walleye and sauger (Hobson et al. 1989).

A review of most diet literature indicates that sport and commercial fish species, on the 

whole, do not contribute substantially to cormorant diet and / or are not negatively impacted by 

cormorants (Taverner 1915; Lewis 1929, 1956, 1957; Mendall 1936; Scattergood 1950; Ross 

1973; Robertson 1974; Karwowski et al. 1994; Campo et al. 1993; Hobson et al. 1989; Glahn et 

al. 1998; Bivings et al. 1989; Roney 1979; Craven and Lev 1987; Milton et al. 1995; Kehoe 

1987; Ludwig et al. 1989, Matteson et al. 1999; Madenjian and Gabrey 1994, 1995; Diana et al. 

1997 or Belyea et al. 1999; Kirsch 1995; Bur et al. 1999; Blackwell et al. 1995; Ross and 

Johnson 1995; Johnson and Ross 1996; Johnson et al. 1999; Ross and Johnson 1999b; Suter 

1995; Weseloh and Casselman 1992; Rail and Chapdelaine 1998). There are, of course, 

exceptions to this finding. Birt et al. (1987) reported that fish densities were significantly lower 

in Prince Edward Island bays used by cormorants for feeding than in those outside their foraging 

range, and suggested that cormorants depleted prey in these bays; however, there is no 

commercial fin fishery in these bays. Cairns (1998), in his review of diet studies spanning 1915 –

1997 in the Atlantic region, stated that commercial and recreational species averaged 73 % of 

reported prey composition during the smolt run, and 44 % outside the smolt run. Blackwell et al. 

(1997) and Milton et al. (1995) found that salmon smolts figure prominently in the diet of 

cormorants in this region in May. However, cormorant predation on smolts in Maine is not 

thought to seriously impact the salmon fishery, because cormorants are only one of a myriad of 

mortality factors for hatchery-raised smolts (R.B. Allen, pers. comm.). In the southern U.S. 

catfish raised in aquaculture ponds contribute significantly to the diet of wintering cormorants 

(Glahn et al. 1995). Additionally, in the Great Lakes yellow perch are sometimes abundantly 

represented in the diet of cormorants, though significant impacts have not been documented 

(Matteson et al. 1999).
FINAL DRAFT Part III. Diet 190

Studies assessing impacts to sport and / or commercial fish populations

As noted above, many studies have been conducted to examine the diet of the DCCO and 

to document effects of predation. Despite these efforts, there are surprisingly few studies 

available worldwide that include enough data for assessing the impact of bird predation, and 

modeling attempts are almost nonexistent (Suter 2000). However, the perception that cormorant 

predation is destroying fisheries is strong, particularly among anglers. To properly manage both 

bird and fish populations, scientifically rigorous studies that quantify the impacts of birds on fish 

population dynamics as well as on fisheries are needed (Suter 2000).

Such studies are difficult to undertake for several reasons. Fisheries biologists caution 

that documenting amounts of fish species X consumed by DCCOs does not readily translate into 

determining the impact on species X’s population. An increase in a single predator species like 

the DCCO can lead to complex interactions among other predators, interspecific competitors, 

and prey populations that can confound the ability to draw simple conclusions about the effects 

of consumption rates on commercial fish population dynamics (S. Correia, pers. comm.). Trapp 

et al. (1999) note that to assess the impacts of DCCOs on fish populations, quantitative 

information about numerical abundance of prey species and detailed knowledge of effects of all 

other factors (both biotic and abiotic) that can affect fish populations are required. To date, no 

studies have been able to thoroughly address all of these criteria. However, some investigators 

have obtained current estimates of prey fish population size and mortality, DCCO numbers, fish 

consumption rates, and sport fish catch to assess potential impacts of DCCO predation. These 

studies are summarized below. Particular attention is paid to methodologies used to assess 

cormorant diet, and the scientific rigor with which estimates of fish population size and other 

parameters were obtained.

Great Lakes

In the Great Lakes, three studies incorporate current estimates of abundance of specific 

sport / commercial prey fish species to determine the impact of DCCO predation. These are 

reviewed below. A fourth study (Schneider et al. 1996) examined the impact of DCCO predation 

on smallmouth bass stocks in the eastern basin of Lake Ontario, but used historical data sets to 

estimate smallmouth bass abundance. Because this study utilized data sets that did not provide a 

measure of current conditions and made a number of assumptions from limited data, we did not 

review this study, but citation information is provided in the Literature Cited section. A fifth 

study (Hoyle et al. 1999) examined population trends and declines in smallmouth bass in eastern 

Lake Ontario in relation to water temperature. This study is briefly reviewed to put cormorant 

predation in the context of other environmental factors and ecosystem processes.

The Les Cheneaux Islands, Michigan (Belyea et al. 1999)

To determine impact of cormorants on the yellow perch population in the Les Cheneaux 

Islands area, MI, Belyea et al. (1999) estimated the amount of various fish taxa consumed by 

cormorants in 1995 through an analysis of stomach contents. Diet composition was determined 

by calculating the proportion by weight of each fish taxon in the diet. Weights of fish consumed 

were estimated by using length – weight regressions; the majority (70 %) of yellow perch found 

in stomachs that were measured for length were measured directly, while the remaining 30 % 

were estimated from cleithrum or preopercle lengths. Estimates of daily food consumption of 

cormorants were also calculated, based on published daily calorie intake information, and 

dividing it by the caloric density of fish.
FINAL DRAFT Part III. Diet 191

Estimates of yellow perch population size and mortality rates calculated for 1995 were 

incorporated from Schneeberger and Scott (1997). Schneeberger and Scott (1997) estimated 

yellow perch population size through a mark-recapture effort conducted in 1995. Mortality was 

calculated for yellow perch caught in gill nets during fall 1995 and aged from scale samples. 

Additionally a mortality rate was calculated from a pooled data set (1993 to 1995), and a total 

annual mortality rate of 45 % was estimated for the yellow perch population. Survival was 

calculated from tag-return data. Belyea et al. (1999) then calculated yellow perch mortality from 

cormorant predation by using the daily caloric data together with cormorant population data and 

the proportion by mass of perch in the diet. They estimated that cormorants removed 1 % of the 

yellow perch population that was of legal size in 1995, while angler exploitation removed 2.4 %. 

Belyea et al. (1999) compared their cormorant predation and angler exploitation rates with 

Schneeberger and Scott’s (1997) mortality rates, and concluded that other sources of mortality 

must therefore remove roughly 40 % of legal size perch.

Although cormorant diet contained approximately 48 % yellow perch by weight during 

the spawning season, Belyea et al. (1995) suggested that cormorant predation of perch in 1995 

was, on the whole, not substantial. Additionally, Belyea et al. (1999) reported that cormorant 

predation on yellow perch in the Les Cheneaux Islands area accounts for a greater proportion of 

mortality at younger ages (fish aged 1 – 3 years), while anglers impact essentially age 3 and 

older perch.

Comments on Study. The estimate for yellow perch population size had a 95 % 

confidence level of + 747,597 (27 % of the estimated population). In calculating this estimate, 

there were six assumptions that were implicit to justify the formula used to estimate population 

size. All of these assumptions were met. Assessment of diet composition utilized estimates of 

fish lengths obtained from measurements of mainly intact fish, but also from potentially robust 

key bones (otoliths were avoided, G. Belyea, pers. comm.). While estimating diet from intact 

items alone has been shown to underestimate the proportion of small fish species and 

overestimate the average size for some larger ones (see above, Diet Assessment Methods: 

Advantages and Shortcomings), care was taken to collect birds shortly after they had been 

observed feeding. This was done to minimize the number of birds shot with empty stomachs; it 

may also have increased chances that most fish retrieved from stomachs, even smaller ones, 

would be relatively undigested. Estimates of daily food consumption incorporated published 

daily calorie intake information determined both observationally and experimentally. While the 

estimate for population size had fairly wide confidence intervals, given the level of yellow perch 

mortality due to cormorant predation, it was precise enough to conclude with some certainty that 

DCCOs were not substantially impacting the yellow perch fishery.

Eastern Basin of Lake Ontario (NYSDEC 1999)

The New York Department of Environmental Conservation (NYSDEC) and United 

States Geological Survey (USGS) undertook an intense field effort in 1998 to evaluate the 

impact of DCCO predation on smallmouth bass and other fishes of the eastern basin of Lake 

Ontario (NYSDEC 1999). The area specifically studied included New York waters of Lake 

Ontario from 7 km south of Little Galloo Island to Cape Vincent, and throughout this review will 

be referred to as the eastern basin. The document produced from this work contains 12 papers 

addressing various aspects of the warm water fish community and DCCO diet in this area. Some 

of these papers are summarized here to present the major results of this effort, and impacts of 

DCCO predation in this area.
FINAL DRAFT Part III. Diet 192

In the eastern basin, annual assessments of the warm water fish community, based on gill 

net sampling conducted by the NYSDEC, have reported dramatic declines in smallmouth bass 

abundance since the early 1990s. The mean catch per net gang (or catch per unit effort, CPUE) 

from 1995 – 1997 was more than 50 % lower than the mean CPUE from 1984 – 1986 (Chrisman 

and Eckert 1999). Additionally, Eckert (1999a) reported a decrease in annual survival rate 

among bass ages 6 – 12 since 1991, and a progressive shift in the modal age group of bass 

captured towards younger age 3 and age 4 fish, suggesting decreased survival among younger 

age classes as well. Because increases in DCCOs have corresponded with these declines, efforts 

have been made to determine cormorant predation impacts on smallmouth bass abundance.

Johnson et al. (1999) assessed the diet of cormorants in eastern Lake Ontario by 

collecting pellets from Little Galloo Island and identifying diagnostic structures, primarily fish 

otoliths. They then developed a model to estimate numbers of fish (for each prey species) 

consumed by cormorants in eastern Lake Ontario. One important assumption of the model was 

that cormorant’s produced 1.0 pellets per day. Model results estimated that 1.3 million 

smallmouth bass (the species of greatest concern) were consumed in 1998.

To evaluate the size and age structure of smallmouth bass being consumed by 

cormorants, two studies were carried out: Adams et al. (1999) utilized otoliths from cormorant 

pellets collected in 1993 – 1994, and Schneider and Adams (1999) utilized otoliths and other fish 

parts from cormorant pellets, chick regurgitations and stomach contents collected in 1998. To 

minimize potential bias associated with otoliths of consumed fish, an erosion index scale was 

developed in which otoliths were assigned to various categories by degree of wear; only the least 

eroded and most reliable predictors of fish lengths were used to estimate total length of fish 

consumed (Adams et al. 1999; Schneider and Adams 1999). These studies concluded that the 

smallmouth bass consumed by cormorants were mostly juveniles, between the ages of 3 – 5. 

Adams et al. (1999) reported that the modal age group of bass consumed in 1993 – 1994 was 5 

years; the mean age was 4.4 years. Schneider and Adams (1999) reported that the modal age of 

bass consumed in 1998 was 3 years; the mean age ranged from 2.7 – 3.3 years for the three 

sample types.

The objectives of the final paper in the NYSDEC’s Special Report (Lantry et al. 1999) 

were to determine if cormorant predation significantly increased mortality of smallmouth bass in 

the eastern basin, and if this mortality could cause the declines recorded in the smallmouth bass 

population and fishery. Lantry et al. (1999) synthesized research from several sources. The 

percent of smallmouth bass in the diet was based on the estimate by Johnson et al. (1999). The 

estimate of smallmouth bass abundance and density was based on a 1986 mark-recapture 

population estimate for adult smallmouth bass in the Hardscrabble area (McCullough 1988) and 

expanded to yield a density estimate for the area in the eastern basin between the 0 to 20 m depth 

contours. The population estimate was back-projected for ages 1 to 5 using survivorship values 

for young smallmouth bass obtained at Oneida Lake in the 1970s (Carlander 1977). Age 

distribution of smallmouth bass preyed upon by cormorants was obtained from Adams et al. 

(1999) and Schneider and Adams (1999). Lantry et al. (1999) then calculated the percent of 3 – 5 

year old bass preyed upon by cormorants in the eastern basin, and estimated that cormorant 

predation reduced the 3 – 5 year old age group by 23 % in 1993 – 94, and by 36 % in 1998. The 

main results reported by Lantry et al. (1999) were: 1) the mortality of age 3 to 5 smallmouth bass 

increased substantially after 1988; and 2) loss rates of the magnitude reported above could 

severely limit numbers of adult stock recruited to the fishery; and 3) cormorant predation on 3 –
FINAL DRAFT Part III. Diet 193

5 year old age classes was substantial enough to cause the observed declines in the smallmouth 

bass population.

Eckert (1999b) also identified cormorant predation as one of the factors that may have 

decreased yellow perch survival between the 1970s – 1990s in the eastern basin of Lake Ontario. 

However, more quantitative information is necessary to assess impacts of cormorant predation 

on yellow perch in this area.

Comments on Study. This combined study synthesized a wide range of both cormorant 

and fisheries data. The DCCO diet sampling was especially thorough, covering a broad temporal 

range, utilizing a very large sample size, and combining various assessment methods to 

determine diet composition. Age estimates for consumed fish were included, and efforts were 

made to assess large scale and long-term trends in fish populations. Utilization of such a database 

and the broad perspective brought to the cormorant-fishery interaction in the eastern basin sets an 

important direction for future research. However, a few of the methodologies, assumptions and 

estimates incorporated in the individual papers have associated potential biases that complicate 

interpretation of results. These are noted below.

The conclusion that cormorant predation on 3 – 5 year old smallmouth bass age classes 

was substantial enough to cause the observed declines in the smallmouth bass population rested 

largely on the age distribution of smallmouth bass consumed by cormorants, which was obtained 

through analysis of otoliths retrieved from pellets. Several researchers have questioned this 

method of aging consumed fish (see above, Diet Assessment Methods: Advantages and 

Shortcomings). However, attempts to minimize such biases were made by incorporating an 

erosion index scale for the otoliths used.

The assumption that cormorants produce 1.0 pellets per day may be an inaccurate rate of 

pellet production, and as such may have led to an over- or underestimate of the number of fish 

consumed. Recent field research on the rate of pellet formation for other Phalacrocoracidae 

species suggests that this is too high a rate of pellet production (see above, Diet Assessment 

Methods: Advantages and Shortcomings).

The estimate for smallmouth bass population size and density was calculated in 1986 and 

expanded to the area in the eastern basin between the 0 – 20 m depth contours to produce a 

current smallmouth bass density estimate. However, in 1986, numbers of smallmouth bass 

appeared to be much higher than those in 1998. Given the uncertainty of this model input, it is 

not clear how much confidence can be ascribed to this density estimate.

Survival rates for young smallmouth bass were based on rates from Oneida Lake. This is 

a much smaller inland lake, subject to a different set of ecosystem dynamics. It is not clear how 

applicable these survival rates are to smallmouth bass in the eastern basin.

An important question not addressed in this report is that there were periods when 

smallmouth bass declines corresponded with low numbers of cormorants, and periods when both 

species increased simultaneously. For example, between 1980 – 1985 smallmouth bass 

abundance (measured by CPUE) declined by about 75 % (Chrisman and Eckert 1999); at this 

time cormorant numbers on Little Galloo were relatively small (< 1000 pairs up until 1985, when 

about 1400 pairs were estimated). Then during 1985 – 1990, smallmouth bass abundance 

dramatically increased (Chrisman and Eckert 1999), and cormorant numbers nearly tripled. This 

suggests that other factors might be responsible for smallmouth bass declines. Potential factors 

such as disease, decreased food supply, changing water quality conditions in the 1990s, zebra 

mussel alterations on habitat, fish predators and other bird predators have not been thoroughly 

explored or quantified.
FINAL DRAFT Part III. Diet 194

Diet Studies at Oneida Lake, New York

(Forney 1993; VanDeValk et al. 1998, 1999; C. Adams, pers comm.)

At Oneida Lake, New York, various aspects of DCCO diet and interactions between 

cormorants, anglers, walleye and yellow perch have been studied since 1988 (Forney 1993), but 

no peer-reviewed papers describing the study have been published. Therefore we have 

summarized work conducted there through communications with the researchers involved in the 

work (Connie Adams and Lars Rudstam), descriptions in annual reports prepared by VanDeValk 

et al. (1998, 1999), and through observations on cormorant – fish interactions published in the 

American Fisheries Society Newsletter (Forney 1993). On Oneida Lake, estimates of abundance, 

age composition and survival for yellow perch and walleye were obtained through Miller 

sampler surveys, gillnet catches, catch in trawls, and mark-recapture studies (VanDeValk et al. 

1998, 1999). Cormorant diet was examined 1988 – 1991 through analysis of regurgitations and 

stomach contents (Forney 1993); 1994 – 1997 through analysis of pellets, regurgitations and 

stomach contents (VanDeValk et al. 1999; C. Adams, pers. comm.); in 1998 and 1999, diet was 

examined through regurgitants (C. Adams, pers. comm.). Cormorant diet composition was 

described by % number and % biomass. Numbers of walleye and perch consumed by cormorants 

incorporated daily consumption rates of adult cormorants and chicks, proportion by weight of 

different prey items in the diet, and the number of adults and chicks observed on the lake 

(VanDeValk et al. 1998). Daily consumption was estimated from two methods. In the first, it 

was calculated as the average number of fish found per pellet, and assumed that cormorants 

produced 1.0 pellets / day. In the second method, it was assumed that adult cormorants consume 

20 % of their body weight per day and that chicks consume 0.327 g / day / individual. [Chick 

consumption rates were based on bioenergetics calculations from Fowle et al. 1997 (C. Adams, 

pers. comm.).] Proportion of different age classes consumed by cormorants was estimated 

through analysis of fish remains obtained in pellets, regurgitants and stomach contents. As in the 

1999 NYSDEC study, efforts were made to minimize potential bias associated with aging 

consumed fish through otolith measurements by incorporating an erosion index scale (C. Adams, 

pers. comm.). Exploitation rates of yellow perch and walleye were compared between 

cormorants and anglers. Angler take was estimated through creel surveys (C. Adams, pers. 

comm.).

For both walleye and perch, fishery biologists at Cornell Field Station use the number of 

age 1 fish to predict the number that will be recruited to the adult population. Predation at this 

level and on the pre-recruit populations (age 2 – 3) will have the greatest effect on numbers 

recruited to the fishery, and is believed to represent additive mortality. Essentially, predation on 

age 0 walleye and yellow perch has no impact on the resulting number of adult fish (age at 

recruitment: age 4 for walleye, age 3 for yellow perch). Because it is only from age 1 that the 

number of adults from a year class can be predicted with some confidence, compensatory effects 

are believed to be less pronounced after age 1. Therefore DCCO predation on these larger fish 

will directly decrease the number of adult fish recruiting from that year class (C. Adams and L. 

Rudstam, pers. comm.). In the spring of 1996, a total of 8,821,000 yellow perch aged 1 and older 

was estimated. The amount consumed by cormorants was estimated at 1,123,800 (12.7 %). A 

total of 697,935 walleye aged 1 and older was estimated. The amount consumed by cormorants 

was estimated at 100,600 (14.4 %) (VanDeValk et al. 1999). Creel surveys and pellet analyses 

conducted in 1997 – 1998 indicated that exploitation of adult walleye was solely attributed to 

anglers while adult yellow perch exploitation was divided about evenly between anglers and 
FINAL DRAFT Part III. Diet 195

cormorants. Exploitation of subadult walleye and yellow perch was almost entirely attributed to 

cormorant predation (C. Adams, pers. comm.).

Before cormorants colonized Lake Oneida, the relationship between number of age 1 fish 

and resulting number of adults was tightly correlated. Since the growth of the cormorant 

population, this equation consistently predicts more adults than are estimated with markrecapture counts, suggesting an increase in mortality between age 1 and age at recruitment.

When the number of fish eaten by cormorants is calculated, it is a fair percent of the 

number of fish that are “missing.” Therefore, depredation by cormorants is believed to have an 

impact, as virtually the only other cause of mortality for adult walleye and yellow perch is due to 

angling (C. Adams, pers. comm.).

After reviewing cormorant - fish interactions between 1988 – 1991, Forney (1993) 

reported that cormorant predation does not seriously threaten the Oneida Lake fishery, but noted 

that a continued increase in cormorant abundance may intensify demands for cormorant control. 

The last year of data considered by Forney (1993) was 1991, when 60 DCCO pairs were 

estimated to nest on the lake, and flocks of 300 – 400 fall migrants were common; however, for 

some years, 1,000 – 2,000 cormorants were reported. Since 1991, nesting numbers have 

increased, with 332 nests counted in May of 1998, though through control efforts only 100 were 

allowed to persist. In the same year, about 1,500 birds utilized the lake in August, but quickly 

declined by the end of September to 20 birds following the onset of harassment (VanDeValk et 

al. 1999). The more recent work conducted on Oneida Lake bases a conclusion of effect on perch 

and walleye on three lines of evidence: 1) the timing of the disappearing adults (fewer adults 

than expected from age 1 estimates) coincides with the increase in cormorants; 2) the size of the 

fish eaten by cormorants coincides with the size of the fish that have increased mortality; and 3) 

for walleye at least, the number of fish “missing” is comparable to the number estimated 

consumed by cormorants (C. Adams and L. Rudstam, pers. comm.).

Comments on Study. The work done on Oneida Lake stands out overall in its examination 

and discussion of compensatory and additive processes within fish populations and provides 

some of the best evidence for DCCO impacts on fish populations. A variety of estimates for 

numbers of walleye and yellow perch have been incorporated in an effort to provide as complete 

a picture of fish populations as possible. Assessment of cormorant diet was thorough and utilized 

a combination of methods. Many of the fish consumed by cormorants were sized and aged by 

samples obtained through regurgitants and stomach contents. A few potential limitations of the 

data are noted below.

The population estimates for walleye and perch were current and carefully obtained 

through mark-recapture and other frequently utilized methods. The estimate for walleye had 

narrower confidence intervals than the estimate for yellow perch; however, one of the two 

assumptions discussed for the model used for the walleye estimate was not met. The population 

estimate for yellow perch had sizable confidence intervals because only a small number of 

individuals was recaptured; assumptions for this model were not discussed. Additionally, the 

numbers of walleye and perch taken by cormorants fell largely or completely within these 

intervals (VanDeValk et al. 1998, 1999).

Otoliths retrieved from pellets were used to age some of the consumed fish. For perch the 

researchers believed that the age classes of consumed fish were determined fairly accurately, as 

the otolith size distribution exhibited distinct peaks that could be identified as age 0, 1, 2 and 3 + 

fish. For walleye too few otoliths were measurable, and thus the size of walleye was calculated 

from the size of the otolith using species-specific regressions, which resulted in an underestimate 
FINAL DRAFT Part III. Diet 196

of walleye consumed due to erosion of the otoliths (C. Adams, pers. comm.). It is likely that 

small otoliths were lost, and thus consumption of younger walleye was probably also 

underestimated. Additionally, the authors assumed that 1 pellet per day accurately reflects 

DCCO daily food intake, which may lead to under- or over-estimation of consumption (see Diet 

Assessment Methods: Advantages and Shortcomings, and review of Eastern Basin of Lake 

Ontario (NYSDEC special report 1999).

Modeling predicts that even elimination of the entire cormorant population on Oneida 

Lake would not lead to recovery of the walleye population to its formerly large size. The 

population increases, but then levels off at a fairly low number. Because it does not continue to 

increase, there must be other factors besides cormorant predation holding it at lower levels. One 

element in the model that causes a rapid and steady increase in the walleye population is closing 

the fishery to angler harvest. It appears that if this were done for several years, the walleye 

population would eventually reach the high population of the 1950s. Anglers remain the most 

important predators of walleye on Oneida Lake (C. Adams, pers. comm.). Additionally, other 

recent changes observed in the Oneida Lake ecosystem may also affect recovery of the walleye 

population, including declining phosphorous and other nutrient levels, and invasion of zebra 

mussels with the associated change in water clarity and decline in algal concentrations 

(VanDeValk et al. 1999). In terms of the latter effect, the number of young of year walleye 

caught in the index trawl has been consistently low since 1992, suggesting higher early mortality, 

which could be increasing due to observed higher water clarity. In turn, this would increase 

susceptibility of this age class to predation / cannibalism. If this trend could be reversed, higher 

numbers of young walleye could compensate for the apparent increase in mortality in walleye 

aged 1 – 4 (VanDeValk et al.1999). However, we do not know if factors such as these were 

included in modeling efforts.

Eastern Lake Ontario, Ontario, Canada (Hoyle et al. 1998)

Smallmouth bass abundance in the Canadian waters of the eastern basin of Lake Ontario 

was monitored using gillnets from 1978 to 1998. Age of caught bass was interpreted from scales. 

Annual survival rate was estimated by year-class for fish aged 6 to 10 years old. Smallmouth 

bass year-class strength was estimated by two methods. Age-specific CUEs were summed for 

fish aged 2, 3 and 4 years old by year-class, and provided complete year-class strength 

information for 17 year-classes from 1978 to 1994. Year-class strength was also estimated for 

ages 2 to 11, using proportional year-class strength. Proportional year-class strength for 23 yearclasses from 1973 to 1975 was calculated. Year-class strength estimates were then correlated 

with July / August water temperature data. For water temperatures, two databases were used.

Hoyle et al. (1999) concluded that smallmouth bass year-class strength, as influenced 

primarily by midsummer water temperatures, appeared to be the major factor determining the 

size of the smallmouth bass population in the eastern basin of Lake Ontario between 1978 –

1998. Cumulative gillnet CPUEs for ages 2 to 4 years showed a strong year class in 1995, and 

based on year class strength estimates for the years 1995 to 1998, Hoyle et al. (1999) predicted 

that index gillnet catches should increase over the next few years.

Hoyle et al. (1999) comment on three other factors that may affect size and trends of the 

smallmouth bass population. They state that the annual survival rate for smallmouth bass aged 6 

to 10 years old (mean = 0.62) suggests that overexploitation is not a problem, but they also note 

that one explanation consistent with the observation that survival of strong year-classes (mean = 

0.54) is lower than that for weak year-classes (mean = 0.67) may be that strong year-classes were 

“fished-up” (i.e., exploitation rates were highest in years when bass were most abundant). Noting 
FINAL DRAFT Part III. Diet 197

that cormorant predation has been implicated as influencing survival of young smallmouth bass 

in New York waters of eastern Lake Ontario, Hoyle et al. (1999) comment that if cormorants are 

a major source of smallmouth bass mortality in eastern Lake Ontario, their impact has been too 

recent to distinguish it from expected population trends due to the recent history of summer 

water temperature and low year-class strength. In conclusion, they note that significant 

ecosystem changes in eastern Lake Ontario have occurred since the arrival of Dreissenid mussles 

(Dreissena sp.) in the early 1990s, and state that associated changes (increased water clarity, 

fundamental shifts in food-web interactions) are certain to impact smallmouth bass distribution 

and abundance at some level.

Comments on study. Estimates of smallmouth bass abundance, survival and age 

distribution were calculated through standard methods. These estimates appeared rigorous 

enough to detect population trends over the 20 year period examined, and to document declines 

in abundance similar to those reported for smallmouth bass in New York waters of the eastern 

basin during about the same time period (Chrisman and Eckert 1999). Viewing these population 

trends in relationship to water temperature provides an example of how abiotic factors may 

influence population size, and the importance of examining a variety of factors in efforts to 

determine factors contributing to fish population declines.

Southeastern U.S.

In the southeastern U.S., two studies have been undertaken that incorporate some of the 

data necessary to provide information on impacts to fish populations. The study by Glahn et al. 

(1998) incorporates estimates of abundance of specific sport fish species and DCCOs to 

determine the impact of DCCO predation (Glahn et al. 1998). The study by Simmonds et al. (in 

press) presents a model to evaluate effects of DCCO predation on standing crops and yields of 

reservoir fish species and incorporates various parameters.

Lake Beulah, Mississippi, and Lake Eufala, Alabama (Glahn et al. 1998)

Glahn et al. (1998) compared percentages of prey species in the diet of cormorants at 

Lake Beulah, MS, and Lake Eufala, AL, to the percent availability of these prey species in the 

lakes. Fish availability at Lake Beulah was based on gill net, trammel net and hoop net sampling 

in 1995; availability at Lake Eufala was based on gill net sampling in 1997. Analysis of stomach 

contents was utilized to determine cormorant diet composition. For birds collected at Lake 

Beulah, length measurements of intact and partially digested fish were used in species-specific 

length-to-weight equations to obtain % biomass of each species in the diet. For birds collected at 

Lake Eufala, few intact fish were found in stomachs, and otolith analysis was the primary means 

used to identify prey species and number of fish consumed. To avoid problems associated with 

possible otolith erosion during digestion, fish from Lake Eufala were aged by counting hyaline 

and adjacent opaque bands of sagittal otoliths. From these age classifications median fish length 

was assessed from previous length-frequency data from Lake Beulah. The number of shad and 

bluegill removed from Lake Beulah by cormorants over a 4-month period was also estimated, 

based on average number of cormorants observed foraging on the lake, fish species composition 

by weight of the diet, and average daily cormorant food demand (estimated previously, Glahn 

and Brugger 1995).

The only sport fishes that occurred in the diet in substantial numbers were sunfish 

species, particularly bluegill. Glahn et al (1998) estimated that cormorants consumed only a 

small percent of bluegill available in Lake Beulah, but acknowledged that their estimate of 

bluegill consumed may have been low. They concluded that, with the possible exception of 
FINAL DRAFT Part III. Diet 198

predation on harvestable size bluegill, cormorants do not appear to have an appreciably negative 

impact on sport fisheries at the two lakes studied.

Comments on Study. Assessment of diet composition utilized estimates of fish lengths 

obtained directly from measurements of intact fish, partially digested fish, and indirectly, by 

aging fish through counting hyaline and opaque bands. Estimates of daily food consumption 

were based on an earlier study that carefully calculated average daily fish consumption based on 

bioenergetics data. Estimates of fish abundance in both lakes utilized standard methods; 

however, inconsistencies in results and fish sampling techniques that were possibly biased 

against capturing bluegill relative to other species prevented the authors from concluding 

definitively that cormorants had no appreciably negative effect on sport fish. The apparent 

preference cormorants had for bluegill may have been overstated, though bluegill and other 

sunfishes appeared important in the diet at both lakes.

Because sizes of fish consumed by cormorants in the two lakes were estimated with two 

different methods, there may have been inconsistencies in the results; selectively collecting birds 

observed feeding at Lake Eufala might have reduced the number of birds without intact fish in 

their stomachs, and enabled comparative measurements. Based on the average size of intact 

bluegill consumed at Lake Beulah, cormorants would be in direct competition with anglers. 

However, the estimated size distribution of bluegill from otolith aging suggested that < half of 

the bluegill consumed by cormorants were of harvestable size. From their analysis of intact fish 

at Lake Beulah, the authors estimated that the total number of bluegill consumed by cormorants 

during the winter months was a little more than 100,000. However, the total number consumed 

may be larger if the average size of bluegill, as estimated from their otolith analysis, was smaller. 

The authors note that determining the impact of cormorant predation on bluegills overall is 

difficult because of the wide variation in bluegill population structure between years; additional 

studies involving intensive sampling of bluegill populations concurrent with cormorant diet 

studies need to be undertaken to determine the impact from cormorant predation more 

conclusively.

South-central Great Plains (Simmonds et al. in press)

Simmonds et al. (in press) developed mathematical models to evaluate effects of DCCO 

predation on fish abundances and angling yields in a hypothetical south-central Great Plains 

Reservoir. Specifically, an adaptation of the Ricker equilibrium-yield model (Ricker 1975) was 

used to evaluate effects of DCCO predation on standing crops and yields of individual reservoir 

fish species. Parameters incorporated into the model included: rates of fish growth; instantaneous 

rates of natural fish mortality; instantaneous rates of angling mortality; DCCO induced fish 

mortality; DCCO abundance; and the weight of each fish species consumed per hectare per 

cormorant-use-day. DCCO densities and periodicities of seasonal occurrence were based on 

actual counts conducted at eight reservoirs in Oklahoma during two field seasons, for a total of 

16 reservoir observations. Densities were highly variable among reservoirs and between years, 

but at most reservoirs densities were low. In 14 of the 16 observations, cormorants were present 

only during fall and spring; in the remaining two observations they were abundant throughout the 

migration and wintering periods.

Their analyses suggested that DCCO predation has only a minor or inconsequential 

impact on the recreational fishery of a typical (e.g., low cormorant density) reservoir in 

Oklahoma. At the two reservoirs where high densities of cormorants remained throughout the 

migration and wintering periods, impacts were more severe. However, high predation rates did 

not persist at either reservoir, as high densities of cormorants were only estimated in the first 
FINAL DRAFT Part III. Diet 199

year of the study; in the second year, substantially lower cormorant densities were observed. The 

authors suggested that long-term monitoring of DCCO abundances and temporal use patterns is 

needed to understand predation dynamics and comprehensively model and evaluate impacts on 

reservoir sport fishes in atypical (e.g. high cormorant density) Oklahoma reservoirs. Fisheries 

biologists have drawn similar conclusions, and suggest that impact studies should include 

examination of long-term trends in DCCO abundance, as detecting relations through correlation 

type analyses will require long time-series (S. Correia, pers. comm.).

Simmonds et al. (in press) also noted that effects of DCCO predation may be detrimental, 

beneficial or neutral in reservoirs if effects negate each other. Results of their model indicated 

that DCCO predation reduced standing crops of channel catfish, largemouth bass and white 

crappie, and reduced their yields to the sport fishery; but in many cases density-dependent fish 

growth responses largely mitigated these effects. Thus, while predation may reduce survival, 

standing crop, and yield of a fish population, and consumption of small fish could reduce the 

forage available to large sport fish, it may also decrease competition and thus increase the growth 

rate of the remaining sport fish. Additionally, cormorant predation could conceivably function as 

a biological control of gizzard shad populations, which may lead to increased availability of 

juvenile shad to predatory sport fish.

Comments on Study. This study did not actually assess cormorant diet or impacts to an 

actual fishery. However, it was included because it provided information on predation impacts in 

reservoirs and because it illustrates the utility of modeling to assess cormorant impacts. It also 

incorporated many important estimates (e.g. rate of fish growth, etc.) that most impact studies 

have not included, but are necessary to enhance understanding of how compensatory responses 

of fish populations may buffer predation effects.

Conclusions: Impacts to Sport and / or Commercial Fisheries

To summarize, three of the five studies assessing impacts to sport and commercial 

fisheries reported cormorants did not appear to appreciably impact fisheries (Belyea et al. 1999; 

Glahn et al. 1998; Simmonds et al., in press). Two reported cormorants cause significant impacts 

or declines within a fishery (see Oneida Lake studies above, NYSDEC special report 1999).

Review of these studies highlights the importance of several key pieces of data. One 

parameter that has been difficult to obtain with a high degree of precision is an estimate of prey 

fish population size. This is an essential estimate for determining impact from DCCO predation, 

because it is required to calculate the proportion of the prey fish population consumed by 

cormorants. Another important parameter is an accurate age class distribution of fish consumed. 

This parameter is needed because predation on certain age groups is likely to have a much 

greater effect on recruitment to a fish population and the fishery’s subsequent size than would 

predation on other age groups (e.g., predation on 1 – 3 year old vs. young of the year walleye). 

Studies to determine age distribution of cormorant prey necessarily rely on examination of fish 

parts that have been subjected to the highly acidic environment of the DCCO’s stomach. For an 

accurate age distribution of fish consumed, and thus better assessment of impacts to a fishery, 

methods of diet assessment that have less associated bias should be utilized whenever possible. 

In some cases this may mean relying on a smaller sample size, for example, if fish parts are 

retrieved from stomach contents as opposed to pellets. In some situations, a smaller sample of

birds with stomachs containing fresh food (n = 12 – 15) may be adequate (Carss et al. 1997).

A fundamental question that studies need to address is that of compensatory vs. additive 

mortality within a fish population due to cormorant predation (Hatch and Weseloh 1999). 
FINAL DRAFT Part III. Diet 200

Interactions among mortality factors are not well understood, and releasing fish from the 

predation pressure of one predator (e.g. cormorants), may not necessarily lead to higher catches 

of fish, because these fish may be lost to other causes (e.g. other predators, poor growth, higher 

mortality associated with high fish density, poor overwinter survival) (Draulans 1987; Bayer 

1989; Marquiss and Carss 1997; Carss et al. 1997; Derby and Lovvorn 1997b). Forney (1993) 

noted that “removal of walleye by cormorants could increase survival of perch” on Oneida Lake 

because yellow perch under 200 mm are vulnerable to predation by piscivorous fish and 

mortality is often high. Thus removal of cormorants from this complex system could potentially 

lead to further declines in perch.

Additional information is needed on compensatory mechanisms within fish populations 

that may operate to buffer effects of predation (Carss and Marquiss 1997, 1999; Marquiss et al. 

1998; Rose et al 1999). If fish populations exhibit compensatory responses to predation, and 

mortality due to cormorant predation may not be additive, measuring cormorant predation 

relative to fish abundance may still not allow quantification of cormorant contribution to decline 

in fish stocks. Therefore cormorant predation should be examined in the context of other 

mortality factors.

Overall, comprehensive studies within systems to determine net effect of cormorants and 

the nature and magnitude of cormorant predation have not been conducted, primarily because of 

the difficulties involved in acquiring several key pieces of data. Without better understanding of 

fish population dynamics and responses to predation, it will be difficult, or impossible, to assess 

predation impacts, even when we know the mortality rate of fish due to DCCO predation. 

Studies such as those conducted on Oneida Lake and the work by Simmonds et al. (in press) 

have incorporated, or are incorporating, some data that attempt to deal with these mechanisms. 

In conclusion, rigorously demonstrating whether or not DCCOs negatively impact 

fisheries requires precise estimates of several parameters by both fisheries and cormorant 

biologists. Of the studies reviewed, only one (Belyea et al. 1999) obtained estimates rigorous 

enough to determine impacts of DCCO predation. If this study had found that mortality of the 

adult perch population due to DCCO predation was considerably higher than the estimated 1 %, 

more precise data on fish population size and compensatory processes within the yellow perch 

population would perhaps be necessary to determine the impact such a predation rate has on both 

fish population size and subsequent catch. This observation suggests that the data required to 

determine impacts will vary to some degree depending on the unique conditions of the aquatic 

ecosystem under study. Depending on the results of initial studies, further sampling may be 

needed.

Studies assessing impacts to aquaculture

Most studies to assess aquacultural impacts due to cormorant predation have been 

conducted in the southern U.S., especially in the Mississippi Delta region, which is the center of 

the catfish production industry. Attempts to assess these impacts have utilized a variety of 

techniques: 1) surveys of catfish farmers have been conducted to assess economic losses; 2) 

studies have collected diet data, observed cormorant predation at ponds, and calculated 

cormorant predation rates; and 3) bioenergetics models have been developed to predict 

cormorant predation impacts. While some studies reviewed do not directly assess impacts, they 

were included because they provide relevant information on perception of impacts, and on 

aspects of cormorant foraging ecology that may result in spatial, temporal and sexual variation in 

levels of impact.
FINAL DRAFT Part III. Diet 201

Surveys

Catfish Producers in the U.S. (Wywialowski, 1999)

In January of 1997, the National Agricultural Statistics Service (NASS) surveyed catfish 

producers in the U.S. about wildlife caused losses in 1996 (Wywialowski, 1999). Data were 

analyzed for six regions that included 15 states: CA, KS, MO, OK, TX, LA, MS, AL, AR, KY, 

TN, NC, SC, GA, FL. Fourteen hundred and sixty-five catfish producers were contacted; of 

these, 1,008 completed surveys. Proportionally, the greatest majority of producers were in 

Mississippi (n = 300), followed by Alabama (n = 163) and then Arkansas (n = 117). Producers 

estimated that wildlife-caused losses cost them $12 million in lost production or repair of 

facilities. Additionally, about 68 % of catfish producers spent time and money (nationwide total 

of > $5million) trying to avoid wildlife-caused losses of their catfish. In 1996, the total value of 

losses was about 4 % of the total value of catfish sales, which was $424 million. The 4 % cost of 

wildlife may represent 1/3 – 1/6 of profits.

The main problem caused by wildlife, primarily birds, was feeding on catfish; the DCCO 

was cited most frequently as the primary species causing losses; next most frequently cited birds 

were herons and egrets. No breakdown of the percent of total losses caused by each wildlife 

species was provided. However, Wywialowski (1999) concluded that cormorant-caused losses 

could be anticipated to be a major problem in the Mississippi Delta region, given the growth of 

catfish production in conjunction with growing numbers of DCCOs that winter in the region.

Comments on study: This survey gathered important information on the percentage of 

catfish farmers citing wildlife-caused losses to their stocks, efforts of these producers to prevent 

wildlife-caused losses, and the total cost of sustained losses farmers attributed to wildlife. The 

survey also provided relevant information on the utilization of Wildlife Services by catfish 

farmers and the effectiveness of Wildlife Services in helping farmers prevent wildlife-caused 

losses. An important question raised in discussion of survey results was whether producers can 

actually identify the species and amount of losses caused by wildlife, but the survey did not 

incorporate questions to specifically address these questions.

Catfish Producers in Mississippi (Stickley and Andrews 1989)

In the late 1980s, the USDA/APHIS Science and Technology Research Station at 

Mississippi State University conducted a telephone survey of 281 Mississippi catfish farmers 

about the impact of fish-eating birds on their ponds (Stickley and Andrews 1989). Farmers were 

questioned regarding methods used to repel fish eating birds from their ponds and their 

perceptions of the effectiveness and costs of harassment. Eighty-seven percent of farmers felt 

that fish-eating birds caused problems at their farms and warranted harassment. These farmers 

estimated that they spent an average of 2.6 man-hours per day harassing birds at an average 

annual cost of $7400; the total annual cost of bird harassment according to these farmers was 

$2.1 million. Losses to birds approximated 3 % of total catfish sales.

Moderate to heavy cormorant activity (at least 25 cormorants per day) was reported by 57 

% of Delta farmers, and by 14 % of non-Delta farmers. The average cost per day for cormorant 

harassment statewide was $26.00, which translates into approximately $4700 per farmer for the 

six month period cormorants are present in the state. In addition to reporting survey results, 

Stickley and Andrews (1989) estimated that 35,000 cormorants were present in the Delta in 1988 

from November through April. Based on a conservative estimate of 0.67 lb of fish / day / 
FINAL DRAFT Part III. Diet 202

cormorant at the market price at that time of $0.78 per pound, they estimated that the value of 

fish lost to cormorants in Mississippi in 1988 was roughly $3.3 million.

Comments on Study. Stickley and Andrews (1989) acknowledge that their estimate 

assumes that cormorant diet consists entirely of catfish, while the diet of cormorants in this 

region is known to include other fish, such as shad and bream (Glahn et al. 1995; Glahn and 

Brugger 1995). Additionally, peak numbers of cormorants in the Mississippi Delta do not remain 

constant over winter months as Stickley and Andrews (1989) assume, but gradually increase, 

reaching their peak in February or March (Glahn and Brugger 1995). Furthermore, costs were 

estimated based on value of fish at time of consumption (replacement value), which does not 

necessarily reflect losses to net harvest because of compensatory mechanisms acting within 

catfish ponds. Thus, even though Stickley and Andrews (1989) utilize a conservative estimate for 

daily fish consumption, their assumptions about cormorant diet and numbers over the winter 

likely overestimated the economic impact of cormorant predation to catfish farmers (Glahn and 

Brugger 1995; see review, below).

General Aquaculture in Florida (Brugger 1995)

Brugger (1995) conducted telephone interviews with 38 biologists, extension agents and 

growers in Florida to identify issues of concern and reported that currently DCCOs are not 

obvious pests to aquaculture; overall, little conflict occurs between cormorants and the food-fish 

and game-fish industries in Florida waters. Conflicts occur mainly when wintering cormorants 

feed at inland hatcheries, grow-out ponds, and ornamental fish ponds. Losses to food fish, 

primarily catfish, can be locally severe. Several game fish hatcheries have also sustained high 

losses of fingerlings and stockers in recent years. Cormorants may also cause damages to the 

tropical fish industry near Tampa Bay, but the impact of vertebrate predation in general on the 

tropical fish industry is not known. Because ponds are small in acreage, exclusion, scare tactics, 

or a change in cultural practices were suggested as potentially appropriate methods to reduce fish 

losses to piscivorous birds. 

Comments on study: At the time this study was conducted (early 1990s) there was little 

conflict occurring between cormorants and fisheries in Florida waters. In addition to the survey, 

this study reviewed breeding and wintering population trends. Precise estimates for Florida’s 

breeding and wintering populations are not available. If conflicts in Florida become more 

significant in the future, better estimates of numbers of birds will be needed.

Diet Studies and Observations at Ponds

Mississippi Delta Foraging Behavior Observations (Stickley et al. 1992)

Stickley et al. (1992) conducted an observational study in the Mississippi Delta to 

determine the impact of cormorant depredations on commercial catfish populations. They 

selected 16 different pond complexes for cormorant depredation surveys from December 1989 –

April 1990. Complexes were selected on the basis of growers’ expectations of depredations; 

within the complexes, ponds were identified that were most likely to receive cormorant use. 

During the surveys, observers kept a minute-by-minute count of the number of cormorants on the 

pond. The number and species of fish seen in the bills of cormorants on the pond surface were 

recorded. Approximate lengths of each catfish (based on visual observation) were also recorded. 

Fish catch rates were determined, and feeding rates on a “cormorant-hour” basis were 

summarized [e.g. 10 cormorant-hours (600 cormorant-minutes) would equate to 100 cormorants 

on a pond for six minutes, or 20 cormorants on a pond for 30 minutes, etc.]. The average 
FINAL DRAFT Part III. Diet 203

consumption rate for all ponds surveyed was five catfish per cormorant-hour; rates ranged from 0 

– 28 catfish per cormorant-hour. The average estimated length of catfish consumed was 12 cm. 

Stickley et al. (1992) concluded that, at their highest feeding rates, cormorants could have a 

devastating impact on catfish fingerling populations over time, which in turn would result in 

great financial impacts.

Comments on Study. Feeding rates were estimated for selected ponds based on expected 

levels of high cormorant predation, and ponds experiencing low levels or no predation were 

subsequently excluded from the study. Thus feeding rates were not provided for ponds in 

general. Individual ponds appear to experience various levels of predation, and some seem to be 

much more attractive to cormorants than others. For example, Hodges (1989) conducted 27 

weekly surveys at 50 catfish ponds in Humphreys County, MS, one of the largest catfish 

production areas in the country. During these surveys, cormorants were observed at only nine of 

the 50 ponds, and on only 14 occasions. Hodges found that “cormorants were present more often 

at large defunct ponds with large maximum fish size..." Therefore the predation rates calculated 

by Stickley et al. (1992) should not be extrapolated to ponds in general.

The feeding rate was reported in numbers of catfish per cormorant-hour, and was based 

on observations of entire flocks, in which an interchange of numerous birds throughout the 

observation period was possible. Individual birds were not tracked. Because of the manner in 

which the data were collected, these rates should not be applied to individual birds for extended 

periods of time. Additionally, care should be taken in how these rates are cited. For example, 

Glahn and Stickley (1995) cite the average rate, stating that, “this rate would equate to a loss of 

$400 US during a 9-h foraging day for a flock of 100 birds.” Mott and Boyd (1995) make a 

similar statement. While the researchers did not actually apply this rate to 100 individual birds 

assumed to be feeding for nine hours, the way they report their calculations based on this rate 

gives the impression that individual cormorants may feed at this rate throughout an entire day. 

This is an important distinction because if individual birds did forage at these rates for nine 

hours, they would consume many more fish than necessary for meeting metabolic requirements 

and have a much greater impact than actually occurs.

Finally, this study provides estimates of economic impact based on replacement costs for 

fingerlings at the time of predation. However, the degree to which cormorant predation on catfish 

fingerlings represents additive vs. compensatory mortality is not known. Therefore, it is not 

possible to extrapolate these replacement costs to net impacts at time of harvest, as the impact of 

cormorants relative to other mortality factors has not been determined.

Mississippi Delta Stomach Content Analysis (Glahn et al. 1995)

Glahn et al. (1995) examined food habits of wintering cormorants in the Delta region of 

Mississippi. Birds were collected at catfish farms and night roosts. Stomach contents were 

analyzed, and total lengths of fish consumed were estimated. Channel catfish and gizzard shad 

were the only two important prey species in the diet, but proportions of these species in the diet 

varied dramatically among months, geographic locations, and between the sexes of birds 

collected. Overall, the diet was approximately equal between catfish and shad, though catfish 

was much more predominant in the diet of males, collected at both catfish farms and night roosts, 

than in the diet of females. The diet of birds collected at catfish farms did not vary between 

morning and afternoon collections, while samples from birds collected at roost sites were slightly 

lower in catfish biomass and more diverse in prey species. Thus, the authors assumed that 

stomach contents from birds collected at roost sites would form the best basis for assessing diet 

of wintering cormorant populations.
FINAL DRAFT Part III. Diet 204

For the roost site samples, cormorants were collected from a total of 13 roost sites, six 

western and seven eastern. Spatial availability of catfish appeared to be an important factor 

determining the diet of cormorants among the roosting areas. The western roosts were located in 

five counties bordered by the Mississippi River and contained 6,835 ha of catfish ponds. The 

eastern roosts were located in five counties that contained 7,374 ha of catfish ponds. However,

four of these roosts were within 5 km of Humphreys County, which has the highest catfish 

acreage in the U.S. In cormorants collected from western roosts, gizzard shad was the major prey 

item, while in those collected from eastern roosts, catfish was the major prey item. Glahn et al. 

(1995) assumed cormorants in the eastern roosts were consuming catfish in Humphreys County 

in addition to the other five counties, and because of this cormorants from the eastern roosts 

actually had access to a total catfish availability of 19,844 hectares. Compared with catfish 

availability at western roosts, this is an abundance ratio of 2.9:1. Additionally, differences in the 

diet of males and females may have influenced the difference in diet between regions, as males 

appeared to consume more catfish than females and to be more abundant at eastern roosts.

In both the farm and night roost collections, the size-class distributions of catfish 

recovered from cormorant stomachs fell within common commercial stocking sizes. The authors 

concluded that cormorants could have a negative impact on the commercial catfish industry in 

Mississippi, but that the proportion of catfish in the diet appeared to vary significantly with 

location and time of year (see Table 16, Table 17). The following strategies were suggested as 

potentially useful in reducing catfish losses: 1) increasing availability of shad by stocking these 

fish in areas of heavy catfish predation in the spring; 2) disperse roosting cormorants away from 

areas of high catfish density; 3) limit availability of size classes most frequently consumed (10 –

20 cm), particularly in the spring, possibly by increasing the size of “stocker” fish or delaying 

stocking until after cormorant migration in April. 

Comments on study. While this study did not quantify the impact of cormorant predation, 

it did determine temporal and spatial magnitude of cormorant predation on catfish. It is also the 

first study to identify sex of the bird as an important source of variation in diet (D.V. Weseloh, 

pers. comm.). Such data are important in evaluating the appropriateness of management 

strategies to reduce potential impacts of cormorant predation.

Florida Foraging Behavior Observations (Schramm et al. 1984)

Schramm et al. (1984) conducted an observational study at a south Florida pond during 

September – October, 1980. The pond was stocked with 75,000 channel catfish fingerlings and 

observed by a person with binoculars between 0800 – 1200 and 1300 – 1700 on 20 randomly 

selected days. The mean catch per bird per hour and mean number of birds foraging during each 

hour were estimated. The sum of the mean total catch for the nine 1-hour periods (0800 – 1700) 

was used as a minimum estimate of average daily consumption. Schramm et al. (1984) reported 

13 resident cormorants consumed an estimated 246 channel catfish daily during 0800 – 1700, 

which represents an average daily consumption of 19 catfish per bird, or an average of 304 g of 

catfish daily per bird. The 13 cormorants continued to feed at the pond during the winter and 

nested the following spring in a nearby cypress dome. By November of 1981, approximately 50 

cormorants were feeding in the pond. Schramm et al. (1984) concluded that depredation by the 

Florida DCCO may be an important limiting factor to open-pond fish culture in Florida.

Comments on Study. Because Schramm et al. (1984) did not band or otherwise mark 

individuals, we are unsure of how the authors determined “residency” and breeding status of 

birds observed on the pond. Without marking the birds in some manner to identify individuals, 
FINAL DRAFT Part III. Diet 205

estimates of average consumption by resident birds may have a high degree of inaccuracy 

associated with them.

Controlled Catfish Predation Experiment (J. Glahn, unpubl. data)

J. Glahn (unpublished data from 1999 study) conducted a controlled experiment to assess 

DCCO impacts to gross production (in kg) of catfish. Ponds 0.1 acres in size were divided in half 

and stocked with catfish to mimic typical densities at southern catfish farms (i.e., 5,000 

fingerlings / acre); an equal biomass of golden shiners was also stocked as a potentiall buffer 

prey (as a surrogate for shad). One DCCO was given access to one of the halves, while the other 

half received no predation. After a 10 day period of DCCO foraging, the birds were removed and 

catfish were cultured for the remainder of the 1999 growing season. In the ponds with DCCOs, 

greater biomass production per fish (compensatory growth) resulted; this was attributed to 

possibly greater relative food availability and density dependent growth. But despite 

compensatory growth in catfish, overall pond production was reported, on average, as 20 % less 

in treated versus control pond halves, suggesting that at the pond scale and predation rates 

investigated, DCCO impacts were additive. However, in a 0.1 acre pond where diseases depleted 

fish density, losses due to DCCO predation were insignificant. This study will be replicated in 

2000 to supplement its statistical rigor.

Comments on Study. Preliminary results indicate that mortality due to DCCO predation 

can be additive under certain circumstances and compensatory in others. If these patterns are 

confirmed by data collected in 2000, researchers will be a large step closer to understanding net 

impacts of DCCOs on aquaculture. One important assumption in this study was that golden 

shiners would be preferentially selected by cormorants at a similar frequency as shad based on 

the similarity of handling time for these species; however no studies have been conducted to 

compare cormorant preference for these species. Because this is a controlled study at a much 

smaller scale and with fewer sources of mortality than are normally found in catfish ponds, more 

information will be needed to determine how or if these data should be used to estimate impacts 

at a larger scale. If it is true that 1) DCCO predation at ponds with diseased fish is compensatory 

and 2) DCCOs forage preferentially at ponds with diseased fish, then the influence of these 

factors will need to be addressed / incorporated in estimates of impact.

Bioenergetics Modeling

Mississippi Delta (Glahn and Brugger 1995)

Glahn and Brugger (1995) constructed a bioenergetics model to refine estimates of daily 

food requirements of wintering cormorants and their impact on the Mississippi Delta catfish 

industry. Their model estimated individual energy demands, population energy demands and 

catfish crop losses per month during two wintering periods: November to Apr 1989 – 1990 and 

November to Apr 1990 – 1991. The following parameters were incorporated in the model: basal 

metabolic rate (BMR, seasonally adjusted); daily activity budget; activity-specific metabolic 

rates; thermoregulation; daily energy budget; digestive efficiencies; individual energy demand; 

population estimates; population energy demand; nutrient composition of fish; diet composition; 

catfish mass consumed; percentage of catfish mass (by size class); catfish numbers consumed 

(by size class); and value ($) of catfish (by size class). To transform the monthly energy demand 

of the cormorant population into the monthly populations of fish consumed, the following 

information was obtained: 1) the percent biomass of the diet of major fish groups consumed; 2) 
FINAL DRAFT Part III. Diet 206

the percent of the diet by size class and number of fish by weight in that size class; and 3) the 

energy (kJ) content in each fish group.

To assess economic and numerical impacts, two parameters from the Mississippi Delta 

catfish industry were included in the model: replacement value $ loss of catfish, and “Standing 

crop” % loss. For the calculation of replacement cost of catfish consumed, the authors assumed 

$20 US per thousand fish plus a variable cost of $0.0049 to $0.0069 / cm times the size (5 

through 28 cm) of 12 commercial size classes in 2.54 cm increments. Larger fish (> 28 cm) were 

based on the number / kg times $1.54 / kg. The standing crop of catfish in the size classes most 

often preyed upon by cormorants (10 to 23 cm) was difficult to identify; the number of 

fingerlings > 7 cm or exposed in ponds to cormorant predation was unknown. Therefore, to 

calculate an estimate of the total “standing crop” vulnerable to cormorant predation, the authors 

used the total area of catfish ponds stocked in the Mississippi Delta (c. 32,000 ha) times the 

average stocking rate of 12,350 fish / ha for all months except Mar and April. These latter 

months are the height of the stocking season, so the stocking rate of ponds was increased by 50 

% or 18,525 catfish / ha for these months, resulting in “standing crops” of catfish vulnerable to 

predation of 395 million catfish from November to February and 592.5 million catfish in March 

and April.

Monthly cormorant populations varied (7,000 – 27,000 birds) and peaked in either 

February or March each year. Percent catfish in the diet also varied monthly (0 – 97 %), and diet 

showed a trend similar to populations with low catfish consumption in fall and peak catfish 

consumption February – April. Approximately 4 % (18 – 20 million catfish) of the estimated 

catfish standing crop was consumed by cormorants each year, and annual losses to the catfish 

industry were predicted at $2 million and $1.8 million during 1989 – 1990 and 1990 – 1991, 

respectively. More than half of the average annual consumption of catfish by cormorants was 

predicted to occur in February and March due to increasing cormorant populations and a shift in 

the diet towards more catfish during those months. Glahn and Brugger (1995) note that their 

average annual monetary loss figure of $1.9 million was 40 % less than the annual loss of $3.3 

million estimated by Stickley and Andrews (1989; see above), and that their model would also 

have produced much higher losses if they had assumed a constant high population of cormorants 

subsisting on a diet of 100 % catfish.

Comments on Study. Glahn and Brugger (1995) believe that their model provides the best 

estimate of the impact of DCCO predation on the Mississippi Delta catfish industry during the 

winters of 1989 – 1990 and 1990 – 1991, based on the empirical data assembled to construct the 

model. We agree with this statement. Of all the literature we reviewed that attempted to assess 

impacts to fish populations (including sport and commercial fish populations), this approach 

appeared to be one of the most thorough and inclusive. The authors incorporated several 

important physiological and ecological parameters into the model, data necessary to more 

accurately represent energy flow between predator and prey.

Nevertheless, both estimates of impact (numerical and economic) are based on 

parameters which make it difficult to assess the impacts precisely. The standing crop estimate 

was based in part on an unpublished average stocking rate, which we were unable to judge for 

accuracy. [Estimates of fish stocking densities range widely; Hodges (1989) reported a range of 

fish density, from 5,000 – 150,000 fish / ha reported by farmers.] Economic impacts were 

reported in terms of replacement costs, which, as described above, can not be extrapolated to net 

impacts at time of harvest, because the impact of cormorants relative to other mortality factors 

has not been determined.
FINAL DRAFT Part III. Diet 207

Conclusions: Impacts to Aquaculture

Studies to assess impacts of DCCOs on aquaculture have evaluated many diverse 

techniques; all suggest that DCCOs can consume large amounts of catfish at certain times and 

locations. However, the magnitude of these impacts still needs to be made more clear. Similar 

questions that remain unanswered regarding impacts to sport and commercial fisheries are just 

beginning to be addressed in studies assessing impacts to aquaculture [e.g., compensatory vs. 

additive mortality, influence of other mortality factors, compensatory mechanisms in farm fish 

populations; see Controlled Catfish Predation Experiment (J. Glahn, unpubl. data), above]. A 

more comprehensive understanding of the interrelationships between fish-eating birds and catfish 

farming is needed. More studies of the interactions within this system are necessary to 

understand the net effect of cormorants at catfish ponds, and the nature and magnitude of 

cormorant predation. Jackson and Jackson (1995) suggested that one way to gain a more 

comprehensive understanding of the impact of cormorants on ponds is to randomly sample all 

available ponds, not just those at which cormorants are seen feeding. This is important in trying 

to assess impacts because: 1) there may be characteristics of ponds that attract or discourage 

cormorants (Hodges 1989); 2) by feeding at ponds where fish are weakened or dying (Hodges 

1989), cormorants may actually benefit some farmers (Jackson and Jackson 1995); and 3) 

estimates of losses to cormorants may be inflated as a result of inclusion of losses that would 

have occurred with or without cormorants present.

The bioenergetics modeling and DCCO inclusion / exclusion experiments were two of 

the more promising techniques utilized. If modeling efforts could be expanded to incorporate 

parameters such as fish growth rates in the presence / absence of DCCOs, oxygen stress, other 

predators, and disease, a more comprehensive view of the system and how factors interact might 

be possible, and provide more relevant information on the net effect of cormorants at ponds. 

Such work may also provide valuable insights for impact assessment of open-water fisheries.

Studies on food preferences

In their review of cormorant diet literature, Trapp et al. (1995) found that most studies 

reveal little about food preferences; this requires simultaneous information on the relative 

abundance of all potential prey species at a given site. The only study we found that examined 

food preferences by utilizing quantitative data on prey fish species was the one described above 

by Glahn et al. (1998). At both lakes cormorants fed mainly on shad and sunfishes. For the most 

part, cormorants appeared to consume the fish species most available; but at Lake Beulah, 

cormorants did appear to have a preference for sunfishes, particularly bluegill. But because of 

possible biases in fish sampling techniques (see above) the preference may have been overstated. 

The authors also found that cormorants appeared to prefer gizzard shad over threadfin shad, and 

they speculated that this is because gizzard shad represent a more optimally-sized prey for 

cormorants. At Lake Eufala, where threadfin shad greatly outnumbered gizzard shad, threadfin 

shad were taken proportional to their relative abundance. Glahn et al. (1998) concluded that 

availability, rather than size, is probably the most important factor in prey selection by 

cormorants; another factor may be accessibility of prey fish.

Stickley et al. (1992) reported a preference for gizzard shad over catfish. In the only pond 

that contained both gizzard shad and catfish, observers noted 64 shad and only two catfish in 

cormorant bills over about three hours. Hodges (1989) also reported a possible preference for 

shad over catfish.
FINAL DRAFT Part III. Diet 208

Table 7. Pacific Coast: Occurrence of Salmonids in the diet of Double–crested Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

Location % in diet Time Assessment 

Method

Source Variation

Oncorhynchus spp.

30.3 (N); 23.7 (B) Apr-Aug Boli (n = 64);

Stomach contents

(n = 90)

14.4 (B) Apr Boli, Stomach 

contents

(n = 19 combined)

95.3 (B) May Boli, Stomach 

contents

(n = 19 combined)

6.6 (B) June Boli, Stomach 

contents

(n = 62 combined)

Lower 

Columbia 

River: 

Estuary 

colonies 

combined

4.3 (B) July Boli, Stomach 

contents

(n = 54 combined)

Temporal

L. Columbia 

River: 

Upriver

35 (B)1 Apr-Aug Boli, Stomach 

contents

(n = 120 combined)

Spatial

L. Columbia 

River: River 

mouth

16 (B)1 Apr-Aug Boli, Stomach 

contents

(n = 31 combined)

Roby et al. 

1998

Spatial

British 

Columbia, 

Mandarte Is.

0.2 (N); 0.9 (B) Nesting Boli

(n not provided)

Robertson 

1974

1 Diet composition from East Sand Island, near the Columbia River mouth, was based on a 

small sample size; most stomach content samples were collected farther up river in the Rice 

Island area, thus the diet composition data for cormorants is probably biased toward 

salmonids (Roby et al. 1998).
FINAL DRAFT Part III. Diet 209

Table 8. Pacific Coast: Occurrence of littoral or estuarine fish in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

Location % in diet Time Assessment 

Method

Source Variation

STICKLEBACK

Lower 

Columbia 

River, estuary 

colonies

21.2 (N);

1.6 (B)

Apr-Aug Boli (n = 64); 

Stomach contents

(n = 90)

Lower 

Columbia 

River: Upriver

24 (B) Apr-Aug Boli, Stomach 

contents

(n = 120 combined)

L. Columbia 

River: River 

mouth

14 (B) Apr-Aug Boli, Stomach 

contents

(n = 31 combined)

Roby et al. 

1998

Spatial

British 

Columbia, 

Mandarte Is.

0.4 (N);

0.1 (B)

Nesting Boli

(n not provided)

Robertson 

1974

SHINER PERCH1

L. Columbia 

River, estuary 

colonies

8.6 (N);

3.5 (B)

Apr-Aug Boli (n = 64); 

Stomach contents

(n = 90)

Lower 

Columbia 

River: Upriver

4 (B) Apr-Aug Boli, Stomach 

contents

(n = 120 combined)

L. Columbia 

River: River 

mouth

22 (B) Apr-Aug Boli, Stomach 

contents

(n = 31 combined)

Roby et al. 

1998

Spatial

British 

Columbia, 

Mandarte Is.

15.5 (N);

20.5 (B)

Nesting Boli

(n not provided)

Robertson 

1974

California, 

Central Coast, 

Farallon Is.

78.6 (N) Post-breeding 

season

Pellets (n = 175)

California, 

San Miguel I.

12.3 (N) Breeding 

season

Boli (n = 39)

Baja 

California, I. 

San Martin

7.1 (N) Post-breeding 

season

Pellets (n = 35)

Ainley et 

al. 1981

Spatial, 

Assessment 

method
FINAL DRAFT Part III. Diet 210

Location % in diet Time Assessment 

Method

Source Variation

STRIPED SEAPERCH

British 

Columbia, 

Mandarte Is.

1.3 (N);

3.5 (B)

Nesting Boli

(n not provided)

Robertson 

1974

California, 

Central Coast, 

Farallon Is.

0.1 (N) Post-breeding 

season

Pellets (n = 175) Ainley et 

al. 1981

Spatial, 

Assessment 

method

Other Embiotocids (not including SHINER PERCH and STRIPED PERCH, above)

California, 

Central Coast, 

Farallon I.

13.8 (N) Post-breeding 

season

Pellets (n = 175)

California, 

San Miguel I.

8.9 (N) Breeding 

season

Boli (n = 39)

Baja 

California, I. 

San Martin

2.2 (N) Post-breeding 

season

Pellets (n = 35)

Ainley et 

al. 1981

Spatial, 

Assessment 

method

PACIFIC SAND LANCE1

British 

Columbia, 

Mandarte Is.

20.5 (N);

4.6 (B)

Nesting Boli

(n not provided)

Robertson 

1974

PEAMOUTH

L. Columbia 

River, estuary 

colonies

15.8 (N);

32.9 (B)

Apr-Aug Boli (n = 64); 

Stomach contents

(n = 90)

Lower 

Columbia 

River: Upriver

19 (B) Apr-Aug Boli, Stomach 

contents

(n = 120 combined)

Lower 

Columbia 

River: River 

mouth

8 (B) Apr-Aug Boli, Stomach 

contents

(n = 31 combined)

Roby et al. 

1998

Spatial

1 During summer, shiner perch and Pacific sand lance inhabit shallow water in large schools, 

and are probably caught while in this zone (Robertson 1974).
FINAL DRAFT Part III. Diet 211

Table 9. Pacific Coast: Occurrence of littoral-benthic fish in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

Location % in diet Time Assessment 

Method

Source Variation

Cottidae (SCULPIN SP.)

L. Columbia 

River, estuary 

colonies

13.5 (N);

6.0 (B)

Apr-Aug Boli (n = 64); 

Stomach contents

(n = 90)

L. Columbia 

River: Upriver

8 (B) Apr-Aug Boli, Stomach 

contents

(n = 120 combined)

L. Columbia 

River: River 

mouth

30 (B) Apr-Aug Boli, Stomach 

contents

(n = 31 combined)

Roby et al. 

1998

Spatial

British 

Columbia, 

Mandarte Is.

2.7 (N);

5.9 (B)

Nesting Boli

(n not provided)

Robertson 

1974

California, 

Central Coast, 

Farallon Is.

3 (N) Post-breeding 

season

Pellets (n = 175) Ainley et 

at. 1981

PENPOINT GUNNEL

British 

Columbia, 

Mandarte Is.

23.8 (N);

35.7 (B)

Nesting Boli

(n not provided)

Robertson 

1974

CRESCENT GUNNEL

British 

Columbia, 

Mandarte Is.

22.8 (N);

15.9 (B)

Nesting Boli

(n not provided)

Robertson 

1974

SNAKE PRICKLEBACK

British 

Columbia, 

Mandarte Is.

11.5 (N);

10.2 (B)

Nesting Boli

(n not provided)

Robertson 

1974

SUCKER

L. Columbia 

River, estuary 

colonies

2.3 (N);

8.1 (B)

Apr-Aug Boli (n = 64); 

Stomach contents

(n = 90)

Roby et al. 

1998
FINAL DRAFT Part III. Diet 212

Table 10. Pacific Coast: Occurrence of pelagic fish in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

Location % in diet Time Assessment 

Method

Source Variation

PACIFIC HERRING

British 

Columbia, 

Mandarte Is.

1.3 (N);

2.7 (B)

Nesting Boli

(n not provided)

Robertson 

1974

Lower 

Columbia 

River, estuary 

colonies

0.2 (N)

0.5 (B)

Apr-Aug Boli (n = 64); 

Stomach contents

(n = 90)

Roby et al. 

1998

CLUPEIDS (Opisthonema spp.)

Gulf of 

California, 

Bahia de los 

Angeles 

0.2 (N) Post-breeding Pellets (n = 106)

Sonora, Bahia 

de Kino

11.6 (N) Post-breeding Pellets (n = 208)

Ainley et 

al. 1981

Spatial
FINAL DRAFT Part III. Diet 213

Table 11. Great Lakes: Occurrence of open-fresh water fish in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

F = percent of samples which contain at least one specimen of the taxon of interest

Lake(s) / 

Island

% in diet Time Assessment 

Method

Source Variation

ALEWIFE

L. Huron, 41 (N); 57 (B) May-Aug

Michigan & 

Superior1 100 (F) Aug

Green Bay,

n. Lake 

Michigan

76 total fish 

biomass 

consumed) 

Beaver Is.,

n. Lake 

Michigan

72 (total fish 

biomass 

consumed)

Main body, n. 

Lake Huron

64 (total fish 

biomass 

consumed)

N. Channel / 

Georgian Bay, 

L. Huron

37 (total fish 

biomass 

consumed)

May-Aug

Boli

(n not reported)

Ludwig et 

al. 1989

Spatial

L. Huron and 

Ontario

0-50 (F) Apr/MayJuly/AugPellets (n = 1595)

L. Ontario, 

Little Galloo

47 (F) Apr-July Pellets (n = 938)

Lake Huron, 

Bustard Is.

13 (F) May-Aug Pellets (n = 144)

Lake Huron, 28 (F) Pellets (n = 36)

Bustard Is. 70 (F)

June 20, 1994

Boli (n = 47)

Neuman 

et al. 1997

Spatial, Diet 

Assessment

27 (N) Apr-Sept Pellets (n = 2939)

1 (N) May Pellets (n = 742)

Little Galloo

Is., Lake 

Ontario 68 (N) June Pellets (n = 601)

Johnson et 

al. 1999

Temporal

29 (N); 47 (B) Apr-Oct Stomach contents 

(n = 373)

< 0.1 (N, B) Apr 17-May 16 Stomach contents 

(n not reported)

Les Chenaux 

Is., n. Lake 

Huron

61 (N); 90 (B) Jul 4-Aug 1 Stomach contents 

(n not reported)

Belyea et 

al. 1999

Temporal

w. Lake Erie 0.4 (F) Apr-Oct Stomach contents 

(n = 302)

Bur et al. 

1999
FINAL DRAFT Part III. Diet 214

Lake(s) / 

Island

% in diet Time Assessment 

Method

Source Variation

52.52 (F) Jul 5, 7, 23; Aug 

2, 5, 12, 1993

Cat Is., Lake Boli (n = 507)

Michigan

57.5 (F) Jun 3, 26; Jul 10, 

25, 1997

Boli (n = 400)

Willow Is.,

L. Michigan

50.8 (F) Jun 13-July 25, 

1983

Boli (n = 1090)

Matteson 

et al. 

19993

Temporal, 

Spatial

GIZZARD SHAD

W. Lake Erie 70 (N.); 50 (F); 

48 (B)

Apr-Oct Stomach contents 

(n = 302)

Bur et al. 

1999

RAINBOW SMELT

L. Huron, 

Michigan and 

Superior

7 (N); 8 (B) May-Aug Boli

(n not reported)

Ludwig et 

al. 1989

Apostle Is.,

L. Superior 

(Gull Island)

1 (F) May-Sept / Oct 

1984

Pellets, Boli 

(n = 150)

Apostle Is.,

L. Superior 

(Eagle I.)

0.9 (F) May-Sept / Oct 

1984

Pellets

(n not reported)

Craven 

and Lev 

1987

Apostle Is., 

(Gull Island)

1.8 (F) May-Sept / Oct 

1985

Boli

(n not reported)

Matteson 

et al. 1999

L. Huron and 

Ontario

2 - 65 (F) May-Aug Pellets (n = 1595)

L. Ontario, 

Little Galloo

4 (F) Apr-July Pellets (n = 938)

Lake Huron, 

Bustard Is.

57 (F) May-Aug Pellets (n = 144)

Neuman 

et al. 1997

Spatial, 

Temporal

0.7 (N); 0.2 (B) Apr-Oct Stomach contents 

(n = 373)

< 0.1 (N, B) Aug 2- 19 Stomach contents 

(n not reported)

Les Chenaux 

Islands, n. 

Lake Huron

2 (N); 0.4 (B) Jul 4-Aug 1 Stomach contents 

(n not reported)

Belyea et 

al. 1999

w. Lake Erie 1 (F) Apr-Oct Stomach contents 

(n = 302)

Bur et al. 

1999

1 Food samples collected at L. Superior colonies were too few to be useful. However, of 21 food items 

recovered at a L. Superior colony, only 10 % (n = 2) were alewives. In conjunction with Craven and 

Lev’s (1987) study, this suggests that in L. Superior alewife is much less prominent in the cormorant 

diet than in Lakes Huron and Michigan. This is also true for w. Lake Erie (Bur et al. 1999).

2 Sample represents both alewife and gizzard shad combined (Matteson et al. 1999).

3 This study reports frequency of occurrence of fish in DCCO regurgitation samples from 5 Wisconsin 

studies on L. Superior and Michigan conducted intermittently during 1983 – 1997.
FINAL DRAFT Part III. Diet 215

Table 12. Great Lakes: Occurrence of littoral-freshwater fish in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

F = percent of samples which contain at least one specimen of the taxon of interest

Lake(s) / 

Island

% in diet Time Assessment 

Method

Source Variation

STICKLEBACKS (NINESPINE, THREESPINE)

L. Huron, 

Michigan 

and Superior

41 (N); 3 (B) May-Aug Boli

(n not reported)

Ludwig et 

al. 1989

Apostle Is., 

L. Superior 

(Gull Island)

20.2 (F) May-Sept / Oct 

1984

Pellets, Boli

(n not reported)

Apostle Is., 

L. Superior 

(Eagle I.)

NA1 May-Sept / Oct 

1984

Pellets

(n not reported)

Craven and 

Lev 1987

Spatial; 

Temporal; 

Diet 

assessment 

method?

Apostle Is., 

L. Superior 

(Gull Island)

75.9 (F) May-Sept / Oct 

1985

Boli

(n not reported)

Matteson et 

al. 1999

22 (N) Apr-Sept Pellets (n = 2939)

51 (N) May Pellets (n = 742)

Little Galloo 

Is., Lake 

Ontario 0 (N) Sept Pellets (n = 301)

Johnson et 

al. 1999

Temporal

58 (N); 15 (B) Apr-Oct Stomach contents 

(n = 373)

91 (N); 46 (B) May 17-Jul 3 Stomach contents 

(n not reported)

Les 

Chenaux Is., 

n. Lake 

Huron

0.7 (N); 0.2 (B) Sep 14-Oct 6 Stomach contents 

(n not reported)

Belyea et al. 

1999

Temporal

SMALLMOUTH BASS

L. Huron, 

Michigan 

and Superior

< 1 (N); 1 (B) May-Aug Boli

(n not reported)

Ludwig et 

al. 1989

L. Huron 

and Ontario

2 - 88 (F) May-Aug Pellets (n = 1595)

L. Ontario, 

Little Galloo

6 (F) Apr-July Pellets (n = 938)

Lake Huron, 

Bustard Is.

41 (F) May-Aug Pellets (n = 144)

Neuman et 

al. 1997

Spatial

w. Lake Erie 0.8 (F); 0.3 (B) Apr-Oct Stomach contents 

(n = 302)

Bur et al. 

1999

Cat Is., Lake 

Michigan

0.2 (F) Jun 3, 26; Jul 10, 

25, 1997

Boli (n = 400) Matteson et 

al. 1999
FINAL DRAFT Part III. Diet 216

Lake(s) / 

Island

% in diet Time Assessment 

Method

Source Variation

1.5 (N) Apr-Sept Pellets (n = 2939)

0.3 (N) April Pellets (n = 276)

Little Galloo 

Is., Lake 

Ontario 3 (N) July, Aug Pellets (n = 1316)

Johnson et 

al. 1999

Temporal

ROCK BASS

L. Huron, 

Michigan 

and Superior

< 1 (N.); 4 (B) May-Aug Boli

(n not reported)

Ludwig et 

al. 1989

L. Huron 

and Ontario

0.8 - 14 (F) May-Aug Pellets (n = 1595)

L. Ontario, 

Little Galloo

1.1 (F) Apr-July Pellets (n = 938)

L. Huron, 

Bustard Is.

9.7 (F) May-Aug Pellets (n = 144)

Neuman et 

al. 1997

Spatial

3 (N) Apr-Sept Pellets (n = 2939)

3.6 (N) May Pellets (n = 742)

Little Galloo 

Is., Lake 

Ontario 2.1 (N) June Pellets (n = 601)

Johnson et 

al. 1999

Temporal

PUMPKINSEED

L. Ontario, 

Little Galloo

9.6 (F) Apr-July Pellets (n = 938) Neuman et 

al. 1997

6.8 (N) Apr-Sept Pellets (n = 2939)

18.1 (N) April Pellets (n = 276)

Little Galloo 

Is., Lake 

Ontario 3.1 (N) July Pellets (n = 715)

Johnson et 

al. 1999

Temporal

W. Lake 

Erie

0.8 (F) Apr-Oct Stomach contents 

(n = 302)

Bur et al. 

1999

Centrarchidae (SMALLMOUTH BASS, ROCK BASS, PUMPKINSEED)

0.8 (N); 6 (B) Apr-Oct Stomach contents 

(n = 373)

9 (N); 15 (B) Apr 17-May 16 Stomach contents 

(n not reported)

Les 

Chenaux Is., 

n. Lake 

Huron

0.1 (N); 0.2 (B) Jul 4-Aug 1 Stomach contents 

(n not reported)

Belyea et al. 

1999

Temporal

W. Lake 

Erie2

0.8 (F) Apr-Oct Stomach contents 

(n = 302)

Bur et al. 

1999

YELLOW PERCH

L. Huron, 

Michigan 

and Superior

3 (N); 13 (B) May-Aug Boli

(n not reported)

Ludwig et 

al. 1989
FINAL DRAFT Part III. Diet 217

Lake(s) / 

Island

% in diet Time Assessment 

Method

Source Variation

L. Huron 

and Ontario

4 - 78 (F) May-Aug Pellets (n = 1595)

L. Ontario, 

Little Galloo

20 (F) Apr-July Pellets (n = 938)

Lake Huron, 

Bustard Is.

17 (F) May-Aug Pellets (n = 144)

Neuman et 

al. 1997

Spatial

2 (N); 11 (B) Apr-Oct Stomach contents 

(n = 373)

< 0.1 (N, B) Aug 20-Sep 13 Stomach contents 

(n not reported)

Les 

Chenaux Is., 

n. Lake 

Huron

42 (N); 48 (B) Apr 17-May 16 Stomach contents 

(n not reported)

Belyea et al. 

1999

Temporal

18 (N) Apr-Sept Pellets (n = 2939)

7 (N) June Pellets (n = 601)

Little Galloo 

Is., Lake 

Ontario 48 (N) Sept Pellets (n = 301)

Johnson et 

al. 1999

Temporal

W. Lake 

Erie

0.2 (N); 1 (B);

5 (F)

Apr-Oct Stomach contents 

(n = 302)

Bur et al. 

1999

46.1 (F) Jul 5, 7, 23; Aug 

2, 5, 12, 1993

Cat Is., Lake Boli (n = 507)

Michigan

13.7 (F) Jun 3, 26; Jul 10, 

25, 1997

Boli (n = 400)

Willow Is., 

L. Michigan

38.7 (F) Jun 13-July 25, 

1983

Boli (n = 1090)

Matteson et 

al. 1999

Temporal, 

Spatial

EMERALD SHINER

W. Lake 

Erie

27 (N); 9 (B); 

34 (F)

Apr-Oct Stomach contents 

(n = 302)

Bur et al. 

1999

COMMON SHINER

L. Huron, 

Michigan 

and Superior

2 (N); < 1 (B) May-Aug Boli

(n not reported)

Ludwig et 

al. 1989

SPOTTAIL SHINER

L. Ontario, 

Little Galloo

0.9 (F) Apr-July Pellets (n = 938) Neuman et 

al. 1997

Cat Is.,3

L. Michigan

16.2 (F) Jun 3, 26; Jul 10, 

25, 1997

Boli (n = 400)

Willow Is.,3 

L. Michigan

5.1 (F) Jun 13-July 25, 

1983

Boli (n = 1090)

Matteson et 

al. 1999

Temporal
FINAL DRAFT Part III. Diet 218

Lake(s) / 

Island

% in diet Time Assessment 

Method

Source Variation

Notropsis sp. (SHINER SP.)

L. Huron 

and Ontario

8 – 65 (F) May-Aug Pellets (n = 1595)

L. Ontario, 

Little Galloo

9 (F) Apr-July Pellets (n = 938)

Lake Huron, 

Bustard Is.

58 (F) May-Aug Pellets (n = 144)

Neuman et 

al. 1997

Spatial

Cyprinidae

13 (N) Apr-Sept Pellets (n = 2939)

5 (N) June Pellets (n = 601)

Little Galloo 

Is., Lake 

Ontario 32 (N) Sept Pellets (n = 301)

Johnson et 

al. 1999

Temporal

2 (N); 2 (B) Apr-Oct Stomach contents 

(n = 373)

24 (N); 4 (B) Apr 17-May 16 Stomach contents 

(n not reported)

Les 

Chenaux Is., 

n. Lake 

Huron

< 0.1 (N, B) May 17-Jul 3 Stomach contents 

(n not reported)

Belyea et al. 

1999

Temporal

WHITE PERCH

L. Huron 

and Ontario

7 - 27 (F) May-Aug Pellets (n = 1595)

L. Ontario, 

Little Galloo

18 (F) Apr-July Pellets (n = 938)

Lake Huron, 

Bustard Is.

0 (F) May-Aug Pellets (n = 144)

Neuman et 

al. 1997

Spatial

0.2 (F) Jul 5, 7, 23; Aug 

2, 5, 12, 1993

Cat Is., Lake Boli (n = 507)

Michigan

10 (F) Jun 3, 26; Jul 10, 

25, 1997

Boli (n = 400)

Matteson et 

al. 1999

Temporal

W. Lake 

Erie

0.1 (N); 0.2 (B); 

2.0 (F)

Apr-Oct Stomach contents 

(n = 302)

Bur et al. 

1999

1 Only pellets were collected on Eagle Is., and ninespine sticklebacks could only be identifed in 

boli. However, ninespine stickleback was abundant around this site and probably consumed by 

DCCOs there (Craven and Lev 1987).

2 Unidentified Centrarchid species (Bur et al. 1999).

3 Cat and Willow Islands are about 400 m apart (Matteson et al. 1999).
FINAL DRAFT Part III. Diet 219

Table 13. Great Lakes: Occurrence of bottom dwelling-fresh water fish in the diet of 

Double–crested Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

F = percent of samples which contain at least one specimen of the taxon of interest

Lake(s) / 

Island

% in diet Time Assessment 

Method

Source Variation

SCULPIN (SLIMY, SPOONHEAD)

L. Huron, 

Michigan 

and Superior

4 (N); 1 (B) May-Aug Boli

(n not reported)

Ludwig et al. 

1989

L. Huron 

and Ontario

3-41 (F) Apr/MayJuly/AugPellets (n = 1595)

L. Ontario, 

Little Galloo

5 (F) Apr-July Pellets (n = 938)

Lake Huron, 

Bustard Is.

40 (F) May-Aug Pellets (n = 144)

Lake Huron, 44 (F) Pellets (n = 36)

Bustard Is. 6 (F)

June 20, 1994

Boli (n = 47)

Neuman et al. 

1997

Spatial, 

Diet 

Assessment

4 (N) Apr-Sept Pellets (n = 2939)

8 (N) April Pellets (n = 742)

Little Galloo 

Is., Lake 

Ontario 1 (N) Sept Pellets (n = 301)

Johnson et al. 

1999

Temporal

5 (N); 4 (B) Apr-Oct Stomach contents 

(n = 373)

< 0.1 (N,B) Sept 14-Oct 6 Stomach contents 

(n not reported)

Les 

Chenaux Is., 

n. Lake 

Huron

42 (N); 24 (B) Aug 2-19 Stomach contents 

(n not reported)

Belyea et al. 

1999

Temporal

Apostle Is., 

L. Superior 

(Gull Island)

21.8 (F) May-Sept / Oct 

1984

Pellets, Boli 

(n = 150)

Apostle Is., 

L. Superior 

(Eagle Is.)

58 (F) May-Sept / Oct 

1984

Pellets

(n not reported)

Craven and 

Lev 1987

Spatial; 

Temporal;

Diet 

assessment 

method

Apostle Is., 

L. Superior 

(Gull Island)

1.8 (F) May-Sept / Oct 

1985

Boli

(n not reported)

Matteson et 

al. 1999
FINAL DRAFT Part III. Diet 220

Lake(s) / 

Island

% in diet Time Assessment 

Method

Source Variation

TROUT-PERCH

Lakes 

Huron and 

Ontario

3 - 41 (F) May-Aug Pellets (n = 1595)

Lake 

Ontario, 

Little Galloo

16 (F) Apr-July Pellets (n = 938)

Lake Huron, 

Bustard Is.

28 (F) May-Aug Pellets (n=144)

Neuman et al. 

1997

Spatial

2 (N) Apr-Sept Pellets (n = 2939)

6 (N) April Pellets (n = 276)

Little Galloo 

Is., Lake 

Ontario 0 (N) Sept Pellets (n = 301)

Johnson et al. 

1999

Temporal

Apostle Is., 

L. Superior 

(Gull Island)

9.5 (F) May-Sept / Oct Pellets, Boli 

(n = 150)

Apostle Is., 

L. Superior 

(Eagle I.)

1.8 (F) May-Sept / Oct Pellets

(n not reported)

Craven and 

Lev 1987

Spatial, 

Diet 

assessment

?

WHITE SUCKER

L. Huron, 

Michigan 

and Superior

< 1 (N); 7 (B) May-Aug Boli

(n not reported)

Ludwig et al. 

1989

L. Huron 

and Ontario

1 - 49 (F) May-Aug Pellets (n = 1595)

L. Ontario, 

Little Galloo

1 (F) Apr-July Pellets (n = 938)

Lake Huron, 

Bustard Is.

36 (F) May-Aug Pellets (n = 144)

Neuman et al. 

1997

Spatial

0.2 (N); 4.8 

(B)

Apr-Oct Stomach contents 

(n = 373)

2 (N); 12 (B) Apr 17-May 16 Stomach contents 

(n not reported)

Les 

Chenaux Is., 

n. Lake 

Huron

< 0.1 (N,B) May 17-Jul 3 Stomach contents 

(n not reported)

Belyea et al. 

1999

Temporal
FINAL DRAFT Part III. Diet 221

Table 14. Great Lakes: Occurrence of Salmonids in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

F = percent of samples which contain at least one specimen of the taxon of interest

Lake(s) / 

Island

% in diet Time Assessment 

Method

Source Variation

COREGONUS SP. (LAKE WHITEFISH, LAKE HERRING)

L. Huron, 

Michigan 

and Superior

< 1 (N, B) May-Aug Boli

(n not reported)

Ludwig et al. 

1989

L. Huron 

and Ontario

0.3 - 22 (F) May-Aug Pellets (n = 1595)

L. Ontario, 

Little Galloo

0.3 (F) Apr-July Pellets (n = 938)

Lake Huron, 

Bustard Is.

0.7 (F) May-Aug Pellets (n = 144)

Neuman et al. 

1997

Spatial

Apostle Is., 

L. Superior 

(Gull Island)

1.6 (F) May-Sept 

/ Oct

Pellets, Boli 

(n not reported)

Apostle Is., 

L. Superior 

(Eagle I.)

2.7 (F) May-Sept 

/ Oct

Pellets

(n not reported)

Craven and 

Lev 1987

LAKE TROUT

L. Huron, 

Michigan 

and Superior

< 1 (N,B) May-Aug Boli

(n not reported)

Ludwig et al. 

1989

Apostle Is., 

L. Superior 

(Gull Island)

1 (F) May-Sept 

/ Oct

Pellets, Boli 

(n not reported)

Apostle Is., 

L. Superior 

(Eagle I.)

2 (F) May-Sept 

/ Oct

Pellets

(n not reported)

Craven and 

Lev 1987

L. Ontario, 

E. Basin1

0.2 (N) Apr-mid 

July

Pellets (n=4,848) Ross and 

Johnson 1999a

Salmonidae (BLOATER, LAKE WHITEFISH, ROUND WHITEFISH, SPLAKE)

0.1 (N); 0.9 (B) Apr-Oct Stomach contents 

(n = 373)

< 0.1 (N,B) Apr 17-

May 16

Stomach contents 

(n not reported)

Les 

Chenaux Is., 

n. Lake 

Huron

0.2 (N); 1.4 (B) Jul 4-

Aug 1

Stomach contents 

(n not reported)

Belyea et al. 

1999

1 data provided on % composition of salmonines in diet, but almost all identified were stocked lake trout.
FINAL DRAFT Part III. Diet 222

Table 15. Great Lakes: Locally important fish species in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

F = percent of samples which contain at least one specimen of the taxon of interest

Lake(s) / 

Island

% in diet Time Assessment 

Method

Source Variation

BURBOT

Apostle Is., 

L. Superior 

(Gull Island)

15.5 (F) May-Sept / Oct, 

1984

Pellets, Boli 

(n = 150)

Apostle Is., 

L. Superior 

(Eagle I.)

4 (F) May-Sept / Oct, 

1984

Pellets

(n not reported)

Craven and 

Lev 1987

Spatial; 

Temporal; 

Diet 

assessment 

method

Apostle Is., 

Lake 

Superior 

(Gull island

9.3 (F) May-Sept / Oct, 

1985

Boli

(n not reported)

Matteson et 

al. 1999

FRESHWATER DRUM

W. Lake 

Erie

1 (N); 33 (B); 

34 (F)

Apr-Oct Stomach contents 

(n = 302)

Bur et al. 

1999

LAKE / NORTHERN CHUB

Apostle Is., 

L. Superior 

(Gull Island)

11 (F) May-Sept / Oct 

1984

Pellets, Boli 

(n = 150)

Apostle Is., 

L. Superior 

(Eagle I.)

6 (F) May-Sept / Oct Pellets

(n not reported)

Craven and 

Lev 1987

Spatial; 

Temporal; 

Diet 

assessment 

method

Apostle Is., 

L. Superior 

(Gull Island)

2.8 (F) May-Sept / Oct 

1985

Boli

(n not reported)

Matteson et 

al. 1999
FINAL DRAFT Part III. Diet 223

Table 16. Southern U.S.: Occurrence of Shad in the diet of Double–crested Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

F = percent of samples which contain at least one specimen of the taxon of interest

Location % in diet Time Assessment 

Method

Source Variation

GIZZARD SHAD

TX, inland 

reservoirs

9.9 (N);

14.3 (B)

Nov 1986 –

Mar 1987

Stomach contents

(n = 420)

Campo et al 

1993

THREADFIN SHAD

TX, inland 

reservoirs

69.3 (N);

11.8 (B)

Nov 1986 –

Mar 1987

Stomach contents

 (n = 420)

Campo et al 

1993

SHAD SP. (GIZZARD, THREADFIN)

Lake 

Beulah, MS

89.4 (F); 93.4 

(N)

Dec 1996 –

Mar 1997

Stomach contents

(n = 123)

Lake Eufala, 

AL

70.6 (F);

77.8 (N)

Mar-Apr 1997 Stomach contents

(n = 51)

Glahn et al. 

1998

Spatial

100 (F, B) Oct 1987, 1988 Stomach contents

(n = 5)

0 (F, B) Jan 1988, 1989 Stomach contents

(n = 12)

2 (F); 0.6 (B) Mar 1988, 1989 Stomach contents

(n = 49)

MS Delta 

Region, 

Catfish 

Farms

9.1 (F); 5.3 (B) Oct – Mar 

1987/88

Oct – Mar 

1988/89

Stomach contents

(n = 66)

40.2 (F);

40.8 (B)

Winter 1989-90 Stomach contents

(n = 204)

MS Delta 

Region, 

Night 

Roosts

47.5 (F);

44 (B)

Winter 1990-91 Stomach contents

(n = 257)

Glahn et al. 

1995

Temporal, 

Spatial

Central and 

se. AR, 

aquaculture 

facilities

71.6 (F) Oct 18-Dec 5 

1988

Stomach contents

(n = 135)

Bivings et al. 

1989
FINAL DRAFT Part III. Diet 224

Table 17. Southern U.S.: Occurrence of Catfish in the diet of Double–crested Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

F = percent of samples which contain at least one specimen of the taxon of interest 

V = percent volume is equivalent to % biomass, except that quantities are measured 

volumetrically

Location % in diet Time Assessment 

Method

Source Variation

CATFISH

TX, inland 

reservoirs

1.1 (N);

9.6 (B)

Nov 1986 –

Mar 1987

Stomach contents

(n = 420)

Campo et al 1993

Lake 

Beulah, MS

8.9 (F); 1.8 (N) Dec 1996 –

Mar 1997

Stomach contents

(n = 123)

Lake Eufala, 

AL

29.4 (F);

11.4 (N)

Mar-Apr 1997 Stomach contents

(n = 51)

Glahn et al. 1998 Spatial

0 (F, B) Oct 1987, 88 Stomach contents

(n = 5)

50 (F);

48.6 (B)

Jan 1988, 89 Stomach contents

(n = 12)

91 (F); 95 (B)1 Mar 1988, 89 Stomach contents

(n = 49)

MS Delta 

Region, 

Catfish 

Farms

77.3 (F);

85.3 (B)1

Oct – Mar 

1987/88

Oct – Mar 

1988/89

Stomach contents

(n = 66)

55.4 (F); 50.7 

(B)

Winter 1989/90 Stomach contents

(n = 204)

MS Delta 

Region, 

Night 

Roosts

54.8 (F); 50.1 

(B)

Winter 1990/91 Stomach contents

(n = 257)

Glahn et al. 1995 Temporal, 

Spatial

Central and 

se. AR, 

aquaculture 

facilities

6.8 (F) Oct 18 - Dec 5 

1988

Stomach contents

(n = 135)

Bivings et al. 1989

1 Because 74 % of the samples was obtained in March, and the diet from March collections was 

almost all catfish, the disproportional sampling appeared to skew the overall diet towards 

catfish.
FINAL DRAFT Part III. Diet 225

Table 18. Southern U.S.: Occurrence of Centrarchids in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

F = percent of samples which contain at least one specimen of the taxon of interest

Location % in diet Time Assessment 

Method

Source Variation

SUNFISH

TX, inland 

reservoirs

7.8 (N);

15 (B)1

Nov 1986 –

Mar 1987

Stomach contents

(n = 420)

Campo et al. 

1993

Cent. and se. 

AR, aqua. 

facilities

12.9 (F)1 Oct 18 – Dec 5 

1988

Stomach contents

(n = 135)

Bivings et al. 

1989

Lake 

Beulah, MS

17.3 (F);

3.3 (N)

Dec 1996 –

Mar 1997

Stomach contents

(n = 123)

Lake Eufala, 

AL

17.6 (F);

3.7 (N)

Mar-Apr 1997 Stomach contents

(n = 51)

Glahn et al. 

1998

Spatial

Centrarchidae

Lake Eufala, 

AL

31.7 (F); 6.8 

(N)

Mar-Apr 1997 Stomach contents

(n = 51)

Glahn et al. 

1998

BREAM2

0 (F,B) Oct 1987-88 Stomach contents

(n = 5)

50 (F); 51.4 

(B)

Jan 1988-89 Stomach contents

(n = 12)

8 (F); 3.5 (B) Mar 1988-89 Stomach contents

(n = 49)

MS Delta 

Region, 

Catfish 

Farms

15.2 (F);

9.4 (B)

Oct – Mar 

1987/88

Oct – Mar 

1988/89

Stomach contents

(n = 66)

12.3 (F);

5.2 (B)

Winter 1989-90 Stomach contents

(n = 204)

MS Delta 

Region, 

Night 

Roosts

11.7 (F);

4.5 (B)

Winter 1990-91 Stomach contents

(n = 257)

Glahn et al. 

1995

Temporal, 

Spatial

1 > 47 % bluegill.

2 Includes sunfish and crappie species.
FINAL DRAFT Part III. Diet 226

Table 19. Southern U.S.: Locally important fish species in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

Location % in diet Time Assessment 

Method

Source Variation

BLUE TILAPIA

TX, inland 

reservoirs

5.2 (N);

18.2 (B)

Nov 1986 –

Mar 1987

Stomach contents

(n = 420)

Campo et al. 

1993

Table 20. Atlantic Region: Occurrence of Marine bottom fish in the diet of Double–crested 

Cormorants.

B = biomass of taxon of interest as a percent of total sample biomass

F= percent of samples which contain at least one specimen of the taxon of interest 

N = number of specimens of the taxon of interest as a percent of all specimens in sample

R=relative importance, sum of % of each prey species divided by number of stomachs analyzed

Location % in diet Time Assessment 

Method

Source Variation

COD SP. (92% ATLANTIC COD)

Eastern Shore 

rivers, Nova 

Scotia

37.7 (F);

43.1 (N)

Breeding season, 

1979-80

Pellets, Boli

(n not reported)

Milton et al. 

1995

SCULPIN SP.

Eastern Shore 

rivers, Nova 

Scotia

33.6 (F);

17.5 (N)

Breeding season, 

1979-80

Pellets, Boli

(n not reported)

Milton et al. 

1995

3.4 (F) Late Apr 1986-

88

(n = 66)

0 (F) May 1986-88 (n = 244)

5.3 (F) Early June 1986 (n = 19)

0 (F) Late Apr 1993 (n = 7)

2.6 (F); 1.5 (N) May 1992-93 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

0.85 (F);

0.5 (N)

Jun 1992-93 (n = 73)

Blackwell et 

al. 19971

Passamaquody 

Bay, New 

Brunswick

53.5 (R) May 19-31 1986 Stomach contents

(n = 117)

Kehoe 1987 

(in Cairns 

1998)
FINAL DRAFT Part III. Diet 227

Location % in diet Time Assessment 

Method

Source Variation

8 (F); 2.6 (N) Jun 4-Jul 14, 

1994

Estuary of St. Boli (n = 125)

Lawrence 

River, Québec 4 (F); 1.5 (N) Jun 4-Jul 14, 

1995

Boli (n = 301)

4.3 (F); 1 (N) Jun 28-Jul 20, 

1995

Gulf of St. Boli (n = 69)

Lawrence 

River, St. Mary 

Is., Québec

2.5 (F); 0.5 (N) Jun 28-Jul 20, 

1996

Boli (n = 118)

Rail and 

Chapdelaine 

1998

Temporal; 

Spatial

Bays of Prince 

Edward Island

11.9 (B) Breeding season, 

1985

Stomach contents 

(n not reported)

Birt et al. 

(1987)

GUNNEL SP.

40 (F);

19.9 (N)

Jun 4-Jul 14, 

1994

Estuary of St. Boli (n = 125)

Lawrence 

River, Québec 13.6 (F);

6.7 (N)

Jun 4-Jul 14, 

1995

Boli (n = 301)

10.1 (F);

3.7 (N)

Jun 28-Jul 20, 

1995

Gulf of St. Boli (n = 69)

Lawrence 

River, St. Mary 

Is., Québec

10.2 (F);

3.6 (N)

Jun 28-Jul 20, 

1996

Boli (n = 118)

Rail and 

Chapdelaine 

1998

Temporal; 

Spatial

E. Shore rivers, 

Nova Scotia

12.3 (F);

16.3 (N)

Breeding season, 

1979-1980

Pellets, Boli

(n not reported)

Milton et al. 

1995

Passamaquody 

Bay, New 

Brunswick

4.9 (R) May 19-31 1986 Stomach contents 

(n =117)

Kehoe 1987 

(in Cairns 

1998)

Penobscot R., 

ME, below 

head of tide 

1.7 (F) Early Jun 1992 –

1993

(n = 58) Blackwell et 

al. 1997

CUNNER

Eastern Shore 

rivers, Nova 

Scotia

12.3 (F);

4.1 (N)

Breeding season, 

1979-1980

Pellets, Boli

(n not reported)

Milton et al. 

1995

Penobscot R., 

ME, below 

head of tide

1.3 (F); 1 (N) May 1992 -1993 (n = 68) Blackwell et 

al. 1997

Bays of Prince 

Edward Island

35.5 (B) Breeding season, 

1985

Stomach contents 

(n not reported)

Birt et al. 

(1987)
FINAL DRAFT Part III. Diet 228

Location % in diet Time Assessment 

Method

Source Variation

FLATFISHES (FLOUNDER SP.)

6.9 (F) Late Apr 1986-

88

(n = 66)

0.3 (F) May 1986-88 (n = 244)

10.5 (F) Early June 1986 (n = 19)

0 Late Apr 1993 (n = 7)

1.7 (F) May 1992-93 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

0.85 (F) Jun 1992-93 (n= 73)

Blackwell et 

al. 1997

Temporal

Eastern Shore 

rivers, Nova 

Scotia

4.8 (F); 2.1 (N) Breeding season, 

1979-80

Pellets, Boli

(n not reported)

Milton et al. 

1995

Passamaquody 

Bay, New 

Brunswick

3.2 (R) May 19-31 1986 Stomach contents 

(n =117)

Kehoe 1987 

(in Cairns 

1998)

6.4 (F); 2.6 (N) Jun 4-Jul 14, 

1994

Estuary of St. Boli (n = 125)

Lawrence 

River, Québec 25.9 (F); 9.7 

(N)

Jun 4-Jul 14, 

1995

Boli (n = 301)

10.1 (F); 2.2 

(N)

Jun 28-Jul 20, 

1995

Gulf of St. Boli (n = 69)

Lawrence 

River, St. Mary 

Is., Québec

9.3 (F); 2.0 (N) Jun 28-Jul 20, 

1996

Boli (n = 118)

Rail and 

Chapeldaine 

1998

Temporal; 

Spatial

Bays of Prince 

Edward Island

18.1 (B) Breeding season, 

1985

Stomach contents 

(n not reported)

Birt et al. 

(1987)

1 The study by Blackwell et al. (1997) reported frequency of occurrence and % volume of prey 

taxa for cormorants at and away from dams above the head of tide. It also reported the data in 

biweekly periods. To simplify the presentation of the data, we combined numbers reported at 

and away from dams above the head of tide, and we lumped numbers for biweekly periods 

(early May, late May, etc.). The categories we did not include (particularly “at” and “away 

from dams”) are important. For example, the ability of salmonids to detect and avoid 

predators has been shown to be negatively affected by dams (trying to pass them and 

associated migration delays) (Raymond 1979; Mesa et al. 1994), and cormorants were shown 

to select for use of areas adjacent to mainstem dams on the Penobscot River (Blackwell and 

Krohn 1997). The categories we didn’t include further indicate the influence of spatial and 

seasonal variation of prey populations on cormorant habitat use and diet composition.
FINAL DRAFT Part III. Diet 229

Table 21. Atlantic Region: Occurrence of Estuarine / Diadromous fishes in the diet of 

Double–crested Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

B = biomass of taxon of interest as a percent of total sample biomass

F = percent of samples which contain at least one specimen of the taxon of interest 

R = relative importance, sum of % of each prey species divided by number of stomachs analyzed

Location % in diet Time Assessment 

Method

Source Variation

ALEWIFE

0 (F) Late April 

1986-88

Stomach contents 

(n = 15)

2.4 (F) May 1986-88 (n = 127)

Penobscot 

River, ME, 

above head of 

tide 2.6 (F) May 1992-93 (n = 36)

34.5 (F) Late Apr 1986-

88

(n = 66)

7.8 (F) May 1986-88 (n = 244)

0 (F) Early June 1986 (n = 19)

14.3 (F); 1 (N) Late Apr 1993 (n = 7)

9.4 (F); 11 (N) May 1992-93 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

10.35 (F);

7.5 (N)

Jun 1992-93 (n = 73)

Blackwell et 

al. 1997

Temporal; 

Spatial

Passamaquody 

Bay, New 

Brunswick

12.70 (R) May 19-31 

1986

Stomach contents 

(n = 117)

Kehoe 1987 

(in Cairns 

1998)

Eastern Shore 

rivers, Nova 

Scotia

3.4 (F); 1.6 (N) Breeding 

season,

1979 – 1980

Pellets, Boli

(n not reported)

Milton et al. 

1995

AMERICAN EEL

0 (F) Late April

1986 – 1988

Stomach contents 

(n = 15)

13 (F) May 1986-1988 (n = 127)

Penobscot 

River, ME, 

above head of 

tide 16.7 (F) May 1992-1993 (n = 36)

3 (F) Late Apr 1986 –

1988

(n = 66)

3.9 (F) May 1986-1988 (n = 244)

10.5 (F) Early June 1986 (n = 19)

0 (F) Late Apr 1993 (n = 7)

4.3 (F) May 1992-1993 (n = 68)

Penobscot

River, ME, 

below head of 

tide 

11.1 (F) Jun 1992-1993 (n = 73)

Blackwell et 

al. 1997

Temporal; 

Spatial

Eastern Shore 

rivers, Nova 

Scotia

3.7 (F); 2 (N) Breeding 

season, 1979-80

Pellets, Boli (n not 

reported)

Milton et al. 

1995
FINAL DRAFT Part III. Diet 230

Location % in diet Time Assessment 

Method

Source Variation

RAINBOW SMELT

51.7 (F) Late Apr 1986 –

1988

(n = 66)

7.3 (F) May 1986-1988 (n = 244)

0 (F) Early June 1986 (n = 19)

0 Late Apr 1993 (n = 7)

6.5 (F); 2.5 (N) May 1992-1993 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

0 Jun 1992-1993 (n = 73)

Blackwell et 

al. 1997

Temporal

95.2 (N); 95.9 

(B); 71.4 (F)

May 8-20 1993 Stomach contents 

(n = 14)

Lower Dunk 

River, Prince 

Edward Island 94.5 (B);

94 (F)

May 2-10 1995 Stomach contents 

(n = 151)

Scales Pond1, 

Prince Edward 

Island

89.2 (F) May 8-10 1995 Stomach contents 

(n = 65)

Hill et al. 

1997

4.8 (F); 2 (N) Jun 4-Jul 14, 

1994

Estuary of St. Boli (n = 125)

Lawrence 

River, Québec 3 (F); 1.7 (N) Jun 4-Jul 14, 

1995

Boli (n = 301)

Rail and 

Chapdelaine 

1998

Temporal; 

Spatial

Passamaquody 

Bay, New 

Brunswick

3.7 (R) May 19-31, 

1986

Stomach contents 

(n = 117)

Kehoe 1987 

(in Cairns 

1998)

Eastern Shore 

rivers, Nova 

Scotia

0.8 (F); 0.2 (N) Breeding 

season,

1979-1980

Pellets, Boli

(n not reported)

Milton et al. 

1995

STICKLEBACK

Scales Pond1, 

PEI

75.4 (F) May 8-10 1995 Stomach contents 

(n = 65)

Hill et al. 

1997

Spatial

Passamaquody 

Bay, New 

Brunswick

2.6 (R) May 19-31 

1986

Stomach contents 

(n = 117)

Kehoe 1987 

(in Cairns 

1998)

Eastern Shore 

rivers, Nova 

Scotia

0.4 (F); 0.2 (N) Breeding 

season,

1979-1980

Pellets, Boli

(n not reported)

Milton et al. 

1995

BLUEBACK HERRING

0 Late Apr 1993 (n = 7)

6 (F) May 1992-1993 (n = 68)

Penobscot R., 

ME, below 

head of tide 12 (F); 24 (N) Jun 1992-1993 (n = 73)

Blackwell et 

al. 1997

Temporal
FINAL DRAFT Part III. Diet 231

Location % in diet Time Assessment 

Method

Source Variation

HERRING

0 (F) Late Apr

1986-1988

(n = 66)

1.4 (F) May 1986-1988 (n = 244)

0 (F) Early June 1986 (n = 19)

14.3 (F); 1 (N) Late Apr 1993 (n = 7)

11.95 (F); 

10.25 (N)

May 1992-1993 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

42.85 (F);

10 (N)

Jun 1992-1993 (n = 73)

Blackwell et 

al. 1997

Temporal

ATLANTIC TOMCOD

1.5 (F) Late Apr

1986-1988

(n = 66)

0 (F) May 1986-1988 (n = 244)

0 (F) Early June 1986 (n = 19)

0 Late Apr 1993 (n = 7)

6 (F); 4 (N) May 1992-1993 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

12.6 (F); 3 (N) Jun 1992-1993 (n = 73)

Blackwell et 

al. 1997

Temporal

1 Scales Pond is a freshwater impoundment on the Lower Dunk River.
FINAL DRAFT Part III. Diet 232

Table 22. Atlantic Region: Occurrence of Freshwater fishes in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

F = percent of samples which contain at least one specimen of the taxon of interest

R = relative importance, sum of % of each prey species divided by number of stomachs analyzed

Location % in diet Time Assessment 

Method

Source Variation

SMALLMOUTH BASS 

36.1 (F) Late April 

1986-1988

Stomach contents 

(n = 15)

8.1 (F) May 1986-1988 (n = 127)

Penobscot 

River, ME, 

above head of 

tide 6.3 (F) May 1992-1993 (n = 36)

0 (F) Late Apr

1986-1988

(n = 66)

0.3 (F) May 1986-1988 (n = 244)

0 (F) Early June 1986 (n = 19)

42.9 (F) Late Apr 1993 (n = 7)

0 (F); 0 (N) May 1992-1993 (n = 68)

Penobscot 

River, ME, 

below head of

tide 

0 (F); 0 (N) Jun 1992-1993 (n = 73)

Blackwell et 

al. 1997

Temporal; 

Spatial

YELLOW PERCH 

0 (F) Late April 

1986-1988

Stomach contents 

(n = 15)

9.53 (F) May 1986-1988 (n = 127)

Penobscot 

River, ME, 

above head of 

tide 6.8 (F) May 1992-1993 (n = 36)

0 (F) Late Apr

1986-1988

(n = 66)

0.3 (F) May 1986-1988 (n = 244)

0 (F) Early June 1986 (n = 19)

28.6 (F) Late Apr 1993 (n = 7)

0 (F, N) May 1992-1993 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

0 (F, N) Jun 1992-1993 (n = 73)

Blackwell et 

al. 1997

Temporal; 

Spatial

Passamaquody 

Bay, New 

Brunswick

0.10 (R) May 19-31 

1986

Stomach contents 

(n = 117)

Kehoe 1987 

(in Cairns 

1998)

Eastern Shore 

rivers, Nova 

Scotia

0.8 (F); 1.1 (N) Breeding 

season,

1979-1980

Pellets, Boli

(n not reported)

Milton et al. 

1995
FINAL DRAFT Part III. Diet 233

Location % in diet Time Assessment 

Method

Source Variation

PUMPKINSEED

19.45 (F) Late April 

1986-1988

Stomach contents 

(n = 15)

0.47 (F) May 1986-1988 (n = 127)

Penobscot 

River, ME, 

above head of 

tide 0 (F) May 1992-1993 (n = 36)

0 (F) Late Apr

1986-1988

(n = 66)

0.65 (F) May 1986-1988 (n = 244)

0 (F) Early June 1986 (n = 19)

0 (F) Late Apr 1993 (n = 7)

0 (F, N) May 1992-1993 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

2.6 (F) Jun 1992-1993 (n = 73)

Blackwell et 

al. 1997

Temporal; 

Spatial

CYPRINID SP.

11.1 (F) Late April 

1986-1988

Stomach contents 

(n = 15)

9.8 (F) May 1986-1988 (n = 127)

Penobscot 

River, ME, 

above head of 

tide 11.3 (F) May 1992-1993 (n = 36)

1.5 (F) Late Apr

1986-1988

(n = 66)

1.3 (F) May 1986-1988 (n = 244)

0 (F) Early June 1986 (n = 19)

0 (F) Late Apr 1993 (n = 7)

0 (F, N) May 1992-1993 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

0.85 (F) Jun 1992-1993 (n = 73)

Blackwell et 

al. 1997

Spatial
FINAL DRAFT Part III. Diet 234

Table 23. Atlantic Region: Occurrence of Salmonids in the diet of Double–crested 

Cormorants.

N=number of specimens of the taxon of interest as a percent of all specimens in sample

B=biomass of taxon of interest as a percent of total sample biomass

F=percent of samples which contain at least one specimen of the taxon of interest 

R=relative importance, sum of % of each prey species divided by number of stomachs analyzed

Location % in diet Time Assessment 

Method

Source Variation

ATLANTIC SALMON

0 (F) Late April 

1986-1988

Stomach contents 

(n = 15)

48.3 (F) May 1986-1988 (n = 127)

Penobscot 

River, ME, 

above head of 

tide 70.1 (F);

15.5 (N)

May 1992-1993 (n = 36)

0 (F) Late Apr

1986-1988

(n = 66)

48.26 (F) May 1986-88 (n = 244)

5.3 (F) Early June 1986 (n = 19)

0 Late Apr 1993 (n = 7)

22 (F); 27 (N) May 1992-1993 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

0.85 (F);

0.5 (N)

Jun 1992-1993 (n = 73)

Blackwell 

et al. 1997

Temporal; 

Spatial

4.8 (N); 4.1 

(B); 7.1 (F)

May 8-20 1993 Stomach contents 

(n = 14)

Lower Dunk 

River, PEI

5.3 (F);

4.2 (B);

May 2-10 1995 Stomach contents 

(n = 151)

Hill et al. 

1997

Passamaquody 

Bay, New 

Brunswick

0.10 (R) May 19-31 

1986

Stomach contents 

(n =117)

Kehoe 1987 

(in Cairns 

1998)

BROOK TROUT

Lower Dunk 

River, PEI

1.3 (F); 1.2 (B) May 2-10 1995 Stomach contents 

(n = 151)

Hill et al. 

1997

SALMONIDS (mostly ATLANTIC SALMON)

13.8 (F)1 Apr-Jun

1980-1981

Stomach contents 

(n = 65)

Eastern Shore 

rivers, Nova 

Scotia 39.8 (N) Apr-Jun

1986-1987

Stomach contents 

(n = 83)

Milton et al. 

1988

1 Only wild salmon stocks were available in 1980-1981 because hatchery stocks were not 

released in sampled rivers (Milton et al. 1995).
FINAL DRAFT Part III. Diet 235

Table 24. Atlantic Region: Occurrence of Pelagic fishes in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

F = percent of samples which contain at least one specimen of the taxon of interest

Location % in diet Time Assessment 

Method

Source Variation

AMERICAN SAND LANCE

Estuary of St. 0 (F); 0 (N) Jun 4-Jul 14, 1994 Boli (n = 125)

Lawrence R., 

Québec

6.3 (F); 6.5 (N) Jun 4-Jul 14, 1995 Boli (n = 301)

68.1 (F); 85.3 (N) Jun 28-Jul 20, 

1995

Gulf of St. Boli (n = 69)

Lawrence R., 

St. Mary Is., 

Québec

52.5 (F); 59.3 (N) Jun 28-Jul 20, 

1996

Boli (n = 118)

Rail and 

Chapdelaine 

1998

Temporal; 

Spatial

E. Shore 

rivers, NS

5.2 (F); 6.2 (N) Breeding season,

1979-1980

Pellets, Boli

(n not reported)

Milton et al. 

1995

ATLANTIC HERRING

E. Shore 

rivers, NS

7.8 (F); 4.6 (N) Breeding season,

1979-1980

Pellets, Boli

(n not reported)

Milton et al. 

1995

Estuary of St. 1.6 (F); 0.4 (N) Jun 4-Jul 14, 1994 Boli (n = 125)

Lawrence R., 

Québec

14.3 (F); 4.8 (N) Jun 4-Jul 14, 1995 Boli (n = 301)

1.4 (F); 0.2 (N) Jun 28-Jul 20, 

1995

Gulf of St. Boli (n = 69)

Lawrence R., 

St. Mary Is., 

Québec

7.6 (F); 1.4 (N) Jun 28-Jul 20, 

1996

Boli (n = 118)

Rail and 

Chapdelaine 

1998

Temporal; 

Spatial

CAPELIN

Estuary of St. 48.8 (F); 41.9 (N) Jun 4-Jul 14, 1994 Boli (n = 125)

Lawrence R., 

Québec

47.5 (F); 53.8 (N) Jun 4-Jul 14, 1995 Boli (n = 301)

7.2 (F); 1.5 (N) Jun 28-Jul 20, 

1995

Gulf of St. Boli (n = 69)

Lawrence R., 

St. Mary Is., 

Québec

55.9 (F); 30.9 (N) Jun 28-Jul 20, 

1996

Boli (n = 118)

Rail and 

Chapdelaine 

1998

Temporal; 

Spatial

ATLANTIC MENHADEN

0 (F) Late Apr 1986-88 (n = 66)

0 (F) May 1986-88 (n = 244)

0 (F) Early June 1986 (n = 19)

0 (F) Late Apr 1993 (n = 7)

5.6 (F) May 1992-93 (n = 68)

Penobscot 

River, ME, 

below head 

of tide 

15.3 (F) Jun 1992-93 (n = 73)

Blackwell et 

al. 1997

Temporal
FINAL DRAFT Part III. Diet 236

Table 25. Atlantic Region: Occurrence of Crustaceans in the diet of Double–crested 

Cormorants.

N = number of specimens of the taxon of interest as a percent of all specimens in sample

F = percent of samples which contain at least one specimen of the taxon of interest 

R = relative importance, sum of % of each prey species divided by number of stomachs analyzed

Location % in diet Time Assessment 

Method

Source Variation

SAND SHRIMP

0 (F) Late Apr 1993 (n = 7)

43.3 (F) May 1992-93 (n = 68)

Penobscot 

River, ME, 

below head of 

tide

56.7 (F) Jun 1992-93 (n = 73)

Blackwell et 

al. 1997

Temporal

SHRIMP SP.

17.2 (F) Late Apr

1986-1988

(n = 66)

7.8 (F) May 1986-88 (n = 244)

63.2 (F) Early June 1986 (n = 19)

0(F) Late Apr 1993 (n = 7)

5.6 (F) May 1992-93 (n = 68)

Penobscot 

River, ME, 

below head of 

tide 

0 (F) Jun 1992-93 (n = 73)

Blackwell et 

al. 1997

Temporal

5.6 (F); 1.3 (N) Jun 4-Jul 14, 

1994

Estuary of St. Boli (n = 125)

Lawrence R., 

Québec 5 (F); 2.3 (N) Jun 4-Jul 14, 

1995

Boli (n = 301)

4.3 (F); 1.2 (N) Jun 28-Jul 20, 

1995

Gulf of St. Boli (n = 69)

Lawrence R., 

St. Mary Is., 

Québec

3.4 (F); 0.6 (N) Jun 28-Jul 20, 

1996

Boli (n = 118)

Rail and 

Chapdelaine 

1998

OTHER CRUSTACEANS

9.6 (F); 3.8 (N) Jun 4-Jul 14, 

1994

Estuary of St. Boli (n = 125)

Lawrence 

River, Québec 0.3 (F); 0.2 (N) Jun 4-Jul 14, 

1995

Boli (n = 301)

Rail and 

Chapdelaine 

1998

Temporal

Passamaquody 

Bay, New 

Brunswick

6.3 (R) May 19-31 

1986

Stomach contents 

(n =117)

Kehoe 1987 

(in Cairns 

1998)
FINAL DRAFT Part IV. Impacts to Vegetation and Colonial Waterbirds 237

PART IV. IMPACTS TO VEGETATION AND COLONIAL WATERBIRDS

Through their normal nesting activities, DCCOs have been shown to impact vegetation 

on a local level. These impacts affect not only plants, but can affect the animals that depend on 

them, particularly other colonial waterbirds. Because these impacts have undoubtedly occurred 

throughout the evolutionary histories of co-occurring animals and plants, such interactions 

should be viewed as a natural phenomenon within this particular system. However, in humanaltered ecosystems where habitat is limited, these interspecific dynamics may prove deleterious 

to some species. Additionally, humans may be impacted when DCCOs kill trees valued for 

aesthetics or timber.

Generally, impacts are most concentrated at breeding colonies, but can also occur at 

roosting sites. Concern over impacts to vegetation and other colonial waterbirds has been great 

enough to initiate control programs on islands in the eastern basin of Lake Ontario, Lake 

Champlain, VT (USFWS 1999a,b), the St. Lawrence River Estuary (Bédard et al. 1995), and 

Oneida Lake (R. Miller, pers. comm.). Additionally, there has been much speculation about the 

impacts of DCCOs on other avian species (see also Managing Impacts to Vegetation and Habitat

in PART V. MANAGEMENT OPTIONS and Appendix I. Perceived Impacts Associated With 

Double-crested Cormorants). However, only a few studies have been conducted to assess the 

actual affects DCCO breeding ecology and behavior have on vegetation and other colonial 

waterbirds.

IMPACTS TO VEGETATION

The most important impact to vegetation ascribed to cormorants is the lethal effect their 

acidic guano has on ground vegetation and eventually nest trees. This effect has been well 

documented at Lake Ontario breeding colonies, including Little Galloo Island, New York, 

Hamilton Harbour, Ontario (Weseloh and Ewins 1994; Moore et al. 1995; Weseloh and Collier 

1995), and on islands in the St. Lawrence River Estuary (Bédard et al. 1995). J. Bédard (pers. 

obs. in Bédard et al.1995) reported that DCCO guano deposited on forest soil causes irreversible 

damage to trees in < 3 years. Damage to island property was reported as “ecologically 

measurable” (however, measurements were not provided by Bédard et al. 1995). Moore et al. 

(1995) reported that DCCOs began nesting at Hamilton Harbour in 1984, and beginning in 1986, 

they nested mostly in top branches of cottonwood trees at Pier 27. Since that time, trees have 

gradually died; they were slowly defoliated and by 1993, only 24 % remained alive. Deposition 

of DCCO guano is presumed to have killed them, and the loss of all cottonwood trees at this site 

is inevitable.

Cormorants also directly destroy vegetation by stripping leaves from trees, and the 

combined weight of the birds and their nests can break branches (Weseloh and Ewins 1994; 

Weseloh and Collier 1995). Additionally, in some areas, such as the islands of western Lake 

Erie, the vegetation is of unusual significance or rare, and large DCCO colonies in such areas 

may significantly impact or destroy plants and or plant communities of special interest (Weseloh 

and Collier 1995). The loss of vegetation can also lead to a reduction in general species richness 

at colony sites (Chapdelaine and Bédard 1995).

Through their nesting and roosting habits DCCOs have undoubtedly always had some 

impact on vegetation. However, human induced environmental changes, such as interruptions in 
FINAL DRAFT Part IV. Impacts to Vegetation and Colonial Waterbirds 238

the natural flow of river systems, dredging operations, erosion, deforestation, etc., may magnify 

these impacts as natural areas and breeding bird habitat become more limited (e.g., Kirsch 1995).

IMPACTS TO AVIAN SPECIES

Two potential effects of DCCOs on other colonial waterbirds is site competition and 

habitat degradation (Shieldcastle et al. in press; Weseloh and Ewins 1994; Moore et al. 1995). 

Birds such as Black-crowned Night-Herons (BCNH) may be affected directly through 

competition for nests and nest-sites (Moore et al. 1995; Jarvie et al. 1999), but thus far this has 

not been documented. In some areas, DCCOs may arrive at islands earlier than other species and 

usurp quality nest sites (USFWS 1999b; Jarvie et al. 1999). However, most impacts to other 

birds appear to occur indirectly through habitat degradation. Because DCCOs defoliate trees, the 

trees become less attractive as nest sites to certain species, e.g. BCNH, which prefer concealed 

areas for nesting (Moore et al. 1995).

In the mid-to-late 1980s at Hamilton Harbour, Ontario, DCCOs and BCNHs were 

observed to nest in the same trees until the trees began losing their leaves and dying. The BCNHs 

eventually moved to sandbar willow habitat, while the DCCOs remained in the trees. During the 

time when DCCOs and BCNHs nested in the same trees, they were distinctly separated spatially; 

of the DCCO nests in trees in 1993, 98 % were built > 2 m above the ground, while 94 % of 

BCNH nests occurred in elevations < 2 m above the ground. Guano and debris falling on BCNHs 

and their nests from DCCO nests positioned higher in the same tree may have also precipitated 

the observed habitat shift (Moore et al. 1995). Habitat surveys in 1993 also indicate that DCCOs 

and BCNHs were coexisting in the colony by utilizing distinctly different vegetation types. 

However, as sandbar willows grow larger they may become suitable for DCCOs, which may lead 

to the eventual displacement of BCNHs at this site; DCCOs have displaced BCNHs from nesting 

sites in shrubs at other Lake Ontario colonies (Moore et al. 1995).

On West Sister Island, Lake Erie, DCCOs were first documented nesting in 1992, and the 

first habitat damage to the island was noted in 1995; major leaf loss occurred on trees with 

cormorant nests. Entire trees were nearly defoliated along the island edge where heavy roosting 

by DCCOs occurs. Increases in DCCO numbers on the island have corresponded with declines in 

BCNHs and Great Blue Herons (GRBH). Shieldcastle et al. (in press) predict that once trees die 

and fall, DCCOs will likely become ground nesters, and GRBHs will be displaced from the 

island. Moore et al. (1995) suggest that a switch to ground nesting by DCCOs at Hamilton 

Harbour may also detrimentally impact Herring Gulls already nesting there, but do not explain 

how.

Tree loss by itself appears to have the greatest effect on subcanopy nesters, such as 

BCNHs. On West Sister Island, however, a factor that complicates assessment of DCCO impact 

on BCNHs is that habitat loss for BCNHs also appears to be occurring through successional 

change. As canopy height increases there appears to be a negative effect on nesting BCNHs; 

declines in this species actually began in 1991, the year before DCCOs were first observed 

nesting on the island (Shieldcastle et al. 1999). At Tommy Thompson Park, Lake Ontario, 

increasing numbers of DCCOs correspond with decreasing numbers of BCNHs on peninsulas A 

and B (Jarvie et al. 1999). However, declines in some individual BCNH subcolonies, especially 

on peninsula B, appear to have occurred in areas distinct from those where DCCOs were 

increasing.

While the ability of some species to switch nesting habitat may facilitate co-existence in a 

colony, switching nesting habitat may entail costs. At Hamilton Harbour, several factors may 
FINAL DRAFT Part IV. Impacts to Vegetation and Colonial Waterbirds 239

affect BCNH reproductive success; all are related to the vulnerability of present nesting sites in 

sandbar willows and on the ground (Moore et al. 1995).

Long-term observations of colonial waterbirds suggest that colonial waterbird colonies 

are dynamic, though some species shift colony sites less frequently than others (Ferren and 

Myers 1998b; F. Cuthbert pers. obs.). Colonies of most species experience habitat degradation 

due to deposition and accumulation of guano, causing cyclical patterns of colonization and 

abandonment, followed by a period during which a colony site will lie “fallow”, and then later be 

re-colonized (Ferren and Myers 1998b). A pattern of use, tree die off due to guano deposition, 

abandonment and eventual re-colonization was documented for DCCOs at a colony at Great 

Lake, North Carolina (Pearson et al. 1959; see PART II. POPULATION DATA AND TRENDS, 

North Carolina summary). As this cyclical pattern of use and abandonment occurs, impacts to 

other colonial waterbirds from DCCOs should be insignificant, especially when viewed over 

longer time periods and wider geographical scales. However, if habitat and alternative quality 

sites are limited, impacts may be significant.

In conclusion, observations suggest DCCOs may displace other waterbirds from 

individual colonies; however, in some situations where this appeared to be the case, further 

investigation identified other factors (e.g. vegetational succession) that may be related to shifts in 

nesting patterns or local declines, especially when these declines begin in areas prior to or 

disjunct from cormorant nesting. While displacement may occur locally, no studies have 

demonstrated regional or population level impacts on other colonial waterbirds. Even at the 

colony level changes in parameters such as reproductive success of birds displaced by DCCOs 

have not been measured. More detailed studies are needed to determine the exact nature of 

competitive interactions between DCCOs and other birds. Additionally, the effects of 

interactions on nesting habitat selection and breeding success of other species needs to be 

examined (Moore et al. 1995).

NEWCASTLE DISEASE

(summarized from Kuiken 1999 unless otherwise noted)

Newcastle disease virus is the causative agent of Newcastle disease, an important disease 

for the poultry industry because it causes devastating epidemics and has far-reaching effects on 

trade in poultry products. The disease can affect virtually all avian species, and is commonly 

passed through bird fecal material, though it can also be passed through other sources, such as 

humans who have been handling infected birds. In Double-crested Cormorants, the disease was 

recognized for the first time in 1975 (Québec, St. Lawrence River Estuary), and may cause high 

mortality of juvenile DCCOs. Typically, birds infected with the disease will have wing or leg 

paralysis, loss of balance, and inflammation of the brain and spinal cord.

Between 1990 – 1997, the disease occurred in DCCOs across the full east-west expanse 

of their range in North America, occurring at least four times. In 1990, the disease was 

associated with high mortality of juveniles in several breeding colonies across western Canada. 

In 1992, a more widespread epidemic occurred, and DCCOs from the interior population were 

broadly affected. Weseloh and Collier (1995) estimated that during this outbreak up to 30 % of 

young DCCOs were killed at several colonies, while. Kuiken (1999) cites rates ranging from 10 

– 90 % mortality of juveniles per breeding colony. Outbreaks in DCCOs occurred again in 1995 

and possibly 1996 (Wildlife Health Centre Newsletter 1997).
FINAL DRAFT Part IV. Impacts to Vegetation and Colonial Waterbirds 240

Prior to 1997, all recognized occurrences had been in populations east of the Rocky 

Mountains, although antibodies to Newcastle Disease were found in eggs from nests in British 

Columbia in 1993 (Wildlife Health Centre Newsletter 1997). In May 1997, personnel at the 

Salton Sea NWR, east of San Diego, reported approximately 1,600 dead nestlings and fledglings 

in a mixed colony of DCCOs and Caspian Terns. Most of the dead were DCCOs. A team from 

the National Wildlife Health Center secured samples for diagnostic examination, and Newcastle 

Disease virus was isolated from the affected tissues (Wildlife Health Centre Newsletter 1997). In 

the same year, the disease was also diagnosed in juvenile DCCOs from colonies at the mouth of 

the Columbia River (Oregon) and from Great Salt Lake (Utah) (L.C. Glaser, pers. comm. in 

Kuiken 1999). Mortality of juveniles in these 3 areas ranged from “not normal” to > 90 %.

There have been no reports of extensive mortality to other wild birds cohabiting with 

DCCOs due to Newcastle disease. However, small numbers of American White Pelicans and 

Ring-billed Gulls sharing colony sites with infected DCCOs have been documented as infected 

with the disease, and in 1992, the widespread epidemic was associated with high mortality in 

American White Pelicans and gulls (Larus spp.). In 1995, the virus was also found in a Caspian 

Tern in Ontario in the same area where infected DCCOs occurred. In 1992, Newcastle disease 

may also have been passed from infected DCCOs to commercial range turkeys in North Dakota; 

these turkeys were located < 7 km from a DCCO colony known to be affected with the disease.

In conclusion, infection in other birds from DCCOs with Newcastle disease has not been 

clearly documented to be a significant problem; however, the potential for DCCOs to contract 

and spread the disease to both wild and domestic birds exists.
FINAL DRAFT Part V. Management Options 241

PART V. MANAGEMENT OPTIONS

The J. Paul Getty Museum, Los Angeles

1)

Figure 88

Detail of Hunting on the Lagoon by Vittore Carpaccio

ca. 1490-1495, oil on panel, 75.4 x 63.8 cm.

INTRODUCTION

Cormorant management to reduce conflicts with human interests has taken place in the 

eastern hemisphere for centuries. Possibly one of the earliest records of cormorant control may 

be the late fifteenth century painting by Vittore Carpaccio, “Hunting on the Lagoon,” (Figure 

88). According to Conniff (1991), the scene depicts Venetian archers in lagoon fish enclosures 

shooting at cormorants with pellets. It is not certain whether these archers are trying to prevent 

depredations to fish stocks or are merely hunting cormorants for food or pleasure, and some 

question whether the archers may actually be shooting at fish rather than cormorants (J. Hatch, 

pers comm.). However, the pellet shot by the foremost archer and about to hit a cormorant in the 
FINAL DRAFT Part V. Management Options 242

head is suggestive of lethal intent towards the cormorants. More solid evidence of cormorant 

control appears a century later (1599) when Queen Elisabeth I brandished cormorants as “pests 

of the crown”; in 1603, the last year of her life, she placed a bounty on them as well (Ludwig et 

al. 1995). Here in North America cormorants have long been accused of negatively impacting 

fish populations (e.g. Wood 1634) and have been persecuted because of their fish-eating habits 

(Lewis 1929; Mendall 1936). However, few details about cormorant management to reduce 

impacts to fisheries prior to 1900 exist. Although cormorants and their eggs were eaten by Native 

Americans and early settlers, and nestlings were used as food for dogs and farm-raised foxes 

(Lewis 1929), we found no evidence of active management to increase or maintain cormorant 

numbers prior to 1900.

As DCCOs began to recover from 19th Century declines during the first half of the 20th

Century (see PART II. POPULATION DATA AND TRENDS), cormorant management focused 

on reducing impacts to fisheries, although impacts to timber resources may also have been of 

concern (e.g. McLeod and Bondar 1953). Documented management efforts from this period into 

the early 1950s included official government programs and private citizen actions. The scale of 

these efforts varied considerably, from small-scale, uncoordinated activities of individual 

fishermen to Maine’s multi-year program along its entire coast.

In response to declines in DCCO populations from the 1950s through the early 1970s 

management began to include efforts to increase cormorant numbers. The species was listed as 

endangered in Wisconsin (1972) and Illinois (1960) and as “in need of management” in 

Tennessee (1976). Artificial nesting structures were erected in Wisconsin and Illinois. As a result 

of many factors (see PART II. POPULATION DATA AND TRENDS) DCCO populations soon 

began to recover, and efforts to control the species resumed. Conditions had changed, however, 

resulting in new restrictions to management and new impacts of concern. DCCOs received 

federal protection under the Migratory Bird Treaty Act in 1972, and lethal control activities by 

individual citizens thereafter required special permits from USFWS. Aquaculture in the southern 

U.S. grew considerably in the late 1960s and early 1970s while DCCO populations were very 

low, and as DCCO populations recovered, predation at aquaculture facilities challenged the 

economic assumptions under which the industry had initially evolved (Hodges 1989; Jackson 

and Jackson 1995). Throughout the species’ breeding range, alteration of vegetation from DCCO 

nesting activities led to concerns of impacts to species already threatened by human activities as 

well as impacts to private property.

Currently in the U.S. all lethal take requires permits from the USFWS, except in states 

under the Depredation Order. Depredation permits can be obtained to prevent economic impacts 

or impacts to endangered, threatened or species of conservation concern. Non-lethal harassment 

of birds depredating or about to depredate does not require permits. In some states additional 

permits are required for lethal and / or non-lethal control, including harassment.

In this section we used published and unpublished literature to review and describe the 

techniques available for managing DCCO impacts, we assess numerical and economic 

effectiveness (when data are available), and we provide case studies where appropriate. We 

separated management options into three main categories based on DCCO impacts: 1) 

aquaculture depredations, 2) alteration of vegetation and habitat, and 3) fisheries depredations. 

We also asked survey recipients (see SURVEY RESULTS, below) to provide information on 

control efforts undertaken in their state or province. This information is summarized in Table 28

and discussed at the end of this section. Throughout this section we use the term control to refer 

to both lethal and non-lethal management efforts to reduce cormorant impacts.
FINAL DRAFT Part V. Management Options 243

Table 26. Sources of information on DCCO control at aquaculture facilities.

Citation

Dates 

of study

Geographical 

range

Aquaculture 

facilities

Depredating 

Species

Type of 

study/paper

Lagler 1939 1937-8 United States Fish hatcheries All piscivores Survey

Brémond 1980 NA Global Gen. agriculture Avian Theory & lit. rev.

Inglis 1980 NA Global Gen. agriculture Avian Theory & lit. rev.

Krebs 1980 NA Global Gen. agriculture Avian Literature review

Slater 1980 NA Global Gen. agriculture Avian Theory & lit. rev.

Barlow & Bock 1984 1979-80 Australia Farm dams Cormorants Field study

Moerbeek et al. 1987 1981-3 Netherlands Fish farm (carp) GRCO Field study

Parkhurst et al. 1987 1984 U.S. (eastern) Hatch & nurseries All piscivores Survey

Schramm et al. 1987 NA Florida Gen. aquaculture Avian piscivores Literature review

Hanebrink & Byrd 1989 NA Arkansas Gen. Aquaculture Avian piscivores Survey

Hodges 1989 1987-8 Mississippi Catfish farms Avian piscivores Field study

Stickley & Andrews 1989 1988 Mississippi Catfish farms Avian piscivores Survey

Littauer 1990a,b NA U.S. (southern) Gen. aquaculture Avian piscivores Info brochure 

Conniff 1991 NA U.S. (southern) Gen. aquaculture DCCOs Magazine article

Brugger 1995 NA Florida Gen. aquaculture DCCOs Survey

Duffy 1995 NA North America Gen. aquaculture DCCOs Symposium rev.

Erwin 1995 NA North America Gen. aquaculture DCCOs Literature review

Glahn & Stickley 1995 NA Mississippi Catfish farms DCCOs Literature review

Keith 1995 NA Canada Gen. aquaculture DCCOs Lit. & policy rev.

Mott & Boyd 1995 NA Southeast U.S. Gen. aquaculture DCCOs Literature review

Mott et al. 1995 1992 Mississippi Catfish farm DCCOs Field study

Nisbet 1995 NA North America Gen. Aquaculture DCCOs Symposium rev.

Price & Nickum 1995 NA North America Gen. aquaculture Avian piscivores Literature review

Stickley & King 1995 1992 Mississippi Catfish farm DCCOs Field study

Stickley et al. 1995 1991 Mississippi Catfish farm DCCOs Field study

Thompson et al. 1995 NA Texas Gen. aquaculture DCCOs Lit. rev. & field s.

Pitt & Conover 1996 1993-4 U.S. (western) Fish hatcheries All piscivores Observ. & survey

Spencer 1996 1992-4 Georgia Fish hatcheries DCCO, other avian Survey

Bregnballe et al. 1997 NA Europe Gen. aqua & fish. GRCO Population model

Littauer et al. 1997 NA U.S. (southeast) Gen. aquaculture Avian piscivores Info brochure

Mott & Brunson 1997 NA Mississippi Catfish farms Avian piscivores Literature review

Simmonds et al. 1997 1992-4 Oklahoma Catfish farms DCCOs Field study/survey

Depr. Order; Trapp 1998 NA U.S. (southeast) Gen. aquaculture DCCOs Literature review

Gottfried 1998 1996-7 Louisiana Catfish farms DCCOs Field study

Keller et al. 1998 NA Bavaria, Germany Gen. aqua & fish. GRCOs Field study

Wywalowski 1998, 1999 1997 U.S., primarily S Catfish farms All piscivores Survey

Frederiksen & Lebreton; 

Frederiksen et al. 1999

NA France General aqua. & 

fisheries

GRCOs Population model

Glahn et al. 1999 NA U.S. (southern) Gen. aquaculture DCCOs Literature review

Hatch & Weseloh 1999 NA North America Gen. aquaculture DCCOs Literature review

Keller 1999a 1992-6 Bavaria, Germany Gen. aquaculture GRCOs Field study

Keller 1999b NA Bavaria, Germany Gen. aqua. & fish GRCOs Field study

McKay et al. 1999 NA England & Wales Gen. aqua. & fish. Avian piscivores Field s. & lit. rev.

Reinhold & Sloan 1999 NA Mississippi Gen. aqua./catfish DCCOs Literature review
FINAL DRAFT Part V. Management Options 244

MANAGING IMPACTS TO AQUACULTURE

Aquaculture fisheries impacted by DCCO depredations include food fishes, game fishes 

and bait fishes; exotic fish aquaculture does not appear to be negatively impacted by DCCOs 

(Brugger 1995; Price and Nickum 1995). Of the three impacted categories, catfish farming, 

especially in the Mississippi Delta region, has received the most discussion in the scientific 

literature. Numerous techniques have been devised and tested for reducing DCCO depredations 

at catfish farms and are reviewed below.

Estimates of the economic effectiveness of some of these techniques have been made, 

and in some cases these estimates may be helpful for comparing effectiveness between 

techniques. However, because net impacts of DCCOs to aquaculture remain unknown (see 

PART III: DIET) it is not possible to determine which techniques are economically effective at 

recovering losses due to cormorants.

In a broad review of scientific literature such as this, difficulties arise in attempting to 

summarize papers that not only differ temporally and geographically (important with changing 

DCCO populations), but also vary in application of techniques and even definitions of “success.” 

For example, some investigators have promoted rational economic assessments of impacts and 

control techniques to determine whether and to what extent control is necessary (Parkhurst et al. 

1992; Thompson et al. 1995; Littauer et al. 1997; Reinhold and Sloan 1999). In contrast, others 

may not consider control successful unless all cormorants are removed from the ponds, 

regardless of economics. Variability also arises in surveys in which aquaculturists are asked to 

qualitatively rate the effectiveness of control techniques based on their own definitions of 

success (e.g., Lagler 1939; Parkhurst et al. 1987; Hanebrink and Byrd 1989; Stickley and 

Andrews 1989; Brugger 1995; Spencer 1996; Wywalowski 1998, 1999). To facilitate 

comparisons among studies cited in this section, general information about each has been 

summarized in Table 26.

1) Exclusion Devices: Physical and Functional Barriers

The two most commonly discussed devices to exclude cormorants are nets and wires. 

Nets act as physical barriers to cormorants; overhead wires act as functional barriers (i.e. do not 

exclude cormorants, but make it difficult for them to land on and take off from ponds). Floating 

ropes have been tested as cheaper alternatives to overhead wires. Additional exclusion devices / 

techniques include underwater structures that hinder cormorant access to fish and tall trees 

around ponds that may limit cormorant ability to take flight. Although exclusion devices are 

currently in use at fish hatcheries, Wywialowski (1998, 1999) does not report any catfish 

producers using exclusion/barrier systems in 1996.

Netting

Numerically, nets are the most effective technique for preventing cormorant 

depredations: cormorants that are physically prevented from accessing ponds cannot prey on fish 

within (Littauer et al. 1997; Mott and Brunson 1997). Netting and similar physical exclusion 

devices have been used at fish hatcheries since at least the 1930s (Lagler 1939), and continue to 

be used successfully at aquaculture facilities with small ponds (Parkhurst 1987; Brugger 1995; 

Pitt and Conover 1996). To be completely effective, however, the edges of the nets must extend 

to the ground surrounding ponds. Cormorants have learned to land on pond levies and walk into 
FINAL DRAFT Part V. Management Options 245

the water to bypass netting in Georgia (Spencer 1996) and wire systems in Australia (see Wires, 

below; Barlow and Bock 1984).

Despite their great potential for reducing predation, nets have rarely been used by catfish 

farmers (Stickley and Andrews 1989; Reinhold and Sloan 1999; Wywialowski 1999). The 

reasons given are that 1) large size of ponds typically constructed for raising catfish makes 

netting systems prohibitively expensive, and 2) levies between ponds are typically too narrow to 

hold both net support structures and farm machinery (Lagler 1939; Schramm et al. 1987; 

Hanebrink and Byrd 1989; Brugger 1995; Mott and Boyd 1995; Mott et al. 1995; Price and 

Nickum 1995; Littauer et al. 1997; Depredation Order, Trapp 1998). Price and Nickum (1995) 

state that use of multi age-class stock and other economic arguments precludes reconfiguring of 

ponds to allow netting and wire systems.

Sizes of catfish ponds (in Mississippi) have been described by Price and Nickum (1995) 

as typically ranging from 4 – 14 ha (average = 5 ha), by Glahn and Stickley (1995) as averaging 

8 ha and ranging in depth from 1 to 2 m (with the typical farm having approximately 40.5 ha of 

ponds total), and by Mott and Brunson (1997) as averaging 5 – 6 ha (with the average Delta farm 

totaling 175 ha of water). Littauer et al. (1997) state that typical catfish ponds are 4.9 to 6.1 ha, 

and at an estimated $0.22 / ft2 it would cost approximately $1 million to enclose a 40.5 ha farm. 

Conniff (1991) estimates the cost of covering an 8 ha pond with nets at $22,000. Brugger (1995) 

cites an estimate of $12,141 / ha ($30,000 / acre) for netting in Florida, and concludes that hazing 

and dispersal are more economically effective.

The inconvenience of working around netting systems during maintenance and harvest 

activities is another concern (Lagler 1939; Brugger 1995; Mott and Boyd 1995; Littauer et al. 

1997); Lagler (1939) discussed the possibility that increased labor demands incurred with netting 

systems might be offset by reduced harassment and other control activities.

No economic analyses appear to have explicitly compared costs of erecting netting over 

catfish ponds to potential losses to cormorants. Additionally, the data and calculations used in the 

cost estimates above were not provided. Therefore, it is not possible to determine whether this 

control technique has been sufficiently investigated to make conclusions about its lack of 

economic effectiveness. An analysis that more thoroughly compares costs of various materials 

and configurations, economies of scale, adaptation to (and of) maintenance and harvest activities, 

net losses at harvest due to predation, and various levels of government subsidies is needed to 

determine if this technique deserves more consideration.

Wires

Because cormorants require relatively long distances of open water to take flight (Duffy 

1995) wires (or other filaments) over aquaculture ponds may make them less attractive feeding 

locations, thereby reducing predation. As with netting, wire systems have been used at fish 

hatcheries since the 1930s (Lagler 1939). Surveys of aquaculture facilities report moderate 

effectiveness of wire systems (Pitt and Conover1996; Spencer 1996). Several studies tested use 

of wire systems to protect commercial aquaculture facilities from cormorant predation. 

Moerbeek et al. (1987) studied several wire configurations in the Netherlands to protect fish 

from GRCOs, and found that in general wire systems were successful at deterring large flocks 

from landing, but that individual birds learned to fly between the lines. McKay et al. (1999) 

reported wiring systems may be more effective against GRCOs in England and Wales when 

alternative foraging sites are available. Davis (1990, as cited by Mott and Boyd 1995) also 

reported that various wiring systems used in Texas deterred large flocks of DCCOs and 
FINAL DRAFT Part V. Management Options 246

Neotropic Cormorants, but individual birds were less likely to be deterred. Simmonds et al. 

(1997) found twine stretched over narrow ponds to be effective against most cormorants in 

Oklahoma, but some birds learned to fly under the ends of the twine that were raised to counter 

sagging in the middle. Brugger (1995), on the other hand, reported that wire grids were effective 

in Florida only if few birds were present.

Problems with overhead wire systems include technical and economic problems of 

construction over large catfish ponds (see Netting, above, for sizes), access limitations to 

farmers, and hazards to non-target avian species such as Osprey and swallows (Mott and Boyd 

1995). Birds may also learn to bypass wire barriers by landing on levies and walking into ponds 

(Barlow and Bock 1994) or by learning to fly between the wires (Moerbeek et al. 1987).

Spencer (1996) and Mott and Boyd (1995) both cite Littauer (1990b) to estimate cost of 

covering a 40.5 ha farm with wires at more than $100,000 (at $0.025 / ft2), or $2500 / ha. Smaller 

ponds may be protected more economically: according to Littauer et al. (1997), stringing wire 

over one 3.7 ha pond cost $66 / ha and required 15 working days to install, while smaller ponds 

(up to 0.9 ha) could be protected by wires for as little as $6 / ha and required only 3 working 

hours to install. As was the case for netting, systematic analyses of costs and benefits of wire 

systems to reduce DCCO impacts have not been performed. In Bavaria, Germany, carp ponds of 

up to 4 ha were successfully protected from GRCO predation using various wire systems; 

benefit:cost analyses found some of these systems to be cost effective, and others became cost 

effective when subsidies by the Bavarian government paid for up to 40 % of costs (Keller 

1999a).

The economic effectiveness of this technique will depend on two factors: 1) proportion of 

individual birds that adapts to bypassing the wires and 2) availability of alternative foraging sites 

(see: 5) Provision of Alternative Prey and Foraging Sites, below). The former has received little 

discussion in the literature. The latter will depend on how widely this technique is utilized and 

the proximity of individual farms to either natural or artificial alternative foraging areas.

Floating Ropes

Rather than suspending wires over ponds, ropes with attached floats may be strung across 

the surface of the water to produce a similar impediment to landing and takeoff. This technique 

is less expensive and easier to install than wire systems. Mott et al. (1995) found that floating 

ropes strung parallel to each other 15 – 17 m apart dramatically reduced the number of 

cormorants entering ponds (96 % reductions for each of two test ponds). A few birds appeared to 

habituate to the ropes; however, their numbers remained considerably lower than pre-treatment 

levels, and the addition of Mylar balloons appeared to reduce the number of habituated birds. At 

approximately $100 / ha, the rope system was considerably cheaper than suspended wire systems 

of similar size (see above), and was expected to quickly pay for itself. These results should be 

interpreted with caution, however, as a maximum of 2 ponds was treated within a facility of 21 

adjacent ponds; it is unknown whether the decrease in DCCO activity would be comparable if all 

ponds in a facility or region were similarly treated. Still, it should be noted that the two ponds 

selected for treatment were selected because they had the highest cormorant foraging rates prior 

to treatment, and application of the floating ropes was successful at shifting cormorant foraging 

to apparently less preferable sites. As with wire systems and harassment, proximity of alternative 

foraging sites will likely improve the effectiveness of this technique.
FINAL DRAFT Part V. Management Options 247

Although floating ropes have not been discussed in recent reviews of cormorant 

management techniques (Mott and Boyd 1995; Reinhold and Sloan 1999), this technique shows 

promise and should receive further investigation at larger scales than previously tested.

Other Flight Inhibitors

Duffy (1995) suggested constructing a “small-scale equivalent of the barrage balloons 

that the British used in World War II to discourage low-level bombers,” as another technique for 

making take-off from ponds difficult. This concept has not been tested by itself, but only as part 

of the floating rope system (Mott et al. 1995). Pond size and surrounding tree height may also 

impact cormorant ability to take off from the water, making certain ponds unattractive to 

DCCOs. Some investigators have suggested that such factors should be included in the 

construction of new facilities (Simmonds et al. 1997).

Underwater Exclusion Devices

Barlow and Bock (1984) suggest that cormorant depredations in remote fish dams (stock 

watering ponds) in Australia may be reduced by placing fish refugia in the dams, either to hinder 

cormorant predation or to give fish a place to hide during the day when most cormorant 

predation occurs; however, they did not test this technique. In general, underwater habitat 

modification has received little attention (Hatch and Weseloh 1999). In the Netherlands a small 

exclusion device was placed in the corner of a 10 ha pond in which young carp were reared until 

they were large enough to escape cormorant (P. carbo) predation and were released; the 

technique was “promising” (Moerbeek et al. 1987).

Gottfried (1998) tested the effectiveness of submerged nets as escape cover for catfish in 

aquaculture ponds in Louisiana. Experimental design included both switching treatment and 

control ponds to eliminate pond effects and quantification of DCCO predation success rates 

using behavioral observations. Results showed that DCCOs were equally successful at catching 

catfish in control and treatment ponds; further research on this technique has not occurred. No 

data are available to explain why these nets did not decrease DCCO predation rates, but V. 

Wright (pers. comm.) speculated that hatchery-raised catfish may lack behavior patterns needed 

to utilize escape cover.

Experimental testing of fish refuges in England and Wales demonstrated that these 

devices may reduce fish predation and injury rates by GRCOs (McKay et al. 1999).

2) Non-lethal Harassment at Aquaculture Facilities

There are numerous techniques and devices that have been used to frighten cormorants 

from aquaculture facilities to reduce predation. However, systematic analyses of these techniques 

are limited; most available data are anecdotal or from aquaculturist surveys. These techniques are 

reviewed within three categories: 1) direct human harassment, in which humans attempt to 

frighten, but not kill, cormorants, 2) simulated human harassment, in which static or animated 

devices frighten cormorants by simulating human threats, and 3) other harassment, in which the 

negative stimulus is not necessarily connected to human activity.

Human Harassment

The most common form of direct human harassment is ground patrol with pyrotechnics. 

Patrols may occur on foot or in vehicles and may utilize a variety of pyrotechnics to frighten 

cormorants (and other birds). Pyrotechnics that have been discussed include various 
FINAL DRAFT Part V. Management Options 248

shellcrackers, screamers, whistling projectiles, exploding projectiles, bird bangers, flash / 

detonation cartridges and live ammunition (Lagler 1939; Moerbeek et al. 1987; Hanebrink and 

Byrd 1989; Stickley and Andrews 1989; Littauer 1990a; Brugger 1995; Mott and Boyd 1995; 

Pitt and Conover 1996; Spencer 1996; Littauer et al. 1997; Reinhold and Sloan 1999). Live 

ammunition is often used because it is cheaper and more readily available than pyrotechnics 

(Littauer 1990a; Mott and Boyd 1995; Littauer et al. 1997). Because of potential for accidental 

death, those using live ammunition should also possess kill permits (Littauer 1990a; Littauer et 

al. 1997). Laser guns for startling birds with flashes of light have been marketed for use against 

DCCOs at aquaculture facilities (Desman S.A.R.L., France, Desman@wanadoo.fr); however, 

because their effectiveness is greatest in low light, lasers are more effective at startling DCCOs 

from night roosts (see: 4) Non-lethal Harassment at Night Roosts, below).

Adding labor and materials necessary to patrol a typical catfish farm with cost of 

pyrotechnics and live ammunition, Littauer et al. (1997) estimate harassment costs at $132 / day. 

Apparently referring to Stickley et al. (1992), the authors calculate that a farm with an average of 

100 DCCOs feeding at any time could experience losses of $400 / day (replacement costs); they 

conclude there may be instances in which an aggressive harassment program would be cost 

effective. This appears to be the only attempt at a benefit:cost analysis of harassment techniques. 

In a survey of 281 Mississippi catfish farmers Stickley and Andrews (1989) report an average of 

2.6 person-hours of harassment / day for all bird species, at an annual cost of $7400; harassment 

during the 6 month period when cormorants were present averaged $26 per day, or 

approximately $4700 for the entire six months.

Surveys of aquaculturists reveal that harassment patrols are commonly utilized, despite 

the fact that few consider them very effective. Parkhurst et al. (1987) found that human patrols 

were considered highly successful or better by 20 of 66 hatchery managers (30 %), of limited 

success by 40 (60 %) and not successful by six (9 %). Stickley and Andrews (1989) reported 

that, of 244 survey respondents who reported using harassment techniques, 147 (60 %) used 

vehicle patrols combined with shooting to repel birds; of these 147, 13 % found this combination 

to be very effective, 47 % somewhat effective and 40 % not effective. Of these same 244 

respondents, 21 (9 %) utilized pyrotechnics regularly; of these 21 individuals, 24 % found 

pyrotechnics to be very effective, 57 % somewhat effective and 19 % not effective. In both 

studies above, effectiveness was not defined. Brugger (1995) reported that in Florida human and 

dog patrols frightened birds only briefly. Spencer (1996) reported that two state fish hatcheries in 

Georgia used whistlers and cracker shells, and both techniques were considered partially 

effective against birds in general at each facility. Neither technique was considered effective 

against “resident” DCCOs, however. A survey by Wywialowski (1998, 1999) found that 57 % of 

catfish farmers surveyed shot at cormorants with intent to both kill and harass, and 55 % used 

vehicle patrols of some sort. Moerbeek et al. (1978) reported that flash / detonation cartridges 

were generally ineffective against Great Cormorants, with some cormorants leaving the facility, 

some moving to other ponds within the facility and others unaffected.

Cormorants are often frightened from aquaculture ponds simply because of presence of 

humans, and this behavior potentially may be exploited to reduce cormorant damage. Increasing 

overall human activity levels, keeping most valuable stocks near buildings and activity centers, 

and promoting visitors may decrease predation on aquaculture stocks (Hanebrink and Byrd 1989; 

Hodges 1989; Spencer 1996; and Reinhold and Sloan 1999). Hodges (1989) concludes that the 

presence of humans at aquaculture facilities during critical periods may be the most effective 
FINAL DRAFT Part V. Management Options 249

means of keeping DCCOs off ponds. There do not appear to have been any analyses of this 

technique, however.

Various forms of aircraft have been tested for their ability to frighten cormorants from 

aquaculture facilities. Overall, success rates appear to be high compared to other harassment 

techniques, but the costs associated with this technique have also been considered high. Model 

aircraft have been discussed (Mott and Boyd 1995; Littauer 1990a; Conniff 1991). Littauer 

(1990a) states that one person operating remote controlled aircraft could effectively protect 200-

300 contiguous acres (81 – 121 ha) of ponds, but compared to ground patrols with unskilled 

labor, this technique was more expensive. Conniff (1991) reported that model aircraft were 

effective, but expensive in terms of labor and equipment and that in collisions between 

cormorants and model aircraft, cormorants usually faired better. Moerbeek et al. (1987) tested 

helicopters and ultralight aircraft at an aquaculture facility in the Netherlands and concluded that 

the helicopter was impressive in technical possibilities, but its effects were short lived (i.e., the 

cormorants returned quickly); the ultralight seemed to produce better results, but few data were 

available. They also noted that ultralights were limited to low wind conditions.

Simulated Human Harassment

To reduce labor costs for harassment patrols, various devices, both static and animated, 

have been developed to simulate the threat of human activity near aquaculture ponds. These 

devices range from simple wood cutout scarecrows to elaborate contraptions that create startling 

movements, emit numerous noises and flash lights.

Human effigies / scarecrows have long been used against avian predators at many 

different types of agriculture fields, despite their general lack of success at preventing 

depredations (Lagler 1939; Inglis 1980). Increasing both realism and level of animation in 

scarecrows may improve their ability to scare birds, and combining scarecrows with automated 

sound devices (see below) may enhance the frightening effect (Littauer 1990a; Littauer et al. 

1997). Improving the level of animation can be as simple as adding arms that swing in the breeze 

or as complex as using pop-up or inflatable scarecrows.

Stickley et al. (1995) and Stickley and King (1995) tested an inflatable effigy called 

Scarey (sic) Man © on catfish farms in Mississippi. This device deploys at variable intervals, 

popping up from the ground and bobbing around until deflating a short time later. Deployment is 

accompanied by a shrieking sound. For relatively short lengths of time (10 – 19 days), the device 

significantly reduced the number of DCCOs on the ponds (71 – 99 % reduction in number of 

DCCOs flushed from ponds during ground patrols); compared to replacement cost of catfish 

consumed (based on mean DCCO consumption rate in Stickley et al. 1992), Scarey Man devices 

were considered to be economically efficient (Stickley et al. 1995). However, evidence of 

habituation was reported, especially when day roosts were in view of the Scarey Man devices. 

The longer-term (46 day) study by Stickley and King (1995) considered various enhancement 

techniques (e.g., adding hats and masks to the Scarey Man devices, replacing them on occasion 

with real humans shooting guns, adding propane cannons) to decrease the rate of habituation. 

They found that the devices could be effective for long periods of time, especially if moved 

frequently; they did not find evidence that the propane cannons helped, however. The authors 

suggest that the device be used where DCCO depredations are “serious”.

Another animated scarecrow was described by Conniff (1991). This device was described 

as a jack-in-the-box scarecrow with inflatable plastic arms, revolving strobe lights, and amplified 
FINAL DRAFT Part V. Management Options 250

sounds (130 dB) of horns honking, people shouting, shotguns firing and birds screaming. The 

device was declared “ineffectual”, indicating that “a cormorant can get used to almost anything.”

Unattended vehicles such as Trucks, ATVs and boats have been tried and recommended 

as means to simulate human threats (Stickley and Andrews 1989; Reinhold and Sloan 1999). 

There are few data on the effectiveness of this technique. Stickley and Andrews (1989) reported 

that one survey respondent used a parked vehicle with a scarecrow and found it very effective.

To simulate the threat of gunfire, various noisemakers have been developed. One is the 

rope firecracker, which sets off explosions as the rope burns (Littauer 1990a). More commonly 

discussed are propane / butane / acetylene cannons (Lagler 1939; Moerbeek et al. 1987; 

Hanebrink and Byrd 1989; Stickley and Andrews 1989; Littauer 1990a; Conniff 1991; Brugger 

1995; Mott and Boyd 1995; Stickley and King 1995; Pitt and Conover 1996; Spencer 1996; 

Littauer et al. 1997; Simmonds et al. 1997; Reinhold and Sloan 1999). Some cannons can be 

programmed to go off at varying intervals and variable numbers of times. The reported 

effectiveness of cannons is itself variable. Stickley and Andrews (1989) reported that, of 244 

survey respondents, 97 (40 %) used propane cannons. Of these, 60 % found them to be either 

somewhat or very effective against DCCOs (the same approval rating as for ground patrols); 

however, ground patrols were found to be slightly more effective (13 % of those using ground 

patrols found them to be very effective, whereas only 9 % of those using cannons did). Brugger 

(1995) reported initial success with cannons, but with relatively quick habituation. Conniff 

(1991) reported that butane cannons eventually became perches for DCCOs. Individuals using 

propane cannons in Oklahoma and Georgia reported them to be ineffective (Spencer 1996; 

Simmonds et al. 1997). Moerbeek et al. (1987) found gas cannons generally ineffective against 

Great Cormorants in the Netherlands. Littauer (1990a,b) suggests that the cannons be moved 

every 1 – 3 days to limit habituation, and that their use be stopped when the number of 

habituated birds increases, otherwise future effectiveness of the devices might be compromised.

Perhaps more importantly, use of cannons in conjunction with other harassment 

techniques improves their effectiveness (Stickley and Andrews 1989; Littauer 1990a; Littauer et 

al. 1997; Reinhold and Sloan 1999).

Other Harassment

Other means to startle birds into flight have been developed, and many have been used 

against DCCOs. These include amplified DCCO distress calls; sirens and other electronically 

generated noises; tin plates, mylar balloons, reflecting tape and other reflectors; eyespot balloons 

and raptor silhouettes (Lagler 1939; Barlow and Bock 1984; Moerbeek et al. 1987; Parkhurst et 

al. 1987; Stickley and Andrews 1989; Littauer 1990a; Brugger 1995; Mott and Boyd 1995; Price 

and Nickum 1995; Spencer 1996; Littauer et al. 1997; Reinhold and Sloan 1999.)

Reviewers have generally found distress calls ineffective against DCCOs for long periods 

of time. Hanebrink and Byrd (1989) mention that, while the ADC recommended using amplified 

distress calls, all the calls did was move birds to different ponds. Littauer (1990a) listed distress 

calls among other electronically generated noises whose effectiveness was “uncertain,” but did 

note observations of DCCOs apparently being attracted to distress calls. Mott and Boyd (1995) 

and Reinhold and Sloan (1999) both lump distress calls with other static and automated 

harassment devices that, together, varied in effectiveness. Stickley and Andrews (1989) reported 

that of 244 survey respondents, six (2 %) used distress calls, with two reporting them to be 

somewhat effective and four reporting them not effective.
FINAL DRAFT Part V. Management Options 251

Because of non-uniform application of distress calls among aquaculture facilities, 

conclusions of ineffectiveness may be premature. Brémond (1980), Krebs (1980) and Slater 

(1980) discuss the use of auditory devices to frighten birds from agricultural areas, and conclude 

that effectiveness can be hampered if attention is not paid to bird behavior. Simply recording 

distress calls and playing them back, ignoring the context in which the birds hear the sound, may 

not be the most effective means for frightening birds. Brémond (1980) suggests analyzing 

distress calls to determine what parts cause the most agitation, and manipulating these parts to 

produce “super-normal” stimuli (i.e. unnatural stimuli that have greater effects than natural 

stimuli). Slater (1980) hypothesizes that the benefits of properly-applied distress calls (whether 

normal or super-normal) may include 1) effectiveness at lower volumes than pyrotechnics, 

making them less annoying to humans, 2) slower habituation rates because of prior genetic 

adaptation to behavioral responses and 3) less disturbance to non-target wildlife. These 

suggestions should be approached with some caution as they are based largely on theoretical 

ethological arguments; more research with DCCOs is needed in this area.

Effectiveness of audio and visual scare tactics in this category (specifically distress calls, 

electronically generated noises, tin plates, mylar ribbon, flash tape, flagging, helium balloons, 

inflatable eyespot balloons and hawk silhouette kites) have generally been found to be low when 

deployed by themselves or over long periods of time (Littauer 1990a; Spencer 1996; Reinhold 

and Sloan 1999). Barlow and Bock (1984) reported that, in Australia, noise makers and visual 

scare devices were not compatible with livestock, and that habituation was a problem.

Given the general lack of success with harassment techniques used separately (largely 

due to habituation), many investigators have concluded that, to be effective, 1) a variety of 

techniques must be used, 2) techniques should be applied vigorously, 3) location of static and 

automatic devices should be changed frequently, and 4) the combination of techniques should be 

altered frequently (Moerbeek et al. 1987; Littauer 1990a; Mott and Boyd 1995; Mott and 

Brunson 1997; Reinhold and Sloan 1999). The concept that techniques are best applied in

combination (and may in fact have a synergistic effect) appears to be shared with aquaculturists 

as well (Stickley and Andrews 1989). The recommendation by Littauer (1990a) that use of gas 

cannons be stopped once habituation begins to occur (to prevent a decrease in their utility at a 

later date) could probably be applied to all forms of non-lethal harassment. Again, whether an 

intense program of harassment could be economically justified is unknown given the current 

level of knowledge of predation impacts. The ability of DCCO harassment efforts to 

simultaneously reduce predation by other piscivores is another important question that has yet to 

be investigated.

In Florida, border collies have been trained to chase waterbirds form fish-rearing 

facilities (J. Rodgers), however we did not find any additional information on this technique.

3) Lethal Control at Aquaculture Facilities

Reinforcing Non-lethal Harassment Techniques

Recent discussions of lethal control of DCCOs by shooting at aquaculture facilities has 

focused on reinforcement of non-lethal harassment techniques, rather than population reduction 

(Slater 1980; Littauer 1990a; Mott and Boyd 1995; Spencer 1996; Hatch and Weseloh 1999; 

Reinhold and Sloan 1999), including the Depredation Order (Trapp 1998). However, in some 

cases lethal control has also been recommended to remove problem individuals that appear 

immune to non-lethal harassment (Keith 1995, Pitt and Conover 1996, Spencer 1996). The 

observation that lethal control may help prevent habituation to non-lethal harassment techniques 
FINAL DRAFT Part V. Management Options 252

is consistent with the ethological concept of psychological extinction in which individuals may, 

over time, fail to respond to artificial stimuli (non-lethal harassment, in this case) if the 

“punishment” (shooting to kill) is removed (Slater 1980). Few empirical data supporting the 

effectiveness of lethal control of DCCOs exist, however, and no economic analyses comparing 

costs of lethal and non-lethal controls with benefits of avoided predation are available. Hess 

(1994, as cited in Reinhold and Sloan 1999) observed that fewer DCCOs appeared to enter 

treatment areas where lethal control was practiced, suggesting that the control was effective to 

some extent. Reviewing European studies, McKay et al. (1999) found mixed opinions on 

effectiveness of shooting Great Cormorants to reinforce harassment efforts.

Shooting at DCCOs may not be an effective harassment technique by itself and may 

simply move birds from one pond to another within the same facility (Hanebrink and Byrd 1989; 

Simmonds et al. 1997). Simmonds et al. (1997) conclude that lethal control is controversial and 

often counter-productive; they do not explain in what ways it may be counter-productive, 

however.

Prior to March 1998, anyone wishing to practice lethal control of DCCOs at aquaculture 

facilities in the U.S. was required to obtain an individual depredation permit from the USFWS 

(as well as state permits, where applicable). Applicants were required to provide evidence that 

non-lethal techniques had been tried and found ineffective, and to estimate economic losses if 

DCCOs were not controlled with lethal means (Trapp et al. 1995). To reduce paperwork and 

speed up response rate for aquaculturists seeking permission to shoot cormorants, USFWS issued 

the DCCO Depredation Order on 4 March 1998 (Trapp 1998). The goal of the Depredation 

Order was not to effect large scale population reduction through lethal control. In fact, the Order 

predicted that the expected increase in mortality would not have a negative impact on DCCO 

populations. Rather, the Order was intended to reinforce and supplement non-lethal control 

efforts (Trapp 1998). Although the Service continues to support development of non-lethal 

alternative technologies for reducing cormorant impacts, it concludes that lethal control “will 

always have to be considered as a viable option in an effective, integrated strategy for 

minimizing the deleterious effects of cormorants on aquaculture” (Trapp 1998:10557-8).

Using available survey data, it is difficult to determine how frequently lethal control is 

used among aquaculturists. Wywialowski (1998, 1999) reports that 57 % of respondents shot at 

DCCOs. Stickley and Andrews (1989) reported that 60 % of respondents (147 of 244 surveyed) 

patrolled their ponds, shooting at DCCOs. In neither survey was shooting to harass and shooting 

to kill differentiated.

Reducing Local Populations

For lethal control at aquaculture facilities to effectively reduce local DCCO populations 

and predation levels, the augmented mortality rate must be higher than the immigration rate. This 

level of lethal control may be difficult to achieve. In Bavaria, Germany, the number of Great 

Cormorants reported shot during the winters of 1995/96 to 1998/99 was equal to between 50 and 

100 % of the average Bavarian winter cormorant population. Despite this high level of mortality, 

mean winter numbers of cormorants did not substantially decrease. Shooting did not remove 

birds from water bodies that were supposed to be protected, but simply killed migrant birds 

which were rapidly replaced by newly arriving individuals. Because mean GRCO winter 

numbers did not decrease substantially, shooting was considered an inappropriate management 

tool for reducing overall fish depredation in Bavaria. No conclusions were drawn concerning use 

of shooting to reinforce local harassment efforts, however (Keller et al. 1998; Keller 1999b). In 
FINAL DRAFT Part V. Management Options 253

the Mississippi Delta, numbers of DCCOs increase over the course of the winter, reaching peaks 

in February and March (J. Glahn, pers. comm.). King (1996) found that three of 25 cormorants 

fitted with radio transmitters in the Delta flew to the Gulf Coast approximately 350 km away 

(one of these making the trip in two days), and concluded that “wintering birds may wander 

extensively and rapidly travel great distances.” Thus, in this region, similar outcomes of lethal 

control efforts to those in Bavaria may occur due to winter movement patterns of cormorants 

through and within the Delta.

Additionally, anecdotal evidence indicates that DCCOs are wary birds and difficult to kill 

at aquaculture facilities (Hanebrink and Byrd 1989; Conniff 1991; Price and Nickum 1995). 

Some empirical evidence on the difficulty of killing cormorants at aquaculture facilities appears 

in Hess (1994, cited in Reinhold and Sloan 1999). During 3000 person-hours of effort in which 

maximum take was not limited, only 290 DCCOs were killed at two aquaculture facilities in the 

Mississippi Delta. The Depredation Order (Trapp 1998), also citing Hess (1994), reports that 

investigators were able to kill only 12 % of the number of DCCOs authorized (2500) over a 19-

week period. These observations are important for evaluating the feasibility of population 

reduction by shooting birds during the winter. Some have suggested that shooting DCCOs at 

aquaculture facilities may not be an effective population control technique by itself (Thompson 

et al. 1995; Simmonds et al. 1997).

Accidental Killing of Other Species

The issue of by-kill has also arisen within discussion of lethal control options for 

DCCOs, as has the problem of abuses of lethal control permits. For example, Conniff (1991) 

reports one aquaculture facility in California was convicted of taking 15,000 birds over five 

years when it had only a 50-bird permit; minimum-wage labor “ill equipped to distinguish 

between a water turkey [Anhinga] and a Bald Eagle” had killed not only DCCOs and herons, but 

also hawks and ducks that do not eat fish. Simmonds et al. (1997) reported that in Oklahoma, 

Anhinga, Cattle Egrets and Little Blue Herons were shot during DCCO lethal control efforts. On 

Prince Edward Island, legal hunting of DCCOs was found to have little impact on that species’ 

numbers; however, inadvertent killing of the less common and protected GRCO resulted in a 50 

% decrease in the population; as a result, the PEI government began working to eliminate the 

DCCO hunting season (Korfanty et al. 1997). In addition to active hunting of cormorants, 

passive lethal control techniques (e.g. leg-hold traps) have been deemed unacceptable because of 

great potential for by-kills (Barlow and Bock 1984).

The DCCO Depredation Order (Trapp 1998) does not consider by-kill a significant 

problem, stating that 1) few threatened or endangered species are expected to be confused with 

DCCOs and 2) mortality to other cormorant species and Anhinga was not expected to increase 

significantly. [However, see PART II. POPULATION DATA AND TRENDS, Louisiana

summary, for potential impacts to Neotropic Cormorants; the Anhinga is declining in parts of its 

range (W. Howe pers. comm.)]. Predictions of impacts to non-target species do not appear to 

have been based on calculations from empirical data or modeling. The Order does not call for 

monitoring of the impacts of by-kill beyond tallying self-reported take. The USFWS plans to 

decrease the level of by-kill resulting from the depredation order by 1) restricting shooting to 

daylight hours and 2) working with USDA/WS and NGOs to develop educational materials.
FINAL DRAFT Part V. Management Options 254

4) Non-lethal Harassment at Night Roosts

Harassing DCCOs at night roosts with pyrotechnics and other techniques until the 

cormorants leave has been suggested as a means of discouraging DCCOs from a geographical 

area on a scale larger than the typical aquaculture facility. The goal of this approach is to keep 

DCCOs from roosting in the area at night and subsequently to decrease the number of 

depredating cormorants within the harassment area during the day. Most discussion of this 

technique has focused on the Mississippi Delta region where it has been practiced since 1992 

(Mott et al. 1998). A survey of catfish producers in southern U.S. (Wywialowski 1998, 1999) 

revealed that 14 % reported using this technique. Most were in Mississippi (32 % of all 

Mississippi catfish producers), whereas only 6 % of producers outside Mississippi reported using 

this technique.

Mott et al. (1998) and Reinhold and Sloan (1999) provide the most recent summaries of 

the Mississippi Delta program, and are excellent resources; we briefly summarize their 

conclusions here. In a cooperative effort among aquaculturists, hunters, state and federal 

agencies, and other “concerned individuals” organized by USDA/WS, the Mississippi Delta is 

monitored with ground and aerial surveys, and cormorant roosts located during these surveys are 

harassed. Harassment occurs from two hours before sunset to half an hour after, and is repeated 

nightly until ≥ 90 % of cormorants have abandoned the roost, typically within 1 – 2 days. 

USDA/WS organizes simultaneous harassment of all known night roosts within the Delta once 

per month, and individuals are encouraged to practice their own harassment efforts at other 

times. Harassment includes pyrotechnics such as screamer sirens, bird bombs and shell crackers.

Laser guns have been marketed to startle DCCOs in low-light conditions (Desman 

S.A.R.L., France, Desman@wanadoo.fr, Hatch and Weseloh 1999). McKay (1999) reports that 

laser guns have been used effectively against GRCOs in England, Wales, France and Italy; this 

technique effectively reduced numbers of birds at night roosts as well as numbers feeding in 

nearby ponds in England and Wales. These investigators recommended this technique be utilized 

especially in regions where pyrotechnics will disturb other species.

Assessments of the effectiveness of the Mississippi Delta program are limited, but tend to 

suggest that the program may be numerically and economically effective. Surveys within 

harassment areas during the winter of 1993/94 counted 70 % fewer birds than the previous 

winter (1992/93) when there was no harassment; surveys from 1994/95 showed a 71 % decrease 

from number of birds detected in 1992/93 (Mott et al. 1998). Survey data from aquaculturists in 

1994 revealed that 62 % within the harassment zone reported fewer problems with DCCOs than 

in previous years, whereas 38 % outside the zone had the same sentiments; data from 1995 

revealed little change, with 74 % of aquaculturists within the harassment zone reporting fewer 

problems and 38 % outside reporting the same (Mott et al. 1998). These perceptions are reflected 

in the amount of money spent on harassment at individual aquaculture facilities: within the 

harassment zone, aquaculturists reported an average $1406 decrease in expenses for harassment 

at their facilities in 1994, and $3,217 in 1995. (Whether some of this decrease was due to 

reallocation of resources to night roost harassment, in addition to changing perceptions of DCCO 

impacts is not reported.) Cost of the program was $16,757 in 1994 and $32,302 in 1995. If these 

costs were divided equally among participating aquaculturists, each would have paid $419 in 

1994 and $557 in 1995. Based on a comparison between cost of the night roost harassment 

program and reductions in harassment expenditures at individual aquaculture facilities, the 

control program was considered economically effective.
FINAL DRAFT Part V. Management Options 255

Outside harassment zones, expenditures for harassment at individual aquaculture 

facilities increased on average $845 in 1994 and $741 in 1995, suggesting that the harassment 

program moved DCCOs out of the harassment zone and into surrounding areas (Mott et al. 

1998). King (1996) attempted to analyze the effect of the Delta harassment program on DCCOs 

using radio telemetry; however, the data are difficult to interpret with respect to the control 

program. Overall, night roost harassment was not considered to be sufficient for eliminating the 

need to practice other forms of DCCO control at aquaculture facilities, though it may be effective 

at decreasing losses (Littauer 1997; Mott et al. 1998; Reinhold and Sloan 1999).

Jackson and Jackson (1995) posed two questions about potential counterproductive 

impacts of night roost harassment based on bioenergetics: 1) might harassment cause more

depredations because harassed cormorants expend more energy, and therefore must eat more, 

and 2) might the cormorants stay longer in the Delta, consuming more catfish, because it is more 

difficult for them to build up reserves for their northward migration? There are no energetics data 

to answer these questions; however a study (Wendeln and Becker 1999) on nocturnal disturbance 

of Common Terns reported that increased energy expenditure was compensated by increased 

foraging. The apparent success of the Delta control program suggests that these issues may not 

be important on a local (within the control region) scale. Also, if harassment shifts some of 

cormorant predation efforts to natural feeding areas such as the Mississippi River, then any 

increases in predation caused by harassment will be economically inconsequential. Preliminary 

telemetry studies suggest this may be the case (J. Glahn, pers. comm) Questions that remain 

about the Delta night roost harassment program include analysis of net economic impact. How 

do costs of the program compare with predation avoided as a result of the program? Do 

perceptions of decreased impact, as measured by changes in expenditures for harassment at 

individual aquaculture facilities, provide accurate indices of changes in DCCO impacts on net 

profits?

Some investigators have begun to question the long-term effectiveness of night roost 

harassment (Glahn et al. 2000). The number of known night roosts within the Delta has increased 

recently, making monitoring and harassment more difficult. Whether this increase in number of 

night roosts is due to harassment, to increasing numbers of DCCOs wintering in the Delta, or to a 

combination of factors, is unknown.

5) Provision of Alternative Prey and Foraging Sites

Provision of Alternative Prey

Fish species other than those intended for harvest may comprise a significant proportion 

of biomass within aquaculture ponds. These “trash fishes” may be preyed upon by DCCOs 

preferentially to stocked species, and some investigators have suggested active management of 

these species in aquaculture ponds to decrease predation on fish destined for harvest (Barlow and 

Bock 1994; Glahn et al. 1995; Nisbet 1995; Simmonds et al. 1997). Most discussion of this topic 

has focused on gizzard shad in catfish ponds (Hodges 1989; Stickley et al. 1992; Glahn et al. 

1995; Glahn et al. 1998), although it has been observed that green sunfish may have a similar 

effect on predation in catfish ponds (Hodges 1989; Glahn et al. 1995). The observation by 

Stickley et al. (1992) that average handling time was lower for shad than for catfish (avg. = 7.2 s 

and 47 s, respectively) suggests one reason for the preference. Glahn et al. (1995) also suggest 

that schooling behavior of shad may make them more attractive prey for DCCOs.

Glahn et al. (1995) recommended stocking shad in catfish ponds in the spring to offset 

the increase in the percentage of catfish in DCCO diets observed during this time of year. 
FINAL DRAFT Part V. Management Options 256

However, preliminary results from an experimental study of DCCO predation on catfish suggest 

that alternative prey (shiners, rather than shad, in this case) may not completely buffer the 

impacts of DCCO predation under certain conditions (J. Glahn, pers. comm.). Problems with this 

technique may include: attracting more DCCOs to ponds, increasing disease and oxygen stress to 

catfish, difficulty in managing shad (which may be less tolerant to low temperatures and oxygen 

levels than catfish), and lower value of catfish harvests that include “trash fishes” (Hodges 1989; 

Glahn et al. 1995; Mott and Boyd 1995; J. Glahn, pers. comm.). There appears to have been little 

investigation of these concerns, however.

In Australia, mortality rates for fish in ponds with crayfish did not differ significantly 

between ponds with and without cormorant predation. Fish in ponds with cormorant predation 

and without crayfish suffered higher mortality rates. Therefore, crayfish may have provided a 

preferred alternative food source for cormorants, protecting fish from predation; however sample 

sizes were small (Barlow and Bock 1994).

Alternative Foraging Sites

Natural wetlands, restored wetlands and defunct ponds stocked with preferred prey such 

as shad may shift cormorant foraging away from catfish ponds (Erwin 1995; Mott and Boyd 

1995; Price and Nickum 1995) and may improve non-lethal harassment efforts by providing 

cormorants a place to forage when driven from aquaculture facilities (Lagler 1939; Krebs 1980; 

Slater 1980; Mott and Boyd 1995; McKay et al. 1999). Hodges (1989) reports that DCCOs 

showed a statistically significant tendency to fish in large, defunct catfish ponds (although the 

power of the analysis was low). As with providing alternative prey within active aquaculture 

ponds, however, providing alternative feeding sites may increase the survival of depredating 

species, allowing populations to increase and put further pressures on agricultural crops (Krebs 

1980).

It appears that alternative foraging sites will function less effectively at reducing 

predation by themselves than in conjunction with on-site exclusion and / or harassment 

techniques. For these ponds to be selected preferentially by foraging cormorants, one would 

expect that the alternative sites would have to be of higher quality than the catfish ponds 

themselves, a quality that may be difficult and costly to maintain.

Although some farms may benefit from the buffering effect of shad, and the maintenance 

of alternative feeding sites may improve the effectiveness of other control techniques, neither of 

these control techniques appears to have been applied consciously within catfish production 

regions: Wywialowski (1999) did not report any catfish producers using alternative prey in 1996, 

although this technique may have been included in the category “other.”

6) Alteration of Aquaculture Practices

This is a catch-all category for the many ways aquaculturists may decrease predation 

losses by altering how they run their facilities. Wywialowski (1998, 1999) found that 10 % of 

catfish farmers surveyed reported using techniques under this heading. Wywialowski also 

reported that 7 % of respondents used techniques in the category “other”; some of these 

techniques may be included in this section. Much of the discussion of these techniques for 

cormorant control is based on theoretical arguments, and evidence of effectiveness is mainly 

anecdotal.
FINAL DRAFT Part V. Management Options 257

Decrease Fish Densities

Although very little is known about DCCO selection of foraging habitat, some 

investigators predict aquaculture ponds with high densities of fish may be more likely to be 

detected by DCCOs as potential feeding sites. Fish densities in catfish ponds can be as high as 

150,000 / ha (Hodges 1989). By decreasing density of catfish in ponds, aquaculturists may 

decrease the chance of their ponds being detected and selected as feeding sites (Lagler 1939; 

Barlow and Bock 1984; Duffy 1995; Jackson and Jackson 1995; Mott and Boyd 1995; 

Simmonds et al. 1997). Obviously, decreasing stocking rates, while possibly decreasing 

predation, may also reduce production. However, reducing stocking rates may solve other 

aquaculture problems (oxygen stress, disease, parasites, etc.) common to high stocking densities, 

and may have multiple benefits leading to increased production (Duffy 1995). More research is 

needed to evaluate whether this technique is effective for managing DCCO impacts and to 

determine the interplay among stocking densities, fish growth rates, disease, oxygen stress, 

parasites, DCCO foraging site selection, and DCCO foraging rates. Factors that stress fish and 

limit their growth (disease, oxygen stress, parasites, etc.) may also impact predation rates 

because they affect fish behavior; stressed fish may be preferentially hunted because they are less 

capable of escaping predation (B. Vondracek, pers. comm.).

Among catfish producers the trend has been towards higher stocking densities despite the 

proposed problems summarized above, suggesting that among catfish farmers the benefits of 

higher stocking are perceived as greater than the costs. Increased stocking rates may in fact be a 

response to losses due to cormorants (J. Glahn, pers. comm.).

Pond Dyes

In a similar vein, it has been suggested that cormorant predation might be reduced by 

adding dye to the ponds to decrease the chance that fish will be detected by cormorants in flight, 

or to make it more difficult for cormorants to hunt once in the water (Barlow and Bock 1984; 

King et al. 1995; Mott and Boyd 1995; Spencer 1996; Hatch and Weseloh 1999). There appears 

to have been no systematic testing of this technique, but theoretical arguments and anecdotal 

observations suggest that this technique may be of limited use. Spencer (1996) reports that dyes 

have been used but found ineffective in Georgia. J. Glahn (pers. comm.) notes that catfish ponds 

are naturally turbid, yet cormorants have adapted well to foraging in them. Barlow and Bock 

(1984) observed that various cormorant species in Australia took fish from waters with Secchi 

disk transparency depths of less than 10cm. Van Eerden et al. (1995) state, “The habit of mass 

fishing in [P. carbo] is only a recent phenomenon, thought to have evolved in response to turbid 

underwater conditions with a high stock of pelagic fishes.” Hatch and Weseloh (1999) suggest 

that tactile feeding in DCCOs may render dyes ineffective. On the other hand, experimental work 

with GRCOs in Israel found that prey detection was visual, and that shiny were more readily 

detected than dark-colored fishes(Strod et al. 1999).

Water Turbulence

Surface turbulence (e.g. by water sprayed over ponds) may reduce visibility of prey from 

the air, potentially influencing DCCO foraging site selection. This technique has successfully 

reduced predation by some avian species, but effectiveness against DCCO predation is unknown 

(Littauer 1990). This technique was not reported in any surveys reviewed here for DCCOs.

J. Glahn (pers. comm.) suggests that turbulence may not be effective against cormorants 

with this form of aquaculture because catfish are already difficult to see due to turbidity. 
FINAL DRAFT Part V. Management Options 258

However, if the density of catfish in the topmost layer of the water column (that which is visible 

from the air) is correlated with the total number of fish in the pond, and if cormorants utilize this 

visual information when selecting foraging sites, then this might still be an effective technique.

Aquaculture Facility Location

Several investigators have suggested that future aquaculture developments consider 

potential cormorant impacts when selecting facility location (Jackson and Jackson 1995; Price 

and Nickum 1995; Thompson et al. 1995). From a strictly predation-centered point of view, 

geographic areas along major migration flyways, such as the Mississippi Delta, are poor areas to 

locate aquaculture facilities because of the numbers of DCCOs and other fish-eating birds that 

pass through the area during migration. As early as the 1930s Lagler (1939:174) recommended 

“locating fish-cultural establishments outside the usual limits of the major flyways of fish-eating 

birds and beyond the flight range of large nesting colonies” to avoid severe predation. Because 

large aggregations of aquaculture facilities may be more readily detected by DCCOs than 

isolated ones, it has also been suggested that new facilities be located away from preexisting 

concentrations of aquaculture facilities (Price and Nickum 1995). Apparently this preventive 

strategy has not been frequently utilized, as Thompson et al. (1995:185) state, “Siting decisions 

often do not appear to consider all the benefit:cost factors, such as the potential for predation.”

In the past, when cormorant numbers were low, location of aquaculture sites was based 

largely on physical features (e.g., proper soil types, availability of water) and social features 

(e.g., proximity to processing plants and markets). Today, in the face of increased predation by 

growing cormorant populations, it seems advisable to consider the economic costs of predation 

within the site selection process.

Improved Facility Design

Retrofitting existing aquaculture facilities to incorporate control techniques such as 

exclusion has been dismissed as economically unjustifiable (Price and Nickum 1995). However, 

recommendations that facilities be designed to incorporate improved management techniques 

have been around for decades (Lagler 1939), presumably unheeded (Jackson and Jackson 1995). 

Mott and Boyd (1995), Price and Nickum (1995) and Pitt and Conover (1996) all suggest that 

aquaculture ponds be designed to make netting and wiring more economically efficient. 

Simmonds et al. (1997) recommend smaller ponds because larger ones have higher predation 

rates, perhaps because they are easier to detect from the air. These authors also recommend 

maintaining trees around ponds not only to decrease their delectability from the air, but also to 

make taking off and landing difficult for DCCOs. The trend of increasing pond sizes suggests 

that the costs of cormorant predation and control continue to be overlooked when decisions are 

made of the most “economically efficient” pond size.

Alter Stocking Practices

Because young fish may be more vulnerable to DCCO predation, biologists have 

suggested that aquaculturists in DCCO wintering areas delay stocking of young fish until later in 

the spring, when DCCOs have returned to their breeding territories (Moerbeek et al. 1987; 

Brugger 1995; Glahn and Stickley 1995; Glahn et al. 1995; Mott and Boyd 1995; Simmonds et 

al. 1997). However, J. Glahn (pers. comm.) states that “delaying stocking later in the spring is 

probably not a viable option because changing water temperature during spring increases the risk 

of stress-related mortality and disease of fish transfer.” No systematic analyses appear to have 
FINAL DRAFT Part V. Management Options 259

been performed comparing the benefits of reduced DCCO predation to the costs of these other 

sources of mortality. Glahn et al. (1995) also suggest stocking larger fish.

Passive Harassment

Human activity may reduce DCCO numbers, and it has been suggested that daily 

activities at aquaculture facilities be managed to more effectively discourage predation. This can 

be as simple as scheduling regular activities around ponds, encouraging tourism, and stocking 

most vulnerable fish near areas of high human activity (Moerbeek et al. 1987; Brugger 1995; 

Mott and Boyd 1995; Pitt and Conover 1996; Simmonds et al. 1997). Age-class segregation of 

fish may also allow for more efficient harassment efforts because efforts could be focused on the 

ponds with the most vulnerable and / or valuable age classes. (Brugger 1995). Currently, 

multiple age classes commonly occur in aquaculture ponds, making application of this technique 

difficult (Price and Nickum 1995). There appears to have been no systematic analysis of this 

technique.

Alteration of Fish Behavior

Hatch and Weseloh (1999) suggest that alteration of fish behavior may reduce predation 

impacts, but they do not give any details about how this might be accomplished. One potential 

strategy may be to solve disease and oxygen stress problems (see: 8) No Control, below). McKay 

et al. (1999) suggest that naïve, hatchery-raised fish might be taught to better avoid predators, but 

mention that attempts to use this technique have been unsuccessful thus far.

Chemical Avian Repellants

Avitrol has been used on Herring Gulls in Florida, however there is no information on 

use at aquaculture facilities, and it may not be ideal for hatcheries (Brugger 1995).

Conditioned Taste Aversion

For individuals of some vertebrate species, feelings of illness associated with 

consumption of a particular food item may lead to future avoidance of that food type. This 

natural response may be manipulated to reduce predation on specific agricultural and aquaculture 

crops. Preliminary research in England found that captive GRCOs fed trout fingerlings treated 

with carbachol developed a strong and long-lasting aversion to trout. However, more research is 

needed to determine if this technique can be successfully applied to wild cormorants (McKay et 

al. 1999).

Manage Other Avian Species

Other species of birds may act as visual cues to DCCOs, drawing them to aquaculture 

facilities; managing them may result in reduced DCCO impacts (Hanebrink and Byrd 1989; 

Hodges 1989; Price and Nickum 1995). For example, Hanebrink and Byrd (1989) point out that 

non-piscivorous birds such as Cattle Egrets may attract piscivorous birds. More research is 

needed to determine if this would be an effective control technique. Legal protection of these 

other avian species is also a concern with this technique.

7) Population Control

There has been some discussion in the literature of controlling DCCO population growth 

to decrease predation pressures on aquaculture regions during the winter. Glahn et al. (1999:30) 
FINAL DRAFT Part V. Management Options 260

conclude that currently available DCCO control techniques “are of limited effectiveness and are 

becoming increasingly difficult to implement.” They predict that increases in DCCO population 

sizes will further reduce the effectiveness of these techniques and ultimately propose 

management of DCCO populations that utilize the Mississippi flyway (including shooting birds 

at winter roosts and breeding colonies through an expansion of the Depredation Order). Reinhold 

and Sloan (1999) suggest that “management implications should focus more on the long term 

goals of managing DCCO populations,” and state that a cormorant management plan 

cooperatively developed by USFWS, USDA/WS and state wildlife fisheries agencies is one way 

of meeting this goal. Aquaculturists have also requested population control of DCCOs 

(Depredation Order, Trapp 1998).

By analyzing band-recovery data, Dolbeer (1991) found that birds in the southern 

Mississippi region originated from breeding areas as far west as Alberta and as far east as New 

England. The data suggest that, while lakes Michigan and Superior were the most important 

sources of DCCOs wintering in the lower Mississippi Valley, significant numbers of DCCOs 

also came from the region between Saskatchewan and Eastern Lake Ontario (Table 27). Hatch 

(1995:19), citing Dolbeer (1991), states that “wintering birds that eat a Mississippi farmer’s 

catfish could come from anywhere across the 3000 km breeding range of the populations that 

winter there.” Based on these data, controlling breeding DCCOs to reduce numbers of birds on 

the wintering grounds would require a tremendous effort to be successful (Depredation Order, 

Trapp 1998). No attempts have been made to estimate the costs of such a program. Because not 

all birds in these breeding areas utilize the same wintering areas, population level control would 

also impact DCCOs that are not causing depredations. Currently, satellite telemetry is being used 

to further investigate the migration patterns of cormorants wintering in Alabama, Arkansas, 

Louisiana and Mississippi. Preliminary results of this research also indicate that birds wintering 

in the Delta region originate from across the Interior U.S. and Canadian population’s breeding 

range (S. Werner pers. comm.).

Table 27. Percent of winter band recoveries (1923-1988) that occurred in the Lower 

Mississippi Valley for birds originally banded during the nesting season in various 

geographic regions (Dolbeer 1991).

Banding region No. recovered 

in winter

% winter recoveries 

in Lower MS Valley

Alberta 149 9

Saskatchewan 120 46

Manitoba 25 40

Dakotas/Western Minnesota 134 46

Lakes Michigan and Superior 50 70

Lake Huron 93 42

Eastern Lake Ontario 16 38

New England Coast 215 11

St. Lawrence River 67 6

Gulf of St. Lawrence 16 0

Control of cormorant populations through shooting at winter roost sites in aquaculture 

areas has only recently been introduced (Glahn et al. 1999); there has been no discussion in the 

scientific literature of the effectiveness of this technique. Observations on the lack of success of 
FINAL DRAFT Part V. Management Options 261

winter hunting to reduce cormorant populations in Bavaria, Germany, due to the migratory 

nature of this wintering population, may be applicable here (see discussion in Reducing Local 

Populations, above).

In discussion and modeling of management scenarios for European GRCOs, it has been 

suggested that reducing the size of the overall population may not result in equivalent reductions 

in the number of cormorants occurring at high quality foraging areas, such as aquaculture 

facilities (Bregnballe et al. 1997; Van Eerden and van Rijn 1997). Because foraging habitats vary 

in quality and cormorants are efficient at detecting high quality foraging sites, overall population 

reductions may first cause birds to disappear from least preferred or low quality areas, and 

declines may be less marked in high quality areas (Hodges 1989; T. Bregnballe, pers. comm.). 

Additionally, scale of damage caused by cormorants may not be directly related to total numbers 

in local areas (Bregnballe et al. 1997) For example, approximately 60,000 cormorants are 

estimated to winter in the Mississippi Delta region; however, not all individuals may feed at 

aquaculture facilities, and of those that do, feeding rates may vary (e.g. Glahn et al. 1995).

According to J. Glahn (pers. comm.) the increase in numbers of DCCOs wintering in the 

Delta has been proportionally larger than the increase in numbers of catfish ponds in the area. 

This is an important observation because it indicates that cormorant foraging patterns can not be 

explained by simply referring to concepts such as density dependence. Changes in aquaculture 

practices, such as increasing stocking densities or altering harassment regimes, as well as 

changes in cormorant populations, likely influence cormorant foraging habitat selection and 

therefore affect density dependent mechanisms. A better understanding of the relationship 

between these factors and total population size is needed if population goals are to be set.

Even large-scale reductions in DCCO population are unlikely to eliminate the need to 

continue local exclusion and harassment efforts. Aquaculture ponds, with high stocking rates and 

lack of escape cover for fish, will remain high quality foraging sites (and therefore will continue 

to be subject to predation by the remaining cormorants) unless exclusion and / or harassment are 

used to make these ponds unattractive or unavailable. Whether population control is cost 

effective will depend on cost of implementing population controls relative to savings in reduced 

on-site controls. If, for example, population controls effectively reduced the Delta cormorant 

population by half (back to early 1990s levels), would the avoided predation and the reduced 

expenditures for on-site and night roost harassment be more than that spent on population 

reduction efforts?

Density-dependent effects may determine whether population control will lead to 

population stabilization below natural carrying capacity, as was indicated through modeling of 

wintering GRCOs in France (Frederiksen and Lebreton 1999, Frederiksen et al. 1999). 

Bregnballe et al. (1997) modeled several scenarios for controlling European GRCOs (e.g. 

preventing establishment of new colonies, cutting nesting trees and disturbing nesting birds, 

reducing reproductive output, introducing hunting season, and combining these techniques). 

While all management techniques led to model equilibria lower than that predicted without 

management, prevention of colonization in new habitat was found to be the only technique that 

was not buffered by density dependent effects. (For further discussion of this model, see PART 

VI. POPULATION MODELING, Great Cormorants in Europe) Density dependence has been 

identified in several population parameters for the northwest European GRCO population (Van 

Eerden and Gregersen 1995).
FINAL DRAFT Part V. Management Options 262

8) No Control

The decision to not control DCCOs must be included on the list of management options 

available at aquaculture facilities, even though this option often appears to have been overlooked 

(Nisbet 1995). Individual aquaculturists have been encouraged to compare economic impacts of 

DCCO predation to costs of control and then to make rational decisions on appropriate level of 

control, accepting that no control may be the most appropriate option (Parkhurst et al. 1992; 

Thompson et al. 1995; Littauer et al. 1997; Reinhold and Sloan 1999). Some researchers have 

also observed that DCCO depredations are often considered economic losses while other sources 

of mortality (e.g. disease and oxygen stress) are considered just a cost of doing business, and 

they question whether the focus on cormorant predation is appropriate (Conniff 1991; Parkhurst 

et al. 1992; Jackson and Jackson 1995).

Analyses of DCCO control should consider cost effectiveness of cormorant control 

relative to preventing other sources of loss. Citing USDA data, Conniff (1991) and Price and 

Nickum (1995) report that, although DCCO predation is highest on the list in terms of human 

emotion, disease and oxygen stress are larger sources of loss to the catfish industry as a whole. 

Solving these problems may prove more economically efficient at reducing losses than

controlling DCCOs, however there appears to be no economic analyses comparing the two 

options. Additionally, stressed and diseased fish may be more vulnerable to DCCO predation; 

resolving these problems may have a secondary benefit of reducing DCCO impacts (B. 

Vondracek, pers. comm.). Some investigators have even suggested that DCCO predation may 

benefit some farms by removing diseased fish and reducing stocking rates to more suitable levels 

(Jackson and Jackson 1995). Wywialowski (1998) reports that aquaculturists who reported 

oxygen or disease problems were more likely to report wildlife-caused losses; the correlations 

were significant, but did not establish causality. Hodges (1989) also observed a strong tendency 

for DCCOs and other piscivores to feed in catfish ponds where fish were weakened or dying due 

to disease, stress or weather. In Israel, experimental work with captive GRCOs found that dead 

fish were chosen significantly more than live fish (Strod et al. 1999).

The USFWS has stated that more economic information is needed on the relative impacts 

of avian predation compared to other aquaculture losses (Trapp et al. 1995). Therefore, a more 

systematic approach to DCCO predation should be taken, and if DCCOs are not found to be the 

greatest source of aquaculture mortality, research should be redirected towards more effective 

means of impact abatement (Duffy 1995).

9) Conclusions: Managing Impacts to Aquaculture

Despite lack of data on DCCO impacts to aquaculture, predictions can be made on 

effectiveness of some techniques. Local control techniques vary in ability to reduce predation. 

Exclosure devices can completely eliminate DCCO predation; therefore, efforts should be 

continued to make this technique less costly. Depending on levels of impact, harassment may 

also be cost effective, and combinations of harassment techniques are more effective than any 

such technique used alone. Lethal control may reinforce non-lethal harassment techniques, but 

without sufficient education, monitoring and enforcement, this technique may lead to negative 

impacts to other species. Many other local control techniques may be available, but more 

research is needed to determine which are the most cost effective. Exclosure and other 

techniques would be more cost effective if aquaculture facilities were designed to better 
FINAL DRAFT Part V. Management Options 263

accommodate them. Many local DCCO control techniques reduce predation impacts from all 

avian piscivores, an economic benefit not shared with DCCO population control.

Cost effectiveness of DCCO population control, even if predation impacts are large, is 

hampered by several factors: 1) control of breeding cormorants to reduce predation in main 

aquaculture regions would require intense, ongoing efforts, coordinated between the U.S. and 

Canada across an extremely large geographical area; 2) population control during the winter in 

aquaculture regions may be hampered by migration patterns and high mobility of DCCOs and by 

difficulty of shooting birds at aquaculture facilities; 3) density-dependent effects may counteract

population reduction efforts; 4) relationships between overall population size and local predation 

rates may not be direct; even large reductions in the DCCO population may not eliminate need 

for continued local control; 5) benefits of population control are limited to reduced predation by 

cormorants only, whereas exclosures and harassment reduce predation pressures by multiple 

species. In a review of European GRCO control options, van Eerden and van Rijn (1997:42) 

discuss important components of population dynamics and conclude that “large-scale 

interference in breeding colonies seems unjustified for solving reported problems on the flyway. 

Local protection measures for fish stocks seem more effective...”

“No control” may be the best management option if net losses to DCCO predation are 

found to be lower than costs of control. Additionally, management techniques to reduce other 

sources of aquaculture losses may be more cost effective than managing DCCOs. For example, 

diseased and oxygen stressed fish not only grow poorly, but may also be hunted preferentially by 

avian piscivores; managing these sources of loss may also reduce predation impacts.

Who pays for cormorant control was not addressed in any of the studies we reviewed; 

nevertheless, this is an important consideration. When the catfish industry first began to grow, 

cormorant populations had been reduced by a number of factors, especially pesticides. After 

these pesticides were banned (and DCCOs became a protected species) populations increased, 

and the cost of control shifted to aquaculturists themselves. Population control would likely shift 

the cost of control to the government and ultimately to taxpayers, especially if undertaken at 

breeding colonies. If the government decides that such a subsidy to the catfish industry is 

warranted, it should seriously consider whether the money would be better spent subsidizing 

local control efforts. Analysis of such subsidies should consider whether techniques reduce 

impacts of one species (e.g. population control) or multiple species (e.g. exclusion). The 

Bavarian government, for example, subsidizes wire exclosure systems, but not population control 

(T. Keller, pers. comm.).

MANAGING IMPACTS TO VEGETATION AND HABITAT

Through normal nesting activities, DCCOs impact vegetation on a local level, affecting 

both plants and animals that depend on vegetation for cover and nesting, such as other colonial 

waterbirds. Although DCCO impacts on habitat have occurred throughout the evolutionary 

histories of these species, some interspecific colony dynamics may prove deleterious to some 

species in human-altered ecosystems where habitat is limited. Humans themselves may be 

impacted when DCCOs kill trees valued for aesthetics or timber (see Part IV. Impacts to 

Vegetation and Colonial Waterbirds). This section reviews methods for reducing these impacts.

Management may occur at two levels: 1) local control, in which specific impacts are 

addressed and only those cormorants involved are managed, and 2) regional / population control, 

in which attempts are made to hold entire DCCO populations below their natural carrying 

capacities, thereby limiting the potential for cormorants to cause specific damages. Within this 
FINAL DRAFT Part V. Management Options 264

section, local will generally refer to individual colonies or islands, while regional will refer to 

geographical assemblages of colonies such as those occurring within large lakes or within 

individual states’ boundaries.

Techniques for managing local or regional cormorant populations to prevent impacts to

fisheries are similar to those used to prevent impacts to vegetation and habitat. Therefore, data 

from past attempts to manage DCCO populations to reduce impacts to fisheries are included here 

when discussing effectiveness of population management techniques at reducing DCCO 

numbers. Based on these combined data, effectiveness of population management at reducing 

impacts to vegetation and habitat are discussed. Effectiveness of population management at 

reducing impacts to fisheries is discussed in Managing Impacts To Fisheries, below.

Techniques

Harassment of Adults

Non-lethal harassment techniques such as those used to keep DCCOs away from 

aquaculture ponds (see: 2) Non-lethal Harassment at Aquaculture Facilities, above) have 

occasionally been suggested as a means of disrupting existing DCCO colonies or preventing new 

ones from forming. Although some conservation organizations and private individuals have 

called for more investigation of non-lethal harassment (see Appendix F in USFWS 1999a,b), we 

found no record of this technique being used by itself; when non-lethal harassment techniques 

have been used, they have always been accompanied by lethal techniques such as nest 

destruction (with the killing of eggs and young). Harassment has also been an inevitable 

consequence of other control techniques, and it is difficult to separate out its relative contribution 

to overall effectiveness.

Potential problems with harassment include impacts to non-target wildlife and lack of 

effect on total population size. Because both audio and visual scare devices are generally not 

specific for DCCOs, they may scare away individuals of other species as well. If the goal of 

control is to protect nesting birds from habitat alteration by DCCOs, such an effect would 

obviously be counterproductive. This technique may be effective at protecting plants at specific 

locations where other nesting birds are not of concern, but it is expected that benefits would be 

local, shifting DCCO impacts elsewhere.

Although harassed birds may be prevented from reproducing during a specific breeding 

season or suffer reduced reproductive success because of time and energy expenditures involved 

in finding new nesting areas, this technique is generally believed to have negligible impacts on 

local populations when practiced on a relatively small scale (USFWS 1999a,b). However, if 

prevention of colonization is practiced intensely, annually and over a large enough area, this 

technique may be effective at managing regional populations (Bregnballe et al. 1997).

Bixby Island, Vermont:

When DCCOs were first observed nesting in 1996, the owner of the island, assisted by 

USDA/WS and VDFW, removed nests with chicks and eggs (238 total nests, 1996 – 1997), 

effectively preventing reproduction those years (see Nest / Tree Removal, below). In 1998, 148 

DCCOs were reported loafing on the island, but were displaced using pyrotechnics, propane 

cannons, distress calls, and removal of 15 nests (USFWS 1999a,b).
FINAL DRAFT Part V. Management Options 265

Oneida Lake, New York:

From August to September 1998, USDA/WS used pyrotechnics, nest destruction and 

lethal take to move DCCOs off the lake and reduce predation on sport fishes. The combined 

effort was considered effective (USFWS 1999a,b). Harassment was also used successfully in 

1999 (R. Miller, NYSDEC, pers. comm.).

Lake Ontario, New York:

DCCOs were first observed nesting on Calf Island in 1997, and all 75 nests were 

destroyed that year. In 1998 non-lethal harassment and destruction of 175 nests with eggs was 

used. “Terror balloons” and habitat manipulation (cutting of large nesting trees) are the only nonlethal techniques mentioned, though apparently others were used. The combined techniques were 

considered successful at preventing colonization (USFWS 1999a,b). R. Miller (pers. comm.) 

notes that harassment has been used successfully on Gull and Bass islands since 1994 to prevent 

DCCO colonization.

Nest / Tree Removal

This technique has been used to manage DCCOs locally (to eradicate colonies) and 

regionally (to reduce populations). The technique has been used on both ground and tree nests, 

and often coincides with destruction of eggs, nestlings and adults.

Nest destruction on the ground simply involves the physical breakup of nest structures. 

The means by which nests can be removed from trees is rarely discussed. Bédard et al. (1999) 

suggest that dislodging nests with long poles could be used in the St. Lawrence Estuary, Québec

control program (below), but apparently the technique has not been tried. Nest removal activities 

on Bixby Island (see Lake Champlain, Vermont (USFWS 1999a), below) involved two 300-

gallon gas-powered pumps hooked in tandem, pumping water through fire hoses at 400 gallons / 

minute, 120 pounds / square inch (provided by the Grand Isle Fire Department when state 

equipment proved inadequate). Such pressure was said to be able to sweep a man off his feet, but 

barely able to dislodge a cormorant nest 50 feet up in a tree. One hundred of 112 nests were 

successfully removed during two days of spraying (Pyne 1996). In 1996 and 1997 combined, 238 

nests were removed with this method on Bixby Island, and even though not all nests could be 

removed, no reproduction was documented in either year (USDA/WS 1998). Entire trees have 

been removed, both in private (Anderson and Hamerstrom 1967) and official control efforts 

(USFWS 1999a,b). However, because DCCOs frequently nest on the ground, tree removal may 

only be effective where the substrate is inappropriate for nesting or the threat of mammalian 

predation is high.

Effectiveness of nest / tree removal appears to be variable, depending on local conditions 

and goals of control. In many private actions the goal of population control appears not to have 

been very successful. Only in the Wisconsin: case studies (below) was tree removal implicated in 

complete colony abandonment (without evidence of re-nesting near by). For other private 

actions, re-nesting appeared to take place among about half of disturbed pairs; in these cases, 

control activities may not have eradicated colonies, but they did reduce reproduction somewhat. 

In the Lake Champlain, Vermont (USFWS 1999a), and Lake Ontario, New York (USFWS 

1999b), case studies (below) the goal of preventing colonization on specific islands by DCCOs 

was achieved through these techniques.

One of the problems mentioned with nest removal, even when successful at preventing 

colonization attempts, is that effects cannot be considered permanent; control must be repeated 

each time cormorants attempt to nest in areas of concern. No control technique offers permanent 
FINAL DRAFT Part V. Management Options 266

solutions, however. Another concern with nest tree removal is that it may eliminate nest 

substrates for other species. Where DCCOs have already made trees unsuitable for nesting by 

other species, this may not be an issue; however, removing nest trees may shift DCCO nesting 

and move them into conflict with other species.

Wisconsin:

Anderson and Hamerstrom (1967) report that DCCO nest trees were cut down in colonies 

on the Okee Flowage around 1919, on the Fox River prior to 1955, and on the Petenwell 

Flowage in the late 1950s. In all cases colonies were reported be been permanently abandoned as 

a result of these activities.

The Hat Island, Green Bay, DCCO colony was heavily persecuted by fishermen who 

destroyed ground and tree nests and many of the birds (Matteson 1983). The colony disappeared 

sometime around 1956 or 1957 (Lound and Lound 1956; Anderson and Hamerstrom 1967), but 

had apparently been reestablished by the 1990s (Kaiman and Nelson 1995). Persecution may 

have caused a shift from ground to tree nesting in this colony (Kaiman and Nelson 1995).

Lake of the Woods, Ontario:

Omand (1947) reports that fishermen destroyed 127 DCCO nests in this colony 

(apparently all present) in June 1944. Upon return in July of the same year 150 nests were found, 

suggesting that the initial control effort did not have the intended effect of reducing the local 

population and did not prevent reproduction that year.

Lake Winnipegosis, Manitoba:

McLeod and Bondar (1953) report 943 occupied nests on Whisky Jack Island reef, 20 % 

containing recently hatched eggs on 20 June 1945. All eggs and young were destroyed, as 

apparently were nests. A second visit on 13 July of the same year revealed 495 nests had been 

rebuilt, each containing two or three eggs within. The authors also report that 1617 nests with 

eggs and / or young were completely destroyed on Cormorant Island on 24 June 1945. A second 

visit on 26 July of the same year revealed 643 nests had been rebuilt, each containing 1 – 2 eggs.

Blackbill Islands, Lake Huron:

At a colony off Pointe au Baril, bass fishermen destroyed 181 eggs, broke up 80 nests and 

shot six adults in May 1946. The colony apparently rebuilt nests on adjacent rocks using the 

same sticks; on 8 August 1964, 43 nests were counted, all of which appeared to have been used 

that summer (Baillie 1947).

Prince Edward Island:

“...over the last four years, about 4,000 – 5,000 birds have resided on Ram Island, 

accounting for 80 – 85 % of all cormorants on PEI. In 1996, no cormorant nests were found on 

this island. On the other hand, the adjacent island, which had few cormorant nests prior to 1996, 

was found to have 4,500 cormorants in 1996. The cormorants moved from Ram Island to the 

adjacent island because of persecution, harassment, and nest disturbance. Double-crested 

cormorants, which are resilient birds, ‘got up and left.’” (Korfanty et al.1997:28)

Egg and Nestling Destruction

DCCO eggs and nestlings have been destroyed in attempts to reduce recruitment into 

populations and to eliminate colonies at specific locations (in conjunction with other forms of 
FINAL DRAFT Part V. Management Options 267

harassment). Few specific conclusions can be drawn of effectiveness of these techniques at 

reaching either goal because most available data are anecdotal. In general, egg and nestling 

destruction may not be effective at completely eliminating reproduction for individual DCCOs 

during a nesting season because of their tendency to renest after disturbance. Effectiveness at 

regulating populations with this technique are inconclusive. Ability of this technique to remove 

colonies appears variable; newly established colonies appear to be more easily eliminated than 

well established ones.

When specifically discussed, techniques for destroying eggs and nestlings have included 

crushing with rocks, feet or other objects (Price and Weseloh 1986; Ewins and Weseloh 1994; 

Sheppard 1994/5), as well as throwing both eggs and nestlings into the water (Sheppard 1994/5). 

Older young have been clubbed, strangled and shot (Hobson et al. 1989; Sheppard 1994/5). In 

1999, the Vermont Department of Fisheries and Wildlife requested permission from USFWS to 

asphyxiate by CO2 all chicks on Young island that survived oiling efforts (see Lake Champlain, 

Vermont (USFWS 1999a), below). USFWS denied this request, citing a lack of data to show that 

survival of these young would hinder the overall control effort (USFWS 1999a). These 

techniques appear effective at killing eggs and nestlings, however costs appear not to have been 

analyzed.

Observations suggest that when renesting occurs within the same nesting season, colony 

size (and presumably reproductive output) may be reduced by about half (see Nest / Tree 

Removal, above). For Lake Winnipegosis in general, McLeod and Bondar (1953) concluded that 

destroying the first eggs or young of the season resulted in about 50 % of reproducing pairs 

renesting immediately. However, exceptions occur (see Lake of the Woods, Ontario:, above).

Birds that do renest may also experience reduced reproductive success, but this effect 

appears not to have been investigated. However, Ludwig and Summer (1995) do report one 

instance in which egg destruction had little noticeable impact on clutch sizes and fledging rates. 

Factors that determine which individuals or colonies renest appear not to have been investigated, 

but timing may play an important role: renesting is generally more likely to occur during the 

beginning of the nesting season than at the end.

Some cases are known where local DCCO populations continued to rise despite egg and 

nestling destruction efforts. Hobson et al. (1989) report increasing DCCO populations on Lake 

Winnipegosis, Manitoba, despite destruction of eggs, nestlings and adults by local fishermen. 

Number of nesting DCCOs on Lake of the Woods (both Minnesota and Ontario sides) increased 

from an estimated 50 pairs in 1929 to at least 472 pairs in 1939 despite heavy persecution by 

local fishermen (Baillie 1939, 1947). DesGranges and Reed (1981) report that the DCCO colony 

on Ile aux Pommes, St. Lawrence Estuary, had grown considerably since 1963, and at a faster

rate than the St. Lawrence Estuary population as a whole, despite control efforts 1978 – 1980, in 

which all eggs and young were destroyed, and almost annual disturbance by researchers occurred 

(which resulted in increased egg and chick predation by gulls). The authors also report that on 

Iles Rasades local residents destroyed many clutches several times over the previous 20 years, 

but the colony had quadrupled in size during the last 10 of these 20 years.

In contrast, some investigators have suggested that egg and nestling destruction on a large 

scale (often in conjunction with killing adults) may have slowed the growth of populations 

(Weseloh and Collier 1995), stabilized populations or contributed to declines (Ewins and 

Weseloh 1994; Sheppard 1994/5; USFWS 1999b). However, in some regions such as the Great 

Lakes effects of persecution are difficult to separate from those of environmental contaminants 

(e.g., Ewins and Weseloh 1994). McLeod and Bondar (1954) concluded that consistent 
FINAL DRAFT Part V. Management Options 268

destruction of eggs and young appeared to reduce the breeding population fairly effectively on 

Lake Winnipegosis in the 1940s and 1950s (see Lake Winnipegosis, Manitoba, below). 

Chapdelaine and Bédard (1995) state that there is no doubt illegal control on the Great Lakes 

(which has included egg and nestling destruction) prevented some colonies from expanding. 

Anderson and Hamerstrom (1967) discuss the Lac Du Bay colony in Wisconsin which persisted 

(though apparently undergoing a decline) despite high levels of persecution 1956 – 1963, the 

highest eggshell pesticide levels of all colonies tested in the state (48.9ppm DDT+DDE+TDE, 

mostly DDE*), and considerable paper mill effluent so bad that “the water has become scummy 

and darkened, sulfite wastes abundant, and poor fishing has discouraged the fisherman.”

Effectiveness at causing colony abandonment appears to vary. Most recently, egg and 

nestling destruction, in conjunction with nest destruction and sometimes harassment, has been 

found to be effective at preventing renesting for a year or more at newly established colonies 

(USFWS 1999a,b). Price and Weseloh (1986), citing Quilliam (1973), state that unspecified 

activities by fishermen near Kingston, Ontario probably caused DCCOs to abandon colonies on 

Snake and Salmon islands. Weseloh and Collier (1995), on the other hand, describe egg 

destruction efforts on Georgian Bay, Lake Huron, as unsuccessful because birds relayed, often 

on different islands. Baillie (1947) reports that on the Mink Islands, Lake Huron, 150 adult and 

young cormorants were shot at a colony with 40 nests in 1945; in 1946 the colony was still active 

and had grown to 50 nests. Baillie (1947) also discusses the DCCO colony on Mouse Island, 

Lake Huron. Of the three colonies in the North Channel, Mouse Island received the most 

attention from fishermen who made special trips there to destroy eggs and young. The colony 

persisted despite their efforts, leading them to register an official complaint to the Ontario 

Department of Lands and Forests requesting that the government control the colony. The request 

was denied. Finally, on an island off the coast of Maine, one colony was visited seven times 

during 1968, with cormorant eggs and young destroyed on each visit. Attempts to eradicate the 

colony continued annually until 1972, apparently without success (Krohn et al. 1995).

There appear to be few technical problems with accomplishing the goal of killing eggs 

and nestlings. Other than asphyxiation with CO2, equipment needs are minimal. However, all 

techniques except shooting are limited to ground nests because climbing trees is not only time 

consuming, but it can be hazardous in trees dying from the effects of cormorant activities. One 

solution proposed to bypass the latter problem was to use trained monkeys to destroy eggs in 

Great Cormorant tree nests, but this technique does not appear to have been used (Van Dam 

1997).

Given a lack of systematic investigations of these techniques, it is not possible to assess 

the economic effectiveness of these techniques at reducing cormorant impacts.

Egg Oiling / Spraying

Various substances have been applied to cormorant eggs to kill embryos. White mineral 

oil (variously: Daedol 50, Daedol 50 NF and DAEDOL 55 USP®) has been used most recently 

(Blokpoel and Hamilton 1989, Christens and Blokpoel 1991, Shonk 1998, Bédard et al. 1999) 

because of its reported lower toxicity and fewer environmental impacts compared to other 

substances that have been used, such as oil/water/formalin (Gross 1951) or acetic 

 

*

Ludwig and Summer (1995) state that the Great Lakes DCCO population in general could not reproduce at 

replacement rates until egg DDE levels fell to <10ppm, and population growth could not be sustained until levels 

dropped to 6-8ppm.
FINAL DRAFT Part V. Management Options 269

acid/formalin/cresylic acid/glycerine/water (McLeod and Bondar 1953). Other substances 

applied to eggs include “Gelfa” (an alcohol-soluble plastic solution), “Krylon” (a plastic aerosol) 

and shellac (DesGranges and Reed 1981). Although some of these substances may kill embryos 

through toxic effects, most are expected to prevent the exchange of gasses through the shell, 

causing asphyxiation (Blokpoel and Hamilton 1989; Christens and Blokpoel 1991; Shonk 1998). 

All citations above refer to control of DCCOs except Blokpoel and Hamilton (1989) who tested 

effects of white oil on chicken and Ring-billed Gull eggs.

Effectiveness at killing embryos is high, with mortality rates approaching 100 % when 

the oil / solution is applied correctly (Gross 1951; DesGranges and Reed 1981; Blokpoel and 

Hamilton 1989; Christens and Blokpoel 1991; Shonk 1998; NYSDEC 1999; Bédard et al. 1999). 

Although laboratory tests found oiling ineffective when applied only to part of an egg (Blokpoel 

and Hamilton 1989), field tests in which only the tops of eggs were sprayed were highly 

successful, indicating that careful application to the entire egg surface may not be necessary 

(Christens and Blokpoel 1991; Bédard et al. 1999). Egg-rolling activities by parents may assist in 

covering the entire surface (Christens and Blokpoel 1991).

Egg oiling / spraying has essentially the same result as that of killing eggs and nestlings, 

but with the added benefit that cormorants are less likely to abandon nests and lay replacement 

clutches, making the technique more effective as an annual treatment. Bédard et al. (1995a:81) 

report, “None of the eggs in 427 experimentally treated nests hatched in 1987 and nearly half 

were still tended by adults 49 to 59 days after laying (the remainder having been abandoned and 

scavenged).” Shonk (1998) reports, “Of the eggs treated with oil, 49 % were incubated past the 

expected hatching date. The remainder of the eggs were lost during the incubation.” Human 

disturbance during oiling increases predation by gulls (Shonk 1998), which may cause some 

cormorants to renest. However, no data are available on the rate of renesting after oiling, either 

in the same colony or elsewhere.

Effectiveness at population control through egg oiling is more uncertain. Because DCCO 

egg laying is not synchronous, only one spraying / summer may not treat enough of the eggs laid 

that nesting season to have a significant impact on the population, and multiple oiling efforts to 

overcome this problem may increase the cost of control beyond acceptable levels. Both problems 

have been cited as reasons for the failure of the Maine: case study (below) by Dow (1953). 

Modeling of the St. Lawrence Estuary, Québec (below and PART VI. POPULATION 

MODELING) suggested that egg oiling alone would not be sufficient to bring DCCO numbers to 

target levels within the desired time frame, leading to the selection of a combined strategy of egg 

oiling and shooting adults (Bédard et al. 1995a); however, effectiveness of the combined 

techniques was greater than predicted by the model (Bédard et al. 1999). Additionally, most tree 

nests are inaccessible to spraying by conventional techniques, limiting control to ground nests 

(Bédard et al. 1995a). Even for ground nests, meeting oiling goals may be logistically difficult: 

Bédard et al. (1999) report the St. Lawrence Estuary, Québec control program goal was 7000 

eggs oiled / year, but the average number oiled was 5019. Lack of inter-colony migration rates 

also makes it difficult to predict the minimum scale of control necessary to overcome effects of 

immigration.

Christens and Blokpoel (1991) also mention egg shaking as a technique that may kill 

embryos without destroying the egg, but conclude that egg oiling is just as effective and less 

labor intensive. Hobson et al. (1989) and Sheppard (1994/5) also mention that fishermen have 

used flame throwers to kill eggs without breaking them, apparently with success.
FINAL DRAFT Part V. Management Options 270

Maine:

From 1944 to 1954 USFWS sprayed over 180,000 DCCO eggs with oil and formalin to 

kill embryos (Hatch et al. 1995) and “to reduce [DCCO] numbers until they no longer prove to 

be a serious menace to the weir fishermen” (Gross 1944:536). According to Gross (1951:536), 

“We confidently predict that in a relatively short time the number of gulls and cormorants will be 

reduced by the control measures to a point where they will no longer be an economic problem.” 

Not everyone agreed that economic impact had been demonstrated, however: Dow (1953:1) 

stated, “It should be noted that there is also no definite evidence indicating that these populations 

should be controlled; or, if control is needed, to what extent it is desirable.” Despite the fact that 

the goal of the program was to reduce the economic impact of DCCOs to fisheries, economic 

impacts do not appear to have been monitored. Rather, effectiveness of the control program was 

measured by number of eggs sprayed (as an index of population size) and by how many of ten 

monitored colonies were abandoned. Because there was no clear downward trend in number of 

eggs sprayed, and only one of these colonies was abandoned during treatment years, the control 

program was declared ineffective and oiling ceased in 1954 (Hatch et al. 1995).

Prior to this control program, New England DCCO populations experienced a rapid 

increase in size (Drury 1973). While the control program did not reduce population size as 

hoped, it may have helped slow the growth rate. Gross (1944) conservatively estimated 20,000 

individual DCCOs nesting in Maine in 1943; Dow (1953) estimated a minimum 22,470 adults in 

Maine in 1952. DCCO breeding populations subsequently grew slowly, if at all (Drury 1973). 

Recent population estimates for Maine are around 17,000 to 18,000 nesting pairs (see PART II. 

POPULATION DATA AND TRENDS, Maine).

Avian Contraceptives

In 1999 USFWS received a request from VDFW to administer avian contraceptives 

DiazaCon and Nicarbazin to DCCOs in addition to other control activities on Young Island (see 

Lake Champlain, Vermont (USFWS 1999a), below). Results were expected to be similar to egg 

oiling. Permission was not granted to VDFW because neither contraceptive was registered for 

use on DCCOs (USFWS 1999a).

Killing Adults

Private DCCO control efforts have often included shooting adults in addition to 

destroying eggs and young (Matteson 1983; Hobson et al. 1989; Ewins and Weseloh 1994; 

Sheppard 1994/5; Carter et al. 1995; Jackson and Jackson 1995; Ludwig and Summer 1995; 

Weseloh and Collier 1995; USFWS 1999b). The goals of these private actions are unknown, but 

they have probably focused on reducing cormorant population levels directly by increasing 

mortality rates for adults and preventing further growth of populations by disrupting breeding. 

Shooting adults has also been proposed to prevent colonization at specific sites: NYSDEC was 

permitted by USFWS to take 30 (1994) to 40 (1995, 1996) adult DCCOs on Bass and Gull 

Islands on Lake Ontario in conjunction with other colony eradication efforts. No cormorants 

were reported taken these years, and USFWS subsequently stopped permitting this take because 

colony eradication had been successful without it (see Lake Ontario, New York (USFWS 

1999b), below).

The effectiveness of shooting to kill adults may be limited in some cases. As is discussed 

in 3) Lethal Control at Aquaculture Facilities (above), DCCOs are wary and difficult to shoot at 
FINAL DRAFT Part V. Management Options 271

aquaculture facilities. During the control program for breeding DCCOs on the St. Lawrence 

Estuary, Québec (below), Bédard et al. (1999:4) observed,

Depending on the terrain, nest density, and behavior of the birds, the operator could shoot as many 

as 100 (75 birds/h) from a single position before moving on as the area became devoid of birds. In 

colonies where forest cover was sparse and / or damaged by prolonged cormorant occupation, the 

operator could not hide as easily, and lower density of nesters and wariness of the birds reduced 

success considerably. This situation prevented us from applying a rigorously proportional culling 

scheme; on some islands, it was impossible to reach the allocated goal while relative tameness of 

the birds on others led us to exceed it.

The goal of the program was to kill 10,000 adult DCCOs (2,000 / yr * 5 years); culling 

lasted only four years, however, and in total 7,917 were reported shot. Bédard et al. (1999) also 

note that adult male DCCOs were more vulnerable to shooting than adult females, with a ratio of 

203:100 males to females shot during this control program.

Limited data are available on the effectiveness of killing adults on controlling local 

populations / colonies. Matteson (1983) notes that shooting DCCOs at pound nets by fishermen 

did not prevent the nearby Mink Island, Lake Ontario population from increasing from 40 nests 

in 1945 to 50 nests in 1956. Ludwig and Summer (1995) analyzed leg band recovery data for 

DCCO colonies in the Les Cheneaux region of Lake Huron; based on the level of immigration 

into this region, these authors concluded that lethal control of adults would have to be Great 

Lakes-wide to be effective at controlling local populations (see also Korfanty et al. 1997).

Shooting adults at colonies may have impacts beyond killing individual breeders because 

of harassment effects on survivors. Ewins and Weseloh (1994) report the shooting of > 50 adults 

on Pigeon Island, Lake Ontario, in 1993 (when the colony had 818 pairs) reduced reproductive 

output for that year: fledging rates were 0.3 vs. 1.6 young / nest on a nearby island that was not 

subject to shooting. Long-term impacts on the colony were not reported. 

Killing adults may be a successful technique for controlling regional DCCO populations, 

but may require that large numbers be killed and geographic range of the program be sufficiently 

wide. Hobson et al. (1989) noted that illegal killing of adult DCCOs, in conjunction with illegal 

destruction of eggs and nestlings on Lake Winnipegosis, Manitoba was not enough to prevent 

this population from increasing. Other investigators have stated that human persecution has 

contributed to declines in DCCO numbers over large regions during the mid 1900s (Ewins and 

Weseloh 1994; USFWS 1999b), that decreased shooting as a result of legal protection in 1972 

contributed to the subsequent population increases (Carter et al. 1995), or that shooting and other 

techniques may have slowed the growth of populations in the 1940s and 1950s (Weseloh and 

Collier 1995). However, it is difficult to separate out the effects of control from the effects of 

pesticides or changes in fish populations.

The St. Lawrence Estuary, Québec, control program (below) provides an example of a 

control effort intensive and expansive enough to reverse DCCO population growth. The 

effectiveness of shooting adults alone may be investigated if the tree-nesting DCCO population 

behaves independently from the ground-nesting population, as is believed by Bédard et al. 

(1999). The model used to decide which control techniques were necessary (see PART VI. 

POPULATION MODELING) predicted 6014 tree nests in 1991 and 4845 in 1993; observed 

numbers were 3865 and 2555 respectively. Bédard et al. (1999) discuss several possibilities for 

the higher realized effectiveness of shooting than predicted by the model, and conclude that the 

unequal sex ratio in adults killed probably accounted for the faster than predicted drop in DCCO 

numbers.
FINAL DRAFT Part V. Management Options 272

Effectiveness of killing adults relative to other forms of population control are unknown, 

although it is believed that, individual for individual, killing adults that have survived to breed is 

more effective at reducing populations than destroying eggs, nestlings or fledglings. Because 

first-year birds frequently have low survival rates, a significant portion of control on eggs and 

nestlings is probably “wasted effort” exerted on individuals that would not have survived to 

breed anyway. Ludwig and Summer (1995:40) estimate, “From the whole population 

perspective, killing a young adult just before first nesting will have a 3 to 6 fold greater effect on 

population control than killing fledglings, chicks or eggs.” Few economic data are available to 

compare techniques. Bédard et al. (1999) provide a “very rough estimate” for the St. Lawrence 

Estuary, Québec (below) control program of $10 / adult shot and $3 / nest sprayed with oil. 

Without better economic estimates on the costs of DCCO impacts, the cost effectiveness of 

population control by any means can not be determined.

Shooting adult DCCOs also raises the potential for by-kill, especially in regions where 

Great Cormorants exist. For example, on Prince Edward Island a hunting season on DCCOs had 

no significant impact on this species, but it reduced GRCO populations by 50 %. Nova Scotia 

does not permit the harvest of DCCOs because of potential impacts to Great Cormorants. Legal 

shooting of DCCOs in New Brunswick has also led to killing of Great Cormorants there (Milton 

et al. 1995; Korfanty et al. 1997).

No Control

As in the case of impacts to aquaculture, “no control” should be included on the list of 

management options for impacts to vegetation and habitat. Not only may the costs of controlling 

cormorants by all available methods be higher than the benefits of avoided impacts, but also 

impacts to other species or ecosystems may be better managed by focusing on other impacts than 

those caused by DCCOs. For example, on the St. Lawrence Estuary DCCOs may be an 

immediate threat to trees on some islands, and therefore need immediate local controls, but if 

DCCOs are exacerbating prior over-browsing impacts by herbivores (Bédard et al. 1995), control 

of the herbivores should also be considered in long-term management plans. As another 

example, concerns about potential impacts of DCCOs to habitat on the Upper Mississippi River, 

where cormorants likely altered vegetation prior to European settlement, stem from the fact that 

the dynamics of the river system and its islands have changed due to regular dredging and 

development of locks and dams (Kirsch 1995). Managing DCCOs to reduce impacts by other 

species, including humans, does not solve the original problem, may be less effective than 

dealing directly with the original problem, and places biologically unjustified blame on the 

species.

It has also been suggested that landowners be educated about the naturalness of habitat 

alteration by DCCOs, resolving the problem of DCCO impacts by changing the perception and / 

or understanding of DCCO impacts (Milton et al. 1995).

Case Studies: Multiple Techniques

The following case studies illustrate the use of multiple management techniques 

(discussed above) to control DCCO impacts to vegetation, colonial waterbirds and / or fisheries.
FINAL DRAFT Part V. Management Options 273

Lake Winnipegosis, Manitoba

Interpretations of cormorant control on Lake Winnipegosis in the 1940s and 1950s have 

varied, even though all citations led ultimately to the description of this program by McLeod and 

Bondar (1953), who described it as thus:

Control

The cormorant population of the area appeared to be excessive under existing conditions 

and attempts were made to bring the birds under control without threatening their extinction. To 

what extent earlier control measures had been applied or had been effective the writers were 

unable to determine. Certainly the eggs and young had been destroyed sporadically by fishermen 

for many years. During our visits to the rookeries in 1943, 1944 and 1945 at the height of the 

breeding season all eggs and young were destroyed.

A recheck of the effectiveness of this procedure on two occasions showed that at least 

30% to 50% of the pairs had renested. How successful the attempts at the late season would be is 

not known but a considerable portion, if not all, the young would be large enough to make the fall 

migration.

It was decided in 1951 that the killing of the embryos without egg destruction as 

mentioned by G[r]oss (1944) would prolong the brooding period and remove the stimulus to lay a 

second time, thus forming a much easier and more effective method of control... (p.9-10)

Control efforts made during visits to two colonies in 1945, and subsequent renesting, are 

detailed in a discussion of DCCO reproduction. The only other instances of DCCO control or 

take prior to 1951 presented are observations of egg destruction by lumbermen on Skunk Bay 

Reef and fishermen on Wade Point Reef, harvest of young from Bachelor Island each year by a 

mink rancher for mink food, and the collection of “very few” adults for diet analysis.

In 1951 twenty nine different solutions were tested on chicken eggs, five of which 

consistently killed the embryos. Of these, one solution was applied in the field to DCCO eggs, 

with an apparent reduction in effectiveness, and the authors concluded that “the method is 

considered to be not more than 50 % efficient at best” (p.10). No mention is made of further egg 

treatments or official control after 1951.

Prior to 1943, “information gathered from personal observations and from local 

fishermen indicated that the Double-crested Cormorant had been increasing quite rapidly” (p.9). 

Complete surveys of Lake Winnipegosis in 1945, 1950 and 1951 revealed a definite downward 

trend in the number of observed nests, with 9862, 5949 and 4656 reported, respectively. Based 

on uncited sources, a correction factor of four was multiplied by the number of nests to estimate 

the total number of breeding and nonbreeding cormorants on the lake for each year of survey, 

leading to total population estimates for 1945, 1950 and 1951 of 39,448, 23,796 and 18,624, 

respectively.

Conclusions relevant to control were (as numbered):

6. If the first eggs or the first young of the season are destroyed at an early age about 50% of the 

reproducing pairs will renest again almost immediately.

8. Consistent destruction of the eggs and young appears to reduce the breeding population fairly 

effectively but it was not possible to determine how many were driven to seek new nesting areas 

or what the mortality from natural causes was.

9. Five different chemical solutions were found to give 100% kill of hen embryos but the only one 

tried on cormorant eggs in the field gave not more than 50% efficiency.

10. The cormorant population on Lake Winnipegosis is now at a level where the problem in 

connection with fish predation is less acute than formerly but still requires attention. (p.11)
FINAL DRAFT Part V. Management Options 274

Conclusions relevant to future population control efforts from the data available in 

McLeod and Bondar (1954) are limited because it is not possible to determine the level of private 

control efforts that took place in addition to official ones. The combined efforts appeared to be 

successful at lowering lake-wide breeding populations, although it is always possible that other 

unknown factors may have contributed to the decline. Important questions remain unanswered. 

Would official egg destruction activities have had a significant impact on the population without 

concurrent private control activities? Because “this decline merged with a much wider fall in 

numbers retrospectively attributed to pesticides,” (Hatch 1995:13) how much of the observed 

decline can be attributed to the combined control efforts? If the population had been nearly 

halved from its peak in 1945, yet still required attention, what population goal would have met 

the objective of bringing the birds “under control without threatening their extinction”?

Illegal controls have continued on Lake Winnipegosis, and some believe that these 

disturbances have resulted in more breeding on the lake because adults are forced to pioneer new 

nesting islands which are typically large and treed (B. Koonz, pers. comm.).

St. Lawrence Estuary, Québec

DCCOs nesting on the St. Lawrence River Estuary were subjected to a five-year control 

effort using both egg oiling and shooting adults to reduce impacts on unique insular forest 

ecosystems (Bédard et al. 1995a). The program was based on the results of a model of various 

control techniques (see PART VI. POPULATION MODELING). The goal of the program was 

to decrease the breeding DCCO population to 10,000 pairs by killing 2,000 adults and oiling 70 

% of all accessible nests each year. Although the model was considered conservative, the control 

program had a much more dramatic impact than was predicted, decreasing the population to 

approximately 12,000 pairs in only three years.

The control effort was initiated by island landowners concerned about damage to 

vegetation who lobbied the Ministère du loisir de la chasse et de la pêche. Sport and commercial 

fishermen also voiced complaints about DCCO impacts to fisheries, and while Bédard et al. 

(1995) suggests that these interests were considered by the Ministère, Bédard et al. (1999) 

specifically state that the fishing lobby was not involved in the decision to control cormorants. 

The goal of the control program was to reduce the DCCO population to lessen impacts to 

vegetation, but without eliminating the population altogether: “Maintaining a level of 10,000 

pairs in the estuary was deemed adequate to supply opportunities for bird watchers while at the 

same time maintaining substantial colonies.” (Bédard et al. 1995:79) How this numerical goal 

was obtained is not explained.

The model (see PART VI. POPULATION MODELING) predicted that neither egg oiling 

nor shooting adults alone would be successful at meeting the numerical goal within five years, 

and that even when combined both techniques would have to be applied at what was considered 

their maximum feasible levels to reach the goal. Therefore it was decided to attempt to shoot 

2000 adult DCCOs and oil eggs in 70 % percent of all accessible (ground) nests. Control efforts 

began in 1989. Shooting was found to be more effective at reducing the tree-nesting segment of 

the population than was predicted by the model (see Killing Adults, above) and was discontinued 

after the fourth year. Despite the noted wariness of cormorants, the annual shooting quota of 

2000 was apparently not difficult to meet: from 1989 to 1992 the reported number shot was 

1993, 2078, 1778 and 2058, respectively. Because access was denied to some oiling efforts, the 

goal of oiling 70 % of accessible eggs was not met, and ranged from 53 % to 62 % during the 

five years of oiling efforts (Bédard et al. 1995).
FINAL DRAFT Part V. Management Options 275

Reasons for the apparent over-effectiveness of the selected control techniques relative to 

model predictions are discussed in PART VI. POPULATION MODELING. Because impacts by 

DCCOs were not quantified, the effectiveness of the control program at reducing damages can 

not be determined in a traditional benefit:cost sense. Whether damage to island vegetation has 

been reduced, or even avoided, has not been discussed. Bédard et al. (1999) estimate total cost of 

the control program was $38,000 / yr, and that, as a very rough estimate, this was about $10 / 

adult shot and $3 / nest oiled. They note, however, that these estimates are site-specific and 

cannot be generalized.

Long-term effectiveness of the program is uncertain, especially because the population 

has not been censused since 1993. Measures that could be taken to maintain the population at the 

desired level (e.g., egg oiling in the largest ground-nesting colonies, disturbances to tree-nesting 

colonies, dislodging tree nests with long poles) have been discussed, but do not appear to have 

been modeled or tested (Bédard et al. 1999).

Lake Champlain, Vermont (USFWS 1999a)

Prior to 1999 the State of Vermont Department of Fisheries and Wildlife (VDFW) held 

USFWS depredation permit MB-751275 to control DCCOs and several gull species through nest 

removal on Lake Champlain. This permit was used to protect vegetation and wildlife (especially 

Common Terns) on certain islands from alteration through colonization by these species. In 1999 

VDFW requested an amendment to this permit to allow DCCO population control activities 

intended to reduce impacts to vegetation and wildlife by reducing colonization rates. Requested 

activities included oiling eggs, killing nestlings and administering avian contraceptives on Young 

Island, the largest DCCO colony on Lake Champlain; on other islands, permission was requested 

to continue nest destruction activities. After reviewing the request, USFWS granted VDFW 

permission to oil eggs on Young Island and to continue nest destruction on other islands. Killing 

nestlings was not permitted because insufficient evidence was presented that this control was 

necessary for meeting population goals. Administration of avian contraceptives was not 

permitted because none was registered for use with DCCOs. The effectiveness of egg oiling was 

determined through two means: 1) modeling by Fowle (1997) (see PART VI. POPULATION 

MODELING) and 2) the presumption that reducing local population growth would reduce local 

colonization rates. Under this control program, virtually all DCCO reproduction was permitted to 

be prevented on Vermont waters of Lake Champlain, a roughly 64 % decrease in lakewide 

reproduction.

Documentation of significant impacts by DCCOs to plant or animal species is difficult to 

find within the USFWS assessment of the VDFW proposal. Although dramatic changes in plant 

and avian diversity had occurred on Young Island as a result of colonization by DCCOs, none of 

the three management options assessed (nest removal, egg oiling and nest removal, and all 

activities proposed by VDFW) was expected to benefit fauna or flora on this island. On other 

islands, species of conservation concern were known or expected to exist, but no specific threats 

had been identified. Even on Bixby Island, where the Canadian Milk Vetch (a State-listed 

endangered species) was known to exist, no data were presented demonstrating that DCCO 

colonization attempts occurred on parts of the island where this plant grew. Direct interactions 

between DCCOs and Common Terns, the other State-listed endangered species mentioned in the 

assessment, were described as uncertain, and difference in nesting habitat was suggested as a 

possible explanation for limited interactions between these species.
FINAL DRAFT Part V. Management Options 276

The USDA/WS environmental assessment prepared in conjunction with the VDFW and 

NYSDEC for their proposal to the USFWS (Garland et al. 1998) provides slightly more 

information on DCCO impacts. Cormorant roosting is believed to have shifted gull and tern 

nesting locations on Popasquash Island and to have “affected” the number of terns using the 

island. La Barr and Rimmer (1996) are cited after a statement that establishment of a cormorant 

colony on this island might result in the loss of nesting terns there. Successful colonization of 

Bixby Island is said to likely result in the loss of Canadian Milk Vetch there, but no details are 

given. L. Garland, VDFW (pers. comm.) has indicated that the vetch occurs on Bixby Island 

under the drip edge of cottonwood trees in which DCCOs have attempted to nest.

Past removal of newly-established colonies through nest removal and / or harassment was 

found to be successful at preventing colonization of new islands on Lake Champlain by DCCOs 

(see also Nest / Tree Removal, above.). Effectiveness of this technique varied among islands, 

with some requiring more frequent treatments than others. In general, nest removal was found 

not to have long-term effects, but required ongoing monitoring and management. Nevertheless, 

this technique could be used by itself to prevent impacts to vegetation and wildlife.

Effectiveness of egg oiling is more difficult to assess. Based on previous research, ability 

of this technique to prevent reproduction is well established. Ability of egg oiling to prevent 

local impacts to vegetation or wildlife is less clear, however. If the Lake Champlain DCCO 

population received no immigration from other populations, then reductions in this population 

might lead to reduced colonization rates. However, some level of immigration into the Lake 

Champlain DCCO population must exist (otherwise, the lake would not have been colonized in 

1982), and if high enough, immigration may decrease the effectiveness of local population 

control at reducing colonization rates. As growth continues in DCCO colonies on New York 

waters of the lake and in the Missiquoi NWR, and as these colonies become saturated with 

nesting birds, then colonization rates would be expected to increase. Additionally, holding the 

lake-wide population below its natural carrying capacity may actually increase the immigration 

rate if surrounding populations continue to grow to higher densities. Even in the more thoroughly 

studied European GRCO, factors influencing colonization dynamics are speculative (van Eerden 

and Gregersen 1995). As with nest removal and harassment, egg oiling requires ongoing 

management efforts to be effective.

Effectiveness of egg oiling at influencing local population levels is difficult to interpret 

from the assessment. When discussing effects of egg oiling on this population as predicted by the 

model constructed by Fowle (1997), the assessment predicts that oiling would not prevent the 

population from increasing through 2004, but that the rate of growth may begin to decrease after 

2002. In response to public comment on the draft assessment, the model is described as 

predicting that oiling would stabilize the population by 2004. In the ultimate Finding of No 

Significant Impact, egg oiling was predicted to lead to declines in the lake-wide population 

through 2004.

In conclusion, the addition of egg oiling to Lake Champlain DCCO management efforts 

will probably not threaten this population. However, because previous nest removal and 

harassment efforts were successful at preventing colonization, egg oiling was not necessary for 

meeting VDFW goals (preventing colonization of additional islands by DCCOs). Both 

management strategies will require ongoing management efforts, and no data were presented to 

suggest that egg oiling was a more cost effective technique. Without immigration data, 

effectiveness of population management at reducing colonization rates could not be predicted.
FINAL DRAFT Part V. Management Options 277

Lake Ontario, New York (USFWS 1999b)

Prior to 1999 the New York State Department of Environmental Conservation 

(NYSDEC) held USFWS depredation permit MB-828930 to prevent colonization of DCCOs on 

certain islands in the Eastern Basin of Lake Ontario with nest and egg destruction and lethal 

control. The goal of these control activities was primarily to protect other colonial waterbirds 

from negative impacts due to DCCO nesting. In 1999 NYSDEC requested an amendment to this 

permit to allow DCCO population control activities intended to reduce impacts to colonial 

waterbirds and to improve the Eastern Basin smallmouth bass population. Requested activities 

included oiling eggs and shooting adults on Little Galloo Island, the largest colony on the 

Eastern Basin; on other islands, permission was requested to continue nest destruction activities. 

After reviewing the request, USFWS granted NYSDEC permission to oil eggs on Little Galloo 

island and to continue nest destruction on other islands. Shooting of adult birds on Little Galloo 

was not permitted “because insufficient information is available to justify this take in accordance 

with USFWS depredation policies.” Because USFWS is not permitted to issue depredation 

permits to benefit sport fisheries, the decision to permit egg oiling was based on impacts to birds 

and vegetation. Under this control program, all DCCO reproduction was permitted to be 

prevented on U.S. waters of the Eastern Basin, a roughly 75 % decrease in reproduction.

In addition to legal NYSDEC control efforts, this population was subject to illegal control 

activities in 1998, during which approximately 2000 DCCOs, mostly juveniles, were shot on 

Little Galloo Island. Disturbance by researchers was cited as the probable cause of observed 

declines in DCCO colony size on Little Galloo Island since 1996.

More detailed biological information is provided in this environmental assessment than in 

the VDFW assessment (above), but specific impacts are still difficult to find. Although dramatic 

changes had occurred on Little Galloo Island as a result of colonization by DCCOs, none of the 

three management options assessed (nest destruction, egg oiling and nest destruction, and all 

activities proposed by NYSDEC) was expected to benefit fauna or flora on this island. On other 

islands, species of conservation concern were known to exist, but no specific threats were 

identified. Common Terns were State-listed threatened; however, impacts of DCCOs on groundnesting terns in general were described as minimal, direct interactions between DCCOs and 

Common Terns were uncertain, and difference in nest-site selection was suggested as possible 

explanation for limited interaction between these species. Black-crowned Night-herons were 

described as rare and local nesters in northern New York, and past interactions with DCCOs 

suggested cormorants could exclude BCNHs from some sites, but DCCOs impacts on regional 

BCNH populations were unknown. Caspian Terns were also mentioned within the assessment, 

but no negative interactions had been documented between this species and DCCOs (even on 

Little Galloo Island where both species were increasing), and negative impacts from DCCOs 

were predicted on Little Galloo Island only if the Caspian Tern colony grew to an even larger 

size.

Past removal of newly-established colonies through nest removal was found to be 

successful at preventing colonization of new islands on the Eastern Basin by DCCOs. As on 

Lake Champlain (above), effectiveness of this technique varied somewhat among islands, and 

the effects were not long-term, but in an ongoing monitoring and management program this 

technique could be used by itself to prevent impacts to vegetation and wildlife.

Assessment of the effectiveness of egg oiling at reducing population levels and 

colonization rates is similar to that of egg oiling on Lake Champlain (see above). Immigration 

data are not available for this population. Predictions of effectiveness of egg oiling at managing 
FINAL DRAFT Part V. Management Options 278

populations in this assessment appear to be based on the Lake Champlain model by Fowle 

(1997), and in this assessment, these predictions showed the same pattern of multiple 

interpretations as in the Vermont assessment.

In conclusion, addition of egg oiling to Eastern Basin DCCO management efforts will 

probably not threaten this population. However, because previous nest removal efforts were 

successful at preventing colonization, egg oiling was not necessary for meeting NYSDEC goals 

of preventing impacts to vegetation and wildlife. Both management strategies require ongoing 

management efforts, and no data were presented to suggest that egg oiling was a more cost 

effective technique. Without immigration data, the effectiveness of population management at 

reducing colonization rates could not be predicted.

Conclusions: Managing Impacts to Vegetation and Habitat

In cases where DCCO nesting activities may impact threatened or endangered species in 

habitat-limited regions, prevention of colonization has been demonstrated as a successful tool for 

preventing these impacts without resorting to population control. If cases are found where these 

local control techniques are insufficient, population control of nesting DCCOs may be successful 

at reducing population levels under two conditions: (1) scale of control is large enough to 

overcome the effects of immigration and (2) the control effort is well coordinated, long-term, and 

sufficiently rigorous to overcome density-dependent compensation mechanisms. Few data are 

available to predict what levels of control are needed to be successful, however. Modeling of 

European GRCO population control techniques suggests that preventing the establishment of 

new colonies is the only technique not buffered by density-dependent mechanisms (Bregnballe et 

al. 1997); currently this population control technique is being utilized in Denmark (Bregnballe 

and Asbirk 1995).

MANAGING IMPACTS TO FISHERIES

Site-specific DCCO impacts to fisheries generally occur in three situations in which fish 

are aggregated at high densities: 1) large-scale releases of hatchery-raised lake trout and other 

lake fish, 2) salmon smolt runs, and 3) fish harvest methods such as pound nets and weirs that 

congregate fish in enclosed areas that are open to cormorant predation. Management techniques 

for these impacts are discussed below.

Techniques for managing entire DCCO populations are available and may be successful 

at reducing DCCO numbers if applied on the proper scale (see Managing Impacts to Vegetation 

and Habitat, above). However, because many uncertainties remain regarding impacts of DCCOs 

on fisheries (see PART III: DIET), effectiveness of population reductions at reducing impacts to 

fisheries can not be predicted.

Hatchery-raised Trout Releases

Various simple techniques are available for preventing released trout from remaining in 

large aggregations that are vulnerable to preferential predation by cormorants. Fish can be 

released during the night, providing them with time to disperse before cormorants begin feeding 

the following morning. Fish dispersal can also be aided by releasing them from barges in deep 

water, rather than from the shore. Studies in 1993 and 1997 by the NYSDEC found that bargestocked trout survived better than those stocked from shore. Fish releases in streams may be 
FINAL DRAFT Part V. Management Options 279

improved by stocking earlier in the season, before cormorants return from migration (NYSDEC 

1999, NYSDEC Special Report 1999).

The criticism by Conniff (1991) that night releases were unnecessary on Lake Ontario 

because DCCO diet was found to be primarily alewives in that lake appears to overlook the 

temporal differences in diet studies versus stocking events. Although the proportion of trout to 

alewives in DCCO diet may be small over an entire season, the proportion could be much higher 

on days that trout are stocked if cormorants feed preferentially on these fish. If such a change in 

feeding behavior were observed in DCCOs, and if this feeding behavior has a significant impact 

on the survival of the released trout, changing to night releases would be beneficial.

Lethal and / or non-lethal harassment may also reduce predation on swarms of recently 

released fish. As is discussed in the Lake Ontario, New York (USFWS 1999b) case study, 

NYSDEC has held permits to shoot up to 40 DCCOs at fish stocking locations 1997-1999. 

However, no DCCOs have been reported taken under this permit, suggesting that the above 

alterations to stocking methods may be successful without harassment (USFWS 1999b).

Salmon Smolt Releases / Runs

Bayer (1989) presents a detailed review of salmon hatchery management techniques that 

may help reduce losses of smolts to predators in general on the Pacific Coast. Most investigators 

have addressed the topic of DCCO predation on salmon smolts, focusing on techniques for 

relieving DCCO predation pressures only. These DCCO-specific techniques all attempt to move 

migrating smolts past areas of cormorant predation (especially river segments near DCCO 

colonies) as quickly as possible and with smolts highly dispersed. Milton et al. (1995) and Price 

and Nickum (1995) both cite personal communication with G. Farmer that changing the time of 

releases improves survival of smolts. Releasing during times of higher river discharge rates and 

later in the year appears to reduce recovery rates of salmon tags at nearby DCCO colonies. 

Releasing smolts higher upstream from regions near DCCO colonies or above lakes upstream 

from cormorant feeding areas also breaks up congregations of smolt that might otherwise attract 

cormorant predation. Matteson (1983) citing Bissonette et al. (1982) and Roby et al. (1998) also 

suggests altering the timing and location of salmon releases.

Nets / Weirs

Although largely anecdotal, data exist that strongly suggest cormorants may have an 

impact on fish catches in purse-seine nets, pound nets and weirs. These are all fish-capture 

techniques that passively trap fish in enclosures that are open to the sky, rendering them 

vulnerable to avian predation until the fish are harvested. Although direct losses through 

predation may be of concern, the impact discussed in the greatest detail appears to be gilling: 

behavioral responses to cormorant feeding by fish (even those too large to be eaten by DCCOs) 

may result in the fish gilling themselves in nets. Gilled fish have a lower market value than nongilled fish (Wilson 1941; Omand 1947; Matteson 1983; Craven and Lev 1987). Craven and Lev 

(1987) reported that an increase in the DCCO population on the Apostle Islands, Lake Superior 

was associated with increased gilling rates, and that increased gilling rates were also associated 

with fledging DCCOs first beginning to forage for themselves. Site-specific, non-lethal control 

techniques appear to have been discussed most frequently in the literature. Population control 

was considered in the Apostle Islands, but not attempted because the colonies involved were on 

National Park Service lands, where such control was not permitted.
FINAL DRAFT Part V. Management Options 280

Equipment Modification

Covering these nets with netting or wires has been proposed to prevent DCCO access to 

fish trapped within. However, DCCOs appear to adapt readily to entering the nets through the 

same entrances as the fish, easily exiting by the same route and leaving gilled fish behind 

(Omand 1947; Matteson 1983). Modifying nets and the poles that support them to prevent 

perching has been suggested not only because these perches might serve as vantage points for 

foraging cormorants, but also because the mere sight of predators may frighten fish into gilling 

themselves (Matteson 1983; Craven and Lev 1987). For pound nets, metal cones or spikes have 

successfully prevented perching on poles, and electric wire has prevented perching on the nets 

themselves (Craven and Lev 1987; S. Craven pers. comm.). Reducing the size of the mesh in 

nets has been suggested as a means to reduce gilling (Matteson 1983), but this technique appears 

not to have been investigated.

Harassment

The harassment techniques suggested and tested at nets are similar to those used at 

aquaculture facilities, as are the conclusions drawn from them. Craven and Lev (1987) tested the 

effectiveness of owl decoys, mylar balloons, hanging scarecrows, untended boats, and an audio 

device called Av-Alarm. They found that DCCOs habituated to all devices over time, but that the 

combination of a boat with a scarecrow was successful at preventing cormorants feeding in the 

nets for at least five weeks of a six week trial. Success of the techniques was generally measured 

in terms of numbers of cormorants observed feeding in the nets, but the combined scarecrow and 

boat technique was reported to have reduced gilling rates as well. The Av-Alarm not only was 

unsuccessful in that DCCOs were observed perching within seven feet of the speaker, but also 

was incompatible with National Park Service policies. Craven and Lev (1987) ultimately suggest 

a combination of harassment (boat, scarecrow, mylar balloons) and equipment modification 

(metal cones or spikes on poles and electric wires on nets).

Matteson (1983) also discusses harassment techniques that were tried by fishermen 

themselves in the Apostle Islands, none of which were reported to have worked: rubber snakes, 

wind wheels, brightly-colored flags, eagle decoys, pieces of metal, and gas cannons. Not only 

was the gas cannon incompatible with National Park Service policies, but it also was reported to 

keep fish away from the nets. Matteson (1983) also mentions techniques that were suggested, but 

apparently not tested: gas cannons, sonic horns, distress calls, hanging dead cormorants from 

posts, and suspending Bald Eagle or Gyrfalcon decoys over nets in conjunction with recorded 

vocalizations of these birds.

Wilson (1941:11) reports discussions with Door County, Wisconsin fishermen who 

“claimed that after the cormorants appeared in late summer no fish were caught in the pound 

nets. Where catches of several hundred pounds of fish had been made, lifts dropped to 15 – 20 

pounds. In desperation scare crows were nailed onto the net stakes to frighten the cormorants. 

Good lifts resulted.”

In Nova Scotia cormorant predation was described as a site-specific problem for weir and 

gill-net harvests of Mackerel, Atlantic herring and alewife. Scare and kill permits are reported to 

have resolved the problem of cormorant predation, though details are not presented (Milton et al. 

1995; Keith 1995).
FINAL DRAFT Part V. Management Options 281

Alternative Perches and Prey

Craven and Lev (1987) note that DCCOs perch on poles used to support pound nets even 

when nets are not in place (natural perches being scarce in the area), and suggest that erecting 

perches away from nets, but still in suitable foraging habitat, may help control efforts by 

“diluting the problem” and by providing cormorants with alternative perches from those at which 

harassment is being practiced. These alternative perching sites could be combined with the 

suggestion by Matteson (1983) that DCCOs be provided with an alternative foraging site stocked 

with potential prey near the main Apostle Islands colony. Matteson (1983) admits, however, that 

supplemental feeding may be a short-term solution that ultimately leads to larger long-term 

problems associated with population growth.

Reduce Time Fish Spend in Nets / Weirs

Reducing the amount of time that fish spend in nets and weirs would reduce their 

exposure to DCCO predation and presumably impacts to these fisheries (Matteson 1983). No 

economic analyses have been reported, however, that compare the costs of increased harvesting 

frequency relative to avoided losses due to cormorants.

4) Conclusions: Managing Impacts To Fisheries

Effective techniques are available for reducing local DCCO impacts to fisheries through 

changes in management techniques, equipment modification and harassment. Reducing DCCO 

populations requires large efforts to be effective (see Managing Impacts to Vegetation and 

Habitat, above) and may not result in significant benefits to fisheries populations if predation is 

compensatory (Bayer 1989; Marquiss and Carss 1997).

SURVEY RESULTS

Only 11 of the 37 states and provinces for which surveys were completed reported that 

cormorant control methods were currently being employed (Table 28). Of these, all employed 

lethal and non-lethal measures except Québec, which employed only lethal control, and 

Massachusetts, which only employed non-lethal control. Previous control efforts (shooting and 

egg destruction) were reported for Rhode Island, but the program was discontinued due to a lack 

of personnel to conduct work.

Six of the states employing control measures were in the southern U.S. These states were 

conducting control programs because of depredations at aquaculture facilities and fish hatcheries. 

All of these states incorporated lethal and non-lethal control measures.

In the Northeast, New York and Vermont are employing control measures due to habitat 

destruction and impacts to other colonial waterbirds in the Great Lakes. Massachusetts has 

undertaken limited control measures at specific sites in the 1990s. At Ram Island, a tern 

restoration project was underway and non-lethal cormorant control was employed in 1990 and 

1991. Efforts were described as a “worrying program”: 2 individuals went to the island everyday 

and harassed nesting DCCOs, broke up their nests and destroyed eggs. These activities were very 

successful in discouraging DCCOs from nesting. In 1991, activities were begun on April 29, and 

all birds abruptly abandoned the site on June 7; they have not since returned. Other measures had 

previously been employed at this site, including the installing of a bathel—a 4 foot wire fence 

run throughout the colony area, and topped with strands of twine. This was done to make 

takeoffs and landings more difficult, but was a total failure. Birds perched on the stakes, 
FINAL DRAFT Part V. Management Options 282

incorporated twine into their nests, and braced their nests against the fencing. Harassment is 

being employed at a night roost/staging area in Orleans, MA and includes pyrotechnics. Virginia 

did not provide information about why cormorant control was employed.

In Canada, management of cormorants falls under provincial jurisdiction. The 

government of Manitoba does not undertake any control activities, but much illegal control 

occurs around Lake Winnipegosis. These activities are undertaken principally because 

cormorants are perceived to impact fish populations, even though a recent study by Hobson et al. 

(1989) indicated that commercially valuable species do not contribute significantly to the diet of 

cormorants breeding on Lake Winnipegosis. In fact, data collected between 1945 – 2000 indicate 

declines in commercially valuable fish populations on Lake Winnipegosis are due to overfishing 

(W. Lysack, pers. comm.). Cormorants are also killed in this area because of impacts to trees, but 

to a lesser degree. Québec undertakes cormorant control because of impacts to unique insular 

habitats and privately owned islands (see above).

Table 28. States and provinces practicing Double-crested Cormorant control.

State Lethal measures Non-lethal measures

AL Shooting Harassment

AR Shooting Harassment, noise-making, decoys

LA Shooting Multiple harassment techniques

MA None Harassment 

MS Shooting Harassment; Night roost dispersal 

program

NY Egg destruction, egg oiling Nest destruction

OK Shooting Hazing

QC Shooting, egg oiling none

TX Shooting Harassment

VA Yes1 Yes1

VT Egg oiling Harassment; nest destruction

1 Both lethal and non-lethal measures are undertaken, but details on specific measures 

employed were not provided.
FINAL DRAFT Part VI. Population Modeling 283

PART VI. POPULATION MODELING

INTRODUCTION

In his overview of the symposium “The Double-crested Cormorant: Biology, 

Conservation and Management” held in 1992, Nisbet (1995:248) provides an excellent summary 

of knowledge of DCCO population dynamics and modeling efforts:

In comparison with other widespread and common colonial waterbird species, the breeding 

biology and population dynamics of the Double-crested Cormorant are very poorly known. The 

comparative neglect of this species by biologists may be due in part to prejudice against an ugly, 

black, smelly bird (Hatch 1995), but is more attributable to difficulties in studying it. Cormorants 

are very sensitive to disturbance when nesting, commonly nesting in trees, and are difficult to 

catch or to observe in winter. With one notable exception in California (M. M. Bishop, unpubl. 

data), no long-term studies of breeding birds were presented or cited [in the symposium], and no 

studies of individually-marked or known-aged birds have been conducted. Although data on 

productivity have recently been generated (Weseloh et al. 1995, Chapdelaine and Bédard 1995, 

Stenzel et al. 1995), there is little or no information on age at first breeding, recruitment, dispersal 

or age-specific mortality. Consequently, the parameters used in population models (Bédard et al. 

1995) are quite speculative. More important, we do not know how these population parameters 

vary with population density, colony size or environmental conditions. Hence, we do not know 

what factors may limit the populations, either now or in the future. Data presented in several 

papers suggest that some populations may have started to level off in the late 1980s. However, 

even if this proves to be true, it is not clear whether the limiting factors would be those acting in 

the breeding area or in the winter quarters. At present, there is no way to predict population trends 

even in the immediate future (five years or less). The appearance and rapid spread of Newcastle 

disease among breeding cormorants in 1992 increases the uncertainty about future trends.

The Birds of North America account for DCCOs (Hatch and Weseloh 1999:20) indicates 

that there has been little improvement in knowledge of DCCO life table parameters since the 

above-mentioned symposium, stating,

Compared to other widespread colonial waterbirds, population dynamics of Double-crested 

Cormorant poorly studied; no life table constructed. Key demographic parameters have been 

established at only 1 colony, in a 3-yr study of P. a. albociliatus on Mandarte I., British Columbia, 

where numbers were increasing by 8.4% / yr (van der Veen 1973). Large demographic differences 

are likely between populations that are resident or migratory, expanding or stable, but such 

differences have not been examined.

Their Appendix 2 cites 16 sources of DCCO annual reproductive success estimates, 

ranging from 0.01 young / nest (Gress et al. 1973), to 2.59 young / nest (McNeil and Léger 

1987); the most recent of these estimates came from Bédard et al. (1995). When discussing 

annual reproductive success, Hatch and Weseloh (1999:20) also caution, “All studies probably 

are subject to observer effect and subsequent predation...and all figures are much lower for 

DDE-contaminated populations.” Discussion of population regulating / density dependent factors 

remains speculative (e.g. Ludwig and Summer 1995). Despite lack of information on DCCO life 

table parameters, informative models can be built to investigate cormorant population dynamics 

and to determine which parameters need to be obtained with greater accuracy to reduce 

uncertainty in predictions of population trends.
FINAL DRAFT Part VI. Population Modeling 284

Many types of population growth models exist. Some are deterministic (i.e., all 

parameters are fixed values and therefore models predict the same outcome every time). Other 

models are stochastic (i.e., some or all parameters vary, causing the model to behave differently 

with each trial). Deterministic models are sometimes subjected to sensitivity analyses (in which 

parameters are varied and observations are made of subsequent changes in model behavior) 

because life history parameters are either poorly known or have been shown to vary. These 

analyses are important for determining how much confidence can be placed in predictions made 

by deterministic models. Stochastic models, in contrast, incorporate variation internally, and 

statistical analyses are performed on the models’ multiple trials to measure overall levels of 

variation. To our knowledge, all DCCO models have been deterministic. Models can also be 

classified as exponential (parameters are constant/density independent) or logistic (parameters 

are variable / density dependent:). Not all density dependent models behave logistically, 

however. Other population model types exist, but will not be discussed here. Also, we do not 

review non-population growth models, such as DCCO bioenergetics models (e.g. Cairns et al. 

1991, Glahn and Brugger 1995, Madenjian and Gabrey 1995).

Selection of the proper model to construct depends on data available, questions the model 

is intended to answer, and external factors likely to influence population dynamics over time 

period to be modeled. For example, although exponential models are biologically unreasonable 

in their assumption that populations are not limited by food, physical space, disease, or other 

factors, for many applications they 1) may be the only models possible to construct given 

available data, and 2) may approximate the dynamics of populations well below their carrying 

capacity. Additionally, the scale of the population to be modeled is an important consideration, 

especially for predicting the effects of emigration / immigration.

For DCCOs, models have generally been constructed to perform one of three tasks: 1) 

estimate past growth rates, 2) predict future DCCO population levels (and presumably impacts) 

in the absence of control, and 3) predict the effects of various management techniques on DCCO 

population levels. Another interesting use of a population model (see Lake Ontario, below) 

attempts to determine whether observed growth in the Lake Ontario DCCO population could 

have occurred without immigration.

In this section we review the basic population models available, focusing on those used 

for DCCOs, and discuss case studies where appropriate.

DETERMINISTIC EXPONENTIAL MODELS

Basic Exponential Growth Models

The basic exponential growth model is one of the simplest models of population 

dynamics. Essentially, this model assumes a constant per capita rate of increase (or decrease) 

over time, which results in populations growing (or declining) by a fixed percentage of their size. 

Modeling specific populations involves measuring past growth rates and then applying these 

rates to current population levels to project future growth. There are two basic equations, one 

used primarily for discrete time intervals (usually years):

Nt = Noλ

t

and one for continuous time:
FINAL DRAFT Part VI. Population Modeling 285

Nt = Noe

rt

where No is initial population size, Nt is predicted population size at time t, λ is the annual 

population growth factor (or annual finite rate of increase) and r is the instantaneous, per-capita 

growth rate (or simply rate of increase) of the population. The relationship between these growth 

measurements is er = λ, and (λ - 1) * 100 is the mean annual percent increase in the population. 

The instantaneous growth rate, r, incorporates the four factors that can change population size: 

birth, death, immigration and emigration. Thus, given an initial population size and that 

population’s past growth rate, it is possible to estimate future population levels if that growth rate 

remains constant by simply plugging the numbers into these equations. Predicting how long 

DCCO population growth rates will remain constant (unaffected by density-dependent factors 

such as food, space, disease, etc.), and therefore how far into the future populations growth can 

be reliably predicted, is difficult because of the lack of data for DCCOs.

Of the many studies that report exponential growth rates, relatively few have used this 

model explicitly to predict future DCCO population trends. Those that have done so have limited 

their projections to relatively short lengths of time. USFWS (1999a,b) project DCCO population 

growth on Lake Champlain, Vermont and on the Eastern Basin of Lake Ontario, respectively, 

over one year to assess the impact of proposed control measures on these populations. Fowle 

(1997) predicts growth in the Lake Champlain population over four years (1996 – 2000) in the 

absence of control. Bédard et al. (1995) predict population growth in the St. Lawrence Estuary 

population over five years (1987 – 1992) in the absence of control.

If a given population’s size is known for two points in time (No and Nt), the above 

equations can be used to estimate the average growth rate of the population, under the 

assumption that the population grew exponentially during this interval. Solving for either r or λ

produces the following equations:

r = ln Nt – ln No and λ = (Nt / No)

1/t

t

These numbers can easily be converted into the often-cited “annual percent increase” 

format, as described above. We note that some investigators appear to have reported r as annual 

percent increase (e.g., Ewins and Weseloh 1994; Bédard et al. 1995; Weseloh et al. 1995). While 

the two numbers are similar for most common growth rates, growth rates for DCCO populations 

have often been measured at 20 % or more, and in these cases the difference is more significant 

(Figure 89).

Because growth rate estimates calculated from the simple exponential model are based on 

only two data points, they should be approached with caution. Anyone using the exponential 

model to calculate a population’s growth rate makes the assumption that that population grew 

exponentially between the two selected points, even though the population may have made the 

transition linearly, sigmoidally or by any other trajectory. Some argument ought to be made to 

justify the selection of the exponential model over others when describing population growth. 

Otherwise, the impression may inadvertently be given that the population is growing 

exponentially, when in fact there are no data to indicate this. The Pigeon Island, Ontario DCCO 

colony (Ewins and Weseloh 1994) illustrates this point: while population growth rates were 

reported in terms of annual percent increase (based on the exponential growth model and nest 

counts for 1978 and 1993), nest count data for the years in between reveal that population growth 
FINAL DRAFT Part VI. Population Modeling 286

was more linear than exponential, adding approximately 57 nests each year on average (Figure 

90). This case study also illustrates another potential problem with the use of the basic 

exponential model for calculating growth rates: due to variation in population growth rates 

between years, the overall rate calculated may vary considerably depending on the two years 

selected for analysis. In this case study, annual percent increase calculated from nest counts in 

1978 and 1991 is 52 % (r = 0.420), whereas calculated from 1979 and 1990 data it is 24 % (r = 

0.216).

Figure 89. The relationship between annual percent increase and r.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 10 20 30 40 50 60 70 80 90 100

Annual % population increase

r

Figure 90. Numbers of Apparently Occupied Nests (AON) on Pigeon Island, Ontario with 

population growth estimates.

0

200

400

600

800

1000

1200

1400

1600

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998

Year

Number of AON

Ew ins and Weseloh (1994) r = 0.355 (42.6% annual grow th)

C. Weseloh (pers. comm.) Linear regression

y = 57.0x - 112671

R^2 = 0.82
FINAL DRAFT Part VI. Population Modeling 287

Because of these potential problems, this form of the exponential growth model should 

not be used to calculate growth rates when data are available for more than two years. To do so 

ignores valuable data that may reveal population trends other than exponential growth and also 

wastes the often considerable efforts made to gather these data. Instead, techniques that 

incorporate all available data, such as regression analysis, should be used (e.g. Matteson et al. 

1999).

One additional complicating factor is the contribution of immigration to population 

growth and the effect it may have on projections of population growth into the future. Initial 

growth of a newly established colony or population may rely more on immigration than on 

reproduction. For example, the Pigeon Island, Ontario colony grew from four nesting pairs in 

1978 to 40 pairs the next year, an increase of 900 % (Figure 91). For DCCOs, such growth is 

obviously not possible by reproduction alone, even if all fledglings survived to reproductive age, 

and therefore must have included a large immigration component. This large second-year 

immigration event had a considerable impact on the overall growth rate calculated in this study: 

whereas annual growth from 1978 (four nests) to 1993 (818 nests) was 43 % based on the 

exponential model, making the calculation with 1979 (40 nests) and 1993 data reduces the 

annual growth rate to 24 %. Because the first growth estimate was influenced by a large initial 

immigration rate (relative to colony size) that has subsequently decreased, this estimate would 

not be appropriate for predicting future population growth.

Figure 91. Annual percent change in number of apparently occupied nests (AON) in Pigeon 

Island, Lake Ontario.

-100

0

100

200

300

400

500

600

700

800

900

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998

Year

Annual % change from previous year
FINAL DRAFT Part VI. Population Modeling 288

Age-Structured Exponential Growth Models

Age-structured models attempt to replicate natural population dynamics more accurately 

than basic exponential models by incorporating within them differences in parameters that exist 

between age (and sometimes sex) classes. These models predict exponential growth (or decline) 

because they assume that all parameters are constant (density independent) for their respective 

age and sex classes. Therefore, these models are similar to the basic exponential model in the 

number of years they can reliably predict future population trends.

Differences in mortality rates between young of year, subadults and adults appear to be 

most commonly included in DCCO models (see case studies, below). As is discussed in the 

following case studies, interpretation of age-structured models, when age-specific parameters are 

poorly known, is difficult and investigation of the confidence levels of model predictions is 

important. Differences in mortality rates between the sexes may be modeled, but this does not 

appear to have been included in any DCCO models. Age-specific fecundity rates are not known 

for DCCOs and appear not to have been modeled. Other poorly known parameters such as 

immigration and emigration have not been included in age-structured DCCO models.

We found only three age-structured DCCO population models. Bédard et al. (1995; St. 

Lawrence Estuary, Québec, below) and Fowle (1997; Lake Champlain, Vermont, below) 

constructed models of this type to investigate the effectiveness of various DCCO control options. 

Even though few age-specific data are available for DCCOs, age-specific models were necessary 

in both of these studies to assess effectiveness of egg oiling and / or shooting adults at 

manipulating population levels. Each study used different techniques to estimate age-specific 

parameters, to establish the initial age class distribution (unknown for both studies), and to 

analyze model output. Only Fowle (1997) performed sensitivity analyses of model output. The 

third age-structured model (Price and Weseloh 1986; Lake Ontario, below) did not investigate 

DCCO control, but rather the question of whether observed growth in the Lake Ontario 

population (1974 – 1982) could have occurred exclusively through reproduction of Lake Ontario 

colonies or whether immigration was necessary to produce observed population growth rates. 

This study used yet another technique for estimating population age structure. All three studies 

assumed all breeding-age adults nest every year, an assumption that has not been investigated in 

the field for DCCOs. In contrast, van Eerden and van Rijn (1997) estimated that 40 % of 

potentially available breeders (> 3 yrs) in the Oostvaardensplassen, The Netherlands, GRCO 

population did not breed (see also PART II. POPULATION DATA AND TRENDS, Nonbreeding Birds and Total Population Size).

Lake Ontario

Price and Weseloh (1986) constructed an age-structured, deterministic, exponential 

model to investigate the question of whether observed 65 % annual growth in nesting DCCOs on 

Lake Ontario (1974 – 1982) could have occurred without immigration. The model included agespecific mortality rates, equal fledging rates for all breeding birds (i.e., no age-specific 

fecundity), age of first breeding at either two or three years, and all adults breeding each year. 

Not discussed are model structure in terms of year or life-stage classes. Initial age class 

distribution appears to have been set at 22 adult males and 22 adult females (based on 22 nests 

observed in 1974) with pre-breeding age classes based on assumed fledging and survival rates 

for nestlings from 22 nests each of the previous few years (and equal sex ratios).

Initially, the model was run under conditions that led to a stable population: first breeding 

at three years, annual fledging rate of 1.0 young / nest, 70 % pre-breeding mortality and 15 % 
FINAL DRAFT Part VI. Population Modeling 289

annual adult mortality. Pre-breeding mortality rates appear not to have been calculated annually, 

but for the entire period between fledging and first breeding at either years two or three. Under 

these conditions annual growth rate was 0 %.

Given hypothesized changes in model parameters that might occur during periods of 

rapid population increase (as this population appeared to be exhibiting) the model was re-run 

with altered parameters: breeding at two years, annual fledging rate of 2.5 young / nest (based on 

unpublished data averaged over the Great Lakes), pre-breeding mortality of 50 % and 10 % 

annual adult mortality. This model produced an annual growth rate of 37 %, considerably lower 

than the observed 56 % annual increase. Parameters were adjusted again until the goal of 

reproducing 56 % annual growth was met. One successful combination consisted of breeding at 

two years, 2.8 fledglings / nest / year, 31 % pre-breeding mortality and 10 % annual adult 

mortality. The other successful combination consisted of breeding at two years, 3.0 fledglings / 

nest / year, 40 % pre-breeding mortality and 5 % annual adult mortality. These parameters were 

considered plausible for DCCOs, suggesting that immigration was not necessary for the observed 

56 % annual growth between 1974 and 1982.

To further investigate the data, annual percent increases were calculated for years 

between 1974 and 1982, and these levels of increase were compared to estimates of numbers of 

young that might have entered the population from breeding two years prior (assuming breeding 

at two years, 2.8 young / nest, 31 % pre-breeding mortality, 10 % annual adult mortality, all 

breeding-age adults breed). According to this analysis, observed growth in years 1976, 1978 and 

1979 (171 %, 110 % and 56 %, respectively) was higher than predicted based on estimated 

reproduction in previous years, and it was concluded that in these years “substantial migration 

(up to 55 % of the year’s growth) must have occurred, unless other colonies had existed to 

produce the extra young two years earlier.” However, in discussing this model, Duffy (1995) 

notes that the population may contain non-breeding adults that recruit into colonies during 

“exceptional years” and that not necessarily all of the increase in the population beyond that 

predicted by recruitment needed to have been due to immigration alone.

St. Lawrence Estuary, Québec

Impacts by DCCOs to vegetation in the St. Lawrence estuary were considered to be high 

enough that the population should be reduced. The goal was to reduce the population to 10,000 

nesting pairs from a high of 14,662 in 1987 (see also St. Lawrence Estuary, Québec in PART V. 

MANAGEMENT OPTIONS). Bédard et al. (1995) constructed an age-structured, deterministic, 

exponential growth model to determine what level of control would be needed to meet this goal. 

The two techniques selected for control were oiling eggs to kill embryos (a technique only 

feasible for ground-nesting colonies) and shooting adults (a technique generally limited to treenesting colonies). Although investigators wanted to avoid killing adults because public opinion 

does not favor this technique, the model predicted that only a combination of the two techniques 

would be successful at meeting the population goal and both were used.

The model was constructed with POP-II software (Fossil Creek Software 1990). Number 

of age classes was not reported. Four pieces of information were needed to construct the basic 

model: 1) age at first breeding, 2) age-specific mortality rates, 3) fledging rate and 4) population 

age structure. Age at first breeding was assumed to be three, although it was acknowledged that 

some two-year old DCCOs may breed. Though not explicitly stated, it appears that all birds ≥ 3 

yr were assumed to breed. Initial age-specific mortality rates were taken from van der Veen 

(1973). The initial fledging rate of 2.0 fledglings / female / year appears to have been arbitrarily 
FINAL DRAFT Part VI. Population Modeling 290

chosen from a range of reported values and was applied equally to all breeding-age birds. To 

estimate population age distribution, a model was constructed with the above three parameters 

and a simulated founding population of 100 males and 100 females. The model was run for 24 

years and the age class distribution at that point was used to represent the age-class distribution 

in the St. Lawrence population. Why this technique was used, and why 24 years was selected 

was not discussed. The above combination of parameters could not reproduce a stable, let alone 

growing, population, so the first three parameters were altered until a combination was found 

that successfully reproduced the estuary’s observed growth of 6155 pairs in 1979 to 14,662 pairs 

in 1987. The final combination of parameters consisted of first breeding at three years, 50% year 

1 mortality, 25 % year 2 mortality, 20 % mortality in years 3 and above, and a fledging rate of 

2.0 juveniles / female / year. The resulting parameters were then changed to simulate the various 

control techniques. Throughout modeling, tree-nesting and ground-nesting components of the 

population were assumed to be independent of each other and no birds moved between them 

from year to year. The above parameters were applied equally to both populations. Immigration 

into and emigration from the St. Lawrence Estuary were assumed not to occur.

The maximum level of egg oiling considered feasible was 70 % of all ground nests. This 

control method was presumably simulated by reducing breeding output in 70 % of the groundnesting birds to zero, but details are not presented. The oiling of 50 % of all ground nests was 

also modeled. An unstated assumption of this model is that none of the adults in nests subjected 

to oiling renested and that oiling was 100 % effective at killing embryos. Although evidence is 

presented that the latter assumption is valid, little is known about the activities of the roughly 50 

% of DCCOs that abandon oiled eggs (see Egg Oiling / Spraying in PART V. MANAGEMENT 

OPTIONS).

Modeling effects of shooting adults was more complex. It was assumed that 1) shooting 

one individual from a nesting pair would cause their nest to fail, 2) 75 % of adults shot would 

belong to different pairs (thus, 25 % of the shooting was “wasted effort” with respect to reducing 

reproduction), and 3) nest predation by gulls, crows and ravens as a result of disturbance during 

shooting episodes would cause an additional 15 % reduction in reproduction. No empirical data 

were available for these three assumptions. The combined effect of these impacts on recruitment 

was calculated to be an overall reduction of 40 % at colonies where shooting was to take place 

(15 % due to direct losses, 15 % through predation losses, and 10 % through loss of a breeding 

partner). This effect was modeled by reducing fledging rates to 1.2 young / female / year under a 

2000 / year culling strategy and 0.92 young / female / year under a 3000 / year culling strategy. 

These calculations implicitly assumed culling would have an equal effect on both sexes.

Results of modeling control strategies in different combinations showed the overall 

DCCO population continued to increase when either egg oiling or shooting adults was practiced 

separately. Only combination of both techniques was reported to be successful. Results are 

difficult to interpret, however, as some figures and captions in Bédard et al. (1995:81-83) do not 

match. Furthermore, no sensitivity analyses were performed to determine how uncertainty in

model parameters might have influenced these conclusions. From these results investigators 

decided the best control strategy would be to oil eggs and shoot adults at maximum feasible 

levels: oiling 70 % of ground nests and shooting 2000 adults / year. Although oiling 50 % of 

ground nests was apparently predicted to meet control program goals, 70 % was selected because 

“the only risk in doing so was to reach our goal more rapidly.”

Effectiveness of the control program was more dramatic than the model predicted, and 

led to the decision to stop shooting after four years, rather than the five that had been planned 
FINAL DRAFT Part VI. Population Modeling 291

(see St. Lawrence Estuary, Québec in PART V. MANAGEMENT OPTIONS). Why might 

reality have produced results so different from those predicted by the model? Given lack of data 

on DCCO life-table parameters and lack of model sensitivity analyses, it is difficult to answer 

this question. All parameters were based on best guesses and tweaking of the initial model to 

reproduce a historical pattern consisting of two points. Important assumptions were also made 

that may not be valid. Analyzing the control program, Bédard et al. (1999) state that unequal sex 

ratios in DCCOs shot (203 males: 100 females) “probably accounted for the faster than predicted 

drop in numbers.”

The model predicted that control would not be successful without shooting adults, a 

strategy those involved wanted to avoid because of negative publicity it was expected to draw. 

(After completion of the control program, it was reported that, despite attempts to manage public 

opinion, “press coverage was acerb and negative.”) Given that the selected control program was 

considerably more effective than predicted, it is possible that the goal of reducing the population 

to 10,000 pairs might have been achieved without shooting adults. In that case, the model would 

not be considered successful at predicting the most appropriate control strategy under current 

social conditions. However, even in hindsight it is not possible to make definitive judgments 

without more data.

Lake Champlain, Vermont

Fowle (1997) constructed a deterministic, age-structured, exponential model of DCCOs 

on Lake Champlain, Vermont with the stated goals of 1) comparing the model’s predicted 

growth rates to observed DCCO growth rates on Lake Champlain, and 2) evaluating proposed 

DCCO control methods. Sensitivity analyses were performed on two of the model’s variables 

(mortality and reproduction rates), but the importance of other unknowns / assumptions remained 

untested. The model’s predicted rate of increase, r, at stable-age distribution in the absence of 

control was found to be 0.21, which is reported as comparable to observed growth rates. Of the 

three control strategies modeled, reducing adult survival by 75 % was found to be the most 

effective strategy at lowering r. Preventing reproduction on all islands except the Four Brothers 

and Shad Islands (described as the control technique “with the least amount of disturbance”) was 

the next most effective technique, while reducing adult survival by 10 % reduced r by only a 

small amount (see below).

This modeling effort appears to be based on two separate models: an age-structured 

exponential model and a basic exponential growth model. The age-structured model used life 

table parameters (both known and unknown for DCCOs) to estimate r without control and under 

various control strategies. The basic exponential model was then used to project 1996 population 

estimates into the future given various r values. Effectiveness of control strategies was measured 

by how close each strategy’s predicted r value came to zero.

The age-structured model consisted of 15 age classes (0 – 14). A reproductive rate of 

2.54, based on fledging rates observed on Young Island, Lake Champlain (1995 – 1996), was 

applied to all age classes 3 – 14 (i.e. no age-specific fecundity). All adults were assumed to breed 

every year (despite control efforts to prevent colonization on some islands on Lake Champlain). 

Age at first breeding was assumed to be three years. Age-specific survival rates (58 % for age 0, 

75 % for 1 and 2, and 80 % for 3 and above) were not based on DCCO data. Rather, they were 

taken from a model by Madenjian and Gabrey (1995) who used Herring Gull mortality rates 

reported by Paynter (1947) for DCCO hatchlings (here age 0) and non-breeding birds (ages 1 and 

2), and Cairns et al. (1991) for breeding birds (ages 3 and above). The 80 % survival rate of 
FINAL DRAFT Part VI. Population Modeling 292

Cairns et al. (1991) is reported as a “typical literature value” for the general class of inshore 

seabirds. Because population age distribution was unknown, r was calculated from the model 

when it reached a stable age distribution. (Age-structured exponential models reach a point at 

which r and the proportion of individuals in each age class no longer change from year to year; 

these proportions are the stable age distribution.)

Sensitivity of the model was tested by varying survival and fledging rates by 10 % and 

observing effects on r. These tests revealed that the model was more sensitive to variation in 

survival than reproduction: 10 % change in survival rates led to 40 % change in r, whereas 10 % 

change in fledging rates resulted in 10 % change in r. Thus, of these two parameters, the model 

was most sensitive to the one that was unknown and had been estimated from other species, and 

least sensitive to changes in the variable that was calculated directly from the population in 

question. Variation in other unknowns such as age at first breeding and proportion of nonbreeding adults was not tested.

Three control strategies were modeled. The effects of oiling all eggs on Young Island 

(see Lake Champlain, Vermont (USFWS 1999a) in PART V. MANAGEMENT OPTIONS) was 

simulated by reducing reproduction (in an unexplained manner) to simulate reproduction 

occurring only on the Four Brothers and Shad Islands. To simulate two levels of lethal control of 

adults, adult survival rate was reduced by 10 % and 75 %. The simulated egg oiling resulted in 

an r of 0.06 at stable age distribution. Reducing survival of adults by 10 % resulted in a r at 

stable age distribution of 0.18, while reducing adult survival by 75 % resulted in an r of 0.02. 

The level of lethal control necessary to reduce adult survival rates by 75 % is reported as equal to 

killing over 12 % of the breeding population.

Using the above r values to predict future population growth makes the assumption that 

the population is currently at a stable age distribution. This assumption may not be valid. Fowle 

(1997) notes that the model predicted birds of age 1 and 2 would comprise 34 % of the 

population at stable age distribution, whereas observations of foraging flocks found these age 

classes comprising only 10 %. This discrepancy could have resulted from several scenarios. The 

Lake Champlain DCCO population could be at a stable age distribution that is different from that 

predicted by the model because the reproduction and mortality parameters in the model are 

incorrect. Or, the model’s predictions could be correct, but the population may not be at a stable 

age distribution. Or, the model could be correct and the population could be at the predicted 

stable age distribution, but observations of the population’s age classes may be incorrect. Given 

available data, it is not possible to determine if any of these scenarios explains the discrepancy, 

however the accuracy of the sub-adult to adult ratio estimate may be questionable. It appears that 

authors assumed the sub-adult : adult ratio in foraging flocks is equal to the same ratio in the 

entire population. This may not be the case. Because breeding adults must provide food for both 

themselves and their young, energy requirements for breeding adults are greater than for nonbreeders; therefore, foraging effort of breeding adults should be higher than that of non-breeders. 

(The bioenergetics model in this study predicted a higher food intake for breeding cormorants 

compared to non-breeding cormorants.) In growing populations, a large proportion of nonbreeders generally will be sub-adults. (The model assumed all non-breeders were sub-adults.) If 

foraging efficiency of breeders is similar to that of non-breeders, and if non-foraging birds do not 

associate with foraging flocks, then non-breeders would be expected to appear in foraging flocks 

less frequently and for shorter durations than breeders. Therefore, the sub-adult : adult ratio in 

foraging flocks (as calculated from observations throughout the nesting season), may be lower 

than this ratio in the population as a whole.
FINAL DRAFT Part VI. Population Modeling 293

As with the St. Lawrence Estuary, Québec, model (above), this model assumes that 

mortality to adults through lethal control will be equal for both sexes. Because unequal mortality 

to males and females during the St. Lawrence Estuary control program may have contributed to 

the higher effectiveness of the control program than predicted (Bédard et al. 1999), the 

effectiveness of lethal control on r values reported here should be interpreted with caution. 

Nevertheless, the sensitivity analyses performed on this model are instructive and highlight the 

need for better age-specific DCCO mortality rates.

DETERMINISTIC LOGISTIC MODELS

Basic Logistic Growth Models

Logistic models attempt to incorporate the density-dependent factors that limit population 

growth. In the basic logistic growth model, all factors are combined into one number, the 

population’s carrying capacity, or K. As the population grows, its growth rate, r, is reduced such 

that as the population approaches its carrying capacity r ‡ 0 and growth eventually stops. These 

models make the assumptions that a fixed carrying capacity exists and that the population will 

increase to this level in a sigmoidal fashion. Mainly, these models are used to estimate what a 

growing population’s carrying capacity will be, and not to predict the population’s trajectory per 

se. This goal is accomplished by fitting population data to various logistic growth curves using 

regression analyses, and then analyzing how close the fit is.

The only application of the basic logistic growth model to DCCO data appears to be 

Craven and Lev (1987), who apply this model and nest count data for Gull Island, Lake Superior 

(1978-85) to predict island carrying capacity. They fit their data to the logistic equation

Nt = ___k____

1 + Ae-rmt

and found that the best fit curve was described when k = 324, A = 18.041 and rm = 0.818. 

Thus, their model predicts a carrying capacity of 324 nests. However, nest count data after 1985 

(S. Matteson, pers. comm.) reveal island carrying capacity is much higher, possibly yet to be 

reached (Figure 92). This example reveals that logistic curve fitting may be sensitive to recent 

population counts (e.g., 1984 and 1985 in Figure 92), and that carrying capacity predictions for 

populations just beginning to show signs of slowed growth are subject to great uncertainty.

Suter (1995) uses the basic logistic curve to analyze growth trends in the GRCO 

population wintering in Switzerland from 1967 to 1992. Population data were fit to three 

different models (logistic, power and exponential) and statistical analyses were applied to each to 

determine which gave the best fit. Although the correlation coefficients for all three were high (r 

= 0.996, r = 0.986 and r = 0.977, respectively) the power and exponential curves “were inferior 

to the logistic model, as shown by the high autocorrelation of the residuals (logistic: r = 0.079, 

n.s.; power: r = 0.626, p < 0.001; exponential: r = 0.715, p < 0.001).” Two carrying capacity 

values were predicted by the model depending on whether a low 1991 count was included in the 

analyses: 8800 cormorants with 1991 data and 9400 without. Although it would be instructive to 

compare these predictions with data collected after 1992, we have not acquired these data. 

Neither of the above logistic curve fitting examples gave confidence intervals around predictions 

carrying capacities.
FINAL DRAFT Part VI. Population Modeling 294

Figure 92. Nesting population change on Gull Island, Lake Superior and logistic model 

based on 1978 – 1985 nest counts.

0

100

200

300

400

500

600

700

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

Year

Number Nests

Craven & Lev 1987 data Craven & Lev 1987 Model S. Matteson (pers. comm.)

Age-Structured Logistic Growth Models

If the effect of density-dependence on various life history parameters such as survival and 

reproduction are known, it is possible to incorporate these interactions into age- and / or sexstructured models. Such models could be useful for predicting a population carrying capacity in 

the absence of trend data. These models could also be valuable for investigating effects of 

density dependence on attempts to control population levels. For example, density-dependent 

effects on reproduction may increase per capita reproductive output at low population levels, 

thereby increasing effort necessary to manipulate populations to lower and lower levels in a 

nonlinear fashion. An understanding of such interactions would be valuable for deciding what 

levels of population control might be economically feasible.

Data on effects of population density on life table parameters are unavailable for DCCOs. 

For example, Ludwig and Summer (1995) discuss ways in which human factors, disease, 

parasites, availability of nesting habitat and availability of food may limit DCCO populations, 

and they provide anecdotal evidence that some of these factors may be acting on the Great Lakes 

population, but they provide no quantitative data that could be used to model these effects.
FINAL DRAFT Part VI. Population Modeling 295

GREAT CORMORANTS IN EUROPE

Bregnballe et al. (1997) constructed a deterministic, age structured model with density 

dependence for the European GRCO sinensis population to investigate the ability of various 

management techniques to manipulate population levels. The model was created by modifying a 

preexisting model originally constructed for oystercatchers with parameters derived from GRCO 

research. The parameters in which density dependence was incorporated were the proportion of 

non-breeders, chick mortality and winter survival. Mortality was density independent for eggs as 

well as for juveniles and adults during the spring, summer and autumn. Clutch size was not age 

structured or density dependent. Parameters such as initial population size, age class distribution 

and area of breeding and wintering habitat were mentioned but not discussed.

Similar to DCCO modeling efforts, many of the parameters necessary for constructing 

this model were either variable, poorly known, or unknown; selection of parameter values 

consisted largely of educated guesses. The model was calibrated by adjusting the strength of 

density dependence and availability of resources “until the unmanaged population behaved in a 

way that was within the range of what [the authors] found would be likely to occur,” and so that 

the population stabilized “before reaching an unrealistically large size” (Bregnballe et al. 

1997:77). In other words, the model was constrained a priori by assumptions of overall 

population limitations.

Because of the level of uncertainty in the parameters, sensitivity analyses were run to 

determine how this uncertainty affected model output. Variables were changed individually and 

observations were made of subsequent changes in model behavior. Assumptions about density 

dependence were found to be very important in determining model output, with 1) amount of 

available habitat and strength of competition within breeding habitat affecting the proportion of 

adults breeding and 2) fledging rate and survival of young birds affecting final population size. 

Age of first breeding was found to affect size of the non-breeding population, rate of approach to 

equilibrium, and final population size.

Four different management scenarios were modeled independently, and then in various 

combinations. Because of low level of confidence in many model parameters, little confidence 

was held in the predicted final population sizes. Effectiveness of management techniques was 

measured by comparing extent by which these techniques lowered the estimated equilibrium 

levels when compared to model predictions without any management of the cormorants. The 

four management scenarios were: preventing establishment of new colonies, cutting nesting trees 

and disturbing breeding birds, reducing reproductive output, and introducing a hunting season in 

Europe.

Considerable detail is given in discussing the results of each management simulation, 

both in quantitative and qualitative terms. Overall, it was concluded that each management 

scenario could be effective at holding the entire sinensis population below its natural carrying 

capacity. The authors “conclude that shooting adult cormorants would be more efficient than nest 

destruction and egg culling partly because of compensatory mechanisms and partly because of 

the direct and indirect effects of hunting on mortality. Preventing new colonies from being 

established could be efficient when and where colonisation of areas of intermediate or high 

quality could be hindered.” All combinations of management techniques were found to be more 

effective than the techniques applied separately. The authors end with the following caveat:
FINAL DRAFT Part VI. Population Modeling 296

Although the model predicts that all the above management measures would lead to stabilisation 

at a lower population level than if no actions were taken, a best professional judgment suggests 

that a substantial reduction in population size does not necessarily lead to a substantial reduction 

in the number of cormorants foraging in so-called problem areas. Furthermore, economic losses 

would not necessarily decline proportionately with a decline in the number of cormorants foraging 

in a problem area.

CONCLUSIONS

Review of these studies indicates that prediction of future DCCO population trends and 

analysis of control methods is hampered by a lack of age- and sex-specific data for this species. 

Even when proper models are selected for available data and sensitivity analyses are performed 

on assumptions and unknowns, the questions that can be answered by these models are limited to 

the more basic components of cormorant population dynamics. Until better DCCO life table data 

are available, sensitivity analyses of assumptions and unknowns will be very important for 

assessing confidence levels in population model predictions.
FINAL DRAFT Part VII. Research Needs 297

PART VII. RESEARCH NEEDS

Research needs have been identified by multiple researchers and through our DCCO 

Status Assessment. Additionally, an interagency meeting was convened March, 2000, in 

Sandusky, Ohio, to discuss Research Needs for North American Cormorant Management, and 

ideas from this meeting have been incorporated. In our assessment, we have specifically focused 

on research necessary for managing and resolving human/cormorant conflicts, and have 

attempted to group research needs in order of priority. With the exception of number 25, all of 

the needs presented below were discussed at the Sandusky meeting, and each was ranked as a 1, 

2 or 3 in terms of importance for management of the DCCO problem. A “1” designated the 

highest importance, while a “3” designated not important for management. Number 25 was later 

suggested by John Trapp, and ranked by a few individuals present at the Sandusky meeting and 

Jeremy Hatch.

Research needs were identified and ranked in the following categories: 

• Distribution

• Demographics

• Diet

• Impacts to Fisheries and Aquaculture

• Impacts to Flora and Fauna

• Management Techniques

DISTRIBUTION INFORMATION

1) Information is needed on proportion of total population utilizing aquaculture facilities, and 

on how and where winter numbers are distributed (winter distribution as it relates to “local 

features”; Trapp et al. 1995). (2)

2) Information is needed on distribution, numbers and habitat use of non-breeding birds 

during the breeding season. (1)

3) Better delineation of interior, southern and northeast Atlantic Coast breeding populations; 

boundaries are becoming blurred. Genetic studies would be useful to examine 

distinctiveness and extent of gene flow between populations (Hatch and Weseloh 1999). (3)

4) Better utilization of historic distribution information to help distinguish range expansion 

from re-colonization. (3)

DEMOGRAPHIC INFORMATION 

5) To model management scenarios and effectiveness, to predict future trends, and to 

determine factors that may limit populations, more information is needed on population/life 

table parameters (Brugger 1995; Erwin 1995; Nisbet 1995; Hatch and Weseloh 1999), such 

as:
FINAL DRAFT Part VII. Research Needs 298

• age of 1st breeding (1)

• reproductive output (LRS and age-specific fecundity) (1)

• frequency of breeding (annual?) (3)

• mortality sources (3)

• age-specific survival/mortality (1)

• emigration and immigration: local (1); regional (2)

• philopatry in DCCOs (important for local population control effectiveness): local (1); 

regional (2)

Long term studies (banding with hardmetal and field-readable bands1, etc.) will be 

necessary to acquire these data. Until these DCCO data are available, known parameters 

for European GRCO could potentially be used for DCCO modeling. Additionally, 

effectiveness of various management scenarios has been modeled for European GRCOs 

(Bregnballe et al. 1997).

6) Better estimates of numbers of breeding birds within each zone (e.g. lack of surveys, 

sampling inconsistencies). To produce population estimates with high confidence, both 

improved geographic coverage and a standardized sampling method are necessary. (1)

7) Better estimates of numbers of wintering birds in specific locations (1) and overall (3).

8) Studies to ascertain how population processes are affected by changes in density, as in the 

European Great Cormorant population (e.g., Van Eerden and Gregersen 1995; Frederiksen 

and Bregnballe in press). (1)2

9) Information on factors influencing colony size, formation and movement [e.g., dispersal 

behavior as it relates to selection of new areas for colonization (Bregnballe and Gregersen 

1997)]. (2.5)

DIET

10) Experimental feeding trials should be undertaken to assess the biases associated with pellet 

and stomach content analyses. Trials could be used to determine the most appropriate 

(species-specific) key bones and other hard parts to be used for analysis. Then attempts 

could be made to quantify size- or species-related differential recovery (Carss et al. 1997). 

(2)3

11) Need standardized methods for analysis of pellets and stomach contents to make studies 

more comparable (Carss et al. 1997). (2)3

12) Better information on rate of pellet formation and influencing factors (Carss et al. 1997). 

(2)3

IMPACTS TO FISHERIES AND AQUACULTURE

13) Assess mortality due to DCCOs relative to other mortality factors at the local level (Price 

and Nickum 1995; Trapp et al. 1995; Erwin 1995). Better estimates of numbers and 

densities of fish populations will be required to do this. (1)
FINAL DRAFT Part VII. Research Needs 299

14) Better information is needed to explain mechanisms within fish populations that buffer 

effects of DCCO predation (i.e. compensatory vs. additive mortality due to DCCO's) 

(Krohn et al. 1995; Erwin 1995; Nisbet 1995; Carss et al. 1997). Care should be taken to 

investigate whether different life-stages or species complexes of fish are more or less 

vulnerable to cormorant predation (i.e. differential effects on young-of-year vs. adult fish; 

forage vs. predator fish species). (1)

15) More information is needed on DCCO foraging behavior and predator-prey interactions 

(Carss et al. 1997; Hatch and Weseloh 1999). (2)

16) Information is needed on intra-specific competition, dynamics between individual birds and 

possible density dependent effects on foraging choices, behavior and patterns (Nisbet 1995; 

Bregnballe pers. comm.). (3)

17) Studies to examine the potential beneficial role of the DCCO at aquaculture ponds (e.g., 

remove dead and diseased fish; reduce densities of uncontrolled fish; eliminate stocked fish 

fry predators; indicator of undesired conditions at ponds; Jackson and Jackson 1995). See 

Ashkenzai and Yom-Tov (1996) for similar information on the role of herons at fish ponds 

in Israel. (2)

IMPACTS TO FLORA AND FAUNA

18) Research should be conducted to quantify impacts to vegetation and to other waterbirds. 

Sites where DCCO induced vegetation changes may harm rare plant communities or 

animals should be identified, and impacts to survival / reproduction of avian species should 

be examined. (1)

19) Research is needed to determine how Newcastle disease may be transmitted from DCCOs 

to commercial poultry and to identify risk factors involved. Research is also needed to 

determine whether DCCOs transmit Newcastle disease to other wild birds (Kuiken 1999). 

(3)

MANAGEMENT TECHNIQUES

20) States where the Depredation Order is in effect should closely monitor DCCO numbers to 

determine impacts of and to DCCOs. The number of birds taken under the Depredation Order 

and effectiveness of the Order in terms of buffering DCCO impacts should be assessed. A 

comprehensive study of birds killed under the Depredation Order to determine age and sex of 

birds depredating at aquaculture facilities, relative to the population as a whole, should be 

conducted. (1)

21) More research is needed on how to make control measures more cost effective. Many 

techniques are poorly tested and should be investigated more thoroughly. (1)

22) Studies to address the human dimensions of DCCO conflicts and possible solutions through 

education should be undertaken. (1)
FINAL DRAFT Part VII. Research Needs 300

23) Encourage communication between fisheries biologists and ornithologists to determine 

how piscivorous birds can best be incorporated into fisheries management objectives (e.g., 

stocking sportfish species or sizes less vulnerable to DCCO predation; adjust stocking rates 

to accommodate some degree of predation; in areas where there is conflict, create 

alternative habitat to minimize potential competition for fish resources; change the time of 

stocking to avoid cormorant predation; encourage monitoring of return rates of adult fish or 

creeled fish to determine if cormorant predation has a significant impact on overall fish 

returns) (Ottenbacher et al. 1994). (1) 

24) For species that may be taken accidentally in lethal control efforts, such as GRCO, 

Anhinga and Neotropic Cormorants, more research is needed about how losses may impact 

these species on a regional or population level. For example, observations at ponds to 

determine proportions of each species present and taken may provide useful data on losses 

and impacts. (3)

ECONOMIC IMPACTS

25) Research on economic impacts, especially as it relates to the sport fishing industry, 

tourism, etc., should be conducted. (1)

______________________

1 Importance of hardmetal bands should be emphasized; cormorants outlive bands made of aluminum, so that 

data for birds more than a few years old are suspect. Use of appropriate bands is essential for accurate measures 

of adult survival. Additionally, large bands that are readable through a telescope, such as though used in 

Denmark and elsewhere, will be necessary for measuring survival and movements, and can provide important 

information on the dynamics of colony formation (J. Hatch, pers. comm.).

2 After discussion with Jeremy Hatch, number 8 was re-written and as re-stated is more important than in its 

earlier form, so its rank was changed from “1.5” to “1”.

3 We originally listed number 11 before number 10. Additionally, numbers 11 and 12 were originally ranked as 

“1” while number 10 was ranked “2”. However, after discussion with D. Carss and J. Hatch, we made the above 

changes in order and rank for the following reasons: Ranking number 10 as a “2” while numbers 11 and 12 

were ranked as “1” implies that the pellet method yields representative samples for establishing daily food 

intake, but this method is unsuitable for estimates of DFI (see Diet Assessment Methods: Advantages and 

Shortcomings); Pellet analyses are not a priority research need for management of cormorants, since potentially 

more reliable methods (e.g., stomach contents analysis) may be used to assess cormorant diet. If biases 

associated with pellet analyses remain unknown, numbers 11 and 12 aren’t meaningful.
FINAL DRAFT LITERATURE CITED 301

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FINAL DRAFT APPENDIX I. Perceived Impacts 325

APPENDIX I. PERCEIVED IMPACTS ASSOCIATED WITH DOUBLECRESTED CORMORANTS

To determine if states and provinces perceive DCCOs as important impacts to biological 

resources and human interests, we included questions regarding perceptions of DCCO impacts in 

our DCCO survey. Recipients of the survey were asked to provide information on impacts to the 

following resources: commercial and or sport fishes, aquaculture, vegetation, other vertebrate 

species (birds, mammals, herps, etc.), private property, and water quality. We also requested 

information on disease transmission between cormorants and domestic or wild birds, and impacts 

to public health. Table 29, Table 30 and Table 31 summarize perceived impacts to each resource 

by region. We also developed a survey for fisheries biologists to obtain information on impacts 

to fisheries from the fisheries community perspective; these data are summarized in Table 32 and 

Table 33.

The range of knowledge about DCCOs and their possible impacts varied greatly among 

survey recipients. Additionally, recipients were simply asked to give their own or their agency’s 

opinion on the questions being asked. Therefore, it is important to note that the information in 

this section is in no way intended to document impacts. The data presented here are mainly 

qualitative, and are included only to provide a broader perspective on attitudes of wildlife and 

fisheries agencies and biologists towards cormorant issues.

SURVEY RESPONSE

Overall, we obtained a good response to our survey requesting information on DCCO 

populations, impacts and management. We sent surveys to 61 individuals, and of these, about 80 

% (49 individuals) responded by either completing the survey or by contributing partial data. In 

most states and provinces, one individual was contacted to complete the survey. However, in 

some cases, two or more individuals were sent surveys. This was done when we determined 

through telephone contacts that more than one person would be necessary to supply all requested 

information; or because DCCO problems were documented in an area and specific individuals 

could contribute important information and or a wider perspective. Data are summarized in 

Table 29, Table 30, Table 31 and below.

For our survey requesting information from fisheries biologists, we sent surveys to 35 

recipients, and of these about 66 % (23 individuals) completed surveys. This information is 

summarized in Table 32, Table 33 and below. Additionally, some individuals completing our 

DCCO survey consulted with fisheries biologists to complete sections regarding impacts to 

fisheries and aquaculture.

Impacts to commercial / sport fish (reported by wildlife biologists, etc.)

1) Respondents were asked to rank the importance (minor, moderate or major), in the opinion 

of their agency, of Double-crested Cormorant predation in limiting economically important 

(game and / or commercial) fish species in their state / province.

The majority of states and provinces reported that DCCO predation was perceived to be 

of minor importance to sport / commercial fish populations. Only two states and one province 
FINAL DRAFT APPENDIX I. Perceived Impacts 326

reported that DCCO predation was perceived to be of major importance in limiting fish 

populations.

Southern U.S.: Two states reported that DCCO predation was perceived tobe of major 

importance in limiting fish populations in the southern U.S., Texas and Arkansas. However in 

Arkansas, two persons completed surveys, a USDA/WS District Supervisor, who reported 

DCCO predation was perceived to be of major importance, and a Nongame / Endangered 

Wildlife Program Coordinator, who reported predation was perceived to be of moderate 

importance. Three states reported that DCCO predation was perceived as moderately important. 

In addition to Arkansas, Louisiana had two individuals complete surveys, a USDA/WS biologist, 

who reported that impacts to fish were unknown, and a nongame biologist who reported that 

DCCO predation was perceived as moderately important. Respondents from the other seven 

states in the region reported that DCCO predation was perceived to be of minor importance. 

Oklahoma reported that importance of DCCO predation on fish populations was unknown.

U.S. and Canadian Interior: All survey respondents reported that DCCO predation was 

perceived to be of minor importance, with the exception of Manitoba, where predation was 

perceived to be of major importance.

North Atlantic: Twelve respondents from eight states and two provinces reported that 

DCCO predation was perceived to be of minor or no importance; five respondents from six states 

reported that it was perceived to be of moderate importance (the same individual responded for 

CT, MA and RI). There were two respondents from Rhode Island; the USDA/WS biologist 

reported impacts to be of moderate importance while the state biologist reported that importance 

of DCCO predation on fish populations was not known. Though a survey was not completed for 

New Brunswick, Korfanty et al. (1997) reported that Double-crested Cormorants are not viewed 

as a problem in this province with respect to impacts on commercial fisheries.

2) Respondents were asked if there were any current studies underway to assess impacts in their 

state / province.

Current studies were reported in each region in a total of 10 states and provinces. These 

studies are presented in Appendix II. Current Research, Table 34.

Impacts to commercial / sport fish (reported by fisheries biologists)

1) Respondents were asked if commercially valuable fish species had declined in their area, and 

if so, to list the species in decline and factors thought responsible for the declines.

Declines were reported in 16 states and provinces. Fish species reported in decline and 

factors thought responsible are listed in Table 33. Overfishing and habitat degradation (loss of 

spawning habitat, pollution, dams, poor water quality, phosphorus abatement, etc.), were the 

most frequently reported causes of decline. Other factors reported included exotics (zebra 

mussels, sea lamprey, etc.), predation (by other fish and DCCOs in 5 states), white perch 

invasion, global and local environmental change, recruitment failures, lack of zooplankton, 

weather, temperature, and the natural cycle. 
FINAL DRAFT APPENDIX I. Perceived Impacts 327

2) Respondents were asked if fish declines could be linked to Double-crested Cormorant 

increases, and if so, to please supply data to support this link.

Five individuals from five states reported declines in fish populations that they believed 

were linked to DCCO predation. Four of the five areas where links to DCCOs were reported 

were in the Great Lakes; the other was in the Northeast. Maine and Michigan reported only 

anecdotal data linking DCCOs to fish declines. In Vermont, declines in yellow perch were 

correlated with DCCO increases; trends in creel survey estimates versus growth of Lake 

Champlain cormorant populations from 1990 through 1997 are being analyzed, but at the time of 

this writing no concrete data were available to support this link. Wisconsin provided a report that 

included rough estimates of the number of yellow perch consumed by DCCOs during the 1983 

breeding season at Willow and Cat islands. The report also included estimates of the potential 

amount of perch in pounds lost to DCCOs, but did not incorporate data on the total number of 

perch present. Since that report was written, a model has been developed that has allowed the 

reconstruction of annual biomass estimates for yellow perch in southern Green Bay, and the 

amount consumed by DCCOs, estimated at 5.96 % in 1983. New York also provided recent 

reports that included estimates of fish abundance and percentages consumed by DCCOs 

(VanDeValk et al. 1999; NYSDEC 1999 1999), and felt that significant impacts to smallmouth 

bass and possibly to walleye were occurring (See PART III: DIET, studies assessing impacts to 

sport and or commercial fish populations, for limitations of these type of data in assessing 

relationship between fish population declines and DCCO predation).

3) Respondents were asked if there was any information on the percentage of commercially 

important fish populations consumed by DCCOs, and to include any important reports that 

address this type of information for their state / province.

Respondents from New York and Wisconsin provided us with reports or data (see #2 

above) that provided information on the percentage of commercially important fish populations 

consumed by DCCOs. The respondent from Michigan referenced the NYSDEC’s Final Report 

(1999), noted above, and a report by Diana et al. (1997). Data from the latter report was 

discussed and analyzed by Belyea et al. (1999), and summarized earlier in (PART III: DIET). 

Belyea et al. (1999) and Diana et al (1997) reported that DCCOs removed about 1 % of the legal 

sized yellow perch population in the Les Cheneaux Islands area, Michigan, and were not 

believed to be causing substantial declines in the population.

Impacts to Aquaculture

1) Respondents were asked to rank the degree (minor, moderate, major) to which DCCO 

predation at aquaculture facilities was perceived to be a problem in their state / province.

DCCO predation at aquaculture facilities was perceived as a major problem only in the 

southern U.S. and in Manitoba. In the Great Lakes, the northeastern U.S., and Canada, it was 

mostly perceived to be a minor problem. A total of five states, Illinois, Connecticut, 

Massachusetts, Rhode Island and Maine, reported it to be perceived as a moderate problem. In 

the northeastern states, both USDA/WS personnel and state agency wildlife biologists completed 
FINAL DRAFT APPENDIX I. Perceived Impacts 328

surveys; USDA/WS personnel reported it perceived as a moderate problem while state agency 

biologists reported it perceived as a minor problem.

2) Respondents were asked to estimate the economic loss due to DCCO predation at aquaculture 

facilities in their state / province.

Only four states provided actual dollar amounts associated with DCCO predation at 

aquaculture facilities. In the southeastern U.S., LA, AL, and MS estimated dollar amounts in the 

millions. In the northeast, VA estimated losses of about $15,000. In most states and provinces, 

the dollar amount of loss was unknown.

3) Respondents were asked how many (most, some, few) aquaculturalists in their state had taken 

DCCOs under the provisions of the DCCO cormorant depredation order.

Cormorants were taken around aquaculture facilities mainly in the southern U.S. We 

received replies of “most” only in Arkansas and Mississippi. Replies of “some” were given for 

Texas, Louisiana, Alabama, and Florida. In general, most states responded with few, none or 

unknown to this question.

In Canada, the DCCO is protected and managed under provincial law. Scare and kill 

permits have been issued to “some” aquaculturalists in Manitoba, and to “few” on Prince Edward 

Island. The other provinces we surveyed did not report issuing any permits.

Impacts to Vegetation 

1) We asked respondents if vegetation (categorized as herbaceous layers, trees, and rare or 

unique plant species / communities) had been affected by DCCOs in their state / province.

Impacts to vegetation, mainly to trees, were reported in each zone, but most frequently in 

the northeastern U.S. and Canada. Eight northeastern states, CT, ME, MD, NH, NY, RI, VT, and 

VA, reported impacts either to trees (7), the herbaceous layer (5), or both (4). All five Canadian 

provinces reported impacts to trees, while Manitoba and Ontario also reported impacts to 

herbaceous layers. In the southern U.S., impacts to trees were reported in OK, AL, AR, FL, and 

NC; impacts to herbaceous layers were also reported in NC. In the Great Lakes, impacts to both 

herbaceous layers and trees were reported in IA, OH, MI and WI. No states or provinces reported 

any known impacts to rare plant species or communities, with the exception of Ontario, which 

reported that the unique Carolinian habitat on East Sister Island in Lake Erie might be affected 

by DCCO nesting. Ontario plans to assess the site this year if funds are available (J. Harcus, pers. 

comm.).

Impacts to trees were reported to be due mainly to guano deposition, and resulted in tree 

die off at breeding colonies and roost sites. Arkansas reported that guano deposition at roost sites 

stunted growth of buds and leaves in spring; Mississippi reported a similar concern over impacts 

to spring tree growth at roost sites. Other reported impacts included “tree pruning” by DCCOs 

when building their nests; on some islands in Iowa, all small branches were removed from trees 

for nesting material.

Impacts to the herbaceous layer were also reported due to guano deposition, and often 

this layer was reduced or eliminated from the colony site. In some areas (MB, VT), the only 

herbaceous vegetation remaining around cormorant breeding colonies were nitrogen tolerant 
FINAL DRAFT APPENDIX I. Perceived Impacts 329

species, such as stinging nettles and thistles. Michigan reported that in some breeding colonies, 

diverse native island plants were replaced with “barnyard” species. Virginia reported that 

destruction of the herbaceous layer made low islands subject to flooding.

Impacts to avian species

1) Respondents were asked if the recent increase in DCCOs had impacted avian 

species in their state / province, and if there were such impacts, to describe them..

Impacts to avian species were reported in each zone, and were reported to occur mainly 

through habitat degradation (effect of DCCO guano on trees and ground cover) and competition 

for nest sites. Impacts to other colonial waterbirds, particularly herons and egrets, were reported 

most frequently. In the Great Lakes, Ohio reported that DCCOs might displace Great Blue and 

Black-crowned Night Herons and Great Egrets on West Sister Island. Michigan reported that 

DCCOs may have caused the relocation of Great Blue and Black-crowned Night Herons and 

Caspian Terns on some islands. Illinois reported that Black-crowned Night Herons may have 

been displaced from one rookery by DCCOs, but that Great Blue Herons had also moved in and 

thus may have caused Black-crowned Night Heron dispersal. Wisconsin reported that Herring 

Gulls were displaced from some areas on islands, but were not eliminated or forced out. Iowa 

reported that Canada Geese and Mallards nested on islands on the Mississippi River that were 

later denuded by DCCOs, resulting in increased predation and abandonment by these species.

In the southern U.S., Arkansas reported that DCCOs had excluded Cattle Egrets and 

Little Blue Herons from nesting areas at a rookery in Grassy Lake, and that egrets, herons, and 

other waterbirds had been replaced by DCCOs at a swamp in the southwestern portion of the 

state. Mississippi reported speculation that large concentrations of night roosting DCCOs may 

displace wintering waterfowl from roosting, feeding and loafing sites.

In the northeastern U.S., several states reported impacts or potential impacts. New York 

reported that DCCOs nesting at Little Galloo Is., Lake Ontario, displaced Black-crowned Night 

Herons. Vermont reported impacts to Black-crowned Night Heron, Cattle Egret, Gadwall, 

Mallard and American Black Duck. All of these species previously nested on Young Island, but 

vegetation was impacted by DCCO guano deposition. Most of the island is now denuded, and 

characterized by DCCOs and gull species; none of the species mentioned above currently nests 

on the island. Potential or possible impacts were reported in ME, MD and MA. Massachusetts 

reported that “it is reasonable to conclude that there has been a small amount of displacement of 

Herring Gulls in their nesting areas, but this has not been quantified.” Maine reported that 

DCCOs nesting on islands with Common Eiders reduced the available habitat in the vicinity of 

the DCCO colony, but that these changes are temporary and not limiting to other colonial nesters 

in most cases. Maryland reported that in the late 1990s, DCCOs moved into a heronry where 

they may eventually exclude herons because of guano impacts. The island also supports 

Common Terns, Herring Gulls and Great Black-backed Gulls, which could be impacted in 5-10 

years.

In Canada, Manitoba reported that DCCOs trample Common Tern colonies, and kill trees 

used by Great Blue and Black-crowned Night Herons. Additionally, they compete for habitat 

with Ring-billed and California Gulls, Caspian Terns and American White Pelicans. Prince 

Edward Island reported that DCCOs probably have an impact on Black-crowned Night Herons, 

crowding out herons and killing nesting trees at a greater rate than herons would have done. 
FINAL DRAFT APPENDIX I. Perceived Impacts 330

Ontario simply reported impacts to Black-crowned Night Herons, but didn’t explain what these 

impacts were.

A few respondents also reported positive impacts to avian species associated with DCCO 

nesting habits. Bill Koonz reported that DCCOs in Manitoba create habitat for themselves and 

for other species, such as plovers, terns, avocets, etc., by setting back vegetation and providing 

open areas. Similarly, David Allen noted that the tendency of DCCOs to reduce herbaceous 

vegetation is probably beneficial for some of the other colonial nesters in North Carolina that 

require bare sand for nesting.

Another important observation was that illegal cormorant control in Manitoba appeared 

to have substantial impacts for colonial waterbird breeding ecology on Lake Winnipegosis. Bill 

Koonz reported that during the 1999 colonial waterbird survey, most traditional cormorant 

colonies along the main travel lanes of Lake Winnipegosis had been disturbed, and cormorants 

had abandoned these sites. Ring-billed and California gulls, and Caspian and Common Terns 

also nest at these sites, but their nests had not been disturbed. Cormorants appeared to have a 

buffering effect within this community, previously nesting among and between these species. 

However, without DCCOs to buffer nesting cells between the gulls and terns, these species nest 

together, and considerable predation on nests occurs when these islands are visited by humans 

(where the nesting cells overlap, all the nests of Common Terns and Ring-billed Gulls are 

quickly destroyed). Additionally, when cormorants and pelicans are not on these islands, the 

islands begin to re-vegetate and the spaces used by the remaining species overlap. The result is 

under utilization of the available nesting space, destruction of nests, young and eggs by 

competing species, and a gradual loss of habitat on the colony due to vegetation encroachment. 

A number of islands on Lake Winnipegosis appeared to demonstrate this phenomenon.

Impacts to other fauna

1) Respondents were asked if the recent increase in DCCOs impacted any other terrestrial 

fauna (mammals, herps, etc) in their state / province, and if so, to describe any impacts.

Iowa was the only state to report impacts. Nesting DCCOs were believed to have reduced 

the diverse fauna that formerly occupied five islands on the Mississippi River, but no additional 

information was provided.

Damage to Private Property

1) Respondents were asked to provide information on damages to private property by DCCOs 

(type of damage, estimated dollar loss, etc.).

Only four states (SC, CT, ME and MA) and two Canadian provinces (MB and QC) 

reported damages to private property. South Carolina reported that damages to private property 

were minor and that affected land had not been reduced in value. Connecticut reported that a 

landowner complained about damages to expensive pinion oak trees and he actively discouraged 

DCCOs from nesting on his land. All trees survived and the affected land was not reduced in 

value. The Maine State Director of USDA/WS reported that he receives several calls a year 

about DCCOs eating trout out of private ponds. Loss varies from $1000-25,000 at trout ponds 

and bait operations and may result in reduced land value. The Massachusetts State Director of 

USDA/WS reported that she receives 2-3 calls per year of DCCOs roosting on docks and boats 
FINAL DRAFT APPENDIX I. Perceived Impacts 331

and making a mess with their droppings. She also reported that non-nesting DCCOs roost on 

power lines in Orleans, MA, and damage the herring run, pond water quality and residences 

around the lake, which has resulted in landowners in this area being unable to sell their homes. 

Manitoba reported that fish stocks are impacted but had no more specific information. The 

respondent from Quebec referred us to a paper by Bédard et al. (1995) which reported damages 

to unique forest habitats on several privately owned islands in the St. Lawrence River Estuary. 

No information was provided on dollar amounts of damage, etc. (see Part IV. Impacts to 

Vegetation and Colonial Waterbirds, for further information on this study). 

Disease Transmission 

1) Respondents were asked if there were any documented cases of disease transmission between 

DCCOs and wild or domestic birds.

Respondents from Michigan reported documented cases of disease in DCCOs. One 

reported that there had been a few cases of Newcastle disease in the past but not recently and not 

in any large numbers. The other reported a continental wide outbreak of Newcastle disease that 

occurred in 1992, and affected many DCCOs (mostly chicks), some White Pelicans, and a few 

domestic turkey flocks. The outbreak occurred during the summer, affecting breeding birds from 

the Rocky Mountains to Quebec, and is thought to have been picked up on the wintering grounds 

(J. Ludwig, pers. comm.). (See Part IV. Impacts to Vegetation and Colonial Waterbirds for 

further information on Newcastle Disease).

Impacts to Water Quality

1) Respondents were asked if there were any concerns in their state / province over increasing 

DCCO numbers and possible impacts to public health due to water quality and disease 

transmission.

Seven states and one province expressed concerns over possible impacts to water quality. 

In the Great Lakes, one respondent from Michigan reported that the Center for Disease Control 

was interested in human influenza viruses being carried by DCCOs and possibly transmitted 

through the water, but this has not been investigated and no additional information was provided. 

Ontario reported concern over the possibility that Newcastle Disease could be spread to domestic 

poultry. In the southeast, respondents from Alabama and Arkansas expressed concern over 

transfer of disease and parasites in aquaculture ponds, and that waste products from DCCOs may 

deplete oxygen and lower water quality. South Carolina reported that concerns about DCCO 

impacts on public health via water quality or disease transmission were slight but increasing. In 

the northeast, respondents from Maine reported concerns over DCCO and gull guano at lakes 

and ponds. The Massachusetts State Director of USDA/WS completed a survey for MA, CT and 

RI and reported that there was concern but did not specify in which states or the reasons for 

concern.
FINAL DRAFT APPENDIX I. Perceived Impacts 332

Impacts to Public Health

1) Respondents were asked if any instances of public health problems had been documented..

One Massachusetts respondent reported public health problems associated with DCCOs. 

in Orleans, where a resident was reported to have asthma and allergies caused by odor of DCCO 

guano. Whether this was properly documented is not known.
FINAL DRAFT APPENDIX I. Perceived Impacts 333

Table 29. Impacts in Zone 3 (U.S. and Canadian Interior)

l = minor, m = moderate, h = major, p = possible, n = none, y = yes, 

u = unknown (states with > 1 response had > 1 respondent)

Resource Impacted IL IN IA MB MI MO OH ONT SK VT WI

Degree of 

impact

Sport & l l l h l, l l l l l l l

Commercial 

Fish Studies n n n u n, u n y n y n n

Degree of 

impact

m l l h l, l l l l l l l

Economic loss N/A n l u u, u l n/u n n n u

Aquaculture

Legal take n n few some n, u l n n n n n

Herbaceous n n h h l, h n y u y h y

Trees n n m h l, h n y l y h y

Vegetation

Rare/unique n n n u n, u n p u n n u

Other species Avian ? n y y y, y n y n y y y

Nonavian n n y n n, n n n n n u u

Private property n n u y u, u n n u n n n

Disease transmission n n n u y, * n n n n n n

Water quality n n n u n, ? n n y n n n

Public health n n n u n, u n n n n n n

* see text

Table 30. Impacts in Zone 4 (Southern U.S.)

l = minor, m = moderate, h = major, p = possible, n = none, y = yes,

u = unknown (states with > 1 response had > 1 respondent)

Resource Impacted AL AR FL GA KY LA MS NC OK SC TN TX

Degree of 

impact

Sport & m h, m l l l u, m l u l l h l

Commercial 

Fish studies y n, n n n u u, y n y n n n n

Degree of 

impact

h h, h m u u h, h h u m l h h

Economic 

loss

Millionsh, h u u u Millions, u5,000

,000

u u u u u

Aquaculture

Legal take some most, 

most

s u u some, u most u u u some u

Herbaceous u u, u n u u n, u n n l u u n

Trees h y, l l u u n, u u y l n u n

Vegetation

Rare/unique u u, u n u u n, u n n n u u n

Other Avian n y, y n n/u n n/u, n ? u n u u n

species Nonavian n u, n u u n n/u, n n n n u u n

Private property u u, u n n u n, u n n n l u u

Disease transmission n n, n n n n n, n n n n n u n

Water quality y y, y n n u u, u n n n l u n

Public health n n, n n n u n, n n n n n u n
FINAL DRAFT APPENDIX I. Perceived Impacts 334

Table 31. Impacts in Zone 5 (North Atlantic)

l = minor, m = moderate, h = major, p = possible, n = none, y = yes,

u = unknown (states with > 1 response had > 1 respondent)

Resource impacted CT DE ME MD MA NB NH NJ NY PA PEI QB RI VA WV 

Degree 

of 

impact

l, m l m, m non

e

l, m l l l m l, l l l u, m m non

e

Sport & 

Commercial 

Fish

Studies y, u n u, y n n, u n n n y n, n y y u, u n n

Degree 

of 

impact

l, m l m, l l l, m l u l l n, l l l l, m l n

Economic 

loss

u, u n Substantial, uN/A u, u u u u not 

sig

u, n u n n, u 15,

000

n

Aquaculture

Legal 

take

f, f few n, 

few

few n, 

few

N/A u u N/A n, n few n Few, 

few

u n

Herbaceousu, u n u, l y n, u p y n/u n n, n u n n, u l n

Trees l, y n u, l y n, n y y n/u y n, n y l l, y u n

Vegetation

Rare/ 

unique

n, u n u, n n n, u u n n/u n n, n u/n n n, u u n

Other Avian u, n u n/u, y ? ?, y n n u y u, n ? n/u n, n u n

species Nonavian

u, n u n/u, n n n, u n n u n n, n u n/u u, n u n

Private property l, n n y, n n n, y n n n/u n n, n u/n y u, n n n

Disease transmission n, u n u, n n n, u n n n n n, n n n n, u n n

Water quality n, ? n y, y n u, ? n n n/u n n, n n n n, ? n n

Public health n, n n u, n n n, y n n n/u n n, n n n n, n n n
FINAL DRAFT APPENDIX I. Perceived Impacts 335

Summary of fisheries survey data: states / provinces reporting declines linked to DCCOs .

Table 32. Summary of fisheries survey data: states / provinces reporting declines linked to 

DCCOs.

State/ 

Province

Declines Area of Decline DCCO 

Link

DCCO data DCCO 

Studies

% 

consumed

DE yes Mid-Atlantic n n n

MA yes Inshore (marine) n n n

MA no Inland/Trout 

hatcheries

n n n

AL no n n n

IN no n n n

NJ variable Freshwater n n n

ME yes Marine y anecdotal y (past) n

ME yes Marine n n n

VT yes Freshwater y data being 

analyzed

n n

MI yes Freshwater y anecdotal y (past) y

AR unknown y n

NY yes Freshwater y data y y

WI yes Freshwater y % consumed 

only

y y

MB yes Freshwater n y(past) n

PEI & 

NB

yes Freshwater, Marine n y n

GA yes Freshwater, Marine n n n

IA yes Freshwater n n n

ONT yes Freshwater n fishermen y n

OK no n n n

CT yes Marine n On salmon, no 

impact

n n

OH yes Lake Erie n y n

TN no n n n

MS yes Interior Lakes, MS 

Delta Region

n y small
FINAL DRAFT APPENDIX I. Perceived Impacts 336

Table 33. Summary of Fisheries Survey Data: Fish species reported to be in decline and 

factors thought responsible.

State / 

Province

Species Factors Responsible

DE American shad, black sea bass, scup, 

bluefish

Overfishing, Loss of critical habitat

MA scup, tautog, black sea bass, Ocean Pout, 

Gulf of Maine cod, Cape cod yellowtail, 

longfinned squid, Gulf of Maine 

windowpane, Southern New England 

windowpane

Overexploitation

NJ Shad and alewife populations are variable lake trout predation (alewives), Lack of 

Zooplankton

ME alewife, blueblack herring, rainbow smelt, 

winter flounder, Atlantic cod, pollack, sea 

herring (inshore stocks), Atlantic salmon

Overfishing, Loss of spawing habitat,

Increased predation, Pollution, Dams

VT yellow perch white perch invasion, Cormorant 

predation, Zebra mussel

MI yellow perch, walleye, northern pike, 

bluegill, smallmouth bass

Overfishing, cormorants, sea lamprey, 

Natural Cycle, alewife, Zebra mussel, 

Weather

AR Unknown if any species are declining

NY yellow perch, walleye, smallmouth bass, 

rock bass

DCCOs, reductions in phosphorus, Zebra 

mussel

WI yellow perch, smelt white perch invasion, cormorants, trout, 

salmon and burbot predation

MB walleye, sauger Overfishing (commercial)

PEI & 

NB

Eel, Atlantic salmon Overfishing, Recruitment failure

GA Savannah River striped bass, Atlantic 

sturgeon

Overfishing, Habitat degradation

IA freshwater drum, bullhead Unknown

ONT American eel and lake whitefish (L. 

Ontario); rainbow smelt and white perch 

(L. Erie); yellow perch (L. Erie, Huron); 

lake trout (L. Superior)

Global and local environmental change, 

Temperature, Phosphorus abatement, 

Zebra mussels, Commercial exploitation

CT American shad, alewife, blueback herring, 

winter flounder, tautog

By-catch, Stocked recovery of striped 

bass, Entrainment, Unknown marine 

factors

OH Not provided Fishing, Exotics, Reduced Productivity

MS largemouth bass, redear, bluegill Poor water quality (high silt load in 

water)
FINAL DRAFT APPENDIX II. Current Research 337

APPENDIX II. CURRENT RESEARCH

To identify current research on DCCOs, we asked recipients of our DCCO and fisheries 

surveys to identify ongoing DCCO research and population monitoring efforts in their state / 

province. Recipients were also asked to identify priorities for DCCO research and monitoring. 

Table 34 shows the states and provinces that reported current research / monitoring, and 

identified research priorities. Recently completed research was reviewed in earlier sections.

Table 34. States and provinces engaged in current DCCO or DCCO related research or 

monitoring programs, and or with identified research priorities.

State / 

Province

Ongoing / Future 

Research

Currently Monitoring Research Priority

OK -Foraging efficiency;

-Avg. number/mass of prey 

consumed daily;

-prey selection in small lakes 

and ponds;

-methods to reduce foraging 

efficiency in ponds and fish 

hatcheries (M. Howery, OK 

Dept. Wildl. Cons.)

IA -Colonial nesting bird 

survey (E. Kirsch);

-Volunteer rookery 

survey (K. Bogenschutz, 

IA DNR)

-Continue annual rookery 

survey (IA DNR)

MI -Starting study of 

smallmouth bass population 

around Beaver Is. due to 

residents’ concerns over 

DCCO predation (MI DNR 

Fish. Div. and MI Univ.)

-Monitor nest numbers 

(G. Belyea, MI DNR)

-Identify and locate 

commercial fishermen that 

under report catch (J. Ludwig)

OH -Telemetry study (1999) to 

determine where DCCOs 

are feeding and distances 

flown on foraging flights on 

w. Lake Erie (M. Bur, 

USGS BRD, Sandusky 

Field Office)

-Population monitoring 

(M. Shieldcastle, OH 

Div. of Wildl.)

-Identify problems and 

solutions to effects on 

colonial waders

-Reduce fisherman concerns 

(OH Div. of Wildl.)

WI -Statewide survey every 

5 years, and other sites 

surveyed intermittently 

(S. Matteson, WI DNR)
FINAL DRAFT APPENDIX II. Current Research 338

State / 

Province

Ongoing / Future 

Research

Currently Monitoring Research Priority

AL -Winter surveys for 

monitoring (F. Boyd, 

USDA/WS)

-Continue winter surveys

-Assist with other research 

efforts (USDA/WS)

AR -DCCO impacts on catfish 

and baitfish production;

-Efficacy of harassing 

DCCOs at individual day 

roosts and / or night roosts 

near catfish and baitfish 

farms;

-Determine migratory 

corridors, breeding 

locations, and winter 

movements of DCCOs 

wintering in catfish 

production areas of the se 

U.S. through satellite 

telemetry study;

-Provide information 

necessary for regional 

and/or flyway-based DCCO 

population management 

strategies;

-Quantify and monitor 

enrichment of nutrients in 

aquatic and terrestrial 

environments associated 

with DCCO night roosts and 

breeding colonies in AR;

-Analyze effects of DCCOs 

on bottomland hardwood 

silvics and wetland ecology 

(S. Werner, WSDA/WS)

-Determine extent of 

DCCO nesting in se AR 

(S. Werner, USDA/WS)

-Protect aquaculture

-Determine impact on sport 

fisheries (K. Rowe, AR Game 

& Fish Comm.; M. Hoy, 

USDA/WS))

LA -Monitoring only 

through CBCs, BBSs, 

and waterbird colony 

surveys (B. Vermillion, 

LA Dept of Wildl. & 

Fish.)

-Monitor / count birds in 

Toledo Bend colonies;

-Literature search as part of 

recently developed task-force 

to investigate cormorant 

issues;

-Review of Migratory Bird 

Treaty Act. (LA Dept of 

Wildl. & Fish.)
FINAL DRAFT APPENDIX II. Current Research 339

State / 

Province

Ongoing / Future 

Research

Currently Monitoring Research Priority

MS -Clarifying catfish 

production losses

-Enhancing roost dispersal 

efforts through shooting (J. 

Glahn, USDA WS)

-Satellite tracking study 

monitoring movements on 

wintering grounds and 

tracking birds back to 

nesting colonies (S. Werner 

and Tommy King , USDA / 

WS)

-Monitoring wintering 

population and 

estimating numbers (J. 

Glahn, USDA WS)

-Determine where, when and 

how cormorant population 

control can best be 

implemented to alleviate 

DCCO impact on aquaculture 

along with using cultural and 

barrier strategies to alleviate 

impacts 

-monitor winter numbers and 

numbers taken under the 

USFWS depredation order 

and experimental control 

procedures(J. Glahn, 

USDA/WS)

NC -Survey all inland 

colonies every 5-10 

years (D. Allen, NC 

Wildl. Res. Comm.)

SC -Periodic ground counts 

of nests;

-Annual aerial estimates 

of nest numbers (T. 

Murphy, SC DNR)

ME -Monitor colonial 

seabirds nesting on 

coastal islands (R.B. 

Allen, ME Dept. Inland 

Fish & Wildl.)

-Continue monitoring (R.B. 

Allen)

MD -Periodic population 

census every 3-4 years 

(D. Brinker, MD DNR)

-Population monitoring (D. 

Brinker)

MA -Statewide nesting 

survey at 10 year 

intervals, next in 2004-

2005 (B. Blodget, MA 

Div. of Fish & Wildl.)

-Continue monitoring 

breeding colonies every 10 

years to document trends (B. 

Blodget)

-Measure damage from 

droppings to habitat;

-Measure damage from 

feeding on herring runs;

-Measure damage to water 

quality;

-Measure damage to property 

values;

-Measure damage to human 
FINAL DRAFT APPENDIX II. Current Research 340

State / 

Province

Ongoing / Future 

Research

Currently Monitoring Research Priority

health;

-Study habitat use preferences 

(roosts, nesting areas)

-Study roost dispersal 

effectiveness techniques (L. 

Henze, USDA / WS)

NY -Assessing impacts to 

Oneida Lake yellow perch 

and walleye populations 

through population 

monitoring (fisheries and 

joint effort with DCCO 

population / diet 

monitoring) and fisheries 

assessment of current 

DCCO management 

(reduction in successful 

management and 

harassment of fall migrants) 

(L. Rudstam, Cornell 

Biological Field Station 

315-633-2358

-Assessing impacts to Lake 

Ontario smallmouth bass 

population through 

population monitoring and 

DCCO diet assessment 

(joint effort with wildlife 

staff on DCCO population 

monitoring) and 

effectiveness of egg oiling 

(A. Schiavone, NYSDECFisheries 315 785-2621).

-Determine foraging range

and behavior of DCCOs 

nesting on Oneida Lake (J. 

Coleman, NY Coop Fish & 

Wildl. Res. Unit, Cornell 

Univ)

-Monitor upstate nesting 

colonies annually, and 

Long Island colonies 

every 3 years (R. Miller, 

NYSDEC)

-Continue monitoring nesting 

colonies

-Conduct research to 

determine effects of DCCO 

control efforts at Lake Ontario 

and Oneida Lake islands (R. 

Miller)

PA -Nesting colony is 

monitored (D. Brauning, 

PA Game Comm.)

-Monitor nesting population 

(D. Brauning, PA Game 

Comm.)
FINAL DRAFT APPENDIX II. Current Research 341

State / 

Province

Ongoing / Future 

Research

Currently Monitoring Research Priority

RI -Studies to assess impacts to 

commercial and sport 

fisheries (M. Gibson, 

Wickford Marine Base)

-Annual nest counts (C. 

Raithel, RI Div Fish & 

Wildl.)

-Assess potential impact to 

fisheries (C. Raithel, RI Div 

Fish & Wildl.)

VT -Monitor islands for 

nesting, and annual nest 

counts at Lake 

Champlain (L. Garland, 

VT Dept Fish & Wildl.)

MB -Monitor breeding 

population via nest 

counts (B. Koonz, MB 

DNR)

NB Study on predation of 

Atlantic salmon smolts by 

mergansers, cormorants and 

seals in New Brunswick (F. 

Whoriskey, Atlantic salmon

Federation, 

emerrill@nbnet.nb.ca

ONT -Diet study underway this 

summer at e. Lake Ontario 

colonies (Alastair Mathers, 

Lake Ont Mgmt Unit, Ont. 

Min. Nat. Res. 613-476-

8733)

-Monitor numbers on 

Great Lakes, also some 

periodic inland counts 

(J. Harcus, Ont. Min. 

Nat. Res.)

-Cormorant food studies in e. 

Lake Ontario for 1999 if 

funds are available;

-Studies to assess impacts to 

fish stocks in inland lakes;

-More standardized 

monitoring of DCCO 

numbers across the province 

in areas other than the Great 

Lakes (J. Harcus, Ont. Min. 

Nat. Res.)

PEI -Bioenergetic modeling for 

consumption estimates of 

Atlantic salmon by avian 

predators (cormorants, 

mergansers and kingfishers) 

in the maritime provinces, 

as a percent of standing 

stock (D. Cairns, Dept of 

Fisheries & Oceans, PEI);

FINAL DRAFT APPENDIX III. Scientific Names 343

APPENDIX III. LIST OF COMMON AND SCIENTIFIC NAMES

BIRDS

Common Name Scientific Name Abbreviation

Anhinga Anhinga anhinga

American Avocet Recurvirostra americana

American Black Duck Anas rubripes

American White Pelican Pelecanus erythrorhynchos

Bald Eagle Haliaeetus leucocephalus

Canada Goose Branta canadensis

Common Eider Somateria mollissima

Common Raven Corvus corax

cormorants:

Brandt’s Cormorant Phalacrocorax penicilalatus

Double-crested Cormorant P. auritus DCCO

Great Cormorant P. carbo GRCO

Neotropic Cormorant P. brasilianus

Pelagic Cormorant P. pelagicus

Red-faced Cormorant P. urile

crow sp. Corvus sp. 

egrets:

Cattle Egret Bubulcus ibis

Great Egret Ardea alba

Gadwall Anas strepera

gulls:

California Gull Larus californicus

Greater Black-backed Gull L. marinus

Herring Gull L. argentatus

Ring-billed Gull L. delewarensis

Gyrfalcon Falco rusticolus

herons:

Black-crowned Night-heron Nycticorax nycticorax BCNH

Great Blue Heron Ardea herodias GRBH

Little Blue Heron Egretta caeruela

Mallard Anas platyrhynchos

Osprey Pandion haliaetus

Oystercatcher Haematopus ostralegus

plover sp. Pluvalis sp. and Charadrius sp.

terns:

Caspian Tern Sterna caspia

Common Tern Sterna hirundo

swallow sp. Hirundinidae
FINAL DRAFT APPENDIX III. Scientific Names 344

FISHES

Common names of fishes are not traditionally capitalized in publications (American 

Fisheries Society 1991), whereas standard format for bird common names (American 

Ornithologists’ Union 1998) requires use of capital letters. Use of these two different formats 

respects nomenclature decisions followed by these two professional societies. Species have been 

grouped together when common names reflect taxonomic relationships; otherwise, they are 

alphabetized by first letter of the common name.

Common Name Scientific Name

alewife Alosa pseudoharengus

Atlantic menhaden Brevoortia tyrannus

bass sp. PERCIFORMES

largemouth bass Micropterus salmoides

rock bass Ambloplites rupestris

smallmouth bass Micropterus dolomieu

striped bass Morone saxatilis

bloater Coregonus hoyi

bluegill Lepomis macrochirus

bluefish Pomatomus saltatrix

bream bluegill or pinfish

bullhead sp. Ictaluridae (Ameiurus sp.)

burbot Lota lota

capelin Mallotus villosus

carp sp. Cyprinidae

common carp Cyprinus carpio

catfish, sp. Ictaluridae

channel catfish Ictalarus punctatus

chub sp. Cyprinidae

lake (northern) chub Couesius plumbeus

cisco (also lake herring) Coregonus artedi

cod sp. Gadidae

Atlantic cod Gadus morhua

crappie sp. Centrarchidae (Pomoxis sp.)

black crappie Pomoxis nigromaculatus

white crappie Pomoxis annularis

cunner Tautogolabrus adspersus

drum sp. Sciaenidae

freshwater drum Aplodinotus grunniens

eel sp. ANGUILLIFORMES

American eel Anguilla rostrata

flounder sp. Bothidae and Pleuronectidae

winter flounder Pleuronectes americanus
FINAL DRAFT APPENDIX III. Scientific Names 345

herring sp. Clupeidae

Atlantic herring Clupea harengus

blueback herring Alosa aestivalis

lake herring see “lake herring (also cisco)”

Pacific herring Clupea pallasi

sea herring see Atlantic herring

gunnel sp. Pholidae

crescent gunnel Pholis laeta

penpoint gunnel Apodichthys flavidus

lake herring (also cisco) Salmonidae, Coregonus artedi

lamprey sp. Petromyzontidae

sea lamprey Petromyzon marinus

mackerel Scombridae

northern pike Esox lucius

peamouth Mylocheilus caurinus

pinfish Lagodon rhomboides

pollock Pollachius virens

pout sp. Zoarcidae

ocean pout Macrozoarces americanus

prickleback sp. Stichaeidae

snake prickleback lumpenus sagitta

pumpkinseed Lepomis gibbosus

redear see redear sunfish

salmon, sp. Salmonidae

Atlantic salmon Salmo salar

sand lance sp. Ammodytidae

American sand lance Ammodytes americanus

Pacific sand lance Ammodytes hexapterus

sauger Stizostedion canadense

sculpin sp. Cottidae

slimy sculpin Cottus cognatus

spoonhead sculpin Cottus ricei

sea bass sp. Serranidae

black sea bass Centropristis striata

seapearch sp. Embiotocidae

striped seapearch Embiotoca lateralis

scup Stenotomus chrysops

shad sp. Clupeidae

American shad Alosa sapidissima

gizzard shad Dorosoma cepedianum

threadfin shad Dorosoma petenense

shiner perch Cymatogaster aggregata

shiner sp. Cyprinidae

common shiner Luxilus cornutus

emerald shiner Notropis atherinoides

spottail shiner Notropis hudsonius
FINAL DRAFT APPENDIX III. Scientific Names 346

smelt sp. Osmeridae

rainbow smelt Osmerus mordax

splake hybrid trout

(Salvelinus fontinalis x S. namaycush)

stickleback sp. Gasterosteidae

ninespine stickleback Pungitius pungitius

threespine stickleback Gasterosteus aculeatus

sturgeon sp. Acipenseridae

Atlantic sturgeon Acipenser oxyrhynchus

sucker sp. Catostomidae

white sucker Catostomus commersoni

sunfish sp. Centrarchidae

green sunfish Lepomis cyanellus

redear sunfish Lepomis microlophus

tautog Tautoga onitis

tilapia sp. Cichlidae (Tilapia sp.)

blue tilapia Tilapia aurea

tomcod sp. Microgadus sp.

Atlantic tomcod Microgadus tomcod

trout sp. Salmonidae

brook trout Salvelinus fontinalis

lake trout Salvelinus namaycush

trout-perch Percopsis omiscomaycus

or P. transmontana

yellow perch Perca flavescens

walleye Stizostedion vitreum

whitefish Salmonidae

lake whitefish Coregonus clupeaformis

round whitefish Prosopium cylindraceum

white perch Morone americana

windowpane Scophthalmus aquosus

yellowtail Seriola lalandi

MAMMALS

Common Name Scientific Name

arctic fox Alopex lagopus

ground squirrel Spermophilus undulatus

Norway rat Rattus norvegicus

rabbit Oryctolagus cuniculus

red fox Vulpes vulpes
FINAL DRAFT APPENDIX III. Scientific Names 347

ARTHROPODS

Common Name Scientific Name

sand shrimp (also common shrimp) Crangon crangon

shrimp sp. Crustacea

crayfish / crawfish Procambarus clarkii

crayfish / yabbie Cherax destructor

OTHER INVERTEBRATES

Common Name Scientific Name

Dreissenid mussels Dreissena sp.

Longfinned squid Loligo pealei

Zebra mussel Dreissena polymorpha

PLANTS

Common Name Scientific Name

cottonwood sp. Populus sp.

Canadian milk vetch Astragalus canadensis

sandbar willow salix exigua

stinging nettle Urtica dioica

thistle sp. Cirsium sp.

FINAL DRAFT APPENDIX IV. Abbreviations 349

APPENDIX IV. LIST OF ABBREVIATIONS

ADC Animal Damage Control

APHIS Animal and Plant Health Inspection Service

BMR basal metabolic rate

CBC (National Audubon Society) Christmas Bird Count

CPUE catch per unit effort

DFI daily food intake

DNR Department of Natural Resources

EIS environmental impact assessment

kJ kilojoule (measure of energy)

MBTA Migratory Bird Treaty Act

NASS National Agricultural Statistics Service

NGO non-governmental organization

NPS National Park Service

NWR national wildlife refuge

NYSDEC New York State Department of Environmental Conservation

PRBO Point Reyes Bird Observatory

USACE United States Army Corps of Engineers

USDA/WS United States Department of Agriculture / Wildlife Services

USGS United States Geographical Survey

USFWS United States Fish and Wildlife Service

VDFW Vermont Department of Fisheries and Wildlife

WMA wildlife management area

FINAL DRAFT Appendix V. Personal Communications 351

APPENDIX V. LIST OF PERSONAL COMMUNICATIONS

Includes unpublished data and personal observations

Connie Adams

Cornell University, Department of Natural Resources—Cornell Biological Field Station, 

Bridgeport, New York

David Allen

North Carolina Wildlife Resources Commission, Trenton, North Carolina

R. Bradford Allen

Department of Inland Fisheries and Wildlife, Bangor Maine

Daniel Anderson 

University of California, Davis, California

Doug Backlund

South Dakota Department of Game, Fish and Parks, Pierre, South Dakota

Glen Y. Belyea

Michigan Department of Natural Resources, Roselake Wildlife Research Center, East Lansing, 

Michigan.

Brad Blodget

Massachusetts Division of Fisheries and Wildlife, Field Headquarters, Westborough, 

Massachusetts

Kim Bogenschutz

Iowa Department of Natural Resources, Boone, Iowa

Frank Boyd

USDA/WS, Auburn University, Auburn, Alabama

Daniel Brauning

Pennsylvania Game Commission, Montgomery, Pennsylvania

Thomas Bregnballe

National Environmental Research Institute, Dept. of Coastal Zone Ecology, Kalo

Ronde, Denmark

David F. Brinker

Maryland Department of Natural Resources, Annapolis, Maryland
FINAL DRAFT Appendix V. Personal Communications 352

Michael Bur

USGS/ Great Lakes Science Center, Lake Erie Biological Station, Sandusky, Ohio

William Busby

Kansas Biological Survey, Lawrence, Kansas

Vernon Byrd

Alaska Maritime National Wildlife Refuge, Homer, Alaska

Harry Carter

USGS, Dixon Field Station, Dixon, California; Humboldt State University, Department of 

Wildlife, Arcata, California.

John S. Castrale

Indiana Department of Natural Resources, Mitchell, Indiana

Andrea Cerovski

Wyoming Game and Fish Department, Lander, Wyoming

Michael Chutter

British Columbia Ministry of Environment, Land and Parks, Wildlife Branch, Victoria, British 

Columbia

Alan Clark

Julia Butler Hanson/Columbia White-tail deer NWR, Portland, Oregon

Troy Corman

Arizona Game and Fish Department, Phoenix, Arizona

Steven Correia

Massachusetts Division of Marine Fisheries, Pocasset, Massachusetts

Scott Craven

Department of Wildlife Ecology, University of Wisconsin, Madison, Wisconsin

Randy Diblee

Department of Technology and Environment, Fish and Wildlife Division, Charlottetown, Prince 

Edward Island

John Dinan

Nebraska Game and Parks Commission, Lincoln, Nebraska

Eileen Dowd-Stukel

South Dakota Department of Game, Fish and Parks, Pierre, South Dakota
FINAL DRAFT Appendix V. Personal Communications 353

Don Dragoo

Alaska Maritime National Wildlife Refuge, Homer, Alaska

Dennis L. Flath

Montana Fish, Wildlife and Parks, Bozeman, Montana

Doug Forsell

USFWS, Chesapeake Bay Field Office, Annapolis, MD

Lawrence E. Garland

Vermont Department of Fish and Wildlife, Essex Junction, Vermont

Kenneth M. Giesen

Colorado Division of Wildlife, Wildlife Research Center, Fort Collins, Colorado

James F. Glahn

USDA/APHIS/WS, Mississippi Research Station, Mississippi State, Mississippi

Walker Golder

National Audubon Society, Wilmington, North Carolina

Chris Grondahl

North Dakota Department of Game and Fish, Bismarck, North Dakota

Jim Hainline

USFWS, Klamath Basin NWR, Tule Lake, California

Jonh Harcus

Ministry of Natural Resources, Petersborough, Ontario

Robert Hatcher

Tennessee Wildlife Resources Agency, Nashville, Tennessee

Christopher (Kitt) M. Heckscher

Delaware Division of Fish and Wildlife, Smyrna, Delaware

Jim Herkert

Illinois Endangered Species Protection Board, Springfield, Illinois

John Herron

Texas Parks and Wildlife, Austin, Texas

Frank Howe

Utah Department of Natural Resources, Salt Lake City, Utah
FINAL DRAFT Appendix V. Personal Communications 354

William Howe

USFWS, Albuquerque, New Mexico

Mark Howery

Oklahoma Department of Wildlife Conservation, Oklahoma City, Oklahoma

Michael Hoy

USDA/APHIS/WS, Stuttgart, Arkansas

Chuck Hunter

USFWS, Atlanta, Georgia

Drew Jones

West Virginia Nongame Wildlife and Natural Heritage Program, Elkins, West Virginia

Rich Kane

New Jersey Audubon Society, Bernardsville, New Jersey

John Kanter

New Hampshire Fish and Game Department, Concord, New Hampshire

Thomas Keller

Technische Universitaet Muenchen, Angewandte Zoologie, Freising/Weihenstephan,

Germany

Bill Koonz

Manitoba Department of Natural Resources, Winnipeg, Manitoba

Kathy Kuletz

U.S. Fish and Wildlife Service, Migratory Bird Management, Anchorage, Alaska

Meg Laws

USFWS, Malheur NWR, North Harney, Oregon

Harry LeGrand 

North Carolina State Natural Heritage Program, Raleigh, North Carolina

Kathleen Leo

West Virginia Nongame Wildlife and Natural Heritage Program, Elkins, West Virginia

Tony Leukering

Colorado Bird Observatory, Brighton, Colorado

Rich Levad

Colorado Bird Observatory, Brighton, Colorado
FINAL DRAFT Appendix V. Personal Communications 355

Roy Lowe

USFWS, Oregon Coastal Islands NWR Complex, Newport, Oregon

Frederick E. Ludwig (deceased)

contact J.P. Ludwig (below) for unpubl. data

James.P. Ludwig

The SERE Group, Ltd., Kingsville, Ontario

Walt Lysack

Manitoba Department of Natural Resources, Fisheries Branch, Winnipeg, Manitoba, Canada

Sumner Matteson

Wisconsin Department of Natural Resources, Bureau of Endangered Resources, Madison, 

Wisconsin

Robert L. Miller

New York State Department of Environmental Conservation, Nongame and Habitat Unit, 

Wildlife Resources Center, Delmar, New York

William Montevecchi 

Biopsychology Programme, Memorial University of Newfoundland, St. John’s, Newfoundland, 

Canada

Patty Morrison

USFWS, Ohio River Islands NWR, West Virginia

Ian Moul

Foul Bay Ecological Research Limited, Cortes Island, British Columbia

Tom Murphy

South Carolina Department of Natural Resources, Green Pond, South Carolina

Maura Naughton

USFWS Migratory Birds and Habitat Programs, Pacific Region, Portland, Oregon

Larry Neel

Nevada Division of Wildlife, Fallon, Nevada

Dave Nysewander

Washington Department of Wildlife, Washington

Brainerd Palmer-Ball, Jr.

Kentucky State Nature Preserves Commission, Frankfort, Kentucky
FINAL DRAFT Appendix V. Personal Communications 356

Richard Peterson

South Dakota Ornithologists' Union, Hot Springs, South Dakota

Will Post 

The Charleston Museum, Charleston, South Carolina

Jean-Francois Rail

Canadian Wildlife Service, Québec

Chris Raithel

Division of Fish, Wildlife and Estuarine Resources, West Kingston, Rhode Island

Philip Robinson

USDA/APHIS/WS, Oklahoma City, Oklahoma

Dan Roby

Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, 

Oregon State University, Corvalis, Oregon

Todd Roden

Minnesota Department of Natural Resources, St. Paul, Minnesota

Jim Rodgers, Jr.

Florida Game and Fresh Water Fish Commission, Gainesville, Florida

Kieth Roney

Royal Saskatchewan Museum, Regina, Saskatchewan

Robert M. Ross

USGS Biological Resources Division, Wellsboro, Pennsylvania

Karen Rowe

Arkansas Game and Fish Commission, Humphrey, Arkansas

Lars Rudstam

Cornell Biological Field Station, Bridgeport, New York

Jorge Saliva

USFWS, Boqueron, Puerto Rico

Don Schwab

Virginia Department of Game and Inland Fisheries, Williamsburg, Virginia

Mark Shieldcastle

Crane Creek Wildlife Research Station, Oak Harbor, Ohio
FINAL DRAFT Appendix V. Personal Communications 357

Dave Shuford

Point Reyes Bird Observatory, Point Reyes, California

Scott Smith

Maryland Department of Natural Resources, Queenstown, Maryland

Shawn Stephensen 

USFWS, Migratory Bird Management, Anchorage, Alaska

Julie Stofel

Washington Department of Fish and Wildlife, Olympia, Washington

Jim Tabor

Washington Department of Fish and Wildlife, Ephrata, Washington

R. Telfair II

Texas Parks and Wildlife Department Management and Research Station, Tyler, Texas

Chuck Trost

Department of Biology, Idaho State University, Pocatello, Idaho

Bill Vermillion

Louisiana Department of Wildlife and Fisheries, Baton Rouge, Louisiana

Julie Victoria

Connecticut Department of Environmental Protection, Franklin Wildlife Management Area, 

North Franklin, Connecticut

Bruce Vondracek

Department of Fisheries and Wildlife, University of Minnesota, St. Paul, Minnesota

Joan Walsh

Cape May Bird Observatory, Cape May Court House, New Jersey

Scott Werner

USDA/APHIS/WS, Mississippi Research Station, Mississippi State, Mississippi

D.V. (Chip) Weseloh

Canadian Wildlife Service, Downsview, Ontario

Sartor (Sandy) Williams

New Mexico Department of Game and Fish, Santa Fe, New Mexico

Jim Wilson

The Point LePreau Bird Observatory (project of Saint John Naturalist’s Club), Saint John, New 

Brunswick 
FINAL DRAFT Appendix V. Personal Communications 358

Jim D. Wilson

Missouri Department of Conservation, Jefferson City, Missouri

Ulrich Wilson

Washington Maritime National Wildlife Refuge Complex, USFWS, Port Angeles, WA

Brad Winn

Georgia Department of Natural Resources, Coastal Nongame Office, Brunswick, Georgia

Donna Withers

Stillwater National Wildlife Refuge, Fallon, Nevada

Vernon Wright

School of Forestry, Wildlife and Fisheries, Louisiana State University, Baton Rouge, Louisiana
FINAL DRAFT Appendix V: Personal Communications 359