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OCS Study MMS 2000-003

Cetaceans, Sea Turtles and Seabirds in the Northern Gulf of Mexico:

Distribution, Abundance and Habitat Associations

Volume II: Technical Report

U.S. Departrnnt of the lrteror US. Department ol the Interior

U$. Geological Survey Minurals Management Service

Bologicar Resources Divisior Gulf of Mexcc Region
Contractor Report

USGS/BRD/CR-- 1999-0006

OCS Study MMS 2000-003

Cetaceans, Sea Turtles and Seabirds in the Northern Gulf of

Mexico: Distribution, Abundance and Habitat Associations

Volume II: Technical Report

Editors

Randall W. Davis

William B Evans

Bernd Wursig

January 2000

Prepared under

U.S. Geological Survey, Biological Resources Division

Contract Number 1445-CTO9-96-0004 and 1445-1A09-96-0009

by

The GulfCet Program

Department of Marine Biology

Texas A&M University at Galveston

5007 Avenue U

Galveston, Texas 77551

In cooperation with the

U.S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region
PROJECT COOPERATION

This study was procured to meet information needs identified by the Minerals Management

Service (MMS) in concert with the U.S. Geological Survey, Biological Resources Division

(BRD).

DISCLAIMER

This report was prepared under contract between the U.S. Geological Survey, Biological

Resources Division BRD), Texas A&M University, and the National Marine Fisheries Service.

This report has been technically reviewed by the BRD and the Minerals Management Service

(MMS), and has been approved for publication. Approval does not signify that the contents

necessarily reflect the views and policies of the BRD orMMS, nor does mention of trade names

or commercial products constitute endorsement or recommendation for use. It is, however,

exempt from review and compliance with the MMS editorial standards.

REPORT AVAILABILITY

Extra copies of this report may be obtained from the Public Information Office at the following

address:

U. S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region

Public Information Office (MS 5034)

1201 Elmwood Park Boulevard

New Orleans, LA 70123-23 94

Telephone: (504) 736-2519 or

(800) 200-GULF

CITATION

Davis, R,W., W.E. Evans, and B. WUrsig, eds. 2000. Cetaceans, Sea Turtles and Seabirds in the

Northern Gulf of Mexico: Distribution, Abundance and Habitat Associations. Volume II:

Technical Report. Prepared by Texas A&M University at Galveston and the National

Marine Fisheries Service. U.S. Department of the Interior, Geological Survey, Biological

Resources Division, USGS/BED/CR-1999-0006 and Minerals Management Service,

Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 2000-003 346 pp.

111
PREFACE

This study entitled "Cetaceans, Sea Turtles and Seabirds in the Northern Gulf of Mexico:

Distribution, Abundance and Habitat Associations", also known as the GulfCet II study, provides

synoptic data and analyses on the species diversity, abundance, and habitat characteristics for

cetaceans, sea turtles and seabirds in the northern Gulf of Mexico. Results of the study are

described in three volumes including this volume ("Volume II: Technical Report"), "Volume I:

Executive Summary" and "Volume III: Data Appendix."

This study was sponsored and administered by the U.S. Department of Interior, U.S. Geological

Survey, Biological Resources Division to provide environmental information to the U.S.

Department of Interior, Minerals Management Service. It was managed by Texas A&M

University at Galveston in partnership with the National Marine Fisheries Service at the

Southeast Fisheries Science Center.

V
ABSTRACT

The Gulf of Mexico is a semi-enclosed, intercontinental sea with a total area of about 1.5 million

square kilometers. As a large marine ecosystem, it has a unique bathymetry, hydrography and

productivity. Cetaceans, sea turtles and seabirds are upper trophic level predators that play an

important role in the pelagic marine ecosystem of the Gulf ofMexico. These are highly valued

taxa, protected by national laws and international agreements, and knowledge of their

distribution, abundance and ecology is vital to their protection. GulfCet II was planned to help

resolve issues concerning the potential impacts of various oil and gas activities on cetaceans, sea

turtles and seabirds that inhabit the northern and eastern regions of the Gulf of Mexico,

emphasizing the continental slope where water depths range from 100 to 2,000 m. The

objectives of the GulfCet II field studies (1996-97) were to: 1) expand the geographical coverage

of ship and aerial surveys that were conducted previously during GulfCet I (1992-94), which

surveyed the north-central and northwestern Gulf of Mexico, 2) estimate the minimum

abundances of cetaceans and sea turtles in areas surveyed during 1 99697, 3) collect

simultaneous hydrographic data and biological samples during the ship surveys to better define

the habitat associations of cetaceans and seabirds, and 4) collect acoustic data on cetacean

sounds and identify and record other natural and man-made underwater sounds to provide

additional insights into cetacean distribution and behavior, particularly in relation to noise from

seismic exploration vessels.

We hypothesized that hydrographic features in the study area had different levels of potential

prey that influence cetacean and seabird distribution. We further hypothesized that these food

stocks would be locally concentrated in nutrient-rich areas offshore from the Mississippi River,

within cyclonic eddies, and along the high-shear edges of cyclonic eddies.

An integrated methodology was used that included visual surveys from ships and aircraft, and

acoustic recordings and hydrographic collections from ships. Near real-time sea surface

altimetry from the TOPEXIPOSEIDON and ERS satellites was used during ship surveys to

determine the location of hydrographic features (e.g., cyclones, anticyclones and confluence

zones). Archival satellite sea surface altimetry data were used to retrospectively determine the

location of hydrographic features for analysis with cetacean sightings collected during GulfCet I.

We measured zooplankton and micronekton biomass derived from both net and acoustic

sampling to indicate the amount of potential food available for higher trophic level foraging by

cetaceans and seabirds.

Nineteen cetacean species were identified in the oceanic northern Gulf of Mexico (398,960 km2)

during GulfCet II surveys. The estimated minimum abundance of all cetaceans in the oceanic

northern Gulf based on shipboard surveys was 86,705 animals. Pantropical spotted dolphins

were the most abundant species with an estimated 46,625 animals, followed by spinner dolphins

(11,251) and clymene dolphins (10,093). Estimates for bottlenose dolphins, striped dolphins,

melonheaded whales, Risso's dolphins and short-finned pilot whales ranged from 4,381 to 1,471

animals. Abundances of all other species were less than 1,000 animals. Cetaceans were sighted

throughout the study area, but fewer were sighted in the western Gulf. There are now sighting

records during three or more seasons for at least 16 cetacean species.

vii
Seventeen cetacean species were sighted in the Minerals Management Service's Eastern

Planning Area (EPA, 70,470 km2). The abundance estimate based on aerial surveys (which were

more extensive than the ship surveys in the EPA) was 38,184 total animals. In general,

cetaceans were found throughout the EPA each season. The most abundant species were

pantropical spotted dolphin (13,649) and spinner dolphin (8,670). Other species with abundance

estimates over 1,000 based on aerial surveys were bottlenose dolphin, Atlantic spotted dolphin,

Risso's dolphin, striped dolphin and clymene dolphin. The seasonal abundance of some species

may vary regionally in continental slope waters. For example, dwarf/pygmy sperm whales were

nine-times more abundant in the summer than in the winter.

Cetaceans in the northeastern and oceanic northern Gulf of Mexico were concentrated along the

continental slope in or near cyclones and the confluence of cyclone-anticyclone eddy pairs. Net

tows and acoustic backscatter measurements with an Acoustic Doppler Current Profiler showed

that cyclonic eddies and confluence areas are mesoscale features with locally concentrated

zooplankton and micronekton stocks that appear to develop in response to increased nutrient-rich

water and primary production in the mixed layer. A significant relationship existed between

integrated zooplankton biomass and integrated cephalopod (a major component of cetacean prey)

paralarvae numbers, indicating that higher zooplankton and micronekton biomass may correlate

with higher concentrations of cetacean prey. In the north-central Gulf, an additional factor

affecting cetacean distribution may be the narrow continental shelf south of the Mississippi River

delta. Low salinity, nutrient-rich water may occur over the continental slope near the mouth of

the Mississippi River (MOM) or be entrained within the confluence of a cycloneanticyclone

eddy pair and transported beyond the continental slope. This creates a deep-water environment

with locally enhanced primary and secondary productivity and may explain the presence of a

resident, breeding population of endangered sperm whales within 50 km of the Mississippi River

delta. We suggest that this area may be essential habitat for sperm whales in the northern Gulf.

Overall, the results suggest that the amount of potential prey for cetaceans (and seabirds) may be

consistently greater in the cyclone, confluence areas, and south of the MOM, making them

preferential areas for foraging. Since cyclones in the northern Gulf are dynamic and usually

associated with westward moving cyclone-anticyclone pairs, cetacean distribution will be

dynamic. However, with near real-time satellite remote sensing of sea surface altimetry, these

features can be tracked and used to predict where pelagic cetaceans may be concentrated. The

exceptions are bottlenose dolphins, Atlantic spotted dolphins and possibly J3ryde's whales that

typically occur on the continental shelf or along the shelf break outside of major influences of

eddies.

An acoustic survey was performed using a towed hydrophone array to describe the distribution

of cetaceans based on species-specific vocalizations and to record man-made noise. Sperm

whales and pantropical spotted dolphins were the most commonly identified cetaceans, although

recordings were also made for clymene dolphins, spinner dolphins, striped dolphins, Atlantic

spotted dolphins, false killer whales, bottlenose dolphins, rough-toothed dolphins and Fraserts

dolphins. The whistles of nine dolphin species were characterized based on species-specific

patterns of whistle usage and acoustic structure. A diversity of anthropogenic signals was

recorded, many of which were low frequency seismic exploration signals. Seismic exploration

signals were detected during 21% of recordings, although there was no significant difference in

vi"
the cetacean sighting frequency for low, medium, and high noise levels in different hydrographiC

features.

GulfCet II aerial surveys provided the first assessment of sea turtle abundance and distribution

over a large area of the oceanic northeastern Gulf of Mexico. Three sea turtle species occurred

in the EPA study area: loggerhead, Kemp's ridley, and leatherback. The leatherback and Kemp's

ridley sea turtles are listed as endangered, and the loggerhead sea turtle is listed as threatened.

The overall density of loggerhead sea turtles in the EPA shelf was 20 times that of the EPA

slope. The majority ofloggerheads over the EPA slope were sighted during winter. While many

winter sightings were near the 100 m isobath, there were sightings of loggerheads over very deep

waters (i.e., >1000 m). LeatherbackS were sighted throughout the EPA slope and were about 12

times more abundant in winter than summer. The nearly disjunct summer and winter

distributions of leatherbacks indicates that specific areas may be important to this species either

seasonally or for short periods of time.

Seabird species present in the Gulf of Mexico varied by season. The species composition of the

sightings during late summer reflected a pattern of migration and transition to a winter

distribution. Two of the three most commonly identified species (laughing gull and royal tern) in

late summer are considered year-round residents in the Gulf. Pomarine j aegers, a wintering

marine species in the Gulf, were the third most commonly identified species. During midsummer, the black tern was the most abundant species, followed by band-rumped storrn-petrels

(summer migrant pelagic), frigatebirds (permanent resident), Audubon's shearwaterS (summer

migrant pelagic) and sooty terns (summer resident).

Cyclones had the greatest diversity of seabird species, although habitat use varied among

species. Pomarinejaegers were more likely to be present in the MOM area during late summer.

Audubon's shearwaterS were more likely to be encountered inside a cyclone, while bandrumped

storm-petrels were more likely to be present in the areas other than cyclones, anticyclones or

confluence zones during mid-summer. Black terns were encountered more frequently in the

MOM area during mid-summer. Generalized additive models incorporating indicators of

plankton standing stock (surface chlorophyll and predicted mean biomass of zooplarikton and

micronekton) best predicted seabird presence for five ofthe seven species analyzed. Other

predictive models were: sea surface properties of temperature and salinity for black tern, sooty

tern, and laughing gull; sea surface height for pomarine jaeger; and bathymetry for Audubon's

shearwater. Seasonal surveys are needed to better assess community structure and seabirdhabitat associations.

Eighty-three percent of the crude oil and 99% of the gas production in United States federal

waters occurs in the Gulf of Mexico, primarily along the Texas-Louisiana continental shelf and

slope. By 2003, oil production in the Gulf is projected to increase 43%. Production from

deepwater fields (depth >305 m) will account for about 59% of the daily oil production and 27%

of the daily gas production in the Gulf. In addition to oil and gas exploration and production,

this area has considerable commercial shipping traffic that enters the northern Gulf ports. The

long-term forecast for petroleum transportation is for the total volume to increase into the next

century. The cumulative impact of these multiple, potential impact-producing factors on

cetaceans in the northern Gulf cannot be predicted with certainty. However, it can be anticipated

ix
that cetaceans along the continental slope will encounter increasing oil and gas exploration and

production activities. There are significant uncertainties in our understanding of short and longterm effects of seismic and other loud industrial sounds on the behavior and distribution of Gulf

cetaceans. Against the background of growing oil and gas exploration and development,

continued research and monitoring are needed to assess the potential impacts of these activities

on pelagic cetaceans, sea turtles and seabirds in the Gulf of Mexico.

x
TABLE OF CONTENTS

List of Figures xvii

List of Tables xxxi

List of Abbreviations and Acronyms xxxv

Acknowledgments xxxvii

Contributors xxxix

Chapter 1. Introduction 1 Randall W. Davis and William E. Evans

1.1 Background 1 1 .1.1 Federal Requirement for the Program 1

1.1.2 Program Participants 3

1.1.3 Cetacean Surveys of the Northern Gulf Prior to 1994 3

1.2 General Oceanography of the Gulf of Mexico 7

1.3 Guiftet II Study Area and Objectives 10

1.3.1 Study Area 10

1.3.2 Specific Objectives 14

Chapter 2. Environmental Patterns and Oceanographic Processes: Hydrography and

Mesoscale Circulation of the Continental Margin of the Northeastern Gulf of

Mexico, 1996 and 1997 15

Douglas C. Biggs

2.1 Introduction 15

2.2 Methods 21

2.2.1 Primary Data 21

2.2.2 Between Cruise Differences 26

2.2.3 OtherHydrographic Data 26

2.3 Results 30

2.3.1 Hydrography of the Continental Margin of the MMS Eastern

Planning Area 30

2.3.2 Hydrography of the MOM area 47

2.3.3 Deepwater Cyclone-Anticyclone Pair 50

2.4 Discussion 60

2.5 Conclusions 68

Chapter 3. Biological Oceanography 69

John H. Wormuth, Patrick H. Ressler, Robert B. Cady and Elizabeth 3. Harris

3.1 Introduction 69

xi
3.2 Methods 71

3.2.1 Net Sampling 71

3.2.2 Zooplankton 71

3.2.3 Cephalopod Paralarvae 72

3.2.4 Myctophids 72

3.2.5 ADCP 73

3.3 Results 75

3.3.1 Zooplankton 75

3.3.2 Cephalopod Paralarvae 75

3.3.3 Myctophids 90

3.3.4 ADCP 90

3.3.4.1 October 1996 90

3.3.4.2 August 1997 96

3.4 Discussion 100

3.4.1 Zooplankton 100

3.4.2 Cephalopod Paralarvae 105

3.4.3 Myctophids 105

3.4.4 ADCP 106

3.4.5 Acoustic Predicted Mean Biomass (PMH) 107

3.4.6 Habitat Differences 107

3.5 Conclusions 108

Chapter 4. Visual Surveys of Cetaceans and Sea Turtles from Aircraft and Ships 111

Keith D. Mullin and Wayne Hoggard

4.1 Introduction 111

4.2 Study Areas 112

4.3 Methods 112

4.3.1 Data Acquisition 112

4.3.2 Survey Design 116

4.3.3 Data Analysis Techniques 119

4.3.4 Estimation of Perpendicular Sighting Distance and f(0) 121

4.3.5 Statistical Comparison of Abundance Estimates 123

4.3.6 Sea Turtle Density 125

4.4 Results 125

4.4.1 Study Area Summaries 125

4.4.2 Species Accounts 158

4.4.3 Distribution, Density and Abundance of Sea Turtles 163

4.5 Discussion 163

4.5.1 Temporal Distribution 164

4.5.2 Spatial Distribution 164

4.5.3 Cetacean Abundance 165

4.5.4 Distribution and Abundance of Sea Turtles 166

4.5.5 Bias 169

4.5.6 Conclusions 170

xii
Chapter 5. An Acoustic Survey of Cetaceans in the Northern Gulf of Mexico 173

Jeffrey C. Norris, William B. Evans, and Shannon Rankin

5.1 Introduction

173

5.2 Study Area

175

5.3 Methods

175

5.3.1 Equipment 175

5.3.2 Analysis 177

5.3.3 CharacterizatiOn of Dolphin Whistles 177

5.3.4 Effects of Seismic Exploration Signals on Cetaceans 179

5.4 Results

180

5.4.1 Survey Effort 180

5.4.2 Cetacean Contacts 180

5.4.3 Cetacean Distribution 184

5.4.3.1 Sperm Whales 184

5.4.3.2 Dolphins 184

5.4.4 Ecological Correlates 191

5.4.4.1 Sperm Whales 191

5.4.4.2 Dolphins 191

5.4.5 Whistle Characterization and Identification 200

5.4.6 Effects of Seismic Exploration on Cetaceans 206

5.5 Discussion 212

5.5.1 Comparing Visual and Acoustic Surveys 212

5.5.2 The Effects of Seismic Exploration on Cetaceans 213

5.5.3 Sources of Variance in Acoustic Surveying 214

5.6 Conclusions

215

Chapter 6. Cetacean Habitat in the Northern Gulf of Mexico 217

Randall W. Davis, Joel G. Ortega-Ortiz, Christine A. Ribic, William E. Evans,

Douglas C. Biggs, Patrick H. Ressler, John H. Wormuth, Robert R. Leben,

Keith D. Mullin and Bernd Wtirsig

6.1 Introduction

217

6.2 Methods 218

6.2.1 Cruise Data 218

6.2.2 Cetacean Sightings 218

6.2.3 Hydrography and Satellite Remote Sensing of Sea Surface Height

Anomaly 219

6.2.4 Predicted Mean Biomass (PMB) of Zooplankton in the MLD 220

6.2.5 Statistical Methods 220

6.2.5.1 Cetacean Categories 220

6.2.5.2 GulfCet II Data for the Eastern Gulf of Mexico 221

6.2.5.3 GulfCet I and II Data for the Oceanic Northern Gulf of

Mexico 221

6.3 Results

222
6.3.1 Hydrographic Features 222

6.3.2 Predicted Mean Biomass (PMB) of Zooplan.kton 222

6.3.3 GulfCet TI Data: Eastern Gulf ofMexico (Logistic Regression

Analysis 223

6.3.4 GulfCet I and II Data: Oceanic Northern Gulf of Mexico (Chi-square

Analysis 235

6.4 Discussion 235

6.5 Conclusions 253

Chapter 7. Behavior and Site Fidelity of Sperm Whales in the Northern

Gulf of Mexico 255

Bernd Wflrsig, David W. Weller, Spencer K. Lyim, Joel G. Ortega-Ortiz,

Andrew J. Schiro, Troy D. Sparks and William E. Evans

7.1 Background and Historical Overview in the Gulf 255

7.2 Methods 255

7.3 Results 259

7.3.1 Distribution 259

7.3.2 Group Size 259

7.3.3 Individual Recognition 259

7.3.4 Photogrammetry 261

7.3.5 Behavioral State Time Budgets 261

7.3.6 Orientation of Whales 261

7.3.7 Frequency of Dive Types 261

7.3.8 Blow Intervals 261

7.3.9 Crittercam 269

7.4 Discussion 269

7.4.1 Distribution 269

7.4.2 Group Size 269

7.4.3 Individual Recognition 270

7.4.4 Photogrammetry 270

7.4.5 Behavior, Dive Type, and Respiration Information 271

7.4.6 Orientation of Whales 271

7.4.7 Crittercam 272

7.5 Conclusions 272

Chapter 8. Seabird Ecology 275

Nancy A. Hess and Christine A. Ribic

8.1 Introduction 275

8.2 Methods 275

8.2.1 Cruise Logistics 275

8.2.2 Spring Cruise Survey Methods 276

8.2.3 Mid-summer and Late Summer Cruise Survey Methods 276

8.2.4 Bias 278

8.3 Results 279

xiv
8.3.1 Overall Abundance 279

8.3.2 Species and Diversity by Hydrographic Environment 290 8.3.3 Species' Affinities to Hydrographic Region 294

8.3.4 Seabird Habitat Preferences 294 8.4 Discussion 312

8.4.1 Overall Abundance 312 8.4.2 Role of Hydrographic Environments 313

8.4.3 Seabird Habitat 314 85 Conclusions 315

Chapter 9. Summary, Conclusions and Recommendations 317

Randall W. Davis, William E. Evans, Jeffrey C. Non-is, Bernd Wursig, Keith D. Mullin, Douglas C. J3iggs, John H. Wormuth, and Christine A. Ribic

9.1 Objectives of the GulfCet Program 317 9.2 Cetaceans 318 9.3 Sea Turtles 321 9.4 Seabirds 322 9.5 Potential Impacts of Oil and Gas Exploration and Production 323 9.6 Recommendations for Future Research 325

REFERENCES 329

xv
LIST OF FIGURES

Figure i.i. Location of oil and gas platforms in the northern Gulf of Mexico as of

April 1999

2

Figure 1.2. GulfCet I study area in the western and north-central Gulf between

the 100-2,000 m isobaths

6

Figure 1.3. Contour map of the sea surface height for the Gulf of Mexico

showing major hydrographiC features of the region 9

Figure 1.4. GulfCet II study area including the Minerals Management Service's

Eastern Planning Area and the focal study area for cetacean habitat

surveys

12

Figure 1.5. U.s. oceanic northern Gulf of Mexico 13

Figure 2.1. Location of hydrographic stations made by R/V Gyre and RIV Oregon

II during four GulfCet II cruises that surveyed the northeast Gulf of

Mexico in early and late summer 1996 and early and midsummer

1997

18

Figure 2.2. Location of hydrographic stations made over the continental margin

of the IvIMS Eastern Planning Area and the mouth of the Mississippi

River (MOM) area in water depths of 100-1000 m by R/V Gyre and

RIV Oregon II during the four GulfCet II cruises that surveyed the

northeast Gulf ofMexico in early and late summer 1996 and early

and mid-summer 1997

20

Figure 2.3. Property-propertY plots illustrating relationship between mixed layer

depth (MLD) calculated as [surface temperature - 1°C] and [surface

sigma-theta + 0.5], in late versus mid-summer 22

Figure 2.4. Property-property plots illustrating the influence of surface salinity on

surface sigma-theta and the nonlinear relationship between mixed layer

depth and surface sigma-theta in August 1997 23

Figure 2.5. A negative functional relationship between nitrate and temperature

allowed the 19°C depth to be used as a proxy for 10 jM of nitrate 24

Figure 2.6. PropertY-prOPe1tY plots illustrating use of 15°C depth (m) as a proxy for

deepwater dynamic height (cm) relative to 800 m and for deepwater

dynamic height anomaly (cm) relative to the seasonal mean 25

xvii
Figure 2.7. Stations along seven of the closely-spaced XBT transects made by

RIV Oregon lithe MMS Eastern Planning Area were done at very

similar locations and water depths in early summer 1997 as they had

been done in early summer 1996 27

Figure 2.8. Along-track current velocity at 10 m depth, measured by acoustic

Doppler current profiler (ADCP) during R/V Gyre cruise 96G06 28

Figure 2.9. Gridded ADCP current vectors within the deepwater focal and

continental margin areas of the northeast Gulf of Mexico surveyed

by R/V Gyre cruise 96G06 29

Figure 2.10. Along-track current velocity at 10 m depth, measured by acoustic

Doppler current profiler (ADCP) during R/V Gyre cruise 97G08 31

Figure 2.11. Gridded ADCP current vectors within the deepwater focal and

continental margin areas of the northeast Gulf of Mexico surveyed

byRJV Gyre cruise 97GO8 32

Figure 2.12. Contour map of 15°C depth on the continental margin of the

northeast Gulf of Mexico, as determined from 79 hydro graphic

stations made on RJV Oregon II cruise 220, Leg 3 33

Figure 2.13. Contour map of 19°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 79 hydrographic stations made on

R'V Oregon II cruise 220, Leg 3 34

Figure 2.14. Contour map of mixed layer depth (MLD) on the continental margin

of the northeast Gulf ofMexico, as determined from 79 hydrographic

stations made on R/V Oregon II cruise 220, Leg 3 35

Figure 2.15. Contour map of 15°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 79 hydro graphic stations made on

RJV Oregon II cruise 225, Leg 3 36

Figure 2.16. Contour map of 19°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 81 hydrographic stations made on

R'V Oregon II cruise 225, Leg 3 37

Figure 2.17. Contour map of mixed layer depth on the continental margin of the

northeast Gulf of Mexico, as determined from 81 hydrographic stations

made on R/V Oregon II cruise 225, Leg 3 38

Figure 2.18. Contour map of 15°C depth on the continental margin ofthe northeast

Gulf of Mexico, as deteniiined from 89 hydrographic stations made on

R/V Gyre cruise 96G06 39

xviii
Figure 2.19. Contour map of 19°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 89 hydrographic stations made on

R!V Gyre cruise 96G06 40

Figure 2.20. Contour map of mixed layer depth on the continental margin of the

northeast Gulf of Mexico, as determined from 89 hydrographic stations

made on PlY Gyre cruise 96G06 41

Figure 2.21. Contour map of 15°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 74 hydrographic stations made on

R'V Gyre cruise 97G08 42

Figure 2.22. Contour map of 19°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 74 hydrographic stations made on

RJV Gyre cruise 97G08 43

Figure 2.23. Contour map of mixed layer depth on the continental margin of the

northeast Gulf of Mexico, as determined from 74 hydrographic stations

made during R/V Gyre cruise 97G08 44

Figure 2.24. Between-year variation in a) surface temperature, and b) mixed layer

depth for stations along EPA transect lines A-G 45

Figure 2.25. Property-property plot of data from the two RJV Gyre cruises showing

relationship between mixed layer depth and nitracline depth in late

summer 1996 and mid-summer 1997 51

Figure 2.26. Property-property plot of data from the same two R'V Gyre cruises

showing that stations where nitracline domed closest to surface were

inside the cyclone (26.5-28.5°N) in late summer 1996, and in the

cyclone (27.5 - 29°N) and on the continental margin to the north of

the cyclone in mid-summer 1997 52

Figure 2.27. Differences in average deep chlorophyll maximum (DCM) depth and

average nitracline depth between a) Loop Current Eddy C, and b) the

mesoscale cyclone to the north, from 3 CTD casts made in each feature

during October 1996 on R/V Gyre cruise 96G06 53

Figure 2.28. Dynamic topography (cm, 0 m relative to 800 m) of the deepwater focal

area, as determined from 152 hydrographic stations made on RJV Gyre

cruise 96G06 54

Figure 2.29 Dynamic height anomaly of the shipboard data presented in Figure 2.28 55

xix
Figure 2.30. Gridded upper layer geostrophic velocity (0 m relative to 800 m) of the

deepwater focal area, as computed from the October 1996 dynamic

topography (Figure 2.28) 56

Figure 2.31. Dynamic topography (cm, 0 m relative to 800 m) of the deepwater focal

area, as determined from 107 hydro graphic stations made onlVV Gyre

cruise 97G08 57

Figure 2.32 Dynamic height anomaly of the shipboard data presented in Figure 2.31 58

Figure 2.33. Gridded upper layer geostrophic velocity (0 m relative to 800 m) of the

deepwater focal area, as computed from the August 1997 dynamic

topography (Figure 2.31) 59

Figure 2.34. Contour map of surface temperature, (°C) as measured every 2 minutes

throughout R'V/ Gyre cruise 96006 using the ship's SAIL data logging

system 61

Figure 2.35. Contour map of surface salinity (standard salinity units), as measured

every 2 minutes throughout RN Gyre cruise 96G-06 using the ship's

SAIL data logging system 62

Figure 2.36. Contour map of surface chlorophyll (jig/L), as measured every 2

minutes throughout R/V Gyre cruise 96G06 using the ship's SAIL

data logging system 63

Figure 2.37. Contour map of surface temperature (°C), as measured every 2

minutes throughout R/V Gyre cruise 97008 using the ship's SAIL

data logging system 64

Figure 2.38. Contour map of surface salinity (standard salinity units), as measured

every 2 minutes throughout R!V Gyre cruise 97G08 using the ship's

SAIL data logging system 65

Figure 2.39. Contour map of surface chlorophyll (.tg/L), as measured every 2

minutes throughout R/V Gyre cruise 97G08 using the ship's SAIL

data logging system 66

Figure 3.1. Integrated zooplankton biomass (cc m2) from 1m2 MOCNESS samples

taken during late summer 1996 79

Figure 3.2. Integrated zooplankton biomass (cc m2) from 1m2 MOCNESS samples

taken during midsummer 1997 80

Figure 3.3. Tow locations for the 4 m2 MOCNESS samples taken during late summer

1996 81

xx
Figure 3.4. Tow locations for the 15' Isaacs Kidd Midwater Trawl tows taken

during mid-summer 1997 82

Figure 3.5. A within cruise comparison of integrated zooplankton biomass

(cc m2) 83

Figure 3.6. A within feature comparison of integrated zooplarikton biomass

(cc m2) between late summer 1996 and mid-summer 1997 84

Figure 3.7. Integrated cephalopod paralarvae abundance (# m2) from 1m2

MOCNESS samples taken during late summer 1996 85

Figure 3.8. Integrated cephalopod paralarvae (# m2) from 1m2 MOCNESS

samples taken during mid-summer 1997 86

Figure 3.9. Integrated cephalopod paralarvae abundance (#m2) from the 15'

Isaacs Kidd Midwater Trawl tows taken during mid-summer 1997 87

Figure 3.10. A within cruise comparison of integrated cephalopod paralarvae

abundance (# m2) 88

Figure 3.11. Integrated cephalopod paralarvae (# m2) compared between years

with all environments and samples pooled 89

Figure 3.12. Integrated myctophid abundance from (# m2) from the 15' Isaacs

Kidd Midwater Trawl tows taken during mid-summer 1997 92

Figure 3.13. Relative abundance of common myctophid genera by percentage of total in

three 15' Isaacs Kidd Midwater Trawls taken during mid-summer 93

Figure 3.14. False color running plot of S collected along a north-south transect

line from the MOM region, through the cyclone, and into Loop

Current Eddy C during the late summer 1996 cruise 94

Figure 3.15. Scatterplot of the late summer 1996 MOCNESS displacement volume

biomass as a function of S, 95

Figure 3.16. Integrated MOCNESS biomass and integrated predicted mean

biomass (PMB) from late summer 1996 are compared for matching

locations and depth intervals 95

Figure 3.17. PMB for the depth interval 10-50 m is shown as a function of time

for the late summer 1996 cruise 97

xxi
Figure 3.18. Scatterplot of PIVIB integrated from 10-50 m as a function of the

15°C depth at deepwater hydrographic stations during the late summer

1996 cruise

97

Figure 3.19. Predicted mean biornass (PMIB) (cc rn"2) in the depth interval

10-50 m during daytime is shown by the color contours for

the late summer 1996 cruise 98

Figure 3.20. Predicted mean biomass (PMB) (cc m2) in the depth interval

10-50 m during nighttime is shown by the color contoursfor

the late summer 1996 cruise 99

Figure 3.21. Scatterplot of the mid-summer 1997 MOCNESS displacement

volume biomass as a function of S,, 101

Figure 3.22, Integrated MOCNESS biomass and integrated predicted mean

biomass (PIVIB) from mid-summer 1997 are compared for

matching locations and depth intervals 101

Figure 3.23. PMB for the depth interval 10-50 m is shown as a function of time

for the mid-summer 1997 cruise: 0946 GMT on 6 August to 0513 GMT on 22

August 102

Figure 3.24. Scatterplot of PMB integrated from 10-50 m as a function of the

15°C depth at deepwater hydrographic stations during the

mid-summer 1997 cruise 102

Figure 3.25. Predicted mean biomass (PMB) (cc m2) in the depth interval

10-5 0 m during daytime is shown by the color contours for the

mid-summer 1997 cruise 103

Figure 3.26. Predicted mean biomass (PMB) (cc m2) in the depth interval

10-50 m during nighttime is shown by the color contours for the

mid-summer 1997 cruise 104

Figure 4.1. Aerial survey transect lines in the Eastern Planning Area (EPA)

study areas (EPA Shelf <100 m; EPA Slope, 100-2,000 m) 113

Figure 4.2. Transect lines in the Eastern Planning Area surveyed from NOAA

Ship R/V Oregon II during early summer 1996 and 1997 (EPA She1f,

<100 m;EPA Slope, 100-2,000m) 113

Figure 4.3 Cruise track of PlY Gyre during late summer 1996 and mid-summer

1997. (East of 88.0°W: EPA Shelf, <100 m; EPA Slope, 100-2,000 m) 114
Figure 4.4 NOAA Ship RIV Oregon II ichthyoplanktonlmarine mammal cruise

track in the northern U.S. Gulf of Mexico (oceanic northern Gulf

study area) surveyed during spring 1996 and 1997 115

Figure 4.5 Diagram of perpendicular sighting distance (PSD) and other sighting

parameters for shipboard (A) and aerial (B) surveys (b and 0 angle

between track-line and cetacean group, h= altitude) 117

Figure 4.6 The locations of all cetacean groups sighted during summer 1996 and

1997 aerial surveys 140

Figure 4.7 The locations of all cetacean groups sighted during winter 1997 and

1998 aerial surveys 140

Figure 4.8 The locations of all cetacean groups sighted during the four ship

surveys (spring = spring and early summer 1996, 1997; late summer

1996; mid-summer 1997) 141

Figure 4.9 The locations of all Bryde's whale groups sighted during aerial

surveys 142

Figure 4.10 The locations of all Bryde's whale groups sighted during ship

surveys 142

Figure 4.11 The locations of all sperm whale groups sighted during aerial

surveys 143

Figure 4.12. The locations of all sperm whale groups sighted during ship

surveys 143

Figure 4.13. The locations of all dwarf7pygmy sperm whale groups sighted

during aerial surveys 144

Figure 4.14 The locations of all dwarf'pygmy sperm whale groups sighted

during ship surveys 144

Figure 4.15 The locations of all Cuvier's beaked whale groups sighted

during aerialsurveys 145

Figure 4.16 The locations of all Cuvier's beaked whale groups sighted during

ship Surveys 145

Figure 4.17 The locations of all Mesoplodon spp. groups sighted during aerial

surveys 146
Figure 4.18 The locations of all Mesoplodon spp. groups sighted during ship

surveys

146

Figure 4.19 The locations of all pilot, false killer, and pygmy killer whale

groups, and rough-toothed dolphin groups sighted during aerial

surveys

147

Figure 4.20 The locations of all melon-headed, pilot, false killer, and pygmy

killer whale groups, and rough-toothed dolphin groups sighted

during ship surveys

147

Figure 4.21 The locations of all bottlenose dolphin groups sighted during

aerial surveys

148

Figure 4.22 The locations of all bottlenose dolphin groups sighted during

ship surveys

148

Figure 4.23 The locations of all Risso's dolphin groups sighted during aerial

surveys

149

Figure 4.24 The locations of all Risso's dolphin groups sighted during ship

surveys

149

Figure 4.25 The locations of all Atlantic spotted dolphin groups sighted during

aerial surveys

150

Figure 4.26 The locations of all Atlantic spotted dolphin groups sighted during

ship surveys

150

Figure 4.27 The locations of all pantropical spotted dolphin groups sighted during

aerial surveys

151

Figure 4.28 The locations of all pantropical spotted dolphin groups sighted during

ship surveys

151

Figure 4.29 The locations of all striped dolphin groups sighted during aerial

surveys

152

Figure 430 The locations of all striped dolphin groups sighted during ship

surveys

152

Figure 4.31 The locations of all spinner dolphin groups sighted during aerial

surveys

153
Figure 4.32 The locations of all spinner dolphin groups sighted during ship

surveys

153

Figure 4.33 The locations of all clymene dolphin groups sighted during aerial

surveys

154

Figure 4.34 The locations of all clymene dolphin groups sighted during ship

surveys

154

Figure 4.35 The locations of all leatherback sea turtle sightings during aerial

Surveys

155

Figure 4.36 The locations of all loggerhead sea turtle sightings during aerial

surveys

155

Figure 5.1 The configuration of the linear hydrophone array, including the tow

vessel, tow, cable, and the active segment of the array. The placement and

separation of the hydrophones is also indicated. 176

Figure 5.2 Image of the Rainbow screens 178

Figure 5.3 Acoustic effort transects conducted during late summer 1996 cruise 181

Figure 5.4 Acoustic effort transects completed during the mid-summerl997 cruise 182

Figure 5.5 Location of acoustic contacts from sperm whales during the 1996 late

summer cruise 185

Figure 5.6 Location of acoustic contacts from sperm whales during the 1997 midsummer cruise 186

Figure 5.7 Location of acoustic contacts from all dolphins during the 1996 late

summer cruise 187

Figure 5.8 Location of acoustic contacts from all dolphins during 1997 midsummer cruise 188

Figure 5.9 Location of acoustic contacts from pantropical spotted dolphins

during the 1996 late summer cruise and 1997 mid-summer cruise 189

Figure 5.10 Location of acoustic contacts: bottlenose dolphins during the 1996 late

summer cruise and the 1997 mid-summer cruise; striped

dolphins during 1996 and 1997; clymene dolphins during 1996;

spinner dolphins during 1997; Atlantic spotted dolphins during

1996 and 1997; and rough-toothed dolphins during 1996 and

1997 190

xxv
Figure 5.11 Location of acoustic contacts from unidentified dolphins during the

1996 late-summer cruise and the 1997 mid-summer cruise 192

Figure 5.12 Distribution of sperm whales relative to hydrographic and bathygraphic data

during the late summer 1996 cruise. 193

Figure 5.13 Distribution of sperm whales relative to hydrographic and bathygraphic data

during the mid-summer 1997 cruise 194

Figure 5.14 Distribution of all dolphin acoustic contacts relative to hydrographic and

bathygraphic data during the 1996 late summer cruise 195

Figure 5.15 Distribution of all dolphin acoustic contacts relative to hydrographic and

bathygraphic data during the 1997 mid-summer cruise 196

Figure 5.16 Distribution of pantropical spotted dolphins relative to hydrographic and

bathygraphic data during the 1996 late summer cruise 197

Figure 5.17 Distribution of pantropicai spotted dolphin acoustic contacts relative to

hydrographic and bathygraphic data during the 1997 mid-summer cruise 198

Figure 5.18 Distribution of all unidentified dolphin acoustic contacts relative to hydrographic

and bathygraphic data during the 1996 late summer cruise 199

Figure 5.19 Distribution of unidentified dolphin acoustic contacts relative to hydrographic and

bathygraphic data during the 1997 mid-summer cruise 201

Figure 5.20 Decision tree for identifying unknown dolphin contacts 204

Figure 5.21 Scatterplot of discriminant function analysis of sine whistles from

bottlenose dolphin, false killer whales, and all other dolphins. 205

Figure 5.22 Scatterplot of discriminant function analysis of upsweep whistles

from rough toothed dolphins, the unidentified contact, and all other

dolphins used to identify a particular contact. 207

Figure 5.23 Spectrogram and waveform illustrating sperm whale pulses and two

forms of seismic signals 208

Figure 5.24 Schematic diagram, not to scale, of the seismic noise sources and

receivers 209

Figure 5.25 Spectrogram of three types of signals used in the seismic exploration

industry 210

xxvi
Figure 5.26 Percentage of effort with and without the presence of seismic

exploration signals for (A) both GulfCet I & II (northern Gulf),

for seven GulfCet I cruises (western and central Gulf), and

three GulfCet II cruises (eastern and central Gulf) 211

Figure 6.1. Visual survey effort for the late summer 1996 cruise. 224

Figure 6.2. Cetacean (Category 1, all identified species) sightings (+) during the late

summer 1996 cruise. 225

Figure 6.3. Sperm whale (Category 2) sightings during the late summer 1996

cruise 226

Figure 6.4. Squid-eater (Category 3, see text for list of species) sightings during

the late summer 1996 cruise 228

Figure 6.5. Stenellid (Category 4, see text for list of species) sightings during the

late summer 1996 cruise 229

Figure 6.6. Bottlenose dolphin and Atlantic spotted dolphin (Category 5) sightings during the late summer 1996 cruise 230

Figure 6.7. Visual survey effort for the mid-summer 1997 cruise 231

Figure 6.8. Cetacean (Category 1, all identified species) sightings during the mid- summer 1997 cruise 232

Figure 6.9. Sperm whale (Category 2) sightings during the mid-summer 1997

cruise 233

Figure 6.10. Squid-eater (Category 3, see text for list of species) sightings during

the mid-sunmier 1997 cruise 234

Figure 6.11. Stenellid (Category 4, see text for list of species) sightings during the mid-summer 1997 cruise 236

Figure 6.12. Bottlenose dolphin and Atlantic spotted dolphin (Category 5) sightings during the mid-summer 1997 cruise 237

Figure 6.13 Cetacean (Category 1, all identified species) sightings during the late

summer 1996 cruise 247

Figure 6.14 Cetacean (Category 1, all identified species) sightings during the mid- summer 1997 cruise 248

xxvii
Figure 6.15 Sperm whale (CategorY 2) sightings and acoustic contacts during the

late summer 1996

249

Figure 6.16 Sperm whale (Category 2) sightings and acoustic contacts during the

mid-summer 1997 cruise

250

Figure 7.1. Map of the focal study areas: (A) denotes the GuliCet I TIO Cruise 8

study area, (B) denotes the GulfCet II RIV Gyre cruise 96G06 study

area

257

Figure 7.2. Waveform of a single sperm whale click 260

Figure 7.3. Frequencies of Raft, Social and Travel behavior as a function of

daylight hour

264

Figure 7.4. Whale orientation relative to survey vessel 265

Figure 7.5. Frequency distribution of angular differences between whale

orientation and swell direction

266

Figure 7.6. Frequency distribution of Slip-under and Unknown and Fluke-up dives

as a function of daylight hour 267

Figure 7.7. Mean inter-blow interval for Fluke-up and Slip-under dives 268

Figure 8.1. Cross-seasonal comparison of sightings per effort hour for (a)

shearwaters, storm-petrelS and frigatebirds, (b) sulids, j aegers

and gulls, and (c) terns 289

Figure 8.2. Distribution of laughing gulls (late summer cruise) 295

Figure 8.3. Distribution of pomarine jaegers (late summer cruise) 296

Figure 8.4. Distribution of Audubon's shearwaterS (mid-summer cruise) 298

Figure 8.5. Distribution of band-rumped storm-petrelS (mid-surfllTler cruise) 299

Figure 8.6. DistributiOn of black terns (mid-summer cruise) 300

Figure 8.7. Distribution of bridled ten's (mid-summer cruise) 301

Figure 8.8 DistributiOn of sooty ten's (mid-summer cruise) 302

Figure 8.9 Presence of laughing gulls (late summer cruise) predicted by (a)

sea surface properties model and (b) plankton standing stock

model

304
Figure 8.10 Presence of pomarine jaegers (late summer cruise) predicted by

sea surface height model. 306

Figure 8.11 Presence of Audubon's shearwaters (mid-summer cruise)

predicted by (a) bathymetry model and (b) plankton standing

stock models 307

Figure 8.12 Presence of band-rumped storm-petrels (mid-summer cruise)

predicted by plankton standing stock model 308

Figure 8.13 Presence of black tems (mid-summer cruise) predicted by sea

surface properties model 309

Figure 8.14 Presence of bridled tems (mid-summer cruise) predicted by

plankton standing stock model 310

Figure 8.15 Presence of sooty terns (mid-summer cruise) predicted by sea

surface properties model 311
Table 1.1.

LIST OF TABLES

GulfCet II participants, including program managers, principal investigators, Scientific Review Board members, government scientists, and their affiliations

Table 1.2. Cetaceans of the Gulf of Mexico

5

Table 1.3. Estimated minimum abundance (N) of cetacean species based on GulfCet I

ship surveys (1992-94) in the north-central and western Gulf ofMexico

between the 100-2,000 m isobaths 8

Table 1.4. Types of data collected by season and survey for the GulfCet II program 11

Table 2.1 Summary of hydrographic stations made early and late summer 1996 and in

early and mid-sunnner 1997 in the northeast Gulf of Mexico 17

Table 2.2 Hydrographic stations from summer survey cruises grouped into five hydrographic environments 19

Table 2.3 Temperature, density, and dynamic topography of the five hydrographic environments surveyed on the late summer R/V Gyre cruise (Gyre96GO6) 48

Table 2.4 Temperature, density, and dynamic topography of the five hydrographic

environments surveyed on the mid-summer R/V Gyre cruise (Gyre97GOS) 49

Table 3.1. Summary of 1m2 MOCNESS tows, 4 m2 MOCNESS tows and 15'IKMT

tows

76

Table 3.2. Myctophid abundance (# m2) by genera within analyzed 15 Isaacs Kidd Midwater Trawl tows taken during mid-summer 1997 91

Table 4.1. Estimate of the parameter f(0) for each species group from shzp surveys 122

Table 4.2. Estimate of the parameter f(0) for each species group and all sea turtles from aerial surveys 124

Table 4.3. Summary of aerial surveys of cetaceans in the Eastern Planning Area

continental shelf and continental slope study areas during summer and

winter 1996-98

Table 4.4. Cetacean species sighted and number of on-effort sightings from aerial surveys of the Eastern Planning Area continental shelf and continental

slope study areas during summer and winter 1996-98 127

126

4
Table 4.5. Group-size, density and abundance estimates of cetacean species from

aerial surveys in the Eastern Planning Area continental shelfand slope

study areas during summer and winter 1996-98 128

Table 4.6. Seasonal group-size, density and abundance estimates of cetacean species

from aerial surveys in the Eastern Planning Area continental shelf and slope

study areas during summer and winter 1996-98 129

Table 4.7. Group-size, density and abundance estimates of cetaceans from aerial

surveys of the Eastern Planning Area continental shelf and slope study

areas during summer and winter 1996-98 130

Table 4.8. Visual survey effort in each study area during each GulfCet II ship

survey 131

Table 4.9. The number of on-effort sightings of cetacean species during each

GulfCet II shzp survey by study area 132

Table 4.10. Group-size, density and abundance estimates of cetaceans from ship

surveys of the Eastern Planning Area continental shelf and slope study

areas during spring 1996-97 133

Table 4.11. Group-size, density and abundance estimates of cetaceans from ship

surveys in the oceanic northern Gulf of Mexico study area during spring

1996-97 134

Table 4.12. Group-size, density and abundance estimates of cetaceans from ship

surveys of the GulfCet I study area during spring 1996-97 135

Table 4.13. Comparison of abundance estimates (N) in the oceanic northern Gulf of

Mexico study area and the GulfCet I Study Area (NW Slope) 136

Table 4.14. Seasonal occurrence of cetacean species in the EPA study areas and in

the northern Gulf of Mexico during GulfCet II aerial or ship surveys 137

Table 4.15. Sea turtle species sighted and number of on-effort sightings during aerial

surveys of the Eastern Planning Area continental shelf and continental

slope study areas during summer and winter 1996-98 138

Table 4.16. Density and abundance estimates ofsea turtles from aerial surveys in the

Eastern Planning Area continental shelf and slope study areas during

summer and winter 1996-98 139
Table 4.17 Comparison of density estimates (D anirnals/100 km2) of cetaceans from

the GulfCet I study area to those from the EPA continental slope study area

(EPA slope) as assessed by shz and aerial surveys 167

Table 5.1. Numbers of cetaceans acoustic contacts by cruise and species 183

Table 5.2 Number of whistles types by species and the Shannon-Weiner index

for whistle diversity for each species 202

Table 5.3. Means for acoustic parameters for all identified dophins from all cruises 203

Table 6.1 Cruises used in the habitat-association analysis 219

Table 6.2. Contingency table of presence/absence of cetaceans (Category 1) in

the on-effort transect segments with respect to ocean depth region 238

Table 6.3. Contingency table of presence/absence of sperm whales (Category 2)

in the on-effort transect segments with respect to ocean depth region 239

Table 6.4. Contingency table of presence/absence of squid-eaters (Category 3) in

the on-effort transect segments with respect to ocean depth region 240

Table 6.5. Contingency table of presence/absence of stenellids (Category 4) in the

on-effort transect segments with respect to ocean depth region 241

Table 6.6. Contingency table of presence/absence of bottlenose dolphins and Atlantic

spotted dolphins (Category 5) in the on-effort transect segments with

respect to ocean depth region 242

Table 6.7. Contingency table of presence/absence of cetaceans (Category 1) in the

on-effort transect segments with respect to hydrographic features 243

Table 6.8. Contingency table of presence/absence of sperm whales (Category 2) in

the on-effort transect segments with respect to hydrographic features 243

Table 6.9. Contingency table of presence/absence of squid-eaters (Category 3) in

the on-effort transect segments with respect to hydrographic features 244

Table 6.10. Contingency table of presence/absence of stenellids (Category 4) in the

on-effort transect segments with respect to hydrographic features 244

Table 6.11. Contingency table of presence/absence of bottlenose dolphins and

Atlantic spotted dolphins (Category 5) in the on-effort transect

segments with respect to hydrographic features 245
Table 7.1. Sperm whale sighting locations, dates, and distance between resightings

for photographically identified individuals 262

Table 7.2. Size estimates for GOM sperm whales using Gordon (1990)

photogrammetry techniques 263

Table 7.3. Descriptive statistics for categories of inter-blow intervals (IBI) 263

Table 8.1. Common and Latin names of seabirds seen during the spring,

mid-summer and late summer cruises 280

Table 8.2 Number and percent of seabird species seen during the spring cruise 281

Table 8.3 Percentages of taxonomic seabird groups for the spring cruises 282

Table 8.4 Seabird numbers tallied by water depth seen while on-effort during

the mid-summer and late summer cruises 284

Table 8.5 Cross-seasonal comparison of taxonomic group sightings per effort

hour 287

Table 8.6 Species tallied by hydrographic environment within 300 m strip

transect for the late summer cruise 291

Table 8.7 Species numbers and diversity for each hydrographic environment

for sightings within the 300 m strip transect 292

Table 8.8 Species tallied by hydrographic environment within 300 m strip transect

for the mid-summer cruise 293

Table 8.9 Number of transects containing species within hydrographic

environments 297

Table 8.10 Percent of total variance in seabird presence or absence in transects

explained using generalized additive models 303

Table 9.1 Cetaceans ranked by abundance with mean body mass, species-total

biomass (percent of total biomass shown in parentheses) and minimum

estimated food requirements for the oceanic northern Gulf of Mexico 319

xxxiv
LIST OF ABBREVIATIONS AND ACRONYMS

The following acronyms and abbreviations are used throughout this report:

ADCP Acoustic Doppler Current Profiler

AVHRR Advanced Very High Resolution Radiometer

BRD Biological Resources Division

CCAR Colorado Center for Astrodynamics Research, University of Colorado

CCR Cold-core Ring

CHL Chlorophyll

CI Confidence Interval

CON Confluence between ring structures

CTD Conductivity, Temperature, and Depth Profiler

CV Coefficient of Variation

dB rd P Decibels relative to 1 micro Pascal

DCM Deep Chlorophyll Maximum

DFA Discriminant Function Analysis

EPA Eastern Planning Area

GIS Geographic Information System

GPS Global Positioning System

GulfCet I Distribution and Abundance of Cetaceans in the North-Central and

Western Gulf of Mexico (199 1-1995)

GulfCet II Cetaceans, Sea Turtles and Seabirds in the Northern Gulf of Mexico:

Distribution, Abundance and Habitat Associations (this study)

HCLS High Chlorophyll, Low Salinity

HPLC High Pressure Liquid Chromatography

IKMT Isaacs Kidd Midwater Trawl

LC Loop Current

LCE Loop Current Eddy

MAR Mississippi-Atchafalaya River

MLD Mixed Layer Depth

MOCNESS Multiple Opening/Closing Net and Environmental Sampling System

MOM Mouth of the Mississippi

MMS Minerals Management Service

NE Northeast

NEGOM Northeastern Gulf of Mexico

NMFS National Marine Fisheries Service

NOAA National Oceanic and Atmospheric Administration

OCS Outer Continental Shelf

PER Periphery of ring structures

PMB Predicted Mean Biomass

PSD Perpendicular Sighting Distance

SAIL Serial ASCII Interface Loop

SD Standard Deviation

SEFSC Southeast Fisheries Sdence Center, NMFS

SSH Sea Surface Height

xxxv
SST

Stenellids

Sv

TAMIJ

TAMUG

TOPEX/ERS

TOPEX/POSEIDON

T-S

UC

USGS

WCR

WCWRU

XBT

Sea Surface Temperature

Dolphins of the genus Stenella

Acoustic backscattering strength

Texas A&M University (College Station)

Texas A&M University at Galveston

Topography Experiment MissionlEuropeari Research Satellite

Topography Experiment MissionlPoseidon

Temperature-salinity Relationship

University of Colorado

United States Geological Survey

Warm-Core Ring

Wisconsin Cooperative Wildlife Research Unit

Expendable Bathythermograph
ACKNOWLEDGMENTS

Many persons contributed to this study, including the crews and graduate students who worked

on the research vessels and aircraft, laboratory technicians, and the data management staff. The

contributors to this report would like to recognize the efforts of Alejandro Acevedo, Ilona Berk,

Steve Berkowitz, Jose Bersano, Robert Blaylock, Ned Brinkley, British Petroleum, Cheryl

Brown, Carolyn Burks, Shane Collier, Melissa Davis, Brad Dawe, Theia DeLong, Captain Dana

Dyer, Wesley Elsberry, Dagmar Fertl, Michelle Finn, Michael Flirm, Holly Fortenberry, Rachel

Gross, Jeff Hagan, Terry Henwood, Shannon Hesse, Lesley Higgins, Markus Horning, Came

Hubard, Clint Jeske, John Lamkin, Kosrow Lashkari, Nicole LeBoeuf, John Longnecker, Steven

Lowder, Kimberly Marks, Greg Marshall, Tony Martinez, Blair Mase, Kathy Maze, National

Geographic Society, Scott Nichols, Steve Nokutis, Dwight Peake, Mike Peccini, Jon Peterson,

Matt Pickett, Robert Pitman, Todd Pusser, Danielle Raha, Logan Respess, Carol Roden, Diego

Rodriguez, Gerald Scott, Man Smultea, Jon Stem, Sarah Stienessen, Jim Tobias, Erika Vidal,

Melany Wiirsig, and Mindy Zuschlag.

Special thanks to Paula Canton, Kate deMedeiros, Jenny Ogrodnick, and Mary Beth Rew for

technical editing. The GulfCet Data Management Office (DM0) also wishes to express its

appreciation to those who served as reviewers of this document.

We gratefully acknowledge the Department of Oceanography, Texas A&M University, the

Texas Institute of Oceanography and the National Marine Fisheries Service for cost-sharing the

use of all research vessels used for the surveys on this project. Texas A&M University at

Galveston provided logistics and material support throughout, including facilities for the DM0.

Two colleagues not on the GulfCet team provided important help with data reduction: Steve

DiMarco (TAMU) carried out the initial quality control on the ADCP data from RV Gyre cruises

96G06 and 97G08 to produce the data sets plotted in Figures 2.8 and 2.10, following protocols

developed to support the MMS-sponsored Northeast Gulf of Mexico Chemical Oceanography

and Hydrography project (NEGOM-COH). Dr. DiMarco then provided the gridded files of

ADCP near surface velocity that are plotted in Figures 2.9 and 2.11. Peter Hamilton (SAIC)

produced the gridded files of geostrophic velocity at 0 m relative 800 m that are plotted in

Figures 2.30 and 2.33.

Special thanks to the Scientific Review Board members Daryl Boness, Roger Hanlon and

Lawrence Pomeroy for their thoughtful review and comments on the goals and results of this

project, and very special thanks to BRD Contracting Officers Technical Representative Gary

Brewer and MMS Contract Inspectors Robert Avent and Dagmar Fertl for their guidance in the

writing of this report.

This study was funded by the U.S. Department of the Interior, Geological Survey, Biological

Resources Division under contracts 1445-CTO9-96-0004 and 1445-1A09-96-0009.
CONTRIBUTORS

Douglas C. Biggs Department of Oceanography, Texas A&M University, College Station,

Texas 77843-3146; dbiggsocean.tamu.edu

Robert B. Cady Department of Oceanography, Texas A&M University, College Station,

Texas 77843-3146; robcady@ocean.tamu.edu

Randall W. Davis Department of Marine Biology, Texas A&M University, 5007 Avenue U,

Galveston, Texas 77551; davisr@tamug.tamu.edu

William E Evans Department of Marine Biology, Texas A&M University, 5007 Avenue U,

Galveston, Texas 77551; evansw@tamug.tamu.edu

Elizabeth J. Harris Department of Oceanography, Texas A&M University, College Station,

Texas 77843-3146; lizh@ocean.tamu.edu

Nancy A. Hess Department of Wildlife Ecology, University of Wisconsin, 226 Russell

Labs, 1630 Linden Drive, Madison, Wisconsin 537061598;

nahess@students.wisc.edu

Wayne Hoggard Southeast Fisheries Science Center, National Marine Fisheries Service,

P.O. Drawer 1207, Pascagoula, Mississippi 39568;

whoggard@triton.pas.nmfs.gov

Robert R. Leben Colorado Center for Astrodynamic Research, University of Colorado,

Campus Box 431, Boulder, Colorado 80309-043 1;

leben@orbit.colorado.edu

Spencer K. Lynn GulfCet Program Office, Texas A&M University, 5007 Avenue U,

Galveston, Texas 77551

Keith D. Mullin Southeast Fisheries Science Center, National Marine Fisheries Service,

P.O. Drawer 1207, Pascagoula, Mississippi 39568;

kmullintriton.pas.nmfs.gov

Jeffrey C. Norris Marine Acoustics Lab, Texas A&M University, 5007 Avenue U,

Galveston, Texas 77551; norrisjtarnug.tamu.edu

Joel G. Ortega-Ortiz Marine Mammal Research Program, Texas A&M University, 4700 Avenue

U, Bldg. 303, Galveston, Texas 77551; ortegajtamug.tamu.edu

Shannon Rankin Marine Acoustics Lab, Texas A&M University, 5007 Avenue U, Galveston,

Texas 77551
Patrick H. Ressler

Christine A. Ribic

Andrew J. Schiro

Troy D. Sparks

David W. Weller

Joim H. Wormuth

Bernd Wtirsig

Department of Oceanography, Texas A&M University, College Station,

Texas 77843-3146; pressler@ocean.tarnu,edu

Department of Wildlife Ecology, University of Wisconsin, 226 Russell

Labs, 1630 Linden Drive, Madison, Wisconsin 53 706-1598;

caribic@facstaff.wisc.edu

Marine Mammal Research Program, Texas A&M University, 4700 Avenue

U, Bldg. 303, Galveston, Texas, 77551

Marine Acoustics Lab, Texas A&M University, 5007 Avenue U,

Galveston, Texas 77551; sparksttamug.tamu.edu

Marine Mammal Research Program, Texas A&M University, 4700 Avenue

U, Bldg. 303, Galveston, Texas, 77551

Department of Oceanography, Texas A&M University, College Station,

Texas 77843-3146; jwormuthastra,tamu.edu

Marine Mammal Research Program, Texas A&M University, 4700 Avenue

U, Bldg. 303, Galveston, Texas 77551; wursigb@tamug.tamu.edu

xl
1

Chapter 1

Introduction

Randall W. Davis and William E. Evans

1.1 Background

1.1.1 Federal Requirement for the Program

The mission of the U.S. Geological Survey (USGS) Biological Resources Division (BRD) is to

provide the scientific understanding and technologies needed to support the sound management

and conservation of the nation's biological resources. The BRD endeavors to meet its goals by:

developing scientific and statistically reliable methods and protocols to assess the status

and trends of the nation's biological resources

. utilizing tools from the biological, physical and social sciences to understand the causes

of biological and ecological trends and to predict the ecological consequence of

management practices

leading in the development and use of the technologies needed to synthesize, analyze and

disseminate biological and ecological information

striving for quality, integrity and credibility of its research and technology by consistently

improving its scientific programs through internal quality control, external peer review

and competitive funding

The U.S. Department of the Interior's Minerals Management Service (MMS) is a client agency of

the BRD. The MMS has the responsibility for leasing, minerals exploration and development of

submerged Federal lands on the U.S. Outer Continental Shelf (OCS) under the provisions of the

OCS Lands Act Amendments of 1978. The National Environmental Policy Act of 1969 requires

that all Federal Agencies use a systematic, interdisciplinary approach that will ensure integrated

use of the natural and social sciences in any planning and decision making that may have an

effect on the human environment.

The BRD administered this study, hereafter called the GulfCet II program. It is a continuation of

research begun with the Gulftet I program (199 1-95). Both programs were designed to help

MMS assess the potential effects of deepwater oil and gas exploration and production (e.g., ship

activity, seismic exploration, pollution) on cetaceans, sea turtles and seabirds in the northern

Gulf of Mexico by providing synoptic data on species diversity, abundance and habitat. As of

April 1999, there were over 3,000 oil and gas platforms in the northern Gulf (Figure 1.1). Most

of these were located on the continental shelf along the coast of Texas and Louisiana. However,

new technology is now allowing oil and gas exploration and production to expand beyond the

continental shelf and onto the upper continental slope.
30- -98 HOU STON]I 4 NEW ORLEANS, ? / -96 -94 -92 -90 -88 Longitude West -86 -8

Figure 1.1. Location of oil and gas platforms in the northern Gulf of Mexico as of April 1999 (Dat
1.1.2 Program Participants

The GulfCet II Program was managed by Texas A&M University at Galveston (TAMUG) in

partnership with the National Marine Fisheries Service (NMFS) at the Southeast Fisheries

Science Centers (SEFSC). The Texas Institute of Oceanography provided support for graduate

student participation at TAMEJG. A list of the program's participants is shown in Table 1.1. The

GulfCet II program had a Scientific Review Board (SRB) composed of three scientists who

reviewed and commented on the project's goals, methodologies, results, analyses and

conclusions.

1.1.3 Cetacean Surveys of tbe Northern Gulf Prior to 1994

There are several sources of information on the distribution, abundance and diversity of

cetaceans in the Gulf of Mexico (for a review see Jefferson and Schiro 1997). Cetacean

stranding information has been systematically collected since the late 1970's. A considerable

amount of research has been conducted on localized populations of bottlenose dolphins (Shane et

al. 1986, Scott and Hansen 1989, Leatherwood and Reeves 1990). From 1983-86, the NMFS

investigated the distribution, abundance and diversity of cetaceans in U.S. Gulf of Mexico waters

less than 200 m deep, which in some areas extended to 280 km offshore (a total area of about

360,000 krn2) (Scott et al. 1989). Other directed studies, historic whaling records, animal

strandings and opportunistic sightings have expanded the list of cetacean species known to occur

in the Gulf (Mullin et al. 1994c, Jefferson and Schiro 1997) (Table 1.2).

Until recently, relatively little was known about cetaceans inhabiting deeper waters of the Gulf

of Mexico. The MMS and U.S. Fish and Wildlife Service supported aerial surveys of birds, sea

turtles and cetaceans in the Gulf from 198 182 (Fritts et al. 1983). From July 1989 through June

1990, the NMFS conducted aerial surveys of cetaceans along the continental slope of the northcentral Gulf of Mexico in waters ranging from 1801,800 m deep (Mullin et al. 1991, Mullin et al. l994c).

The most extensive survey of cetaceans in the offshore waters (100-2,000 m deep) of the northcentral and western Gulf of Mexico (Figure 1.2) was conducted jointly by Texas A&M

University and the NMFS, Southeast Fisheries Science Center beginning in 1992 and called the

GulfCet I program (Davis and Fargion 1996, Baumgartner 1997, Davis et al. 1998). This threeyear study provided synoptic information on the distribution and abundance of cetaceans using

both visual and acoustic survey techniques. It also provided limited information on cetaceanhabitat associations.

During GulfCet I, bottlenose dolphin, pantropical spotted dolphin and sperm whale were the

most commonly sighted species; each was sighted more than 70 times. Risso's dolphin, clymene

dolphin, dwarf sperm whale, striped dolphin and unidentified ziphiids were each sighted 21 to 44

times, with the other species sighted less than 20 times. Average group sizes ranged from 1.2 for

3
Table 1.1. GulfCet II participants, including program managers, principal investigators, Sci

government scientists, and their affiliations. TAMUG = Texas A&M University at Galveston TAMU = Texas A&M University, College Station NMFS, SEFSC = National Marine Fisheries Service, Southeast Fisheries Science Center BRD, WCWRU, UW = BRD Wisconsin Cooperative Wildlife Research Unit, University of W

CCAR, UC = Colorado Center for Astrodynamic Research, University of Colorado Randall Davis Program Manager, Principal Investigator, Editor TAMUG William Evans Deputy Program Manager, Principal Investigator, Co-editor TAMIUG Bemd Wflrsig Principal Investigator, Co-editor TAMUG Gerald Scott Program Manager for SEFSC NMFS, S

Keith Mullin Principal Investigator NMFS, SDouglas Biggs Principal Investigator TAMU Jeff Norris Principal Investigator TAMUG John Wormuth Principal Investigator TAMIJ Christine Ribic Principal Investigator BRD, W

Robert Leben Principal Investigator CCAR, ULawrence Pomeroy Scientific Review Board Institute 

Universit

Daryl Boness Scientific Review Board National 

Smithson

Roger Hanlon Scientific Review Board Marine B

Woods H

Gary Brewer Contracting Officer's Technical Representative Biologic

U.S. Geo

Robert Avent Contract Inspector Minerals Dagmar Ferti Contract Inspector Minerals 
Table 1.2. Cetaceans of the Gulf of Mexico. The (IE) next to the common name indicates that

the species is listed under the Endangered Species Act of 1973 as endangered.

Balaenidae

Northern right whale (E)

Balaenopterklae

Blue whale (E)

Fin whale (B)

Sei whale (E)

Bryde's whale

Minke whale

Humpback whale (B)

Physeteridae

Sperm whale (B)

Kogiidae

Pygmy sperm whale

Dwarf sperm whale

Ziphiidae

Delphinidae

Melon-headed whale

Pygmy killer whale

False killer whale

Killer whale

Short-finned pilot whale

Rough-toothed dolphin

Fraser's dolphin

Bottlenose dolphin

Risso's dolphin

Atlantic spotted dolphin

Pantropical spotted dolphin

Striped dolphin

Spinner dolphin

Clymene dolphin

5

Eubalaena glacialis

Balaenoptera musculus

Balaenoptera physalus

Balaenoptera borealis

Balaenoptera edeni

Balaenoptera acutorostrata

Megaptera novaeangliae

Physeter macrocephalus

Kogia breviceps

Kogia simus

Peponocephala electra

Feresa attenuata

Pseudorca crassidens

Orcinus orca

Giobicephala macrorhynchus

Steno bredanensis

Lagenodeiphis hosei

Tursiops truncatus

Grwnpus griseus

Stenella frontalis

Steneila attenuata

Stenelia coeruleoalba

Stenella ion girostris

Stenella clymene

Cuvier's beaked whale Zihius cavirostris

Blainville's beaked whale Mesoplodon densirostris

Sowerby's beaked whale Mesoplodon bidens

Gervais' beaked whale Mesoplodon europaeus
Longitude West Figure 1.2. GulfCet I study area in the western and north-central Gulf between t
pygmy sperm whales and Cuvier's beaked whales to 141 for melon-headed whales. The

estimated minimum abundance of cetaceans in the GulfCet I study area was 19,145 (CV = 0.12)

animals (Table 1.3). The most common species were pantropical spotted dolphin, bottlenose

dolphin, striped dolphin and melon-headed whale. Clymene dolphin and Atlantic spotted

dolphin were the only other species with estimates of over 1,000 animals. The estimated

minimum abundance of endangered sperm whales was 313 (0.25) animals. Other species with

estimates of more than 200 animals were spinner dolphin, Risso's dolphin and the short-finned

pilot whale.

The GulfCet I program provided limited information on habitat preference, which showed the

strongest correlation of species distribution with ocean depth (Davis et al. 1998). However, this

study failed to establish strong correlations with other hydrographic variables such as sea surface

temperature, salinity, water column structure and distinctive features such as cyclonic (cold-core)

eddies. This may have resulted from the design of the shipboard surveys that was intended to

provide a synoptic view of cetacean distribution and an estimation of minimum abundance.

These surveys were not designed to determine mesoscale habitat preference of cetaceans.

In addition to cetaceans, the GulfCet I program provided synoptic information on the distribution

and abundance of sea turtles using aerial survey techniques. All five species of sea turtles in the

Gulf of Mexico are protected under the Endangered Species Act of 1973. The leatherback

(Dermochelys coriacea), Kemp's ridley (Lepidochelys keinpi) arid hawksbill (Eretmochelys

imbricata) sea turtles are listed as endangered and the loggerhead (Caretta caretta) and green

(Chelonia mydas) sea turtles are listed as threatened. Three sea turtle species occurred in the

GulfCet I study area: loggerhead, Kemp's ridley and leatherback. The leatherback sea turtle

occurred jil significant numbers throughout the year, especially in the region between the

Mississippi and DeSoto Canyons. These canyons are located along the continental slope 85 km

southwest and 220 km east, respectively, of the Mississippi River delta. Finally, over 30 species of seabirds were sighted during visual shipboard surveys of the study area (Davis and Fargion

1996).

1.2 General Oceanography of the Gulf of Mexico

The Gulf of Mexico is a dynamic body of water dominated by two major circulation features.

The Loop Current, formed by the interconnection of the Yucatan and Florida Currents, governs

the circulation of the eastern (east of the ca. 87.5°W longitude) Gulf. In the central (ca. 87.5-

94.0°W longitude) and western (ca. west of 94.0°W longitude) Gulf, anticyclonic eddies and

their associated cyclonic eddies are the primary circulatory features. The Loop Current enters

the Gulf in a nearly annual cycle. TOPEXIERS satellites produce sea surface altimetry maps that

show Gulf anticyclonic eddies originating as pinched-off, northward penetrations of Loop

current meanders (Figure 1.3). After their separation from the Loop Current, these anticyclonic

eddies drift westward until their progress is eventually constrained by shoaling topography over

the northwestern continental slope of the Gulf. They remain in this region, slowly decaying or

coalescing with another approaching eddy. The overall resulting circulation of the Gulf of

Mexico is remarkable because of its inter-annual variability and intensity. The dynamics of the

Gulf are made more complex by the large fresh water inflow. Nearly two-thirds of the U.S.

7
Table 1.3. Estimated minimum abundance (N) of cetacean species based on GulfCet I ship

surveys (199294) in the north-central and western Gulf of Mexico between the 100-

2,000 m isobaths (Davis and Fargion 1996). CV = coefficient of variation.

8

Family and common name Species name N CV

Balaenopteridae

Bryde's whale Balcienoptera edeni 3 0.81

Physeteridae

Sperm whale Physeter macrocephalus 313 0.25

Kogiidae

Pygmy sperm whale Kogia breviceps 19 0.40

Dwarf sperm whale Kogia simus 88 0.34

Ziphiidae

Cuvier's beaked whale Zzphius cavirostris 14 0.41

Unidentified Ziphiidae 124 0.29

Delphinidae

Melon-headed whale Peponocephala electra 2,067 0.34

Pygmy killer whale Feresa attenuata 36 0.64

False killer whale Pseudorca crassidens 10 0.63

Killer whale Orcinus orca 71 0.46

Shortfinned pilot whale Globicephala macrorhynchus 215 0.50

Rough-toothed dolphin Steno bredanensis 177 0.35

Fraser's dolphin Lagenodeiphis hosei 65 1.17

Bottlenose dolphin Tursiops truncatus 2,538 0.26

Risso's dolphin Grampus griseus 529 0.26

Atlantic spotted dolphin Stenella frontalis 1,145 0.37

Pantropical spotted dolphin Stenella attenuata 7,105 0.22

Striped dolphin Stenella coeruleoalba 2,091 0.52

Spinner dolphin Stenella longirostris 840 0.60

Clymene dolphin Stenella clymene 1,695 0.37

Total 19,145
3OTJ

2Th

2'1

21'4

I 8N

cw crw rw arw I ungi dc

1 :1_IT - L4-..-.

3C -2 2C .15 -10 6 3 6 1 '6 ?O 26 30

Se.i surIac heighi cm)

Figure 1 3. Contour map of the sea surIce heIit kr the Gulf of Mcxico showing major

hydrographie features of the gem. E)atd werc colteeted by n1 imeters aboard the

TOPrxPOSFff)oN and iRS-2 satelfls and used to ohjectbl' niip the height

Id tr Augunc 1, 1997. Th northwani pentraticrt f'the loop CurrenL (red) mm

the eastern (jul 1 ht rcultcd in the scptration of an ainkyelone (ai'i u -un eddy),

identified by the large region oI closed contours in rd around a hiih in the sea

surticc height. Cydonic, cold-core cies (biue arc lows in the sea surface heihi

and visible on the periphery of the auticyclone. The eddy kld in the wcstern gulf is

itck up intic]onc-s and companion cyclones that are ietnnairs of an older Loop

Current eddy interacLiru. with i he eontineruL slopc along the I exas and lexicafl

coat. Dat.i for ocean dcpth lcw thnn 200 meters have been mkcd

;4.IJ

2

9
mainland and half the area of Mexico drains into the Gulf. The Mississippi River discharges into

the northern Gulf through the Balize and Atchafalaya delta regions. Approximately 30% of the

Mississippi River enters the northern Gulf through the Atchafalaya, and the remaining 70% goes

through the Balize birdfoot delta. The Mississippi and other rivers with their associated

pollutants, nutrients, and sediment loads have a great impact on all aspects of continental shelf

oceanography in the northern Gulf. The input of nutrients ensures high phytoplankton

production and thus higher zooplankton productivity (Lohrenz et al. 1990). Twenty-eight

percent of the total U.S. commercial fish catch is from the Louisiana'Texas shelf (Walker and

Rouse 1993). Spawning of key species, such as Gulf menhaden, is also concentrated around the

Mississippi delta. River discharge into the Gulf is distinctly seasonal, with the highest flow

occurring from March through May, and the lowest flow occurring from August through October. Wind forcing and shelf currents are major factors controlling the distribution of

Mississippi River outflow onto the continental shelf. Loop Current eddies and filaments provide

the major control of plume circulation over the continental slope and into the northern Gulf. The

fresh water of the Mississippi River affects the spatial and temporal distribution of areas of

higher primary and secondary production that may influence the distribution of cetaceans in the

Gulf of Mexico.

1.3 GulfCet II Study Area and Objectives

In the GulfCet II program, studies of cetaceans in the northern Gulf ofMexico were continued to

determine their seasonal and geographic distribution in areas potentially affected by oil and gas

activities now or in the future. This program included systematic aerial surveys and shipboard

visual and acoustic surveys to document cetacean and sea turtle populations (Table 1.4). This

work was accompanied by data acquisition designed to further characterize habitat and reveal

cetacean-habitat associations. This study was intended as a spatial and temporal extension of the

GulfCet I program.

1.3.1 Study Area

The GulfCet II study area included:

Eastern Planning Area (EPA) continental slope fTom 100-2,000 m deep east of 88°10.OTW

and north of 26°00.O'N (70,470 km2) and the EPA continentalshelf (12,326 km2) located

from 18.5 km offshore to 100 m deep between 88°10.OW and 85°55.OtW (Figure 1.4).

This area was surveyed using both aircraft and ships (RN Oregon II and R/V Gyre).

GulfCet I Study Area (154,621 km2) (Figure 1.2). U.S. waters from 100-2,000 in deep

west of 87°30.0W. This area is a subset of the oceanic northern Gulf study area (Figure

1.4) and was surveyed during spring with the RJV Oregon II.

Oceanic northern Gulf of Mexico (398,960 kin2) (Figure 1.5). Waters within the U.S.

Economic Exclusive Zone (FEZ) greater than 100 m deep. This area was surveyed using

the R/V Oregon II during spring.

(1)

10
Table 1.4. Types of data collected by season and survey for the GulfCet II program. Abbreviatio

casts; XBT = expendable bathythermograph; SST = sea surface temperature; SSS = sDoppler Current Profiler; MOCNESS Multiple Opening and Closing Net and Enviroheight. Survey Marine Mammal Surveys Dates 00 Hydr

H U 

SPRING 1996 RIV Oregon II Cruise 220 Leg I 17 Apr-4 May 1996 

Leg 2 8-25 May 1996 I Leg 3 29 May-8 Jun 1996 / // 

SUMMER 1996 Aerial 9 ll-31Jull996 11 

LATE SUMMER 1996 WV Gyre CruiseGyre96GO6 11-l9Oct 1996 / / / WINTER 1997 Aerial 10 7 Feb-20 Mar 1997 f SPRING 1997 R/V Oregon II Cruise 222 Leg 1 17 Apr-5 May 1997 / 

Leg2 9-25May 1997 / Leg3 29May-9Jun 1997 / / / / 

SUMMER 1997 Aerial 11 15 Jul-6 Aug 1997 1 

WV Gyre Cruise Gyre97GO8 6-20 Aug 1997 11 / f / 

WINTER 1998 Aerial 12 8 Feb-14 Mar 1998 f 
30- 28 0z - 22- 20C 18 F -98 -96 -94 HOU STON NEW ORLEANS I - -92 -90 -88 -86 -

Longitude West Figure 1.4. GulfCet II study area including the Minerals Management Service's Eas

line) and the focal study area for cetacean habitat surveys (dashed line).
2826- 0z - 22-s 20H 18 C, r10 Figure 15. U.S. oceanic northern Gulf of Mexico. -98 -96 -94 -92 -90 -88 Longitude West -86 -

30i HOUSTON' NEW ORLEANS
(4) Focal study area for cetacean habitat surveys (Figure 1.4). This area was surveyed during

late summer 1996 and mid-summer 1997 by the R'V Gyre to assess the relationship

between cetacean distribution and habitat characteristics (i.e., bathyrnetry, hydrography

and biological oceanography).

Both the EPA slope and GulfCet I study areas are within the boundaries of the oceanic northern

Gulf study area.

1.3.2 Specific Objectives

The specific objectives of the study were to:

Obtain data on temporal and spatial patterns of distribution and minimum abundance of

cetaceans using line-transect and acoustic survey techniques directly comparable to those

used in previous surveys. This included incidental sightings of sea turtles and seabirds.

Identifi possible associations between cetacean high-use habitats and the ocean

environment, and attempt to explain any relationships that appear to be important to

cetacean distributions.

Objective 1 was a continuation of surveys in the north-central and western Gulf that began

during the GulfCet I Program and extended into MMS's Eastern Planning Area. To accomplish

this objective, we conducted aerial surveys and simultaneous shipboard visual and acoustic

surveys using line-transect methods.

To characterize cetacean and seabird habitat (Objective 2), we used an integrated approach that

included the analysis of hydrographic and bathymetric features (e.g., anticyclonic and cyclonic eddies, ocean depth). In addition to physical features, we measured zooplankton and

micronekton biomass derived from both net and acoustic sampling to indicate the amount of

potential food available for higher trophic level foraging by cetaceans and seabirds. Although

the diets of most cetaceans and seabirds in the Gulf are poorly known, we hypothesized that

hydrographic regimes in the study area have different levels of potential prey that influence their

distribution. We further hypothesized that these food stocks would be locally concentrated in

nutrient-rich areas offshore from the Mississippi River, within cyclonic eddies, and along the high-shear edges of anticyclonic eddies.

14
Chapter 2

Environmental Patterns and Oceanographic Processes:

ilydrography and Mesoscale Circulation of the Continental Margin

of the Northeastern Gulf of Mexico, 1996 and 1997

Douglas C. Biggs

2.1 Introduction

The physical forcing functions for the circulation over the continental margin of the northeastern

Gulf of Mexico are river discharge, wind stress, and the Loop Current (LC) and its derived

circulation phenomena. The major river system influencing this region is the MississippiAtchafalaya River (MAR). The U.S. Army Corps of Engineers regulates MAR flow so that twothirds of the total discharge enters the Gulf of Mexico at the birdsfoot delta, while the other onethird is shunted to enter via the Atchafalaya Bay. Most ofthe combined outflow is downcoast to

the west (Cochrane and Kelly 1986), although under some wind conditions a substantial fraction

of the discharge of this system may spread eastward to influence the northeastern Gulf (Walker et al. 1994).

The inner shelf flow is largely wind-driven, with contributions due to buoyancy forcing by river

discharge. Seaward of the shelf-slope break, the LC is the major dynamic, circulation feature in

the Gulf of Mexico. The mesoscale circulation of this deepwater region is largely determined by

the changing geometry of the LC and its associated cold-core and warm-core mesoscale eddies.

Once or twice annually (on average), the LC sheds anticyclonic, mesoscale eddies (LCEs), that

are also called warm-core rings (WCRs). These LCEs have potential lifetimes longer than one

year and typically migrate westward; they spawn cyclonic eddies during interaction with one

another and/or with the continental slope. When the LC pushes into the northern part of the

eastern Gulf; warm-surface-temperature filaments often extend fromits northern wall to the

upper continental slope and outer continental shelf of the northeastern Gulf.

Remote sensing data processed at the University of Colorado Center for Astrodynamics Research

(CCAR) show that a broad area of cold-core circulation was located in the northeastern Gulf

throughout calendar year 1996. This cyclonic eddy shows up persistently in weekly and monthly

altimetry maps as a region of negative sea surface height (SSH) anomaly, depressed 20 cm or

more relative to the climatological mean surface (Yi, 1995). This cyclone was there January

through October in the region 27-29°N, 88-84°W, even though for much of the year the northern

edge of the LC extended north of 25°N and the LC shed two LCEs during 1996.

In January 1996, the LC extended north of 26°N in the eastern Gulf; with LC Eddy A to the west

and centered at 90-92°W. This anticyclonic eddy had separated from the LC the previous

August-September 1995, so by January 1996, Eddy A was 4-5 months old. LC Eddy B separated

from the LC between mid-April and mid-May, but since Eddy B remained centered east of 90°W

for several months, its eastern edge apparently interacted with the LC through about mid-July.

LC Eddy C separated from the LC in August-September 1996, about the time that eddies A and

15
B were merging and beginning to spin down in the western Gulf. By year's end, secondary LC

Eddy D was in the process of splitting away from the northeast corner of Eddy C.

Eddy E separated from the LC in June-July 1997. As in 1996, a region of cyclonic circulation

persisted in the northeastern Gulf for several months before and after this eddy shedding event.

This eddy shedding periodicity, combined with the persistence of cyclonic circulation again in the

region 2729°N, 88-84°W, created mesoscale circulation conditions in the northeastern Gulf in

early and late sunimer 1997 that were remarkably similar to those in early and late summer 1996.

The late summer (October) 1996 RIV Gyre cruise (Gyre96GO6) surveyed the deepwater LCE C

and cyclone pair. The mid-summer (August) 1997 Gyre cruise (Gyre97GO8) surveyed the analog deepwater LCE E and cyclone pair. These "focal" surveys of cyclone-anticyclone pairs

were designed using near real-time altimetry maps to determine the approximate dimensions of

and center locations for the cyclone and anticyclone. Cruise tracks were then plotted to cross the

cyclone with five or six lines of closely spaced expendable bathythermograph (XBT) stations, each of which would also extend into the northern part of the anticyclone. Both of these Gyre

cruises also surveyed the continental margin with closely-spaced XBT stations between water

depths of 100-1000 m to the north and east of the cyclone-anticyclone pair, because this is the

region which MMS has designated its Eastern Planning Area (EPA). In the early summer of 1996 and again in the early summer of 1997, the third leg of NMFS cruises 220 and 225 aboard

RJ\T Oregon II also surveyed the EPA, between water depths of 1 00 1000 m, with closely spaced XBT stations.

Together, these four cruises surveyed the continental margin of the northeastern Gulf with 560

closely spaced hydrographic stations at which XBT data were collected. These were

supplemented with conductivity-temperature-depth (CTD) and bottle data that were collected at

32 locations. The cruise-by-cruise distribution of these stations is summarized in Table 2.1 and

Figure 2.1. Table 2.2 provides additional detail by grouping the stations into five hydrographic

"environments": (1) deepwater cyclone, (2) deepwater LC Eddy, (3) flow confluence between

cyclone and LCE, (4) mouth of the Mississippi River (MOM) region of the continental slope

close off the MAR birdsfoot delta, and (5) other margin (EPA). Figure 2.2 shows the subset of hydrostations on all four cruises that were made in EPA and MOM environments.

This chapter describes how the stations on each of the four cruises were partitioned into these

five hydrographic environments, and further describes each environment in terms of: (a) mixed

layer depth, (b) 19°C depth, and (c) 15°C depth and dynamic topography. The goal was to

represent this series of five environments, which form a continuum ranging from convergence

regions (LCEs) to divergence regions (cyclones), by a few summary statistical parameters. In Chapters 6 and 8, these environmental parameters were used in logistic regression and general

linear models to identify cetacean and seabird-habitat associations.

16
Table 2.1. Summary of hydrographic stations made in early and late summer 1996 and in early

and mid-summer 1997 in the northeast Gulf of Mexico.

Hydrographic Total Survey of Continental Margin Focal Survey of Deepwater Survey number of MMS Eastern Planning Area: Cyclone-Anticyclone pair: stations XBT stations + CTD stations XBT stations + CTD stations

June 96

(Oregon II 79 75 + 4 No deepwater survey was done. cruise 220

Leg 3)

Oct 96

(Gyre 241 = 85 + 4 and 144 + 8

96G06)

June 97

(Oregon II 81 = 77 + 4 No deepwater survey was done. cruise 225

Leg 3)

Aug 97

(Gyre 181= 67 +7 and 102 + 5

97G08)

17
30

24

A

A

t

A--

A - A!I - '!IUA.

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j AA A; A A

AA A A A T&UL.II

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4 A'.A

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A A AA AA A ' A

3000m A AA A " A

A AAA A A gA A'A A AA AAAAAA

A A AA

A

.j'u

Ek

A A14 !

-90 -88 -86

Longitude

Figure 2.1. Location of hychographic stations made by R!V Gyre (triangles) and R/V

Oregon II (squares) during four GulfCet II cruises that surveyed the

northeast Gulf of Mexico in early and late summer 1996 (open symbols)

and early and mid-summer 1997 (filled symbols).

18

-84

28

26
Table 2.2. Hydrographic stations from summer survey cruises grouped into five hydrographic

environments.

Hydrographic Environments Surveyed in the NE Gulf

Hydrographic Number of Cyclone Confluence LCE MOM Other Margin Total

Survey Stations

June96 XBTs 8* none none none 67 75

(Oregon II

cruise 220 CTDs 2* none none none 2 4

Leg 3)

Oct96 XBTs 68 26 26 18 91 229

(Gyre

96G06) CTDs 4 0 3 1 4 12

June97 XBTs 3* none none none 74 77

(Oregon II

cruise 225 CTDs 2* none none none 2 4

Leg 3)

Aug97 XBTs 33 26 36 6 68 169

(Gyre

97G08) CTDs 3 1 2 1 5 12

* Oregon II stations were made along the northern periphery of the cyclone (dyn ht anomaly

ito 5 cm, relative to 800 m), but none of the survey lines extended seaward into the interior

of cyclone.

19
Longitude

Figure 2.2. Location of hydrographic stations made over the continental margin of the

MMS Eastern Planning Area and the mouth of the Mississippi River

(MOM) area in water depths of 100-1000 m by RJV Gyre and R/V

Oregon II during the four GulfCet II cruises that surveyed the northeast

Gulf of Mexico in early and late summer 1996 and early and mid-summer

1997. Symbols are as in Figure 2.1.

20
2.2 Methods

2.2.1 Primary Data

Mixed layer depth (MLD) and 19°C and 15°C depths were read from the XBT data after

correcting the raw data for drop-rate error (due to the fact that the Sippican manufacturer's

software underestimates the fall rate of T10 and T7 probes by about 5%) (Biggs 1992). For each

of the four cruises, the bottom of the mixed layer was chosen as the depth at which surface

temperature was 1°C lower than that at the surface. For two of the four cruises in which surface

salinity was routinely measured (see Section 2.2.3), sigma-theta density was also calculated and

MLD was computed, for comparison, as the depth at which sigma-theta density increased 0.5

above its value at the surface. These two measures of MLD agreed extremely well in late

summer 1996 (upper panel of Figure 2.3), when surface salinity and surface sigma-theta were

both high (>34 psu and 22.5-24.0, respectively). In contrast, MLD was overestimated by the

1°C temperature difference method in mid-summer 1997 (see lower panel of Figure 2.3). The

overestimation was largest when surface salinity and surface sigma-theta were both locally low

(Figure 2.4).

The 19°C and 15°C depths are useful as summary statistics of nitracline depth and eddy dynamic

topography, respectively. Bottle samples analyzed from the late summer 1996 fieldwork showed

that at temperatures below about 22°C, there was a highly predictable negative first order

relationship between temperature and nitrate concentration (Figure 2.5). As demonstrated by Atkinson et al. (1978), temperature can thus be a proxy for nitrate concentration. In particular, in

the Gulf of Mexico the depth of the 19°C isotherm is a good estimation of the depth of the 10

jiM nitrate concentration (Figure 2.5). Other fieldwork (Biggs et al. 1988, Biggs et al. 1996)

had documented that the 15°C isotherm of the main thermocline is depressed to >250 m in strong

anticyclones and domes to <170 m in strong cyclones. Over deepwater with bottom depths

>800 m, the 15°C depth in late and mid-summer was well correlated with upper layer dynamic

height (cm) relative to 800 m and with dynamic height anomaly (Figure 2.6).

The basis for the anomaly computation changes seasonally, as the near surface water is heated to

stand about 10 cm higher in mid-summer than in early summer. In early summer, the mean

dynamic height for the deepwater continental margin away from the LC or its associated eddies

was 95 dyn cm relative to 800 m. This mean dynamic height increased to 105 dyn cm by midsummer, and then relaxed to 100 cm by late summer, as the near surface water began to lose heat after passage of atmospheric cold fronts in September and October. Operationally, early summer

stations with dynamic height relative to 800 m of 95 dyn cm were grouped as "cyclone." At

these stations, 15°C depth was shallower than 170 m. However, the dynamic height anomaly of

these stations relative to the average deepwater dynamic height for early summer was just -ito - 5 cm (see spreadsheet hydrographic data table for all four cruises in the Appendix). Therefore,

these stations must have been at the northern periphery of the cyclone rather than in its interior.

Similarly, late summer 1996 deepwater hydrostations with <100 dyn cm were grouped as

"cyclone,' those >125 dyn cm were grouped as LC Eddy C; and those 100-125 dyn cm were

grouped as "confluence," "other margin," or "MOM," depending on their geographic location in

relation to the cyclone-anticyclone. The operational cut-offs for grouping midsummer 1997

21
22

Figure 23. Property-property plots illustrating relationship between mixed layer

depth (MLD) calculated as [surface temperature - 1°C] and [surface

sigma-theta + 0.5], in late versus mid-summer.
60

August 1997

50- Gyre 97G-08

40-

30

20

0 U

U

DED 0

14 16 18 20 22 24

Surface sigma-t

Figure 2.4. Property-property plots illustrating the influence of surface salinity on

surface sigma-theta and the nonlinear relationship between mixed layer

depth and surface sigma-theta in August 1997.

10-

0

23
28

+

24

26

24

22-

- 20

18

16

14

12

++

++

+

+ +

+

Nitrate v Temperature plot

for CTD-rosette casts

Gyre 96G-06 (Oct 96)

+ + +

-H-++ .I:ll 1- +

+ +++ +

+ ++

+

+

+

+

10 T I 0 5 10 15 20 25

Nitrate (tM)

Figure 2.5. A negative functional relationship between nitrate and temperature

allowed the 19°C depth to be used as a proxy for 10 jiM of nitrate.
200

150-

100

50

100

dyn ht anomaly = O213*x 37

r'20.961

Figure 2.6. Propertyproperty plots illustrating use of 15°C depth (m) as a proxy for

deepwater dynamic height (cm) relative to 800 m and for deepwater

dynamic height anomaly (cm) relative to the seasonal mean.

25

200 500
deepwater hydrostations as "cyclone" versus "confluence" versus "LCE-E" were <105 dyn cm,

105-130 dyn cm, and >130 dyn cm, respectively. Stations made in water depths <800 m were

grouped on the basis of 15°C depth. Operationally, stations with 15°C depth <170 m were

grouped as "cyclone"; stations with 15°C depth >250 m were grouped as LCE; and stations with

15°C depth 170-250 m were grouped as "confluence," "other margin," or "MOM," depending on

their geographic location in relation to the cyclone-anticyclone pair.

2.2.2 Between Cruise Differences

Since the two Gyre cruises surveyed the northeastern Gulf in different months in 1996 and 1997,

between-cruise differences in MLD and 19°C depth likely reflect within-season differences

between mid-summer and late summer rather than year-to-year differences. The third legs of the 1996 and the 1997 Oregon II cruises, however, both surveyed the EPA at the same time of year

(Julian days 151-161). Moreover, most of the hydrostations were co-located at the same (or

nearly identical) water depths and latitude-longitude positions, so differences in MLD and 19°C

depth between these cruises do represent year-to-year differences. Figure 2.7 shows the location

of the seven cross-margin transects on which the between-cruise-differences in water depth and

station location are smallest.

Stations on transects A through F in and around DeSoto Canyon were similar in location in

1996 and 1997. Stations on transect G on the West Florida Terrace and West Florida Shelf

were similar in water depth, although there was up to 30 k.rn difference in locations between

1996 and 1997.

2.2.3 Other Hydrographic Data

The Gyre is equipped with a through-the-hull pumped seawater system and a 153 kHz Rowe

Deines Instruments narrowband Acoustic Doppler Current Profiler (ADCP). Data from both

systems were logged during the late summer and mid-summer cruises, and these data will be

summarized in this chapter to supplement the MLD, 19°C depth and 15°C depth data that were collected in common by both ships.

Near-surface temperature, conductivity and fluorescence were logged every two minutes

throughout both cruises with Gyre's Serial ASCII Interface Loop (SAIL) system. Temperature

and conductivity were measured using Sea-Bird Electronics Corporation in-line sensors, and

fluorescence was measured using a Turner Designs model 10 laboratory fluorometer that was

equipped for flow-through operation.

The ADCP data were not collected by the ship's SAIL system, but were instead downloaded

directly to a PC and then binned into five minute ensembles by the manufacturer's software. The

raw binary data were then converted to ASCII format for subsequent processing ashore on UNIX

workstations. The ADCP raw data were analyzed by Dr. Steve DiMarco, following quality

assurance and quality control procedures used to process ADCP data that were collected later in

1997 and in 1998 by the Northeastern Gulf of Mexico Chemical Oceanography and Hydrography

Study (NEGOM). Jochens and Nowlin (1998) give details about this data processing in section

3.3.3 of the first Annual Report for the NEGOM study. Figures 2.8 and 2.9 are plots of the late

26
Longitude

Figure 2.7. Stations along seven of the closely-spaced XBT transects made by R/V

Oregon II in the MMS Eastern Plarming Area were done at very similar

locations and water depths in early summer 1997 as they had been done in

early summer 1996. These are designated as transects A to (i Symbols

are as in Figure 2i.

27
Longitude

Figure 2.8. Along-track current velocity at 10 m depth, measured by acoustic Doppler

current profiler (ADCP) during Ply Gyre cruise 96G06.

28
3&

28

26

24

k k" 4r--

- S lOOm -4 V -

' D 7' '

'1' 7' kV'V1(k 1' - -ic. .c ". 'c5, kJéVkrcç- ';

OS- 77 - 7' S

- - - ' ' '

7----- D D 1 't -'7 ' 7--*--*-' 7'7' -'-

272' 7 - - 2)Y-*--*-'--*3-- 7 7 - v

- ' ,1! - -3 , 3000 m I - ,!-)--* - -' ulu*

,&'A2'

>

50 cm/s

29

Longitude

Figure 2.9. Gridded ADCP current vectors within the deepwater focal and continental

margin areas of the northeast Gulf of Mexico surveyed by R/V Gyre

cruise 96G06.

-90 -88 -86 -84
summer ADCP currents at z = 10 m, the uppermost depth bin for which data are available.

Figure 2.8 shows post sigma QAIQC (grossly anomalous velocity vectors greater than three

standard deviations from the average value removed). In Figure 2.9, the data have been

optimally interpolated and gridded to more clearly illustrate the mean field. Figures 2.10 and

2.11 are similar plots of the mid-summer ADCP currents at z = 10 m.

2.3 Results

2.3.1 Hydrography of the Continental Margin of the MMS Eastern Planning Area

Contour plots of 15°C depth, 19°C depth and mixed layer depth (MLD) for the four cruises that

surveyed the MMS EPA are given as Figures 2.12-2.23. Because these surveys concentrated on

the depth range 100-1000 m while cyclone and LCEs were usually centered seaward of the 1000

m depth, each of these variables on any individual cruise usually had limited dynamic range over most of this margin.

It is evident from the depth of the 15°C isotherm that a secondary warm eddy was present over

this continental margin in early summer 1996, and again in early summer 1997. In Figures 2.12

and 2.15, these warm-core eddies can be seen as elliptical regions about 30 x 60 n miles in size,

where the 15°C depth was >190 m. These secondary eddies form when warm filaments extend

north and then separate from the deepwater LCEs. Although these secondary warm eddies have

an anticyclortic circulation, CTD vertical profiles show that they contain little or no subtropical

underwater; that is, subsurface salinity in these secondary warm eddies rarely exceeded the 36.5

practical salinity units that is diagnostic of LC water of Caribbean Sea origin. These secondary

warm eddies show up in Figures 2.13 and 2.14, and in Figures 2.16 and 2.17, as elliptical regions

in which 19°C depth and MLD are locally deeper than over the adjacent slope.

The depth of the nitracline, as determined from 19°C depth, was more than 20 m shallower in the

northern edges of the cyclones that were surveyed in early summer 1996 and 1997 than in these

secondary wann eddies. However, locally shallow nitracline depths were also found along the

shelf margin. Figure 2.13 shows that the 19°C isotherm domed shallower than 50 m from the

surface along the upper slope of the EPA in early summer 1996. Figure 2.16 shows that such

shelf edge upwelling was also present in early summer 1997, although the 19°C depth did not reach as shallow.

In addition, it is evident that surface temperature over most of the continental margin was 1-2°C

warmer in June 1997 (Figure 2.24a). Similarly, MLD was generally 10-20 m shallower in June

1996 than in June 1997 (Figure 2.24b), and so was 19°C depth (Figure 2.24c). Below the

nitracline, however, interaunual differences were small: 15°C depth was highly similar between

June 1996 and June 1997 at each of the stations along transects A to F (Figure 2.24d).

Later in the summer of both years, shelf-edge upwelling appeared to relax. Based on Figure

2.21, there is no evidence of the 15°C isotherm doming over the upper slope in midsummer

1997, although the 19°C isotherm (Figure 2.22) is domed by 10-30 m shallower. Similarly,

Figure 2.18 and Figure 2.19 shows no doming of either isotherm over the upper slope in late

summer 1996. The strength of this shelf-edge upwelling probably varies seasonally, for it was

30
31

-90 -88 -86

Longitude

Figure 2.10. Along-track current velocity at 10 m depth, measured by acoustic

Doppler current profiler (ADCP) during K/V Gyre cruise 97G08.

-84
2

28t lz -7'

2r

77 72',7

3000m

-90

4- -;

. r.y 1 k - '-5- D L''- £4 \ - 11QQt -kt f- 7

-\' 1 - -+ 'Lt - '' 7- - - 7 -_-I.-- - 7:' -

/

1roo

Longitude

Figure 2.11 - Gridded ADCP current vectors within the deepwater focal and

continental margin areas of the northeast Gulf of Mexico surveyed by

K/V Gyre cruise 97G08.

32

-84
Shelf margin

iwUing

Longitude

Figure 2.12. Contour map of 15°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 79 hydrographic stations (filled

squares) made on RJV Oregon II cruise 220, Leg 3. The gray scale

shading deliniates local areas where isotherrns shoal (lightest gray) and

deepen (darkest gray) relative to their seasonal average depth on the

continental margin.

33
Longitude

Figure 2.13. Contour map of 19°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 79 hydrographic stations made on

RlVOregon II cruise 220, Leg 3. Symbols and shading as in Figure 2.12.

34

-90 -88 -86 -84
Longitude

Figure 2.14. Contour map of mixed layer depth (MLD) on the continental margin of

the northeast Gulf of Mexico, as determined from 79 hydrographic

stations made on RJV Oregon II cruise 220, Leg 3. The gray scale

shading deliniates local areas where MILD shoals (lightest gray) and

deepens (darkest gray) relative to its seasonal average depth on the

continental margin.

35

-90 -88 -86 -84
-90 -88 -86 -84

Longitude

Figure 2.15. Contour map of 15°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 79 hydrographic stations made on

R'V Oregon II cruise 225, Leg 3. Symbols and shading as in Figure 2.12.

36
Longitude

Figure 2.16. Contour map of 19°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 81 hydrographic stations made on

R/V Oregon II cruise 225, Leg 3. Symbols and shading as in Figure 2.12.

37

-90 -88 -86 -84
38

-90 -88 -86 -84

Longitude

Figure 2.17. Contour map of mixed layer depth on the continental margin of the

northeast Gulf of Mexico, as determined from 81 hydro graphic

stations made on R/V Oregon II cruise 225, Leg 3. Symbols and

shading as in Figure 2.14.
31

1)

29-

27

39

/ !

- - /

- 'I

3OOOm

-90 -88 -86 -84

Longitude

Figure 2.18. Contour map of 15°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 89 hydrographic stations made on

R!V Gyre cruise 96G06. Symbols and shading as in Figure 2.12.
-90 -88 -86 -84

Longitude

Figure 2.19. Contour map of 19°C depth on the continental margin ofthe northeast

Gulf of Mexico, as determined from 89 hydrographic stations made on

1FtV Gyre cruise 96G06. Symbols and shading as in Figure 2.12.

40
Longitude

Figure 2.20. Contour map of mixed layer depth on the continental margin of the

northeast Gulf of Mexico, as determined from 89 hydrographic

stations made on R/V Gyre cruise 96G06. Symbols and shading as

in Figure 2.14.

41
Longitude

Figure 2.21. Contour map of 15°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 74 hydrographic stations made on

1JV Gyre cruise 97G08. Symbols and shading as in Figure 2.12.

42

-90 -88 -86 -84
She1j margin

Longitude

Figure 2.22. Contour map of 19°C depth on the continental margin of the northeast

Gulf of Mexico, as determined from 74 hydrographic stations made on

RIV Gyre cruise 97G08. Symbols and shading as in Figure 2.12.

43

-90 -88 -86 -84
-88 -86

Longitude

44

Figure 2.23. Contour map of mixed layer depth on the continental margin of the

northeast Gulf of Mexico, as determined from 74 hydrographic stations

made on R/V Gyre cruise 97G08. Symbols and shading as in Figure 2.14.
35

30

25

20

24 24

Station and Transect

15

10

1996 B

Stiition nd Transect

Figure 2.24. Between year variation in a) surface temperature, and b) mixed layer

depth for stations along EPA transect lines A-G.

45

3

- 31:

- 21:

5

0

29 29
160

140 -

1997

1996

0

20 17,.i.i.I,I,l,l,I,I,I.1,,I,lIIIIIllII'1'T'TllI 20

Station and Transect

240

220

200

180

160

140 120

46

160

-140

-120

100

100 ...................................................... 100 Station and Transect

Figure 2.24. (continued): Between year variation in c) 19°C depth and d) 15°C depth

for stations along EPA transect lines A-G.

8060 -

40 - C

- go

60

-40
strongest in early summer of both 1996 and 1997, present but restricted to temperatures above

15°C in mid-summer 1997, and not evident in late summer 1996. As a corollary, such shelf-edge

upwelling does not appear to have been caused by the presence of a secondary warm eddy,

because a secondary eddy was present over the MMS EPA in late summer.

The ADCP currents at z = 10 m show that surface flow in late suxrnner 1996 was generally from

east to west along most of the continental margin seaward of the 1000 m isobath, at velocities <1

knot (Figure 2.9). Shoreward of the z = 1000 m isobath, there were locally fast surface currents

in the east, over the outer shelf and slope south of Florida's Big Bend, and in the west, in the

MOM area. In contrast, the surface flow field was less uniform in mid-summer 1997. While

flow was downcoast over the inner shelf, the flow was offshelf in the MOM area and strongly

cyclonic over the West Florida Terrace and West Florida Shelf (Figure 2.11). Between the two

regions, there was a zone of high shear in surface current (strong surface flow offshelf, near

89°W, and strong surface flow onshelf, near 88°W.

Auto-correlation function analysis of the surface temperature and salinity data showed that crossmargin length scales for surface temperature and surface salinity over the EPA continental

margin averaged about 18 km (10 nautical miles).

2.3.2 Hydrography of the MOM area

The MOM area was surveyed on both GulfCet II Gyre cruises, since during GulfCet I this had

been the geographic region in which sperm whales were most likely to be encountered. In late

summer 1996, sperm whale cows and calves were seen in this area on 20 and 28 October.

However, at least in its summary hydrographic characteristics, the MOM environment was quite

similar to the continental margin to the east and to the cyclone to the south. Table 2.3 shows that

surface temperature (± one standard deviation) averaged 25.8 ± 0.3 °C. Surface salinity averaged

35.8 ± 0.1 psu, and surface chlorophyll was low and ranged from 0.09-0.11 tg/l. Propertyproperty plots showed little along-track variation (no fronts) in surface temperature, salinity or

chlorophyll. Nor was this a region of particularly shallow MLD, 19°C or 15°C depth, for in late

summer 1996 these depths averaged 10 m, 13 m and 28 m deeper, respectively, in the MOM area

than in the interior of the cyclone to the south. Surface currents in the MOM environment were

low (<1 knot), and flow was along bathymetry rather than cross-slope.

In mid-summer 1997, the clockwise flow field around secondary Eddy D to the west and the

counterclockwise flow around the cyclone to the east turned the MOM area into an environment

of strong cross-slope flow. Low salinity MAR water was entrained into this flow and carried

ESE into the confluence between the cyclone and LC Eddy E. As a result, mean surface sigma-t

was lower here than over the "othermargin" or in the cyclone (Table 2.4). Apart from being

fresher in salinity and higher in chlorophyll concentration, the MOM area in mid-summer 1997

showed no marked differences in subsurface properties (e.g., 19°C or 15°C depth) when

compared with the "other margin" or the cyclone. This time, no local aggregations of sperm

whales were encountered in the MOM area; on the contrary, most contacts with sperm whales

were made in the deepwater cyclone that was centered 100 nautical miles to the east of the

MOM.

47
Table 2.3. Temperature, density and dynamic topography of the five hydrographic environments

surveyed on the late summer PJVGyre cruise (Gyre96GO6).

48

Late Summer 1996 Cyclone Confluence LCE MOM Other Margin

n=72 n=26 n29 n19 11=95

Surface max 27.6 28.4 28.3 26.3 27.5

Temp mill 25.2 26.0 27.3 25.2 24.1

(°C) mean 26.2 27.5 27.7 25.8 26.0

std dev ±0.6 ±0.5 ±0.2 ±0.3 ±0.6

Surface max 23.9 23.9 23.7 24.0 24.0

Sigma-theta mm 23.0 23.3 23.3 23.4 22.5

mean 23.6 23.6 23.5 23.6 23.7

std dev ±0.2 ±0.2 ±0.1 ±0.2 ±0.1

Mixed max 53 80 80 60 78

Layer mm 22 35 54 41 15

Depth mean 39 56 65 49 51

(m) stddev ±6 ±10 ±6 ±5 ±8

19°C depth max 101 162 272 107 160

(m) mm 67 93 160 91 83

mean 85 126 227 98 103

stddev ±8 ±18 ±35 ±5 ±13

15°C depth max 183 256 424 207 247

(m) mill 128 171 262 166 152

mean 155 205 359 183 187

stddev ±15 ±20 ±57 ±14 ±20

Dynamic max 100 122 150 111 120

Height(cm) mill 88 100 125 100 100

relative to mean 95 107 140 104 104

800m stddev ±4 ±7 +9 ±3 ±4

Dyn Height max 0 22 50 11 20

Anomaly (cm)min -12 0 25 0 0

relative to mean - 5 + 7 +40 +4 +4

100 cmmean stddev ±4 ±7 ±9 ±3 ±4
Table 2.4. Temperature density, and dynamic topography of the five hydrographic environments

surveyed on the mid-summer RJV Gyre cruise (Gyre97C+08).

49

Mid-summer 1997 Cyclone Confluence LCE MOM Other Margin

n=36 n=27 n=38 n=7 11=73

Surface max 31.2 31.7 30.9 31.4 31.7

Temp mm 29.4 29.5 30.1 29.2 29.3

(°C) mean 30.1 30.5 30.5 29.9 30.4

std dev ±0.5 ±0.6 ±0.2 ±0.8 ±0.5

Surface max 22.1 22.5 22.5 18.7 20.8

Sigma-theta mm 19.7 15.7 22.3 16.7 16.9

mean 21.1 20.3 22.4 17.6 19.6

std dev ±0.5 ±2.4 ±0.1 ±0.8 ±1.1

Mixed max 27 50 47 22 23

Layer mill 9 8 23 13 5

Depth mean 19 25 35 18 13

(m) stddev ±5 ±10 ±7 ±4 ±4

19°C depth max 105 167 303 106 125

(m) mm 83 91 163 88 57

mean 94 127 240 97 101

stddev ±6 ±21 ±50 ±7 ±15

15°C depth max 172 249 457 190 228

(m) mm 126 151 254 143 155

mean 150 201 347 170 183

stddev ±11 ±28 ±69 ±16 ±15

Dynamic max 105 129 176 109 112

Height(cm) mm 92 105 130 106 106

relative to mean 100 117 154 108 108

800m stddev ±4 ±8 ±16 ±1 ±2

Dyn Height max 0 24 71 4 7

Anomaly (cm) mill -13 0 25 1 1 relative to mean - 5 + 12 + 49 + 3 + 3

105 cm mean std dev ± 4 ± 8 ± 16 ± 1 ±2
2.3.3 Deepwater Cyclone-Anticyclone Pair

From Tables 2.3 and 2.4, it can be seen that MLD and 19°C depth were, on average, shallower in

the cyclone than in the confluence or LCE in late summer 1996 and mid-summer 1997.

Property-property plots show this even more clearly. Stations with locally shallow MLD had

locally shallow 19°C depth (Figure 2.25), and a locally shallow nitracline was characteristic of

stations between 27-28°N Latitude in the interior of the cyclone (Figure 2.26). The Deep

Chlorophyll Maximum (DCM) was located at or just above the depth at which nitrate first was

analytically detectable; this averaged 50 m below the surface in the cyclone, and 100 m in the

LCEs (Figure 2.27). This DCM was not only higher up in the water column in the cyclone, but

subsurface maximum concentrations of chlorophyll were higher there than in the LCEs. The

physics of convergence versus divergence clearly forced the biological patterns that were seen.

On the other hand, when the area under the chlorophyll profiles was integrated between the

surface and 150 m, it was evident that standing stocks of chlorophyll expressed as mg per square

meter were about equal in Eddy C as in the cyclone.

Contour plots of deepwater dynamic topography relative to 800 m and of deepwater surface

current vectors computed by standard geostrophic methods from the dynamic height data are

shown in Figures 2.28-2.33. These plots show that the five transects that were rim in late

summer 1996 between the MOM and the interior of LC Eddy C provide radial as well as

tangential sections through the cyclone. The shallowest 15°C depths were in the interior of the

cyclone (<130 m) and the deepest were in the interior of LC Eddy C (>420 m).

This >290 meter difference in 15°C depth was equivalent to a 62 dyn cm gradient in dynamic

height between the interior of the two features (88 dyn cm in the cyclone to 150 dyn cm in LC

Eddy C). The corresponding upper layer geostrophic volume transport between the interior of

the two features was 24 x 106 m3 s_I (24 Sverdrups). In the flow confluence from 25.5°-26.5°N

that separated the two features, geostrophic current velocity at the surface exceeded 1.5 knots

(>75 cm

Geostrophic flow created by the cyclone-anticyclone pair that was surveyed in mid-summer 1997

was even stronger. The shallowest 15°C depth in the interior of the cyclone was again <130 m,

but the deepest 15°C depth in the interior of LCE-E was now >450 m. This 320 m difference in

the 15°C depth was equivalent to an 84 dyn cm gradient in dynamic height between the interior

of the two features (92 dyn cm in the cyclone to 176 dyn cm in LC Eddy B). The corresponding

upper layer geostrophic volume transport between the interior of the two features was 31 x 106

m3 s (31 Sverdrups). In the flow confluence from 26°-27.5°N that separated these two features,

geostrophic current velocity at the surface exceeded 2 knots (>100 cm

A geostrophic volume transport of 24-31 Sverdrups (Sv) is a large volume of water. For

example, the inflow into the Gulf of Mexico through the Yucatan Channel and the outflow from

the Gulf of Mexico through the Florida Strait both average about 30 Sv relative to 800 m. The

outflow through the Florida Straits feeds into the western boundary current of the North Atlantic

Ocean (i.e., the Gulf Stream). Off Cape Hatteras, where the surface current velocity of the Gulf

Stream may reach 4-5 knots, the geostrophic transport may reach 90 Sv. From a global

50
100

80

60-

40-

- 20

0

0 100 200 300

0

0 100 200

19C depth (m)

Figure 2.25. Property-property plot of data from the two RIV Gyre cruises showing

relationship between mixed layer depth and nitracline depth in late

summer 1996 and midsummer 1997. Note that stations at which the

nitracline domed close to surface had a shallow mixed layer depth.

51

19C depth (m)

70

August 1997

60 - Gyre 97G-08

50 FlFl Fl 40 - Fl!l Fl flD

C Fl 30- Fl Fl Fl 0

FlFl 20 -

10-

300 400
350

300

250

6

200

150

0

50

00 ii IlI' I

24 25 26 27 28 29 30

Latitude

Figure 2.26. Property-property plot of data from the same two R/V Gyre cruises

showing that stations where nitracline domed closest to surface waters were

inside the cyclone (26.5 - 28.5°N) in late summer 1996, and in the cyclone

(27.5 - 29°N) and on the continental margin to the north of the cyclone in

mid-summer 1997.

52

31

350

300:

250:

200:

150-

100-

ID

D

0

October 1996

Gyr 96G-06

25 26 27 28

Latitude

24 29 30 31
0

0.0

20-

40-

60-

80-

iOO

120-

140-

0

20-

40-

60-

80-

100-

120-

140-

0 2 4 6 8 10 12 14 16 18 20

mean NO3 (pM)

Figure 2.27. Differences in average deep chlorophyll maximum (DCM) depth

and average nitracline depth between a) Loop Current Eddy C, and

b) the mesoscale cyclone to the north, from 3 CTD casts made in

each feature during October 1996 on RIV Gyre cruise 96G06.

53

0CM

mean CHL Qt/L) mean CHL QigfL)

0.0 0.1 0.2 0.3 0.4 0_i 0.2 0.3 0.4

2 3 4 5

mean NO3 (pM)

0
Longitude

Figure 2.28. Dynamic topography (cm, 0 m relative to 800 m) of the deepwater focal

area, as determined from 152 hydrographic stations made on R/V Gyre

cruise 96G06.

54
55

-90 -88 -86

Longitude

Figure 2.29. Dynamic height anomaly of the shipboard data presented in Figure 2.28.

Negative height (cyclone) is designated by heavy dash lines; positive

height (confluence and anticyclone) by heavy solid lines.

-84
Longitude

Figure 2.30. Gridded upper layer geostrophic velocity (0 m relative to 800 m) of the

deepwater focal area, as computed from the October 1996 dynamic

topography (Figure 2.28).

56

-90 -88 -86 -84
-90

Longitude

Figure 2.31. Dynamic topography (cm, 0 m relative to 800 m) of the deepwater focal

area, as determined from 107 hydrographic stations made on iVY Gyre

cruise 97G08.

57

-88 -86 -84
Longitude

Figure 2.32. Dynamic height anomaly of the shipboard data presented in Figure 2.31.

Negative height (cyclone) is designated by heavy dash lines; positive

height (confluence and anticyclone) by heavy solid lines.

58

-90 -88 -86 -84
Longitude

Figure 2.33. Gridded upper layer geostrophic velocity (0 m relative to 800 m) of the

deepwater focal area, as computed from the August 1997 dynamic

topography (Figure 2.31).

59
perspective, the upper 800 m of the eastern Gulf of Mexico is characterized by vigorous

circulation that is very dynamic.

The deepwater cyclone was distinguishable from the anticyclone in surface temperature, surface

salinity and surface chlorophyll characteristics, both in late summer 1996 and mid-summer 1997.

Figures 2.34-2.36 are contour plots of data that were collected every two minutes throughout late

summer 1996. Surface water pumped from a depth of 3.5 m into the main laboratory aboard the

Gyre, where some of the flow was shunted through temperature and conductivity sensors and

through a flow-through laboratory fluorometer. Figures 2.34 and 2.35 show that in late summer,

surface waters of the cyclone averaged 1°C cooler and 1 practicalsalinity unit less saline than

surface waters of the anticyclone. Surface chlorophyll was lower than 0.05 j.tg/L in the warm,

nutrient-depleted surface waters of the anticyclone, while within the cyclone and over the rest of

the continental margin, surface chlorophyll concentrations ranged 0.05-0.3 Lg/L. The highest

surface chlorophyll concentrations were encountered on or adjacent to the continental shelf, in

local areas of greatest freshwater input. A comparison of Figure 2.35 with Figure 2.36 shows

that high chlorophyll, low salinity (HCLS) surface water was seen along the ship's track

northeast of the MOM, and along the east side of the delta. A third area of HCLS surface water

was encountered on the upper continental slope southwest of the MOM. Surface salinity

increased and surface chlorophyll concentration decreased rapidly from 18-25 km (10-15 nautical

miles) of these regions.

Figures 2.37-2.39 are contour plots of surface temperature, surface salinity, and surface

chlorophyll concentrations in mid-summer 1997. Although surface temperatures over most of

the continental margin ranged from 30-31°C, the cyclone appeared as a region of cooler surface

temperature (i.e., 29-30°C). Freshwater from the Mississippi River reached much farther

offshore in mid-summer 1997 than it had in late summer 1996. As a result, the amount of high

chlorophyll surface water over the continental margin was also greater in extent in mid-summer

1997. Low salinity Mississippi River water was entrained into the flow confluence between

cyclone and LCE-E, which can be seen in Figure 2.38 as two tongues of locally low salinity

water: one reaching east of the delta and the other reaching southeast from the southern tip of the

delta. Figure 2.38 shows that low salinity patches of this river water are wrapped anti-clockwise

around the periphery of the cyclone. A comparison of Figures 2.39 and 2.38 shows that surface

chlorophyll concentrations in this river water reached 2.0 j.tg/L, and that especially in the

concentration range 0.1-0.4 ig/L, the patches of highest surface chlorophyll correspond spatially

to patches of lowest surface salinity. As in late summer 1996, surface chlorophyll levels in the

interior of the cyclone ranged 0.05-0.15 jig/L, while in interior of LCE-E, surface chlorophyll

concentrations were extremely low (<0.05 jtg/L).

2.4 Discussion

Since Ties and Sinclair (1982) proposed that larval retention zones can be caused by

oceanographic features, the relationships between stocks of phytoplankton, zooplankton, nekton

and frontal zones have been an area of intense research. For example, it is known that local

aggregations of phytoplankton develop along and within weekly-period meanders and eddies in

the Gulf Stream (Lee et al. 1991) and that elevated fish stocks often co-occur in these frontal

60
30

28-

26-

24

-J

lOOm --

-C

3000m

/ O - 2O99

-90 -88

Longitude

23 24 25 26 27 28 29

Figure 2.34. Contour map of surface temperature (°C), as measured every 2 minutes

throughout R/V Gyre cruise 96G06 using the ship's SAIL data logging

system. Small "+" symbols, usually so close together that they appear as

a broad, dark line, indicate data points used to generate the contours.

61

-86 -84
Longitude

iii

63

0.00 0.40 0.80 1.20 1.60 2.00

Figure 2.36. Contour map of surface chlorophyll Q.tg/L), as measured every 2 minutes

throughout RJV Gyre cruise 96G06 using the ship's SAIL data logging

system.
Longitude

64

Figure 2.37. Contour map of surface temperature (°C), as measured every 2 minutes

throughout RJV Gyre cruise 97G08 using the ship's SAIL data logging

system.

29 30 31 32
Longitude

65

25.5 27.0 28.5 30.0 31.5 33.0 34.5 36.0 37.5

Figure 2.38. Contour map of surface salinity (standard salinity units), as measured

every 2 minutes throughout R/V Gyre cruise 97G08 using the ship's

SAIL data logging system.
Longitude

0.00 0.40 0.80 1.20 1.60 2.00

Figure 2.39. Contour map of surface chlorophyll (j.tg/L), as measured every 2 minutes

throughout R'V Gyre cruise 97G08 using the ship's SAIL data logging

system.

66
disturbances (Atkinson and Targett 1983). However, along the Atlantic coast most of the

phytoplankton production tends to be swept back into the Gulf Stream, whereas in the Gulf of

Mexico, if ungrazed, it may move down and along the continental slope.

In the Gulf of Mexico, frontal zones at the periphery of meanders and eddies that are seaward of

the continental margin are typically expressed as sharp gradients in temperature. These may

have secondary expression as gradients in salinity, particularly in local convergences that entrain

low salinity water and transport it off-shelf as plumes or jets. For example, Biggs and MUllerKarger (1994) reported that some cyclone-anticyclone geometries in the Gulf of Mexico create

flow confluence zones that can transport high-chlorophyll shelf water seaward several hundreds

of kilometers. Sharp frontal zones may also be created during periods of northern extensions of

the Loop Current. Lamkin (1997) found a significant positive correlation between the abundance

of larval nomeid fish and the location of the northern edge of Loop Current by analyzing NOAA

annual icthyoplankton survey data from 1983-8 8. Lamkin's data indicate that Cubiceps

pauciradiatus, in particular, is a species whose adult spawning grounds and larval habitat are tied

to sharp temperature gradients. Peak larval abundance was found close to the frontal interface,

with peak abundance occurring just up slope of the region of peak SST gradient. Lamkin went

on to speculate that the extent of the frontal systems in the Gulf of Mexico would be expected to

impact annual recruitment of a species that is tied to a frontal habitat.

On shorter time scales, the biological implications of thermal fronts in the Gulf of Mexico are

widely recognized by fishermen: many of them preferentially fish in areas where sea surface

temperature imagery shows sharp temperature gradients over short (<10 km) distances. Skipjack

(Katsuwonus pelamis), blackfin tuna (Thunnas atlanticus), swordfish (Xiphias gladius), and blue

marlin (Makaira indica) have been reported by fisherman to be locally abundant in these areas

(Roffer's Ocean Fishing Forecasting Service, pers. corn.). Clearly, populations of apex predators

like these are not likely to be sustained by low or infrequent episodes of enhanced secondary

productivity.

The current paradigm in biological oceanography is that the relative amounts of "new" versus

"regenerated" nitrogen ultimately determine the biological carlying capacity of the surface mixed

layer. Eppley and Petersen (1979) hypothesized that any mechanism that accelerates the

introduction of "new" nitrogen into the mixed layer should increase the biological productivity of

higher trophic levels or (if ungrazed within the upper ocean) be available for export from the

system. For the NE Gulf of Mexico, this hypothesis appears to be especially valid, since

anticyclones that were surveyed during the deepwater focal studies were depleted in nitrate and

other nutrients. Just one to two months had elapsed between the time that LCE C separated from

the LC in July-August and when it was surveyed in late summer 1996. Similarly, just one to two

months had elapsed since LCE E had separated in June-July and when it was surveyed in midsummer 1997. High subsurface salinity >3 6.8 in the depth range 150-250 m confirmed the LC

affinity of both of these features. In both LCEs, near surface temperatures greater than 22°C in

the upper 150 m are diagnostic that the photic zone was nutrient depleted.

In contrast, the cyclones that were surveyed in late summer 1996 and mid-summer 1997 had high

nitrate concentrations closer to the surface. The 19°C depths were as shallow as 67 m (mean

depth = 85 m) in late summer 1996 and 83 m (mean depth = 94 m) in mid-summer 1997, so

67
10 M of nitrate should have domed to these depths. Lower but still analytically detectable

nitrate reached well into the photic zone. Although we did not measure primary production

during the GulfCet II cruises (this would have required light bottle and dark bottle incubations),

it is stimulated by the introduction of new nitrogen (Riggs and Sanchez 1997, Gonzales-Rodas

1999), particularly if it continues for weeks-to-months in time.

From the TOPEXTPoseidon altimeter time series of SSH anomaly, we know that cyclones in the

NE Gulf are temporally persistent yet spatially variable. They typically persist in any one region

for several months, but seldom longer than 12 months. Since the cyclone surveyed in late

summer 1996 had been around since January of that year, it eventually spun down. As a result,

the cyclone surveyed in mid-summer 1997 was not the same cyclone that had been surveyed the

year before. In both years, however, the cyclones were visible in the altimetry for more than six

months before they were surveyed by the Gyre. We hypothesize that six months was sufficient

time for nutrients-phytoplankton-zooplankton processes to produce higher standing stocks of

zooplankton and micronekton, which in turn provide food for the vertically migrating larger

nekton that are preyed upon by cetaceans.

2.5 Conclusions

The physical forcing functions for the circulation over the continental margin of the northeastern

Gulf of Mexico are river discharge, wind stress, and the LC and its derived circulation

phenomena. When these forcing functions introduce new nutrients into the upper region of the

water column, they stimulate planktonic plant production at the base of the food chain that is fed

on, in turn, by zooplaiikton, nekton, squid and marine mammals. LCEs are warm-core, nutrientpoor, surface convergence areas. In contrast, the cyclonic, cold-core eddies that are often found

in association with LCEs are regions of local divergence in which locally high nitrate

concentrations dome close to the base of the mixed layer. Deep chlorophyll maxima (DCMs)

occur shallower and chlorophyll reaches higher concentrations within these cyclones than within

the LCEs, and so the cyclones are regarded as biological "oases" while the interior of the LCEs

are biological "deserts."

Ship surveys sampled a deepwater cyclone-anticyclone pair and surface flow confluence in late

summer of 1996 and in mid-summer of 1997. The latter survey found that a 6-10 m thick lens of

low salinity river water was being transported across the shelf arid entrained into the flow

confluence. This water of river origin had locally high surface chlorophyll, resulting in more

chlorophyll within the confluence than would have been predicted from DCM characteristics

alone. The shipboard surveys also found evidence for upwelling at the shelf break based on

doming of the 19°C isotherm in late spring and early summer of both 1996 and 1997. This shelf

break upwelling was less strong in mid-summer and apparently had ceased by late summer.

68
Chapter 3

Biological Oceanography

John H. Wonnuth, Patrick H. Ressler, Robert B. Cady, and Elizabeth J. Harris

3.1 Introduction

Direct sampling of zooplankton (small, drifting animals) and micronekton (fishes, crustaceans

and cephalopods from 1 to 10 cm, Blackburn 1968) biomass with a variety of towed nets can

provide important information on marine ecosystems (Hopkins 1982; Passarella and Hopkins

1991). In this study, net sampling was used to provide estimates of the biomass and taxonomic

composition of zooplankton and micronekton within the study area. However, due to the finite

time available for sample collection and post-cruise analysis, these net measurements were

necessarily limited in number and spatial coverage. Therefore, as a supplement to the net

sampling program, acoustic measurements of volume backscattering strength (Sw) were made

using an Acoustic Doppler Current Profiler (ADCP). ADCPs have been used as a method for

making indirect measurements of zooplankton and micronekton biomass continuously while the

vessel is underway (Ashjian et al. 1994; Zhou et al. 1994; Zimmerman and Biggs 1999). When

both direct and indirect measurements of zooplankton and micronekton are taken over a wide

geographic or hydrographic range, differences in biomass can be interpreted as differences in the

amount of potential food for higher trophic levels, much the same as regional phytoplankton

mapping is used to infer differences in zooplankton biomass. For the GulfCet II Program, the

biological oceanographic sampling with the nets and the ADCP was used to test the hypothesis

that different hydrographic regimes in the study area have different levels of available prey items,

and these prey items influence the distributions of cetaceans and seabirds.

Measurements of zooplankton and micronekton biomass derived from both net and acoustic

sampling were used to provide a summary of the amount of potential food available for higher

trophic level foraging. In addition, the abundance of cephalopods and myctophids, two

important prey items found abundantly in a wide variety of cetacean arid seabird stomachs(Fitch

and Brownell 1968; Perrin et al. 1973; Clarke 1996; Croxall and Prince 1996), was used to

explore the hypothesized link between higher zooplankton biomass levels and richer cetacean

prey resources.

Cephalopods are oceanic fauna that range in size from planktonic to some of the largest nekton

(Architeuthis) (Roper et al. 1984). Both juvenile and adult cephalopods are voracious predators

and sometimes are found in abundance. They are, in turn, preyed upon by many marine

mammals, marine fish, and seabirds (Clarke 1977, 1996; Croxall and Prince 1996). Therefore,

cephalopod distribution and abundance may influence the distribution and abundance of their

predators. However, adult cephalopods, due to their agility and keen eyesight, are extremely

difficult to catch. Consequently, "paralarval" cephalopods were used in this study as the link to

adult cephalopods. A paralarval squid is defined by Young and Harman (1989) as a "cephalopod

of the first post-hatching growth stage that is pelagic in near-surface waters during the day and

that has a distinctly different mode-of-life from that of older conspecific individuals".

69
Among small midwater fishes, myctophids (or lantemfish) dominate the fish remains found in

cetacean stomachs, often comprising 89% or higher of the total otoliths (ear bones) found (Fitch

and Brownell 1968). Because of their worldwide abundance and abundance in the net samples

collected, myctophids were chosen to represent the influence of midwater fishes on cetacean prey

distributions in GulfCet II. There are 32 genera of myctophids, including 17 in the Gulf of

Mexico, which occur in a variety of habitats ranging from the open ocean to the continental shelf

(McEachran and Fechhelm 1998). Almost all species of myctophids are vertical migrators, with

the myctophids in the eastern Gulf of Mexico concentrating in the upper 150 m at night and from

300 to 900 m during the day (Gartner et al. 1987). Myctophids are an important food source for

cephalopods, sea birds, game fish, and cetaceans. Predator fish include salmon, cod, redfish,

tuna, and swordfish (Nafaktitis et al. 1977). Cetacean species with documented evidence of

myctophid remains in their stomachs include dwarf spenn whales, spinner dolphins, paiitropical

spotted dolphins, striped dolphins, clymene dolphins, and Biyde's whales (Fitch and Brownell

1968; Perrin et al. 1973; Perrin and Gilpatrick 1994; Perrin and HoIm 1994; Perrin et al. 1994;

Perrin and Mead 1994; Kawaguchi and Kawarnura 1981). All of these cetacean species have

been observed during GulfCet I andlor GulfCet II. While the presence of myctophid otoliths in

cetacean stomachs does suggest the whales are eating these fish, Fitch and Brownell (1968)

stipulate that at least some of the otoliths could be present because they were first ingested by

cephalopods. Regardless of whether myctophids are being eaten by cetaceans directly or

secondarily through cephalopods, a high abundance of myctophids may indicate a preferred

foraging region for cetaceans.

ADCPs are normally used by physical oceanographers to measure the velocity of ocean currents.

The ADCP transmits a sound pulse into the water, and the Doppler-shifted sound scattered back

from drifting particles in the water is used to obtain estimates of current velocities. However, the

ADCP also measures the intensity of backscattered sound. Since this intensity is proportional to

the number and backscattering cross sections of particles in an ensonified volume of water (Clay

and Medwin 1977; Medwin and Clay 1998), ADCP measurements using sound frequencies of

153 and 307 kHz have been used to estimate the concentration of sound scattering particles such

as zooplankton and micronekton (Flagg and Smith 1989; Roe and Griffiths 1993; Ashjian et al.

1994; Zhou et al. 1994; Griffiths and Diaz 1996; Ressler et al. 1998). Under typical open ocean

conditions, the particles responsible for the backscattering at these frequencies are likely to be

zooplankton and micronekton (Stanton et al. 1994; Wiebe et al. 1997; Medwin and Clay 1998).

Therefore, with a calibrated measure of S (acoustic volume backscattering strength) obtained

from an ADCP and intensive "sea-truth" sampling of the numbers and kinds of potential soundscattering organisms, quantitative estimates of the standing stocks of zooplankton and

micronekton in the ocean can be made. Acoustic methods are currently recognized as an

important way of studying zooplankton and micronekton (Greene and Wiebe 1990; Wiebe et al.

1997), and there is precedent for their use in assessment of zooplankton and micronekton stocks

in cetacean habitat studies (Macaulay et al. 1995; Beardsley et al. 1996; Croll et al. 1998). In

fact, in the eastern Pacific, Fiedler et al. (1998a) recently described the use of a 153 kHz ADCP

to examine spatial and temporal variability in the biomass of potential dolphin prey stocks

consisting of zooplankton, micronekton, and squid.

70
3.2 Methods

3.2.1 Net Sampling

The samples for both zooplankton and micronekton were taken on the October 1996 and August

1997 RIV Gyre cruises (Gyre 96G6 and Gyre 97G8). Three types of sampling equipment were

used. The 1 m2 Multiple Opening/Closing Net and Environmental Sampling System

(MOCNESS) with a mesh size of 333 im was used on Leg II of the October 1996 cruise and on

all of the August 1997 cruise. The MOCNESS is commonly used in several sizes, which refer to

the net's vertical mouth area when towing at a 45° angle to the vertical. The pertinent

characteristics of the MOCNESS system are: (1) it allows up to nine discrete samples to be

collected during one tow, sampling either obliquely or horizontally; (2) it collects water

temperature, depth and salinity data and displays it in real time to allow changes in sampling

strategy during the course of a tow; and (3) it monitors net angle and computes volume filtered

for each individual net (Wiebe et al. 1976). During sampling in October 1996, the first net of the

MOCNESS was used to collect an oblique sample during all tows; in August 1997 the

MOCNESS was fished during descent, yielding nine, depth-discrete samples. The 1 m2

MOCNESS was towed at speeds of 1.5 - 2.0 knots.

On Leg I of the October 1996 cruise, a 4 m2 version of the MOCNESS with a mesh size of 3 mm

was used, allowing collection of seven discrete samples also at a towing speed of 1.5 2.0 knots.

In May 1997, a 15' Isaacs Kidd Midwater Trawl (IKMT) with a mouth opening of.14.7 m2 and a

mesh size of 4 mm was tested. The IK.MT, which collects only one sample, was towed

obliquely. Volume was recorded by a flowmeter suspended in the mouth of the net. Maximum

depth of tow was determined by the wire length and wire angle method. Unlike the MOCNESS

nets, the IKMT was towed at 4.0 - 5.0 knots. The advantage ofthis faster towing speed is a

reduction in the effects of net avoidance by more actively swimming organisms (e.g.,

cephalopods and myctophids). The catches with the IKMT were much larger than those from the

4 m2 MOCNESS. Therefore, despite the loss of depth-discrete data, the IKMT was used instead

of the 4 m2 MOCNESS on the August 1997 cruise. Depth-discrete data for the zooplankton

biomass and cephalopod paralarvae was obtained from the 1 m2 MOCNESS, which was always

towed immediately before or after each IKMT tow.

Locations for all tows depended on the ship's location following daylight cetacean observations.

As a result, sufficient sampling for most statistical procedures in different environmental features

was difficult to obtain. The environment of each tow was characterized by its temperature and

salinity profile as determined by sensors on the MOCNESS, by XBT (expendable

bathythermograph) and/or CTD (Conductivity-TemperatureDepth) sampling during, before, or

after each tow, or by sea surface topography derived from the hydrographic data.

3.2.2 Zooplankton

Samples were preserved in 10% buffered formalin on the ship. On the October 1996 cruise, 199

samples were collected (171 1 m2 MOCNBSS samples and 28 4m2 MOCNESS samples). On

the August 1997 cruise, 177 samples were collected (162 1 m2 MOCNESS samples arid 15

IKMT samples). After a minimum of two weeks preservation, the displacement volumes were

71
determined in the 1 m2 MOCNBSS samples in the following manner: Draining the sample of

formalin, rinsing it into a graduated cylinder, using distilled water to raise the sample volume to

a standard volume, draining it through a concentrating funnel, and shaking the contents of the

concentrating funnel into another graduated cylinder to remove interstitial water. The difference

between the initial and final volume is the displacement volume. Displacement volumes were

determined for all samples. This process took approximately 30 minutes per sample.

Displacement values were divided by the volume of water filtered for each net to give a biomass

per volume expressed in cc m3. When the displacement volume for each tow had been

determined, the interval biomass values were multiplied by the interval thickness and summed to

give integrated biomass (cc ma), a quantitative measure of the zooplankton biomass under one

square meter of sea surface. Due to large differences in mesh size from the 1 m2 MOCNESS,

biomass values from the 4 m2 MOCNESS and the IKMT are not comparable and therefore were

not calculated.

3.2.3 Cephalopod Paratarvae

After the displacement volumes were determined, all samples (1 m2 MOCNESS, 4 m2

MOCNESS, and 15' IKMT) were sorted for paralarval cephalopods. There was a very good

linear relationship between the displacement volume of a sample and the sorting time for

cephalopod paralarvae (r2 = 0.95). For GulfCet II, on average, 25 minutes of sorting were spent

per sample. The cephalopods were then identified to the family taxonomic level. To calculate

number of paralarvae per m2 for the MOCNESS, the number of cephalopods per sample was

divided by the volume of water filtered per net. When each sample for each tow was complete,

the interval concentration values were multiplied by the interval thickness and summed to give

integrated paralarvae or number of paralarvae per m2. However, for the IKMT, the total

paralarvae per tow were divided by the volume of water filtered, and this number was then

multiplied by the maximum depth of tow to give the number of paralarvae per m2. These are all

quantitative measures of the number of paralarvae under one square meter of sea surface.

3.2.4 Myctophids

While myctophids were present in some of the 1 m7 MOCNESS samples, they were smaller in

size and much lower in number than in the IKMT tows. They were not captured at all in many of

the 1 m2 MOCNESS tows. This may have been due to the ability of myctophids to avoid the

smaller, slower MOCNESS, as well as due to the much smaller volume of water filtered by each

1 m2 MOCNESS net relative to the I[KMT. The IKMT tows contained larger and more plentiful

myctophids, and thus were given first priority for analysis. Three of the IKMT tows were chosen

for an initial description of myctophid abundance and generic diversity. Each represented a

specific feature: IKMT#9 from the cyclone margin (NW), IKMT#1 1 from the anticyclone, and

IKMT#7 from the confluence. These three trawls were taken at similar time periods (late

evening) over a two day period. This reduced the possibility of variation due to temporal factors.

The maximum depths of these trawls were similar (153 m, 141 m, and 221 m, respectively). The

number of myctophids was divided by the volume and then multiplied by the maximum depth of

each tow to calculate the number of myctophids per m2. The myctophids were identified to

genus by the location of photophores and other luminous tissue on their bodies.

72
3.2.5 ADCP

A 153 kHz nanowband Rowe Deines Instruments (RDI) ADCP was used to collect acoustic

backscatter data during both cruises. The ADCP was installed in a 'moon-pool' in Gyre's hull,

with its four acoustic transducers facing downward from the bottom of the ship in a concave

configuration. Backscatter data were collected continuously except during data backup. The

signals from all four beams were averaged. These averages were converted from the 'echo

intensity' units, recorded by the ADCP 's automatic gain control (AGC) circuitry, into a

calibrated measure of volume backscattering strength (Sw), using measured system calibration

values and hydrographic parameters affecting the speed and absorption of sound in seawater.

(See R.D. Instruments (1990) and Zimmerman (1997) for details of this procedure.) S was

analyzed over a depth range of 10 - 202 m: 10 m is the upper limit of the data collected, while

202 m was used as the lower limit because the signal to noise ratio decreases appreciably below

this depth. S, was measured during day and night, both while on-station and while underway,

enabling an examination of both temporal and spatial trends in acoustic backscatter.

Biomass estimates of zooplankton arid micronekton were produced using an empirical calibration

of the acoustic signal from the ADCP with samples of zooplankton and micronekton taken from

the 1 m2 MOCNESS (Flagg and Smith 1989; Ashjian et al. 1994; Zhou et al. 1994; Zimmerman

1997; Ressler et al. 1998). For each 1 m2 MOCNESS tow, the mean S., measured at a given

depth during the tow was matched with the measured displacement volume biomass from the

corresponding depth-discrete net sample. Since S., was analyzed from 10 - 50 m (approximately

equal to the mixed layer depth) and 10-202 m (maximum depth range), only MOCNESS samples

from the depth range 10-202 m were included in the calibration. Linear regression of the log10 of

MOCNESS displacement volume biomass (in cc rn * l0) as a function of mean 5., (in

dB re m 47(1) was used as a first-order empirical model of Predicted Mean Biomass (PMB) in

units of cc m3.

During both October 1996 and August 1997, there was a positive fimctional relationship between

biomass and Si,. However, statistical testing of the two regression models indicated that they

were significantly different (see Section 3.4.4). Hence, a different regression equation was used

for each of the Gyre cruises to make predictions of PMB. PMB values (based on S data

averaged every 300 seconds) have a horizontal resolution of 0.3 - 1.5 km, depending on ship

speed, and a vertical resolution of 4 m.

To provide a summary of the biomass in the water column to compare with cetacean or seabird

sightings at a given location, the PMB estimates (cc m3) were vertically integrated (in the same

manner as the 1 m2 MOCNESS displacement volume measurements were integrated, as

described earlier) from 10 - 50 m depth, giving a summary PMB value in units of cc m2 at each

location where an acoustic measurement was made.

The integrated PMB data set was further filtered as follows:

(1) PMH data containing on-station artifacts (anomalously low returns while the ship

maneuvered to hold station during CTD casts), or data collected from locations with

73
74

bottom depths shallower than 200 m (there were no shallow water 1 m2 MOCNESS

calibration samples) were eliminated.

PMB data were separated by night and day before being analyzed for spatial variation or

compared with cetacean or bird sightings, because biomass sampled by the MOCNESS

andlor the ADCP in the upper 200 m is always higher at night than during the day

because of the diel vertical migration of many zooplankton and micronekton. This

phenomenon is common to all phyla of both marine and freshwater zooplankton (Huntley

1985) and is documented for zooplankton, micronekton (Hopkins 1982) and adult squid

(Passarella and Hopkins 1991) in the Gulf of Mexico. However, the diel migration

pattern differs by particular organism, region, and season and therefore is not easily

modeled. To account for the diel pattern in the habitat association analysis, the PMB data

were split into three divisions based on the diel pattern in each cruise. 'Migration' was

defined as the time period during which an apparent upward or downward migration of

high backscatter was seen (which occurred regularly at dawn and dusk). 'Day' was

defined as the time period after the dawn migration but before the dusk migration, and

'Night' was defined as the period between dusk migration and the dawn migration.

Data were separated for each cruise to account for the temporal trends as follows:

October 1996

Day (10.3 hours)-- 0705 1725 CDT (Central Daylight Time)

Migration (1.5 hours)-- 1730 - 1900 CDT

Night (10.8 hours)-- 1905 - 0555 CDT

Migration (1.0 hours)-- 0600 - 0700 CDT

August 1997

Day (11.0 hours)-- 0730- 1830 CDT

Migration (2.3 hours)-- 1835 - 2055 CDT

Night (8.0 hours)-- 2100 - 0500 CDT

Migration (2.3 hours)-- 0505 - 0725 CDT

There is one additional caveat regarding the October 1996 data set. Due to strong breezes

and whitecapped, 8 - 10 ft. seas during the early part of this cruise (morning of 10/12

through morning of 10/14), the ADCP data collected during that period have a lower

signal to noise ratio due to these rough conditions. None of the data used to generate the

empirical model used for that cruise came from this period, nor were there any sightings

of cetaceans or birds.

Ninety-five percent confidence intervals (CI) and 95% prediction intervals (PT) were calculated

for the predictions of biomass generated from the regression. PMB values were then compared

with values of integrated displacement volume biomass from 1 m2 MOCNESS samples to assess

their agreement. In addition, PMB was plotted as a function of time to provide a qualitative

depiction of temporal trends in biomass, and then as a function of 15°C depth (a proxy for

hydrographic regime; see Chapter 2) to visualize spatial patterns only. Also, interpolated contour

maps of daytime and nighttime PMB in the 10 - 50 m depth range were made to show spatial
variation in PMB with respect to geographic location and hydrographic features for each cruise.

Finally, these PMB data were used as a variable in the statistical analysis of cetacean and seabird

abundance and distribution patterns (see Chapters 6 and 8).

3.3 Results

3.3.1 Zooplankton

The data on tow number, location, depth, biomass, number of cephalopod paralarvae, and

environment are shown in Table 3.1. Figures 3.1 and 3.2 show the locations and integrated

zooplankton biomass for the 1 m2 MOCNESS tows during the October 1996 cruise and the

August 1997 cruise, respectively. Figures 3.3 and 3.4 show locations for the 4 m2 MOCNESS

tows during the October 1996 cruise and the IKMT tows during the August 1997 cruise. All

locations are superimposed upon sea surface dynamic height anomalies. A summary of

zooplankton biomass in all environmental categories by year is shown in Figure 3.5. The

zooplankton biomass in the anticyclone for October 1996 is significantly different from that for

either the cyclone or cyclone margin, but not for the confluence tows. Zooplankton biomass in

the confluence for August 1997 is significantly different from the cyclone margin and the

anticyclone but not the anticyclone margin. A summary of zooplankton biomass by those

environments having two or more observations for both cruises is shown in Figure 3.6. This

figure shows that in all possible comparisons, the August 1997 values were significantly higher

than those from October 1996.

3.3.2 Cephalopod Paralarvae

The five most abundant families obtained from the 1 m2 MOCNESS and IKIvIT tows (the 4 m2

MOCNESS samples lacked cephalopods) were: Enoploteuthidae, Ommastrephidae,

Pyroteuthidae, Cranchiidae, and Onychoteuthidae. Within both the 1 m2 MOCNESS samples

and the IKMT tows, the family Enoploteuthidae was by far the most abundant. However, the

rank order of the additional four families varied slightly. These five families constituted

approximately 95% of cephalopods collected (a combined total of 1673 cephalopods) and were

the only families used in calculations (Table 3.1). The 1 m2 MOCNESS samples also revealed

that the paralarvae were generally concentrated in the upper 75 - 150 m (See Appendix for Data).

The integrated water column values for the number of paralarvae per m2 are plotted in Figures 3.7

- 3.9. A comparison of all 1 m2 MOCNESS samples from the different environmental regimes

having two or more observations for both cruises shows there are no statistical differences among

tows during a given cruise (Figure 3.10). Nevertheless, it can be seen that the numbers of

paralarvae per square meter were, on average, three times higher during August 1997 when

compared with October 1996 (Figure 3.11).

In addition, a comparison of all 1 m2 MOCNESS tows from both cruises (n = 36 from 333

samples) was made using cephalopod paralarvae (# m2) and zooplankton biomass (cc m2). The

Spearman Rank Correlation, a nonparametric test which uses only ranks and therefore is less

sensitive to outliers, was chosen. The correlation coefficient was 0.73 (p < 0.00 1). The positive

75
Table 3.1. Summary of 1 m2 MOCNESS tows, 4 m2 MOCNESS tows and 15' IKJVIT tows. Tow # Date Time Position Integrated Integrated Depth (local) Lat (N) Biomass Paralarvae of Depth of 

Isotherm 

Long (W) (cc in2) (# m2) Tow (rn) MOC1-180 10/20/96 1441-1803 28.71 88.81 3.2 0.91 111 178 

MOC1-181 10/20/96 2036-2224 28.71 88.83 4.3 0.85 121 181 MOC1-182 10/21/96 1324-1540 27.05 88.02 4.7 0.50 190 147 MOC1-183 10/21/96 2108-2330 26.23 87.70 5.5 1.05 190 188 MOCI-184 10/22/96 0350-0543 25.65 87.53 2.7 0.57 189 267 MOC1-185 10/22/96 2000-2305 25.19 87.57 1.6 0.04 193 333 MOC1-186 10/23/96 0431-0539 26.00 88.00 2.7 0.09 193 212 MOC1-187 10/23/96 1635-1812 27.10 88.56 3.9 0.42 187 164 MOC1-188 10/23/96 2015-2200 27.30 88.68 4.1 0.53 183 156 MOC1-189 10/24/96 2024-2239 27.23 89.07 5 0.21 385 168 MOC1-190 10/25/96 0336-0508 26.66 88.89 6.5 0.87 385 172 MOC1-191 10/26/96 1353-1523 26.99 87.72 4.3 0.39 393 146 MOC1-192 10/26/96 1944-2121 27.38 87.89 4.4 0.34 393 134 MOC1-193 10/27/96 0350-0440 28.08 88.16 4.3 0.26 191 154 MOC1-194 10/27/96 1950-2050 28.25 88.90 3.9 0.17 183 166 MOC1-195 10/28/96 2353-0109 28.21 89.08 4.7 0.19 181 171 MOC1-196 10/28/96 0347-0440 28.44 89.17 4.3 0.92 187 179 MOC1-197 10/28/96 1300-1430 28.01 89.17 5.3 1.53 190 168 MOC1-198 10/28/96 1618-1702 28.19 89.27 3.5 0.32 189 176 MOC1 -208 8/6/97 2108-2150 26.89 88.40 5.6 1.01 200 351 MOC1-209 8/6/97 2345-0033 26.82 88.37 6.8 5.27 200 393 MOC1-210 8/8/97 0019-0101 27.72 88.02 13 3.67 200 234 MOC1-21 1 8/8/97 1821-1907 28.98 87.93 6.6 1.26 209 192 
Tow # Date Time (local) Position Integrated Integrated Depth Lat (N) Biomass Paralarvae of Long (W) (cc m2) (# m2) Tow (m) Depth of 

Isotherm MOC1-212 8/8/ 97 22 17-233 8 28.63 87.79 6.3 0.73 175 173 

MOC1-213 8/9/97 0123-02l6 28.64 87.80 12.4 1.95 150 148 MOC1-214 8/9/97 164 1-1747 26.96 86.96 6 0.69 200 308 MOCI-215 8/9/97 2006-2128 26.91 86.91 8.9 1.31 200 318 MOC1-216 8/9/97 2317-0031 26.87 86.79 8.8 1.86 200 294 MOC1-217 8/10/97 2306-0028 29.18 86.91 8.9 0.84 200 162 MOC1-218 8/11/97 0158-0317 29.08 86.94 8.6 1.63 175 162 MOC1-219 8/11/97 1653-1836 27.61 87.29 12.5 1.26 400 219 MOC 1-220 8/14/97 2045-2216 28.87 87.56 7.2 0.54 200 183 MOCI-221 8/15/97 0011-0125 28.94 87.45 8.3 1.42 200 183 MOCI -222 8/17/97 0125-0222 28.80 86.63 10.1 1.18 200 171 

MOC1 -223 8/17/97 0420-0514 28.69 86.65 8.6 1.31 200 180 MOC1 -224 8/20/97 0458-0726 27.48 87.05 NA NA 150 187 MOC 1-225 8/21/97 0 150-0241 28.73 88.62 9.4 0.75 200 166 MOC 4-i 10/12/96 0036-0210 29.12 87.35 0.98 ND 400 135 MOC 4-2 10/16/96 2258-0051 29.12 87.39 0.25 ND 400 197 MOC 4-3 10/17/96 0220-0408 29.06 87.36 0.54 ND 400 179 MOC 4-4 10/17/96 1903-2110 27.55 87.44 0.99 ND 400 132 IKMT#5 8/6/97 0505-0600 28.66 89 ND 0.181 212 181 IKMT#6 8/6/97 2234-2330 26.84 88.38 ND 0.023 212 393 IKMT#7 8/7/97 2300-0000 27.7 88.05 ND 0.089 221 234 IKMT#8 8/8/97 0157-0300 27.68 88.02 ND 0.115 229 205 IKMT#9 8/8/97 2358-0 106 28.63 87.83 ND 0.091 153 173 

IKMT#1 0 8/9/97 1801-1900 26.87 86.94 ND 0.071 212 308 

Table 3.1. (continued) Summary of I m2 MOCNESS tows, 4 m2 MOCNESS tows and 15' IKMT t
Table 3.1. (continued) Summary of 1 rn2 MOCNESS tows, 4m2 MOCNESS tows and 15' IKMT 

Tow # Date Time (local) Position Integrated Integrated Depth Depth o

Lat (N) Biomass Paralarvae of Isothen

Long (W) (cc m2) (# rn2) Tow (rn) IKMT#1 1 8/9/97 2147-2300 26.85 86.79 ND 0.048 141 294 

IKMT#12 8/10/97 1601-1647 28.39 86.85 ND 0.000 225 134 IKMT#1 3 IKMT#14 8/11/97 8/12/97 0046-0146 0150-0410 29.13 26.82 86.91 ND 87.4 ND 0.121 0.008 225 153 162 389 

IKMT#1 5 IKMT#1 6 IKMT 17 8/14/97 8/17/97 8/18/97 2229-2352 0253-0410 0330-0523 28.92 28.74 28.67 87.53 ND 86.64 ND 85.48 ND 0.013 0.020 0.023 106 141 141 183 171 193 IKMT# 18 8 /2 0/9 7 0023-0225 27.42 86.99 ND 0.080 354 187 IKMT#1 9 8/21/97 0308-0440 28.76 88.67 ND 0.062 254 166 
3e

28

26

3000 m

I I I

--

I -' ; I- - / -

1 0

20

,0

20

40

40.--

I- --

,0001

Longitude

Figure 3.1. Integrated zooplankton biomass (cc m2) from 1m2 MOCNESS samples taken

during late summer 1996. The diameter of each circle is directly proportional to its

integrated biomass.

0

-90 -88 -86 -84
Longitude

Figure 3.2. Integrated zooplankton biomass (Cc m2) from 1m2 MOCNESS samples taken

during mid-summer 1997. The diameter of each circle is directly proportional to its

integrated biomass.
81

-90 -88 -86

Longitude

Figure 3.3. Tow locations for the 4m2 MOCNESS samples taken during late summer 1996.

-84
82

-90 -88 -86 -84

Longitude

Figure 3.4. Tow locations for the 15' Isaacs Kidd Midwater Trawl tows taken during

mid-summer 1997.
0C) 1210 - 

Environmental Regime and Year Figure 3.6. A within feature comparison of integrated zooplankton biomass (cc m2) between l

summer- 1997. The vertical bars represent the mean values and the brackets repre

confidence limits
lOOm

QQip w .

S - '0

28

_ # 1::\

çc '-. :%j ;/

0

oo 0

26

30

20

/'iOO-'_

(i 29.

--'-S

85

0

20

-90 -88 -86 -84

Longitude

Figure 3.7. Integrated cephalopod paralarvae abundance (if rn2) from 1 in2 MOCNESS

samples taken during late summer 1996. The diameter of each circle is directly

proportional to its integrated abundance.
Longitude

Figure 3.9. Integrated cephalopod paralarvae abundance (# m2) from the 15' Isaacs

Kidd Midwater Trawl tows taken during mid-summer 1997. The diameter of each

circle is directly proportional to its integrated abundance.

87
2.5 2.0 1.5 1.0 0.5 0.0 Figure 3.11. Integrated cephalopod paralarvae (# m2) compared between years with all environ

The vertical bars represent the mean values and the brackets represent theupper an

Late Summer 1996 Mid-
Table 3.2. Myctophid abundance (# m2) by genera within analyzed 15 Isaacs Kidd

Midwater Trawl tows taken during mid-summer 1997.

91

Genus (# m2) Genus (# m2) Genus (# m2)

Diaphus 0.212 Ceratoscopelus 0.161 Diaphus 0.061

Lampanyctus 0.157 Diaphus 0.110 Benthosema 0.037

Ceratoscopelus 0.116 Lampanyctus 0.079 Lepidophanes 0.026

Benthosema 0.09 1 Benthosema 0.055 Diogenichthys 0.013

Lepidophanes 0.071 Notolynchus 0.051 Ceratoscopelus 0.006

Hygophum 0.052 Lepidophanes 0.027 Bolinichthys 0.003

Diogenichthys 0.023 Notoscopelus 0.024 Myctophum 0.002

Bolinichthys 0.020 Diogeniehthys 0.0 14 Hygophum 0.002

Myctophum 0.011 Myctophum 0.014

Notoscopelus 0.009 Lampadena 0.007

Notolynchus 0.007 Bolinichthys 0.003

Lampadena 0.007 Hygophum 0.003

Lobianchia 0.002 Lobianchia 0.003

Symbolophorus 0.003

Unidentified 0.034 Unidentified 0.017 Unidentified 0.002

TOTAL 0.813 TOTAL 0573 TOTAL 0.151

13 Genera 14 Genera 08 Genera

CYCLONE MARGTh CONFLUENCE ANTICYCLONE

(IKMT#9; n 357) (IKMT#7; n = 167) (IKMT#11;n=94)

Volume: 67,203 m3 Volume: 64,404 m3 Volume: 87,733 m3

Max. Depth: 153 m Max. Depth: 221 m Max. Depth: 141 m
Longitude

Figure 3.12. Integrated myctophid abundance (# m2) from the 15' Isaacs Kidd Midwater

Trawl tows taken during mid-summer 1997. The diameter of each circle is

directly proportional to its integrated abundance.

92
45.0 40.0 - 35.0 - 30.0 - 25.0 20.0 15.0 - 10.0 - 5.0 - 0.0 BenthosemaBolinichthysCeratoscopelus Diaphus Diogenichthys Hygophum 

Environmental Regime and Myctophid Genera Figure 3.13. Relative abundance of common myctophid genera by percentage of total in three 

Trawls taken during mid-summer 1997 (data from Table 3.2).
c 0000 V24) !(VZI oo MOC 181 2Lfl2 - 2lZ 0600 1200 1800 0000 Local Tinic (C1)1) w21 1-2 WI1.J% (ILl I tUII OO nkecLuI?.z MOCI-J2 i3 il4-I4 21UI2Uf T T '-1 -'----1--i'''lII 12

0-0 00 11CJC.1.184 CTh.

oils - (l5ii 1U1 - I -92 -37 -*2 -77 -Th 47 -2 S, (dB i-c rn1 4k-i) Figui'c 3,14. FaLse color imuiing plot of S eotJeccd akny a north south ti'ansict lile I'tonu the MOM 

thiu,h the cyclone, and into Looj Cuneut tddv C dunru Lb late surmrir 19% cruise. U'shown cm the y-xi. while both 1mc -rnd locution arc chimgin aIon thc x-axis Pcriods ocorresponding to 1 & MOCNESS tows and othcr cvcnts ot inteiest are indicated below the x1ds and yellows on the plot mdicate higher S. winle blue atii black eolnr indkate less rrturn.' Ntite Ihat hih a Ir11I1xnt ignal (hi1ier S aL iiighl than in tl daytime; und ilso a 

I : hi wr S,, in ihc tyc'liinr ihin in ihc anlie clonic I-Jdy C) wc cvidcnt Mi2bt, ihtq I) NhE. C3rrI..i Ua. C3cIi.r 0011 SI F UI C iddy C Eddy C
12

10

8

6

4

2

0

2.5

2.0 - 1.5 - 1.0

0.5 - 0.0

92 -87 -82 -77 -72 -67 -62

Mean S,, (dB re rn-1 4n')

Figure 3.15. Scatterplot of the late summer 1996 MOCNESS displacement volume

biomass as a function of S. The regression was described by the

equationy=0.0587x+ 5.7692(r2=0.5871, n= 91). Dotted lines

indicate 95% confidence bands.

1:1

prediction overestimates

prediction underestimates

95

0 2 4 6 8 10 12

Integrated PMB (cc rn2)

Figure 3.16. Integrated MOCNESS biomass and integrated predicted mean biomass

(PMB) from late summer 1996 are compared for matching locations and

depth intervals. The error bars are 95% prediction intervals.
fit (Flagg and Smith 1989; Ashjian et al. 1994). Since the model met the required statistical

assumptions of normal residuals and homogeneity of variance, it was considered a valid firstorder model with which to estimate the expected mean zooplankton and micronekton biomass at

a given location. The predicted biomass estimate was denoted as PMB or "predicted mean biomass."

A plot of the integrated MOCNESS displacement volume as a function of integrated PMB values

for the October 1996 cruise shows that the regression makes reasonable predictions of the actual biomass sampled with the net (Figure 3.16). Seven of the points have 95% prediction intervals

that include the 1:1 reference line shown on the plot, and all of the other points agree within a

factor of 2. There is no clear pattern of over- or under-estimation by the regression.

Diel fluctuation due to the vertical migration of zooplankton and micronekton was apparent in a

line plot of PMB integrated from 10 - 50 m as a function of time: PMB was high at night and low

during the day (Figure 3.17). This showed that the acoustic signal has a biological origin, but spatial patterns due to environmental differences were superimposed on this temporal pattern,

making them more difficult to discern. However, when PMB was plotted as a function of the

depth of the 15°C isotherm, spatial patterns became apparent (Figure 3.18). The 15°C isotherm

was used as an index for hydrographic regime: A shallower 15°C depth indicated shoaling

isotherms and cyclonic, upwelling conditions that could cause higher biological productivity.

During day and night, areas with a relatively deep 15°C isotherm (and thus lower near-surface

nutrient levels), such as the anticyclonic LC eddies, were characterized by the lowest values of

PMB. Indeed, statistical testing revealed that integrated PMB varied significantly across the

three hydrographic regimes. Using the operational definition for the three feature classifications

based on 15°C depth given in Chapter 2 of this report, the data shown in Figure 3.18 were

defined as 'cyclone', 'confluence and other margin', or 'Loop Current Eddy C'. A nonparametric analysis of variance by ranks (Kruskall-Wallis test) indicated that integrated PMB

varied significantly across feature classification during day and night (p <0.0001).

To better visualize the spatial patterns in the biomass predictions, PMB was gridded (using

kriging) and contoured with Surfer, Version 6 (Golden Software 1997). PMB during daytime was low throughout the study area (0.2 - L2 cc m2) (Figure 3J9). PMB was lowest in the

anticyclonic eddy C (0.2 - 0.7 cc m2) and in patches at the western and eastern edges of the

cyclone; it was highest (0.7 - 1.2 cc m2) near the Mississippi delta along the continental slope, in

the center of the cyclone, and throughout the confluence between the cyclone and Eddy C.

At night (Figure 320), PMB was higher (as high as 3.2 cc m2) than during the day due to diel vertical migration. Nevertheless, many of the same spatial patterns seen during the day were

apparent at night: PMB was highest in the center of the cyclone, in areas along the continental slope and in the confluence zone. PMB was lowest in LCE-C.

3.3.4.2 August 1997

Linear regression of S, as a function of MOCNESS displacement volume biomass was also used

for the August 1997 cruise (Figure 3.21). A logarithmic transformation was applied to the MOCNESS data before the regression on S, was performed. The residuals from this regression

96
3.5W

30-

4.0

JN.D ND N N D N, DN DND ND,N D N D ND NI D

00N N N N N 00 00 00 00 00

(N

00N (N N N N N N N N N N 00 00 00 00 00 00 00 00 00 00 00

(N

00N N N (N N (N N N N (N N 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

c N N 00 00 C 0 ) (N N (N N N ( (N N 00 00 00 00 00 00 00 00 00 C) C) ON C) (NC) (N

C) (0' (0' C C) ONr 0' 0' 0' b N N 00 0' 0' 000' 0\ 0' 0\ C) N (N N (N N N N N (N (N (N (N (N (N (N (N (N (N (1 (N (N N (N N N (N C) C) C) C) C) C) C) (N (N N (N (N ( (n (

Julian Day

Figure 3.17. PMIB for the depth interval 10 - 50 mis shown as a function of time for the

late summer 1996 cruise: 1846 GMT on 11 October to 1919 GMT 01129

October [Julian days 285.78 - 303.80]. Daytime (D, white) and nighttime

(N, shaded) periods are indicated on the plot. The median 95% CI was +1-

0.01.)

Cyclone

.S.

Confluence and.

Other Margin

. o. -ci P S 0

. S

oo

Loop Current Eddy C

97

.

Night

0 Day

100 150 200 250 300 350 400 450 500

Depth of 15°C isotherm (m)

Figure 3.18. Scatterplot of PMII3 integrated from 10 - 50 m as a function of 15°C depth

at deepwater hydrographic stations of the late summer 1996 cruise. The

median 95% CI was +1- 0.01.

4,0

3.5 3.0

0.5 0.0
1

Longi I ude

0.2 0.7 1.2 1.7 2.2 2.7 3.2 3.7 4.2

Fturc 3.19. Predicted nean bim.ss PMB) ('.e m) in dc depth interval 10-50 m

dw- t! dt I we is shoii by the color contours for the tare sunmier I 99(

cruise. Ihe ver' heavy dark lines indicate thc 1oLaLioTb oF Ihe Jat.i used

to cncratc the biomass cinEurs. fold solid positivc) md boid dashed

(neatie 1ins arc sea siriiicc dinarnic heiuht anomaly (DIIA. cm)

Telattve to the 100 cm rue-art. C:4ntow t1Js re S em. The cvclene is

het%eIi 0 arid -12 cm. confluence bctwccn 0 and 22 cnt and te

tilonc between 25 and SO Cci.

98

-90 -88
99

1L2 0.7 1.2 1.7 2.2 2.7 12 33 4.2

i-;gure 3.20. Iedktett rnen hornass (PMB) (cc rn2) in the depth Lrncrvat 10-50 m

du'Hnj nighninie c shwn by ihc co1r cirnom w the la;e urnrier 199(

cruise !he very heavy iark lines hidicate the Iocatiot of th c diIa used

t cncaie thc b iiva 'utours, l3uld so1id(jxsitve) and bold dashcd

(nga*ivc) Iincs arc sca surfae dynamic heighi unormily (DHA, cm)

relative to ie LOft cm mean. Contour ntervaJs are 5 cm. Thc cyc1on is

kiwieii *nd -12cm, hc Coy lurnLT h ernOrnd 22cm. and the

anticy clone between 25 and 50 cm.

I 1

-86 -84

Lonitu.aie

24

-90 -88
were tested for normality: According to the Kolmogorov D test, the null hypothesis of normally

distributed residuals was narrowly retained for this cruise at the 0.01 level (p = 0.0 156). The

variance of the residuals was also checked and found to be approximately constant. As indicated

by the coefficient of determination, the fit of the August 1997 model (0.3748) was not as good as

that found using the data from the October 1996 cruise (0.5871), but there was still a positive

trend of increasing biomass with increasing S.,. With the same caveats mentioned during the

discussion of the October 1996 regression above, this simple linear model was accepted for firstorder approximation of the PMB at a given location for August 1997.

A comparison of PMB and integrated MOCNESS biomass for August 1997 (Figure 3.22) shows

that although the comparison appears more variable than that from October 1996, again all of the

points agree within a factor of 2 or less with the 1:1 reference line. It should be noted that

integrated PMB values were generally higher during August 1997 relative to October 1996, an

observation confirmed by the net collections. As before, there is no definite pattern of over or

under-estimation by the regression, although there are more under-estimates here than there were

on the October 1996 plot.

The diel fluctuation seen in the October 1996 data due to the vertical migration of zooplankton

and micronekton is apparent in August 1997 (Figure 3.23), again showing that the acoustic signal

is sensitive to the twice-daily migration of zooplankton and micronekton. When integrated PMB

is plotted as a function of 15°C depth (Figure 3.24), spatial patterns are evident. PMB integrated

from 10 - 50 m at deepwater hydrographic stations (as defined in Chapter 2) from August 1997

was lower when the 15°C isotherm was deep during both night and day, which is the same trend

seen in October 1996. When the data were classified by hydrographic feature ('cyclone',

'confluence and other margin', and 'Loop Current Eddy B' as described for the data in Figure

3.18), statistical testing using the Kruskall-Wallis analysis of variance by ranks again indicated

significant variation across these three feature types (p < 0.000 1 and p <0.0002 for day and

night, respectively). Gridding and contouring of the August 1997 data showed that during the

daytime, integrated PMB (Figure 3.25) ranged from 0.7 - 2.2 cc rn2, and again appeared lower in

the LCE (0.7 - 1.2 cc m2) than in the confluence and cyclone (1.2 - 1.7 cc m2) or the continental

slope (1.2 - 2.2 cc rn-2). At night, as in October of the previous year, integrated PMB was greater

than during the daytime (Figure 3.26). Finally, just as during daylight hours, predicted biomass

at night was lowest in the center of the LCE (1.2 - 1.7 cc m2) and highest in the cyclone and

confluence and along the continental shelf (1.7 - 2.7 cc m2).

3.4 Discussion

3.4.1 Zooplankton

There are obvious hydrographic and seasonal differences in integrated zooplankton biomass.

These differences are statistically significant and may have important ramifications for the

distribution and abundance of cetaceans and seabirds. The cyclone and confluence regions have

significantly higher biomass compared to the anticyclone, regardless of season. Mid-summer

values are significantly higher than late summer values, regardless of hydrographic regime

100
12

10

8

6

4

2

2.5 -

2.0 -

1.5 -

1.0 -

0.5

0

0

92 -87 -82 -77 -72 -67 -62

Mean S,, (dB re m' 4ir1)

Figure 3.21. Scatterplot of the mid-summer 1997 MOCNESS displacement

volume biomass as a function of S,,. The regression was described

by the equation y = 0.0386x + 4.4935 (r2= 0.3748, n = 134). Dotted

lines indicate 95% confidence bands.

i,i 1:1

prediction underestimates I.'.

h-1

I I prediction overestimates

2

- .

.. U.. I . . . .- .. - _ .-.- - f 'I!

...-. ..--._. 4. .1111I

III U

I U -I4 6 8

Integrated PIVIB (cc rn-2)

Figure 3.22. Integrated MOCNESS biomass and integrated predicted mean

biomass (PMB) from mid-summer 1997 are compared for matching

locations and depth intervals. The error bars are 95% prediction

intervals.

101

10 12

0.0 - .
40 D ND N D$ DN D N D

3.5

30

2.5

2.0

1.5

1.0

0.5

0.0 O 0 - N N N - N N N N N N N N N N N N N N N N N N N N N N ('1 - N N N N c' N N N N N N N N N N N N N N NN N N N N N N N N N . c r Juhan day

Figure 3.23. PMB for the depth interval 10-50 m is shown as a function of time for the

mid-summer 1997 cruise: 0946 GMT on 6 August to 0513 GMT on 22 August

[Julian days 218.41 234.21]. Day (D) and Night (N) periods are demarcated

by the dotted lines and letters. The median 95% CI was +1- 0.02.

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

100 150 200 250 300 350 400 450 500

Depth of 15°C isotherm (m)

Figure 3.24. Scatterplot of PMIB integrated from 10-50 m as a function of the 15°C

depth at deepwater hydrographic stations during the mid-summer 1997

cruise. The median 95% CI was +1- 0.02.

102

!IIi

4 '

Cyclone Confluence and

Night

o Day

Other Margin

Loop Current Eddy E

.%

00 __________________ f 0O

0 a

00 0

N D N DNDNDN D DND:N:D ND
..:._

i_ 1bc1I

21 _.:;-' ,. -9O -S6

I .oriiTide

a03

02 0.7 1.2 1.7 2.2 2.7 3.2 3.7 4.2

Fieure 3.25. Prdkicd mcmi hioms P\1 ( &) in th dpth inicrvai 10-50 rn

during daytime is shown by the eIor coitcnir thr the mid-s.uinniei 1997

cruist Fh ry hcav dark Irn indic.iic thc 1ocJUons Okhc th.Ld U.Ld tO

gct.rit 11w biomas IMOUUS, BcLi soti (posithc amd bold &shcd

(1Kgative lines ai sea surtice dynamic heht annnialy (DI IA, cm)

rcIaiic in th 105 cm mann. Contour intrvis &c 5 cm. Thc cvclonc i

bctwccn I) and - 3 crn. the confluence Itween 0 and 24 cm, and the

aifliyc1ouc- hctween 2 i.nd 71 m.

L
48

.ongitude

IC'4

-84

Iue 3.2h. Pi-dicIcd mcu bjomss (iMB) ('.. ur) iii Lhe dcpth intcri1 0-50 iii

dur-inz nightnnie is shown by the .i1t,r cinLous for thc nid-mtrncr

l$)7 cruise.. Thc vcry hcvv dark 1incs indicatc thc IcKations of data

uscd b gjt:cratc thc hicmas.s contt-iu R&d solid (pos tic mit1 brild

dashed neathe) lines uxi ca surfcc dynamic hdht oma}r (LflL\.

cin i..ive t tlw 10 cm atcan. Cntou iinea1s e 5 LITT The

cclonis 1eiween 0 and -13 m. thconFucnc heicn Ound 24 cm.

ai the a ce]onc Itwccn 25 and .71 cm.

0.1 0.7 1.2 1.7 2.7 3.2 3.7 4.2

24

90
3.4.2 Cephalopod Paralarvae

According to Clarke (1996), "...cephalopods form an appreciable part of the diet in all the

families and in all [exclusively] marine odontocete genera. . .". Specifically, 28 cephalopod

families are represented in the diet of cetaceans while cephalopods are the main food constituent

of 28 odontocetes. Clarke also found that while the ommastrephids and cranchiids are

dominantly preferred, onychoteuthids, enoploteuthids, and octopoteuthids also form a large

portion of cetacean diets. All of these cephalopod families were found within GulfCet II

samples. A cephalopod species composition study conducted by Passarella and Hopkins (1991)

in the Eastern Gulf of Mexico (in the vicinity of 27°N, 86°W) revealed that the order Teuthoidea,

specifically the families Enoploteuthidae and Cranchiidae, dominate in these waters. Results

from October 1996 and August 1997 reveal similar patterns of abundance. Furthermore, the

samples from GulfCet II illustrate that the paralarvae were concentrated in the upper 75 - 150 m

of the water column in accordance with the vertical distributions of paralarvae as reviewed by

Sweeney et al. (1992).

The most important food items for the sperm whale are squid, followed by fish (Kawakami

1980). Among squids, the numerically important families include the onychoteutbids (globally)

and both the cranchiids and ommastrephids in some regions (Clarke 1996). These three families

are among the five most abundant found in samples collected during this study. An extensive

analysis of cephalopod remains from the stomachs of sperm whales taken in the southeastern

Pacific from 1958 to 1962 showed that larger sperm whales ate larger Humbolt squid (Dosidicus

gigas) and that in areas where larger squid were present, they were eaten preferentially to smaller

ones (Clarke and Paliza, in review). Perrin et al. (1973) analyzed the stomach contents from

Stenella attenuata and ion girostris and found that the families Ommastrephidae,

Onychoteuthidae, and Enoploteuthidae were the most important food items in both volume and

number. Wolff (1982) also concluded that the family Ommastrephidae dominates in terms of

number, frequency of occurrence and weight in the stomach contents of Stenelia attenuata, while

members of the families Enoploteutbidae and Cranchiidae were also listed as prey items. These

families constituted three of the five most abundant families from samples collected during this

study. Ashmole and Ashmole (1967) analyzed the feeding ecology of seabirds and found that the

sooty tern on Christmas Island fed predominantly on Ommastrephid squid, specifically

Sympiectoteuthis. They found that the diet of the tems consisted of 62% squid and 38% fish by

volume, and 40% squid and 60% fish by numbers. Furthermore, Croxall and Prince (1996)

determined that seabirds (terns and petrels) fed on ommastrephids and onychoteuthids while

cranchiids and enoploteuthids also formed an important part of their diets. More specifically,

they found that the principal food component of petrels was the juvenile stages of cephalopods in

the ommastrephid, lycoteuthid, and onychoteuthid families, all of which were collected during

GulfCet II.

3.4.3 Myctophids

The patterns suggested by the limited myctophid data are that the cyclone margins appear to host

the highest abundance of myctophids, with the confluences having the next highest abundance

and the anticyclones having much less abundance. If these patterns are confirmed through

analysis of other IKMT tows, predator species such as cetaceans would be more likely to find the

105
visible surface fronts during the August 1997 cruise. The elevated surface chlorophyll in areas of

low salinity may indicate a phytoplankton growth response to nutrient rich Mississippi River

water that was drawn seaward into the confluence circulation of the cyclone and LCE-E.

A comparison of the contour maps of surface chlorophyll (see Figures 2.36 and 2.39) with those

of PMB (this chapter) does not support the classic inverse relationship between the two

quantities, i.e., high zooplankton biomass after phytoplankton standing stocks have been grazed

down. Instead, the August 1997 data show higher standing stocks of surface chlorophyll

spatially concurrent with higher standing stocks of zooplankton. Surface chlorophyll and

zooplankton biomass as measured in this study may not be closely coupled; some authors have

reported uncertainty and variability in the trophic interactions between zooplankton and

phytoplankton in the northern Gulf of Mexico (Strom and Strom 1996). How long these

conditions had existed before the cruise, or whether they persisted thereafter, is unknown.

However, the October 1996 data do suggest that surface phytoplankton biomass could change on

a time scale of one to two weeks. The first four deepwater transits through the cycloneanticyclone pair encountered higher surface chlorophyll stocks (0.05 - 0.10 pg L' in the

cyclone), while the last transits completed one week later found no chlorophyll concentrations

higher than 0.05 Lg L1 in either feature. Whether this was caused by grazing, sinking of the

surface phytoplankton bloom, or wind-induced mixing is unclear.

3.4.5 Acoustic rredicted Mean Biomass (PMB)

The acoustic sampling was intended to provide an along-track index of the zooplaukton and

micronekton biomass being sampled by the 1 m2 MOCNESS. Cetaceans and birds are not likely

to be feeding directly on the relatively small organisms that make up most of the biomass caught

in those samples. However, larger organisms that cetaceans or birds might take as prey would

depend upon the abundance of animals at these lower trophic levels for their food. Thus, just as

measurements of chlorophyll concentration or primary productivity might be used to evaluate

whether a habitat is rich or poor in terms of food resources that might translate up the food chain

into elevated stocks of zooplankton and micronekton, the biomass of zooplankton and

micronekton sampled by the MOCNESS and ADCP might be used to make inferences about the

potential of an area for supporting the prey of apex predators, such as cetaceans and birds.

3.4.6 Habitat Differences

During October 1996 and August 1997, there was a pronounced diel fluctuation in integrated

PMB, probably due to the vertical migration of sound-scattering organisms. Superimposed on

this were spatial patterns that were most likely due to habitat variation. When the 10 - 50 m

depth interval is considered, integrated PMB showed statistically significant variation across

environments: It was greater in the areas of shallow 15°C depth (e.g. cyclonic features) relative

to areas of deep 15°C depth (e.g. anticyclonic LC eddies) during both cruises. When the contour

plots of PMB from both cruises (day or night) are considered, cyclones in the deepwater part of

the study area appear richer in zooplankton and micronekton biomass relative to the anticyclonic

LCEs. Encouragingly, direct net sampling with the 1 m2 MOCNESS showed a similar pattern

across these environments (Figure 3.1 - 3.2, 3.5). Both lines of evidence lend support to the

hypothesis that cyclonic circulation features in the Gulf of Mexico might be areas of locally high

107
zooplankton stocks, due to increased primary production in the mixed layer supported by the

doming of nutrient rich water within them (Biggs et al. 1988). If nutrient-rich midwater is

indeed being supplied to the surface and allowing increases in phytoplankton stocks, then

zooplankton, fish, and cephalopods may become more abundant as these features persist, thus

providing greater food resources to attract higher-trophic level predators, such as cetaceans and

seabirds.

As mentioned in the comparison of the two cruises at the beginning of this section, there was an

important difference in the pattern of zooplankton and micronekton biomass between cruises.

Both the net and the acoustic sampling demonstrated that the zooplankton and micronekton

biomass was generally higher during August 1997 relative to October 1996, suggesting a

difference in the biological processes in the survey area between cruises. This difference could

simply be due to interannual variability in zooplankton stocks, or, it could reflect seasonal

changes in the zooplankton community. As previously stated, there were environmental

differences between years: August 1997 was characterized by the presence of fresher, higher

chlorophyll surface water relative to October 1996, which may have indicated the presence of

greater nutrient supply and phytoplankton production. Nevertheless, without knowing how long

those conditions existed before the cruise, only speculation is possible regarding whether this

might have supported a greater abundance of zooplankton.

3.5 Conclusions

Both integrated zooplankton biomass and integrated cephalopod paralarvae numbers in the study

area showed higher values in the cyclone and confluence as opposed to the anticyclonic Loop

Cunent Eddy during both October 1996 and August 1997. PMB estimates, derived from the

significant positive relationship between integrated zooplankton biomass (as determined by

direct net sampling and underway measurements of S, using an ADCP), also show that the

cyclone and confluence areas were enriched in integrated zooplankton and micronekton biomass

relative to the anticyclonic Loop Current Eddies. Further, a statistically significant relationship

existed between integrated zooplankton biomass and integrated cephalopod paralarvae (a major

component of cetacean prey) numbers, implying that higher zooplankton and microriekton

biomass do indicate richer concentrations of cetacean prey. Finally, the abundance and diversity

of myctophids, a second important cetacean prey group, appear to be greater in the cyclones and

confluence regions than in the anticyclones. All of these measures suggest that the amount of

prey for cetaceans and seabirds may be consistently greater in the cyclone and confluence

regimes (as opposed to anticyclonic Loop Current Eddies), making these preferential habitats for

foraging.

These observations support the analysis of cetacean distribution for both cruises (see Chapter 6),

which concluded that the presence of cetacean species or groups is correlated with the locations

of these mesoscale hydrographic features. The means by which predatory marine mammals track

these food-enriched hydrographic features as they move remains a topic for further investigation.

In addition, the analysis in Chapter 8 showed that the distribution of predatory seabirds was

influenced by mesoscale hydrography. Presumably, the birds were similarly responding to prey

abundance: The derived PMB estimates described in this chapter were found to be among the

best predictors of seabird distribution in the study area.

108
The analysis of the net sampling and acoustic data collected on these cruises is continuing.

Further investigation of the abundance and distribution of cetacean prey items is needed, and

future field experiments should improve the sampling coverage in the different mesoscale

hydrographic environments that exist in the study area. Low catch numbers of some prey items,

such as cephalopod paralarvae, suggest that larger volume net tows are preferable. However,

since sorting time is positively correlated to the amount of zooplankton biomass caught,

microscope and analysis time would increase. Acoustic sampling can be used to cover a much

larger area than that covered by net sampling and with a much lower level of effort, but net

sampling is a necessary complement in order to identiIky specific prey items and to evaluate the

potential influence of zooplankton and micronekton taxonomic composition on S measurements.

109
Chapter 4

Visual Surveys of Cetaceans and Sea Turtles

from Aircraft and Ships

Keith D. Mullin and Wayne Hoggard

4.1 Introduction

Visual surveys conducted from ships and aircraft during GulfCet I provided information to the

MMS on seasonal abundance and distribution of cetaceans and sea turtles in continental shelf

and slope waters (100-2,000 m) of the north-central and northwestern Gulf of Mexico (Hansen et

al. 1996). Nineteen species of cetaceans were identified in the GulfCet I study area. Pantropical

spotted dolphins, bottlenose dolphins, striped dolphins, melon-headed whales, clyrnene dolphins

and Atlantic spotted dolphins were all estimated to have minimum populations of more than

1,000 individuals, and all were widely distributed in the GulfCet I study area (Figure 1.2), albeit

in different water depth ranges. While less abundant, sperm whales were thought to be

ecologically important because of their large food requirements (Mullin and Hansen, in press).

Seasonal aerial surveys indicated that the abundance of some species, such as Risso's dolphins,

dwarf/pygmy sperm whales and pantropical spotted dolphins, might vary seasonally. The

leatherback sea turtle was the most common sea turtle, and it occurred in similar numbers

throughout the year. Loggerhead and Kemp's ridley sea turtles were also sighted in the GulfCet

I study area.

Because similar seasonal information on cetaceans and sea turtles was lacking for the continental

shelf and slope in the northeastern Gulf of Mexico, aerial and ship surveys were initiated under

the GulfCet II Program to complement those of GulfCet I. Abundance surveys in the

northeastern Gulf of Mexico focused on continental slope waters with bottom depths of 100-

2,000 m and a portion of the continental shelf (waters <100 m). The continental shelf

encompassed a portion of the MIMS Eastern Planning Area (EPA), including the region known as

Destin Dome, as specified by the MMS. Aerial surveys were designed to cover both the warm

and cold oceanographic seasons and were conducted during summer 1996 and 1997, and winter

1997 and 1998 (Table 1.4). Ship-based abundance and distribution surveys were conducted in

the northeastern Gulf during the spring and early summer 1996 and 1997, late summer 1996, and

mid-suninier 1997. In the spring, ship surveyswere also conducted in oceanic waters of the

entire U.S. Gulf of Mexico (waters >100 m deep) that overlapped the GulfCet I study area.

Seasons for both ship and aerial surveys were defined oceanographically as follows: summer,

June - October; fall, November - December; winter, January - mid-March; and spring, midMarch - May. Surveys were not conducted in the fall. The primary objectives of the GuliCet II

visual aerial and ship surveys were as follows:

obtain a minimum abundance estimate for each cetacean and sea turtle species in the

northeastern Gulf to establish a baseline for monitoring trends in abundance over time

study the seasonal abundance and distribution patterns of cetacean and sea turtle species in

the northeastern Gulf

111
compare spring abundance estimates of cetacean species in continental slope waters of the

north-central and northwestern Gulf (GulfCet I study area) to those made by Hansen et al. (1996) for the same area from 1991-94 data

compare ship-based spring abundance estimates of cetacean species in the oceanic northern

Gulf to those made by Hansen et al. (1995) for the same area from 1991-94 data

4.2 Study Areas

The four GulfCet II visual sampling study areas were as follows (area in parentheses):

EPA Continental Shelf (12,326 2) Located in the northeastern Gulf south of the

western Florida Panhandle in waters from 18.5 km offshore to 100 m deep between

88°10.O'W and 85°55.O'W. This area was surveyed using both aircraft and ships (Figures 4.1-4.3).

EPA Continental Slope (70,470 km2). Waters from 100-2,000 m deep east of 88°10.O'W

and north of 26°00.O'N. This area was surveyed using both aircraft and ships (Figures 4.1-4.3).

GulfCetlStudyArea (154,621 j2) U.S. waters from 100-2,000 m deep west of

87°30.OW (Figure 4.4). This area is a subset of the oceanic northern Gulf study area and

was surveyed during spring with a ship.

Oceanic Northern Gulf ofMexico (398,960 1cm2). Waters within the U.S. Economic

Exclusive Zone (EEZ) greater than 100 m deep (Figure 4.4). This area was surveyed

using a ship during spring.

Both the EPA slope and GulfCet I study areas are within the boundaries of the oceanic northern

Gulf study area.

4.3 Methods

4.3.1 Data Acquisition

For all ship surveys, two teams of three observers collected line-transect data during daylight

hours, weather permitting (i.e., no rain, Beaufort sea state <4). Observers used standard ship

survey data collection methods for cetaceans that were similar to those used during GulfCet I

(Buckland et al. 1993, Hansen et al. 1996). Each team had at least two members experienced in

shipboard cetacean observation and identification techniques. Two observers searched for

cetaceans using 25x binoculars mounted on the ship's flying bridge. The third observer maintained

a search of the area near the ship using unaided eye or 7x hand-held binoculars, and recorded data.

The observers rotated through each of these three stations every 30-40 minutes, and each team

alternated two-hour watches throughout daylight. The ship speed was usually 18 km/hr but varied

with sea conditions. Data were recorded on a computer with a global positioning system (GPS)

using a data acquisition program. Data collected included species, group-size, bearing and reticle

112
-88 -86 -84 -82

Longitude

Figure 4.1. Aerial survey transect lines in the Eastern Planning Area (EPA)

study areas (EPA Shelf, <100 m; EPA Slope, 1OO2,000 m).

-86 -84

Longitude

113

-82

Figure 4.2. Transect lines in the Eastern Planning Area surveyed from

NOAA Ship Oregon II during early summer 1996 arid

1997 (EPA Shelf, <lOOm; EPA Slope, 1 O02,000 m).
Late summer 1996

-90

Mid-summer 1997

-86

Longitude

-84

Figure 4.3. Cruise tmck of R/V GyTe during late summer 1996 and mid-summer

1997. (East of 88.00W :EPA Shelf, <lOOm; EPA Slope, lOO-2,000m).

114

-84 -82 -90 -88 -86

Longitude
3O- Figure 4.4. NOAA Ship Oregon II ichthyoplanktonfmarine mammal cruise track in the nort

(oceanic northern Gulf study area) surveyed during spring 1996 and 1997. This a day with line-transect surveys occurring during daylight. The GulfCet I study 2,000 m isobath and west of 87.5° W and north of 26.0° N. -94 -92 -90 Longitude -88 
(a measure of radial distance) of a sighting (Figure 4.5), and data on environmental conditions

(e.g., Beaufort sea state, sun position, etc.) that could affect the observers' ability to sight animals.

The reticle relative to a sighting was measured using an eyepiece with a graduated scale in the

binoculars. The bearing of a sighting relative to the transect line was measured using a 360°

graduated scale attached to the base of the binoculars. Ancillary data were also collected and

included, but were not limited to, time of day, latitude and longitude, behavior, and associated

animals. Typically, the ship was diverted from the transect line to identify species and obtain

group-size estimates. For each sighting, the final group-size estimate was the average of the

independent estimates made by individual observers.

As required by Marine Mammal Research Permits Numbers 738 and 779-1339 issued to the

Southeast Fisheries Science Center (SEFSC) under Marine Mammal Protection Act, data on the

behavioral responses of cetaceans to the survey ships or aircraft were recorded. A complete set of

these responses can be obtained from the SEFSC Pascagoula Laboratory.

During both the ship and aerial surveys, cetaceans and sea turtles were identified to the lowest

taxonomic level possible based on descriptions in field guides and scientific literature (e.g.,

Leatherwood and Reeves 1983, Carwardine 1995). The ability to make an identificationwas

dependent upon water clarity, sea state and animal behavior. Identifications to species level were

not always possible for some genera or groups of species. For example, dwarf and pygmy sperm

whales could not be distinguished from each other from aircraft and could not be reliably

distinguished from each other from ships, and were identified as dwarf'pygmy sperm whales. In

some cases, cetaceans could only be identified as large whales (>7 m long), small whales

(non-dolphin, <7 m), dolphins, odoritocetes, and turtles as unidentified chelonids.

4.3.2 Survey Design

In spring and early summer 1996 and 1997, ship surveys were conducted in three "legs" each year.

The survey platform was the NOAA Ship Oregon II, which has been used extensively since 1990

for cetacean surveys in the Gulf of Mexico (Hansen et al. 1995, Hansen et al. 1996).

Each year during spring, Legs 1 and 2 (19-2 1 days duration each) were conducted in conjunction

with SEFSC bluefin tuna ichthyoplankton sampling (performed by a separate team of scientists).

Ichthyoplankton sampling was conducted along a predetermined trackline at stations uniformly

spaced throughout the oceanic northern Gulf (Figure 4.4). This trackline was transited 24-hours a

day. Line-transect sampling was conducted while traveling between stations during daylight and

was latitudinal or longitudinal, or a combination of both.

For this study, Leg 3 (13-14 days duration) was a dedicated cetacean survey of the EPA shelf and

slope study areas during early summer. The EPA trackline was designed specifically for visual

sampling of cetaceans along transect lines nearly perpendicular to the depth gradient. Surveys

were conducted along nine transect lines that totaled 1,736 km each year (Figure 4.2). The five

western transect lines extended from 18.5 km offshore of the mainland to the 2,000 m isobath.

The four eastern transect lines extended from the 100-2,000 m isobaths. The survey stopped at

night and resumed the following morning at about the same location so that all or most of each

line was visually surveyed.

116
'LA.

Perpendicular

sighting distance

Cetacean

group \

Radial

sighting distance

Ship

Trackline

B1

....- Aircraft

Radial

sighting distance

Cetacean

group

Water surface

Perpendicular

sighting distance

Figure 4.5 Diagram of perpendicular sighting distance (PSD) and other sighting parameters for

shipboard (A) and aerial (B) surveys (b and 0 = angle between trackline and cetacean

group, h = altitude).

117
The R'V Gyre was used to conduct surveys in late summer 1996 (Gyre96006) and mid-summer

1997 (Gyre97GO8). Three areas were surveyed: the EPA shelf, the EPA slope, and a deepwater

focal area in the north-central Gulf (Figure 4.3). The focal area was chosen based on

oceanographic considerations (see Chapter 2). The surveys were conducted in two legs. The

focal area was sampled by five (1996) and seven (1997) tracklines oriented NVT-SE, south of

Louisiana and Mississippi (from 28°30.O'N, 89°00.OW to 25°00.O'N, 88°00.O'W). These

tracklines resulted in tangential, as well as radial sections through the cyclone-anticyclone pair

and the area of flow confluence between them. The EPA trackline (Fig. 4.3) was made up of

eight transect lines. The northern ends of the four western EPA lines each ran from 18.5 km

offshore to the 2,000 m isobath. The four eastern lines began at the 100 m isobath and ended at

the 2,000 m isobath. Transect lines were generally oriented perpendicular to bathymetry. In

general, the trackline was transited 24-hours per day to accommodate acoustic sampling. Linetransect visual sampling was conducted while traveling during daylight hours.

The Gulftet II aerial surveys were a continuation of aerial surveys reported by Hansen et al.

(1996) from GulfCet I. Seasonal sampling intensity in the EPA shelf and slope study areas was

similar to that expended during GulfCet I. Systematic transects with a random start that

generally crossed isobaths orthogonally were uniformly spaced throughout the aerial survey area

(Figure 4.1). This design ensured that transects were randomly situated with respect to cetacean

density and allowed examination of cetacean distribution throughout the study area. Each season

the goal was to survey 58 transect lines (spaced every 13.5 kin) totaling 6,133 km oftransect

effort. They included 42 transect lines (total of 5,220 km) on the continental slope (waters 100-

2,000 in deep) and 16 transect lines (total 913 km) on the continental shelf (waters <100 m

deep). Aerial surveys were conducted during summer 1996 and 1997, and winter 1997 and

1998. As in GulfCet I, effort was based on projected availability of acceptable survey conditions

and flight times to the study area.

A DeHavilland DHC-6 Twin-Otter with large, convex "bubble" windows on each side of the

fuselage was used as the survey platform. The bubble window provided observers an

unobstructed view of the transect line. A period of 45 days and about 100 flight hours were

allocated for each seasonal survey. Survey flights were conducted only on days with good

visibility (i.e., no rain or fog) when there were no or few whitecaps (Beaufort sea state 0-4).

Survey flights typically began at 0800 hours and lasted 6.5 hours. Surveys were conducted from

an altitude of 229 m (750 feet) at a speed of 204 kmlhour (110 knots). A pilot, co-pilot and four

observers participated in each flight. At least two observers on each flight were trained and

experienced in marine mammal aerial survey techniques. The observers were stationed at each

of the two bubble windows and at a computer (data entry) station. Observers searched waters

primarily on and near the transect line and scanned periodically out to the horizon. Only

sightings made from the bubble windows were used in the abundance estimates. To avoid

fatigue, observers rotated through stations about every 30 minutes. Pilots and observers

communicated through headsets with voice-activated microphones. Data were entered on a

computer interfaced with a UPS/LORAN-C navigation receiver using a data acquisition

program. Sighting data included species, group-size, and sighting angle or interval (for

estimating perpendicular sighting distance, PSD). Effort status, observer positions, and a suite of

data characterizing survey conditions (e.g., sea state, weather, visibility and water color) were

118
updated throughout the day whenever a change in any parameter occurred. The date, time, and

location were automatically recorded with each sighting.

The sighting angle between a cetacean group or sea turtle and the transect line was measured

with an inclinometer if the angle was less than 600. Each bubble window was also divided into

seven 10° intervals and one interval >80° corresponding to interval endpoints with PSD equal to 40, 83, 132, 192, 273, 397, 629, 1300 or >1300 m (Figure 4.5). If the inclinometer

malfunctioned or if the sighting angle was greater than 600, the interval was recorded.

When a cetacean group was sighted, the sighting angle or the interval was noted, a dye-marker

was usually dropped to mark the position, and the aircraft was diverted to circle the group.

Before continuing the transect, the species was identified and group-size was estimated by a

consensus of the three observers. The identifying characteristics of each species and any

anecdotal information were noted on a standardized form.

4.3.3 Data Analysis Techniques

Line transect methods (Buckland et al. 1993), implemented with the program DISTANCETM

(Laake et al. 1993), were used to make cetacean and sea turtle abundance estimates. For aerial

surveys, abundance estimates were calculated separately for the EPA shelf and slope study areas

as follows:

for each species for the entire study (i.e., for all four seasonal surveys combined)

for summer and winter (i.e., summer 1996 and 1997 data sets combined, and winter 1997

and 1998 data set combined) for each species with 15 or more on-effort sightings

for all species combined for the entire study, for summer and winter, and for each

seasonal survey (i.e., summer 1996, winter 1997, summer 1997, and winter 1998)

For the spring and early summer ship surveys, cetacean abundance estimates were made for each

species in the EPA shelf and slope for early summer, and the oceanic northern Gulf and GulfCet I

study areas for spring. Although the EPA slope study area is located within the boundaries of the

oceanic northern Gulf study area, only data collected during dedicated surveys of the EPA slope were used in abundance estimates for this area. The surveys were designed specifically to provide

an unbiased sample of the relatively small EPA slope study area. Use of data from the EPA slope

for oceanic northern Gulf abundance estimates would have caused biases. Also note that the

GulfCet I study area is located within the boundaries of the oceanic northern Gulf The data set

used for abundance estimates for the GulfCet I study area is a subset of the oceanic northern Gulf

data set because the saniiplirig designwas similar to that used during 199 1-94.

Data from the late summer 1996 and mid-summer 1997 Gyre surveys were not used for abundance

estimates because very little visual effort occurred in the EPA slope and shelf study areas, and the

effort that did occur was not evenly distributed in either season (Figure 4.3).

119
In each case, the formula used to estimate abundance (N) was:

N A n S f(0)

2L

where A = area of the study area

n = number of on-effort group sightings

S = mean group-size or expected group-size

f(0) = sighting probability density function at perpendicular distance zero

L = total length of transect lines sampled.

Because abundance estimates are a product of estimates, they tend to have skewed distributions.

Therefore, the lognorma1 95% confidence interval was computed for each abundance estimate.

The parameter f(0) was estimated using a hazard-rate, uniform or halfnormal model (Buckland et al.

1993, see below). The program fit the f(0) parameter using a maximum likelihood estimator with

exact sighting distances for the ship surveys and with grouped sighting distances for the aerial

surveys. Model selection of f(0) was determined using Akaike's Information Criterion (AIC, Bucklarid et al. 1993). No attempt was made to estimate the probability of sighting animals on the

transect line, g(0). The resulting estimates do not account for animals that were not sighted due to

observer error (i.e., on the transect but missed) or that may have been unavailable for sighting (i.e.,

on the transect but submerged). Therefore, estimates of abundance are negatively biased (see Discussion).

The variance of N was estimated as:

var(N) 2 var(n) var(S) N var[f(0)}1 + +

[

2

2 ]

and coefficient of variation (CV) was estimated as:

CV(N) VvarN)

The sampling unit for the ship surveys was a day's visual sighting effort, and for the aerial

surveys, a transect. For each abundance estimate, the variance estimate ofn was based on the

variation in the number of on-effort group sightings between sampling units. Since the sampling

units were of variable length, the estimated variance ofn was length weighted. The estimated

variance of S was based on the variation in group-size. The estimated variance of f(0) was based

on the variation between expected versus actual PSD distributions (see below).

In some cases, the group-sizes for some species tended to be inversely related to PSD, a feature

that results from size bias (i.e., larger groups are easier to see at distance than small groups).

Therefore, in these cases, the arithmetic mean of group-size was probably an overestimate of the

true mean group-size and could have lead to positively biased abundance estimates. A regression

N

120
of the group-size by PSD was implemented on program DISTANCETM to generate a mean

"expected group-size." The expected group-size was used in the estimate of abundance if it was

significantly smaller than the arithmetic mean group-size (p<0. 10, Student's t-test, Buckland et

al. 1993).

4.3.4 Estimation of Perpendicular Sighting Distance and f(0)

The PSD was estimated on ship surveys using bearing and reticle measurements (Figure 4.5).

Examination of the bearing and reticle measurements indicated that most were rounded to the

nearest 5 units (5 degrees for bearing, 0.5 for reticle readings). To reduce the potential for

artificial grouping of sighting distances due to rounding of measurements, the bearing and reticle

data for each sighting were smeared by adding a randomly selected value between -5 and 5 for

the bearing, and between -0.5 and 0.5 for the reticle readings. The smeared reticle readings were

converted to radial sighting distance (R) by the method of Lerczak and Hobbs (1998). PSDs

were calculated as: PSD = R sin(b), where b = smeared angle between the sighting and the

transect line.

An exploratory analysis indicated that the use of non-25x binocular sightings (about 11% of all

sightings) made at small radial distances (<0.41 kin) resulted in a spike in sighting probability

density function at PSD = 0. This spike would positively bias estimates of f(0) and N. Many of

these sightings were of animals that were probably attracted to the ship to bowride. One

requirement for unbiased estimates of abundance is that the sighting target(s) should not move in

response to the observer or the observation platform (Bumham et al. 1980, Buckland et al. 1993).

To reduce the potential for positive bias due to attraction to the ship, only sightings made at

radial distances of >0.41 km were included in the data used for estimating abundance.

The sample sizes (number of groups sighted) of most species were considered insufficient to

obtain accurate and precise estimates of f(0). Therefore, sightings of species with similar

sighting characteristics (i.e., body size, group-size, surface behavior) were pooled to estimate

f(0) for five species categories (Table 4.1). Large Whales were species with adult lengths over

7 m. Ciyptic Whales were species with lengths from 2.5 to 7 m that generally do not make

pronounced blows, are not active at the surface (i.e., create very few splashes), and occur in

small groups (e.g., <5 animals). Small Whales/Large Dolphins were species with adult lengths

over 2.5 m that are, in general, moderately active at the surface and occur in groups of about 5 to

40 animals. Small Dolphins were species with lengths <2.5 in that are very active at the surface

and typically occur in groups of>40 animals. Due to the small number of sightings of species in

the Large Whale, Cryptic Whale and Unidentified Dolphin categories during GulfCet II,

sightings from GulfCet I spring ship surveys were included to increase the sample size to

estimate f(0). The abundance for each species was estimated using the pooled f(0) and variance

for its category. By pooling, the variance, CV and confidence interval of each abundance

estimate were probably underestimated because the variance of f(0) was based on an artificially

high sample size.

For aerial surveys, the PSD from the transect line to the group was calculated as: PSD = h tan

(0), where 0 = angle between the transect line and the group, and h = altitude (229 m) (Figure

4.5). The inclinometer could measure distances from 0-60°. Therefore, for sightings from

121
Table 4.1. Estimate of the parameter f(0) for each species group from ship surveys

(n = number of sightings used for the estimate, see text; ESW effective strip width).

f(0) Cv Truncation ESW

Species Group n (-1) [f(0)] (m) (m)

Unidentified dolphins 81 0.450 0.13 5,500 4,444

122

Large Whales 71 0.467 0.12 4,600 4,282 Bryde's whale

Sperm whale

Unidentified large whale

Cryptic Whales 110 0.810 0.10 3,300 2,469 Dwarf/pygmy sperm whale

Cuvier's beaked whale

Mesoplodon spp.

Unidentified Ziphiidae

Unidentified small whale

Unidentified odontocete

Small Whales/Large Dolphins 131 0.556 0.05 5,500 3,597 False killer whale

Killer whale

Short-finned pilot whale

Bottlenose dolphin

Risso's dolphin

Atlantic spotted dolphin

T. truncatus/S. frontalis

Rough-toothed dolphin

Small Dolphins 121 0.598 0.06 5,500 3,344 Melon-headed whale

Pygmy killer whale

Peponocephala/Feresa

Pantropical spotted dolphin

Striped dolphin

Spinner dolphin

Clymene dolphin

Fraser's dolphin
607O0 and from 70-80°, the midpoint of the interval was used as the PSD and was treated as an exact distance. Sightings at angles >800 were excluded from the analysis. As in ship surveys, the number of sightings for most species was too small to obtain an accurate and precise estimate of f(0). Therefore, species with similar sightability from aircraft were pooled into four categories, and an estimate of f(0) was made for each category (Table 4.2). Because of the small number of sightings of species in the Large Whale category during GulfCet II, Large Whale

sightings from GulfCet I aerial surveys were pooled to estimate f(0) for this category. For each

species, the value of f(0) and variance for its category were used in abundance estimates. Again, by pooling, the variance, CV and confidence interval of each abundance estimate were probably underestimated. Exploratory analyses using exact PSDs and various PSD distance interval

combinations were performed to achieve a good fit of the model to the data (i.e., low x2 value and decreased CV[f(0)]. For each f(0) estimate, a model was fit to PSD data grouped into

intervals: 0-132, 133-273, 274-397, 398-629 and 630-1,300 m.

As recommended by Buckland et al. (1993) for both aerial and ship survey species categories, 5 to 10% of the largest PSDs for each category were truncated from the sighting data to improve

the fit of each model. Due to typical sighting characteristics, some species are easier to see at

large PSDs than others and the truncation distance varied by species category (Tables 4.1 and 4.2).

4.3.5 Statistical Comparison of Abundance Istimates

Forney and Barlow (1998) provide an overview of the problems associated with statistical

comparison of animal abundance estimates. Because estimates of animal abundance have

positively skewed distributions, standard t and z tests based on normal distributions are not

appropriate. A test that uses non-overlapping confidence intervals as the criteria for rejecting the null hypothesis of no difference between estimates was shown by Lo (1994) to be extremely

difficult to interpret; alpha levels and power varied unsystematically between normal, log- normal, gamma, and Poisson distributions. Lo (1994) demonstrated that a method based on the

confidence interval (CI) of the difference between means (d) was much more consistent. If the CI of d does not contain zero, the two means are significantly different at the alpha level used to construct the CI. Forney and Barlow (1998) employed this method to compare seasonal abundances of cetaceans in California waters. However, instead of calculating the analytical CI of d, they used a bootstrap method. The bootstrap method is computationafly intensive whereas the analytical CI of d can be calculated with just the abundance estimates and their respective CVs. Given that the bootstrap method is extremely new, we were not able to use it for our

comparisons, and our interest was in the analytical method.

An alpha level of 0.05 was initially used. However, it was discovered that in most cases the

differences were not significant. Since the results were generally similar to Fomey and Barlow's in terms of the range of CVs and the magnitude of the difference between means, their cetacean

abundance and CV results were used to perform the difference test with analytical CIs. Of the six cases where the bootstrap results were significant (p<0.05), the analytical method showed a

significant difference (p<0.05) in only two. In both cases the bootstrap alpha levels were less than 0.005. When the alpha was increased to 0.10, five cases were found to have statistical

significance where the bootstrap found significance at less than 0.05. Clearly, the bootstrap

123
Table 4.2. Estimate of the parameter f(0) for each species group and all sea turtles from aerial

surveys (n number of sightings used for the estimate, see text; ESW = strip width).

124

Species Group

f(0)

n (kni)

CV

[f(0)]

Truncation

(m)

ESW

(m)

Large Whales 39 1.634 0.14 1,300 1,224

Biyde's whale

Sperm whale

Unidentified large whale

Cryptic Whales 111 2.869 0.07 1,300 697

Dwarf/pygmy sperm whale

Cuvier's beaked whale

Mesoplodon spp.

Unidentified Ziphiidae

Unidentified small whale

Unidentified odontocete

Unidentified dolphin

Small Whales/Large Dolphins 211 2.580 0.09 1,300 775

False killer whale

Pygmy killer whale

Short-finned pilot whale

Bottlenose dolphin

Risso's dolphin

Atlantic spotted dolphin

T. truncatus/S. frontalis

Rough-toothed dolphin

Small Dolphins 62 2.508 0.17 1,300 797

Pantropical spotted dolphin

Striped dolphin

Spinner dolphin

Clymene dolphin

Sea turtles 148 3.579 0.11 629 55
method is the more powerful test when dealing with these data. The analytical method was used

to compare non-zero estimates at an alpha of 0.10. In cases where one of the estimates was zero

because there was no variance, only qualitative comparisons could be made.

4.3.6 Sea Turtle Density

Five species of sea turtles are known to occur in the Gulf of Mexico: leatherback, loggerhead,

Kemp's ridley, green, and hawksbill (National Research Council 1990). Sea turtle densities were

estimated from aerial survey data only using the program DISTANCETM. All sea turtle sightings

were of large, probably adult, turtles. Species were identified on the basis of shell shape and

color, and the head size relative to the overall size. Data collection procedures were essentially

the same as those for cetaceans except that sea turtles were not circled. All sea turtle sightings

were pooled to estimate a common f(0) that was then applied to each species. Analytical

methods were the same as those for cetaceans with the following exceptions: 1) data were

right-truncated at 629 m, and 2) data were grouped into intervals of 0-83, 84-192, 193-273, 274-

397 and 398-629 m. Unlike cetaceans, sea turtles do not occur in social groups; each sighting

was usually of a single turtle. If more than one turtle was sighted at a time, a PSD was measured

to each turtle. Therefore, group-size was always one, and there was no variance in group-size.

4.4 Results

To easily find results for cetaceans and sea turtles in each study area, tables are grouped by aerial

and ship survey as follows: Cetaceans, Aerial Surveys (Tables 4.3-4.7); Cetaceans, Ship Surveys

(Tables 4.8-4.13); Cetaceans, Aerial and Ship Surveys (Table 4.14); and Sea Turtles, Aerial

Surveys (Tables 4.15-4.16). The locations of each cetacean and sea turtle species are presented

in the same species order used in the tables, with a separate composite plot for aerial and ship

surveys (Figures 4.6-4.36).

4.4.1 Study Area Summaries

In the following summaries, the coefficient of variation (CV) of the mean group-size estimate

and abundance estimate is shown in parentheses. Both aerial and ship survey results were used

for each of the study area summaries

In total, 3,652 km of transects were sampled from the EPA shelf during the four seasonal aerial

surveys (Table 4.3). All of the proposed transect lines (32 lines) were completed during both

summer (1,826 km) and winter (1,826 km) aerial surveys. In total, 78 cetacean groups were

sighted (36 summer, 42 winter). During spring ship surveys, 449 km of transects were surveyed

and 46 cetacean groups were sighted. Ten and 13 cetacean groups were sighted during fall and

summer, respectively (Tables 4.8-4.9).

Three cetacean species were sighted in the EPA shelf. Bottlenose dolphins and Atlantic spotted

dolphins were sighted during all three seasons. One dwarf/pygmy sperm whale was sighted

during a summer aerial survey. There is no evidence that dwarf or pygmy sperm whales are a

regular inhabitants of continental shelf waters in the Gulf of Mexico. During aerial surveys, 61

125
Table 4.4. Cetacean species sighted and number of on-effort sightings from aerial surveys of the Eastern Planning Area (EPA) continental shelf and continental slope study areas during summer and winter 1996-98 (* one off-effort sighting).

127

EPA CONTINENTAL SHELF

Dwarf/pygmy sperm whale 1 0 1 0 0 0 1 Bottlenose dolphin 13 15 28 15 18 33 61 Atlantic spotted dolphin 2 1 3 2 4 6 9 T truncatus/S. frontalis 0 4 4 1 2 3 7 Total 16 20 36 18 24 42 78

EPA CONTiNENTAL SLOPE

Bryde's whale i 0 1 1 0 1 2

Sperm whale 3 3 6 1 3 4 10

Dwarf/pygmy sperm whale 12 6 18 2 0 2 20 Cuviefs beaked whale 1 1 2 0 1 1 3 Mesoplodon spp. 4 1 5 * 0

*

5 Pygmy killer whale 0 0 0 1 2 3 3

False killer whale 0 1 1 0 0 0 1

Short-finned pilot whale 0 0 0 0 1 1 1

Rough-toothed dolphin 0 1 1 0 0 0 1 Bottlenose dolphin 21 31 52 25 14 39 91 Risso's dolphin 7 0 7 18 6 24 31

Atlantic spotted dolphin 5 4 9 5 1 6 15

Pantropical spotted dolphin 17 17 34 8 11 19 53 Striped dolphin 1 1 2 4 2 6 8

Spinner dolphin 2 2 4 2 1 3 7

Clymene dolphin 3 0 3 0 2 2 5

T. truncatus/S. frontalis 1 2 3 2 0 2 5

Unidentified cetaceans 3 3 6 3 1 4 10

Total 81 73 154 72 45 117 271

STUDY AREA

Summer Winter

Species 96 97 Total 97 98 Total TOTAL
Table 4.6. Seasonal group-size, density and abundance estimates of cetacean species from aerial surveys in the Eastern Planning Area (EPA) continental shelf and slope study areas

during summer and winter 1996-98 (n = number of groups, S = mean group size, D

animals/100 km2, N = abundance estimate, CV = coefficient of variation, LCI and UCI lower and upper limits of log-normal 95% confidence interval; seasonal pairs

in bold are significantly different, p<O.1O).

129

STUDY AREA S CV

Species/Season (S)

D N CV

(N)

LCI UCI

EPA CONTINENTAL SHELF

Bottlenose dolphin

Summer 27 13.7 0.29 26.620 3,281 0.37 1,593 6,757

Winter 31 4.3 0.23 9.075 1,119 0.30 628

1,993

EPA CONTINENTAL SLOPE

Dwarf/pygmy sperm whale

Summer 17 L9 0.20 0.442 3110.33 164 591

Winter 2 1.5 0.33 0.051 360.68 10 135

Bottlenose dolphin

Summer 47 7.3 0.18 4.228 2,980 0.27 1,786 4,971

Winter 36 18.7 0.30 10.346 7,291 0.35 3,692

14,390

Risso's dolphin

Summer 7 12.4 0.35 1.080 7610.51 283 2,042 Winter 24 7.7 0.30 2.848 2,007 0.39 949

4,244

Atlantic spotted dolphin

Summer 9 14.3 0.27 1.597 1,126 0.42 495

2,560

Winter 6 27.0 0.36 2.496 1,758 0.55 611

5,058

Pantropical spotted dolphin

Summer 28 75.7 0.17 25.571 18,020 0.29 10,295 31,540
Table 4.7. Group-size, density and abundance estimates of cetaceans from aerial surveys of the Eastern Planning Area (EPA) continental shelf and slope study areas during summer

and winter 1996-98 (n number of groups, S = mean group size, D = animals/100

1cm2, N = abundance estimate, CV = coefficient of variation, LCI and UCI = lower

and upper limits of log-normal 95% confidence interval).

130

STUDY AREA

Temporal stratum

S CV

(5)

D N CV

(N)

LCI UCI

EPA CONTINENTAL SHELF

Overall 72 9.1 0.19 22.308 2,750 0.24 1,729 4,372

Summer 34 14.4 0.24 34.333 4,232 0.33 2,210

8,103

Summer 1996 15 11.4 0.21 23.104 2,847 0.38 1,352

5,995

Summer 1997 19 16.7 0.36 46.482 5,730 0.48 2,276

14,428

Winter 38 6.2 0.25 15.781 1,945 0.29 1,097 3,450

Winter 1997 16 4.9 0.41 10.149 1,251 0.48 490

3,196

Winter 1998 22 9.7 0.36 29.450 3,630 0.40 1,662 7,930

EPA CONTINENTAL SLOPE

Overall 247 31.3 0.13 51.819 36,517 0.16 26,487

50,346

Summer 140 29.5 0.16 50.080 35,291 0.19 24,214

51,436

Summer 1996 75 35.3 0.19 64.323 45,327 0.24 28,541 71,984

Summer 1997 65 22.9 0.27 35.926 25,317 0.31 13,997

45,791

Winter 107 33.6 0.21 53.980 38,039 0.25 23,401 f-I O')'l 01 ,O.).)

Winter 1997 70 30.5 0.34 58.29 1 41,076 0.37 20,040

84,192
Table 4.8. Visual survey effort in each study area during each GulfCet II shzp survey. (The

GulfCet I study area effort is a subset of the oceanic northern Gulf study area; EPA

Eastern Planning Area; na not applicable).

Spring and early summer 1996 - NOAA Ship Oregon 11(17 April - 09 June 1996)

Spring and early summer 1997 - NOAA Ship Oregon 11(16 April - 10 June 1997)

Late summer 1996 - k/V Gyre (10-29 October 1996)

Mid-summer 1997- RIV Gyre (4-22 August 1997)

131

Study Area

Spring

1996

(kin)

Spring

1997

(km)

Spring

Total

(km)

Late summer Mid-summer

1996 1997

(km) (km)

EPA Continental Shelf 234 215 449 199 205

EPA Continental Slope 1,136 1,450 2,586 711 611

Oceanic Northern Gulf 4,481 4,115 8,596 na na

GulfCetlStudyArea (1,635) (1,961) (3,596) na na

Deepwater Focal Area na na na 703 854

TOTAL 5,851 5,780 11,631 1,613 1,670
Table 4.10. Group-size, density and abundance estimates of cetaceans from ship surveys of the

Eastern Planning Area continental shelf and slope study areas during spring 1996-97

(n number of groups, S = mean group size, D = ariimals/100 km2, N = abundance

estimate, CV = coefficient of variation, LCI and UCI = lower and upper limits of

log-normal 95% confidence interval).

133

STUDY AREA S CV

Species (S)

D N CV

(N)

LCI UCI

EPA CONTiNENTAL SHELF

Bottlenose dolphin 15 8.7 0.13 8.567 1,056 0.33 524

2,125

Atlantic spotted dolphin 12 18.9 0.23 14.822 1,827 0.46 713

4,678

T. truncatus/S.frontalis 10 4.3 0.43 2.815 3470.60 108 1,111

Unidentified dolphin 2 2.5 0.60 0.267 330.95 4 254

EPA CONTINENTAL SLOPE

&yde's whale 2 2.3 0.56 0.041 291.05 4 196

Sperm whale 3 3.0 0.28 0.081 570.57 19 175

Dwarf/pygmy sperm whale 6 1.0 0.00 0.094 660.74 17 263

Falsekillerwhale 1 40.0 - 0.441 3110.97 58 1,680

Rough-toothed dolphin 1 2.1 - 0.023 160.96 3 86

Fraser's dolphin 1115.0 - 1.337 9421.01 163 5,465

Bottlenose dolphin 14 9.6 0.43 1.455 1,025 0.56 349

3,008

Risso's dolphin 9 9.9 0.20 0.964 6790.67 192 2,404

Atlantic spotted dolphin 6 23.1 0.17 1.498 1,055 0.66 304

3,662

Pantropical spotted dolphin 18 50.4 0.24 10.547 7,432 0.40 3,416

16,172

Striped dolphin 2 25.4 0.17 0.591 4160.96 77 2,246

Spinnerdolphin 3216.4 0.51 7.548 5,319 0.75 1,139

24,849

T. truncatus/S.frontalis 2 14.0 0.79 0.303 2131.24 20 2260

Unidentified large whale 1 1.0 - 0.009 61.02 1 37

Unidentified small whale 2 1.0 - 0.03 1 220.88 5 107

Unidentified odontocete 1 1.0 - 0.016 110.96 2 60

Unidentified dolphin 9 2.6 0.23 0.200 1410.54 50 399
Table 4.11. Group-size, density and abundance estimates of cetaceans from ship surveys in the

oceanic northern Gulf of Mexico study area during spring 1996-97 (n number of

groups, S = mean group size, D = animals/100 km2, N = abundance estimate,

CV = coefficient of variation, LCI and UCI = lower and upper limits of log-normal

95% confidence interval).

134

STUDY AREA

Species

S CV

(5)

D N CV

(N)

LCI UCI

OCEANIC NORTHERN GULF

Bryd&s whale 1 2.0 - 0.005 221.07 4 123

Sperm whale 26 1.4 0.15 0.097 3870.45 164 914

Dwarf'pygmyspermwhale 18 2.2 0.17 0.184 7330.53 273 1,970

Cuvier's beaked whale 3 2.8 0.33 0.040 1590.69 44 577

Mesoplodon spp. 7 1.1 0.13 0.038 1500.50 59 384

Unidentified Ziphiidae 3 1.3 0.25 0.019 750.67 22 254

Melon-headedwhale 1125.0 - 0.435 1,734 1.07 304

9,878

Pygmy killer whale 1 12.6 0.044 1751.12 29 1,062

Peponocephala/Feresa 1 5.7 - 0.020 791.10 13 472

Falsekillerwhale 1 63.0 - 0.205 8171.01 152 4,381

Killerwhale 2 2.7 0.62 0.017 681.01 9 497

Short-finned pilot whale 6 19.0 0.73 0.369 1,471 0.86 263

8,224

Rough-toothed dolphin 2 17.5 0.71 0.114 4531.10 46 4,451

Bottlenose dolphin 18 13.1 0.37 0.762 3,040 0.55 1,091

8,470

Risso's dolphin 28 8.4 0.14 0.762 3,040 0.30 1,700

5,438

Atlantic spotted dolphin 3 13.6 0.11 0.132 5280.69 152 1,829

Pantropical spotted dolphin 74 45.4 0.13 11.687 46,625 0.24 29,362

74,039

Striped dolphin 4 79.0 0.11 1.098 4,381 0.55 1,570

12,227

Spinner dolphin 8101.4 0.46 2.820 11,251 0.66 3,283

38,565

Clymene dolphin 8 90.1 0.18 2.530 10,093 0.40 4,665

21,838

Stenellaspp. 1 4.0 - 0.014 550.94 11 273

Unidentified large whale 1 1.0 - 0.003 110.90 2 51

Unidentified small whale 3 1.7 0.40 0.024 940.69 25 352

Unidentified odontocete 6 1.2 0.14 0.033 1310.44 57 304
Table 4.12. Group-size, density and abundance estimates of cetaceans from ship surveys of the

GulfCet I study area during spring 1996-97 (n = number of groups, S = mean group

size, D = animals/100 km2, N = abundance estimate, CV = coefficient of variation,

LCI and UCI = lower and upper limits of log-normal 95% confidence interval).

135

STUDY AREA n S CV D N CV LCI UCI

Species (5) (N)

GULFCET I

Sperm whale 18 1.3 0.14 0.149 2300.56 79 666

Dwarf/pygmy sperm whale 12 2.2 0.20 0.299 4620.64 141 1,512

Cuvier's beaked whale 2 2.3 0.56 0.05 1 780.90 12 519

Mesoplodonspp. 5 1.2 0.17 0.068 1050.61 33 329

Unidentified Ziphiidae 1 1.0 - 0.011 170.80 4 73

Melon-headedwhale 1125.0 - 1.040 1,607 0.97 305

8,473

Short-finned pilot whale 4 47.1 0.33 1.457 2,253 0.58 735

6,904

Bottlenose dolphin 6 30.1 0.47 1.396 2,158 0.76 536

8,685

Risso's dolphin 15 10.0 0.19 1.166 1,802 0.38 860

3,775

Atlantic spotted dolphin 2 13.7 0.19 0.211 3260.77 81 1,309

Pantropical spotted dolphin 33 38.0 0.18 10.439 16,141 0.37 7,847

33,204

Striped dolphin 2 66.8 0.20 1.110 1,717 0.76 433

6,799

Clymene dolphin 5102.0 0.25 4.241 6,557 0.70 1,832

23,463

Unidentified small whale 3 1.7 0.40 0.056 870.78 21 367

Unidentified odontocete 4 1.3 0.20 0.056 870.51 33 232

Unidentified dolphin 9 3.0 0.37 0.172 2660.49 102 695
Table 4.13. Comparison of abundance estimates (N) from ship surveys in the oceanic northern G

GulfCet I study area (NW Slope) (Estimates in bold are significantly different, p<O.l 0

Species Oceanic Northern Gulf of Mexico 199 1-94k I 996-97 N(CV) N(CV) Gulf

199 1-94 

N(CV) 

Bryde's whale 35 (1.09) 22 (1.07) 5 (0.85)

Spermwhale 530(0.31) 387(0.45) 248(0.30) 

Dwarf/pygmy spermwhale 547 (0.28) 733 (0.53) 188 (0.26) Cuvier's beaked whaLe 30 (0.28) 159 (0.69) 23 (0.43) Melon-headedwhale 3,965 (0.39) 1,734 (1.07) 2,569 (0.38) 

Pygmy killer whale 518 (0.81) 175 (1.12) 57(0.37)Killerwhale 277 (0.42) 68 (1.01) 113 (0.48)False killer whale 381 (0.62) 817 (1.01) 17 (0.67)Short-finned pilot whale 353 (0.52) 1,471 (0.86) 232 (0.51) Rough-toothed dolphin 852 (0.31) 453 (1.10) 221 (0.39)Fraser's dolphin 127 (0.89)

0 103 (1.24)

Bottlenose dolphin 5,618 (0.26) 3,040 (0.55) 3,335 (0.30) 

Risso's dolphin 2,749 (0.27) 3,040 (0.30) 803 (0.27) Atlantic spotted dolphin 3,213 (0.44) 528 (0.69) 1,453 (0.42) Pantropical spotted dolphin 31,320 (0.20) 46,625 (0.24) 10,191 (0.23) 

Striped dolphin 4,858 (0.44) 4,381 (0.55) 2,891 (0.54) 

Spinner dolphin 6,316 (0.43) 11,251 (0.66) 1,314 (0.64)Clymene dolphin 5,571 (0.37) 10,093 (0.40) 2,924 (0.38) 

1 - NMFS, Hansen et al. (1995); 2 - GulfUet II, this study; 3 - GuliCet I, Hansen et al. (1996)
Table 4.14. Seasonal occurrence of cetacean species in the EPA study areas, and in the EPA,

focal, and oceanic northern Gulf study areas combined (All Study Areas) during

GulfCet II aerial or shzp surveys (X = one or more on-effort sightings).

137

Species

EPA Shelf

Spr Sum Win

EPA Slope

Spr Sum Win

All Study Areas

Spr Sum Win

Bryde's whale 0 0 0 X X X X X X

Sperm whale 0 0 0 X X X X X X

Dwarf/pygmy sperm whale 0 X 0 X X X X X X

Cuviefs beaked whale 0 0 0 0 X X X X X

Mesoplodon spp. 0 0 0 0 X X X X X

Melon-headed whale 0 0 0 0 0 0 X 0 0

Pygmy killer whale 0 0 0 0 0 X X 0 X

Killer whale 0 0 0 0 0 0 X 0 0

False killer whale 0 0 0 X X 0 X X 0

Short-finned pilot whale 0 0 0 0 0 X X 0 X

Rough-toothed dolphin 0 0 0 X X 0 X X 0

Fraser's dolphin 0 0 0 X 0 0 X 0 0

Bottlenose dolphin X X X X X X X X X

Rissos dolphin 0 0 0 X X X X X X

Atlantic spotted dolphin X X X X X X X X X

Pantropical spotted dolphin 0 0 0 X X X X X X

Striped dolphin 0 0 0 X X X X X X

Spinner dolphin 0 0 0 X X X X X X

Clymene dolphin 0 0 0 0 X X X X X

Total 2 3 2 12 14 14 19 14 14
Table 4.15. Sea turtle species sighted and number of on-effort sightings during aerialsurveys of

the Eastern Planning Area (EPA) continental shelf and continental slope study areas

during summer and winter 1996-98.

EPA CONTINENTAL SHELF

138

Loggerhead seaturtle 18 21 39 26 20 46 85

Kemp's ridley sea turtle 0 0 0 2 1 3 3

Leatherback sea turtle 3 1 4 0 0 0 4

Unidentified chelonjd 4 0 4 3 3 6 10

Total 25 22 47 31 24 55 102

EPA CONTiNENTAL SLOPE

Loggerhead sea turtle 0 2 2 15 10 25 27

Leatherback sea turtle 15 5 20 7 1 8 28

Unidentified chelonid 1 2 3 1 2 3 6

Total 16 9 25 23 13 36 61

Species 96 97 Total 97 98 Total TOTAL

Summer Winter

STUDY AREA
Table 4.16. Density and abundance estimates of sea turtles from aerial surveys in the Eastern

Planning Area (EPA) continental shelf and slope study areas during summer and

winter 1996-98 (n = number of turtles sighted, D = turtles/100 km2, N abundance

estimate, CV coefficient of variation, LCI and UCI = lower and upper limits of

log-normal 95% confidence interval; seasonal pairs in bold are significantly

different, p<ZO. 10.)

STUDY AREA n D N CV LCI UCI

Species/Season (N)

EPA CONTINENTAL SHELF

139

Loggerhead

Overall 84 4.077 503 0.20 339 745

Summer 39 3.891 480 0.30 264 873

Winter 45 4.253 524 0.23 330 832

Kempsridley 2 0.097 12 1.14 2 75

Leatherback 4 0.194 24 0.48 10 60

Unidentified chelonjd 7 0.340 42 0.45 18 98

EPA CONTINENTAL SLOPE

Loggerhead

Overall 21 0.200 141 0.27 83 239

Summer 2 0.034 24 0.77 6 94

Winter 19 0.406 286 0.27 166 492

Leatherback

Overall 25 0.238 168 0.23 107 264

Summer 19 0.327 230 0.58 142 373

Winter 6 0.128 90 0.48 36 224

Unidentified chelonid 5 0.048 34 0.43 15 75
30-

28

U) D

-J

26-

24

4

Longitude

-88 -86

Figure 4.6. The locations of all cetacean groups sighted

during summer 1996 and 1997 aerial surveys.

-82

-88 -86 -84

Longitude

Figure 4.7. The locations of all cetacean groups sighted

during winter 1997 and 1998 aerial surveys.

140
30- 28- 1) -oJ26- 24-98 ++ + Spring Mid-summer o Lasumrrrer -94 -92 +0 00 + U + 9- L Q + + -90 Longitude -88 0 

Figure 4.8. The locations of all cetacean groups sighted during the four ship surveys (sp

summer 19%, 1997; tate summer 1996; mid-summer 1997). (Survey effort eastern Gulf.) 
30-

26-

24

0

-o

24

+ Summer

Winter

142

+ Spring

Mid-summer

o Late summer

Longitude

Figure 4.10. The locations of all Brydes whale groups sighted during ship surveys.

-88 -86 -84 -82

Longitude

Figure 4.9. The locations of all Bryde's whale groups sighted during aerial surveys.

-98 -96 -94 -92 -90 -88 -86 -84
-88 -86 -84 -82

Longitude

Figure 4.11. The locations of all sperm whale groups sighted during aerial surveys.

+ Spring

Mid-summer

0 Late summer

Longitude

Figure 4.12. The locations of all sperm whale groups sighted during ship surveys.

143
0

30-

28

26-

26-

+ Summer

Winter

-82

24 I -88 -86 -84

Longitude

Figure 4.13. The locations of all dwarf/pygmy sperm whale groups

sighted during aerial surveys.

+ Spring

Mid-summer

o Late summer

24

-98 -96 -94 -92 -90

Longitude

-88 -86

Figure 4.14. The locations of all dwarf/pygmy sperm whale groups

sighted during ship surveys.

144

-84
q3 -J

24

-98

+ Summer

Winter

+ Spring

Mid-summer

o Late summer

-94 -92 -O -88

Longitude

145

-86

Figure 4.16. The locations of all Cuvier's beaked whale groups sighted during ship surveys.

-88 -86 -84 -82

Lang itude

Figure 4.15. The locations of all Cuvier's beaked whale groups sighted

during aerial surveys.
30-

26-

24

+ Summer

Winter

+ Spring

E Mid-summer

o Late summer

Longitude

Figure 4.18. The locations of all Mesoplodon spp. groups sighted

during shzp surveys.

146

-88 -86 -84 -82

Longitude

Figure 4.17. The locations of all Mesoplodon spp. groups sighted

during aerial surveys.
=4-.

30-

28-

26-

24

A pilot whale

pygmy killer whale

0 false killer whale

rough-toothed dolphin

-88 -86 -84

Longitude

+ melon-headed whale A pilot whale

pygmy killer whale rough-toothed dolphin 0 false killerwhale * Fraser's dolphin

' Cf <>killerwhale

147

0

0

Figure 4.19. The locations of all pilot, fuse killer, and pygmy killer whale groups,

and rough-toothed dolphin groups sighted during aerial surveys.

-98 6 -94 -92 -90 -88 -86 -84

Longitude

Figure 4.20. The locations of all melon-headed, pilot, false killer, pygmy killer,

and killer whale groups and rough-toothed dolphin groups sighted

during ship surveys.
30-

26-

24

+ Summer

Winter

148

Longitude

Figure 4.22. The locations of all bottlenose dolphin groups sighted during ship surveys.

-88 -86 -84 -82

Longitude

Figure 4.21. The locations of all bottlenose dolphin groups

sighted during aerial surveys.
30-

28-

-J

26-

-88 -86 -84 -82

Longitude

Figure 4.23. The locations of all RissoTs dolphin groups sighted

during aerial surveys.

+ Spring -HMid-summer

(I 0 Late summer

I

I I -96 -94 -92 -90 -88 -86 -84

Longitude

Figure 4.24. The locations of all RissoTs dolphin groups sighted during ship surveys.

+

149

24

-98
24 I I -88 -86 -84 -82

Longitude

Figure 4.25. The locations of all Atlantic spotted dolphin groups

sighted during aerial surveys.

+ Spring

Mid-summer

0 Late summer

Longitude

Figure 4.26. The locations of all Atlantic spotted dolphin groups sighted during ship surveys.

150
+ Summer

E Winter

-86

24

-98 -96 -94 -92 -90

Longitude

Figure 4.28. The locations of all paritropical spotted dolphin gmups sighted during shzp surveys.

151

-88 -86 -84

-84 -82

Longitude

Figure 4. 27. The locations of all pantropical spotted dolphin groups

sighted during aerial surveys.
24

-98

Figui 4.29. The locations of all striped dolphin groups

sighted during aerial surveys.

+ Spring

Mid-summer

o Late summer

Longitude

-92 -90

Longitude

152

-88 -86 -84

Figure 4.30. The locations of all striped dolphin groups sighted during ship surveys.
ii) 28

-o

-J

30-

26-

24

-98

+ Summer

Winter

24

-88 -86 -84 -82

Longitude

Figure 4.31. The locations of all spinner dolphin groups

sighted during aerial surveys.

+ Spring

Mid-summer

o Late summer

-94 -92 -90

Longitude

Figure 4.32. The locations of all spinner dolphin groups sighted during ship surveys.

153

-84
30-

26-

24

-98

24-

I -88 -86 -84 -82

Longitude

Figure 4.33. The locations of all Clymene dolphin groups

sighted during aerial surveys.

+ Spring

Mid-summer

o Late summer

-96 -94 -92 -90 - -88 -86 -84

Longitude

Figure 4.34. The locations of all Clymene dolphin groups sighted during ship surveys.

154
Figure 4.35. The locations of all leatherback sea turtle

sightings during aerial surveys.

155

24 I -88 -86 -84

Longitude

Figure 4.36. The locations of all loggerhead sea turtle sightings

during aerial surveys.
bottlenose dolphin and nine Atlantic spotted dolphin groups were sighted (Table 4.4), and during ship surveys, 31 and 19, respectively (Table 4.9).

The overall abundance from spring ship surveys of bottlenose dolphins and Atlantic spotted

dolphins was 1,056 (0.33) and 1,827 (0.46), respectively (Table 4.10). The overall abundance

from aerial surveys was 1,824 (0.25) and 1,096 (0.50) for bottlenose and Atlantic spotted dolphins, respectively (Table 4.5). The abundance of bottlenose dolphins from aerial surveys during summer, 3,281 (0.37), was significantly different than the Winter estimate, 1,119 (0.30) (p<0.10) (Table 4.6). Too few sightings of Atlantic spotted dolphins were made to examine

seasonal differences in abundance.

The overall abundance of cetaceans from combined aerial surveys of the EPA shelfwas 2,750 (0.24) (Table 4.7). The summer abundance, 4,232 (0.33), was about twice the winter abundance,

1,945 (0.29), but the difference Was not significant (p>0.10). The overall abundance by survey

ranged from 1,251 (0.48) for winter 1997 to 5,730 (0.48) for summer 1997.

Bottlenose dolphins were seen throughout the EPA shelf during spring, summer and winter

(Figures 4.21 and 4.22). Atlantic spotted dolphins were sighted throughout the area, but few

sightings were made in the extreme western portion of the area (Figures 4.25 and 4.26).

In the EPA slope, a total of 18,788 km oftransects were sampled during the four seasonal aerial surveys. All of the proposed transect lines (84 lines) were completed during summer surveys

(10,440 km). During winter aerial surveys, poor weather prevented all of the transects from

being surveyed, and 8,348 km (80%) of the proposed effort was completed (66 lines). In total, 271 cetacean groups were sighted (154 summer, 117 winter) (Table 4.3).

In total, 2,586 km of transects were surveyed during early summer ship surveys of the EPA slope

(Table 4.8). During early summer, 99 cetacean groups consisting of 12 species were sighted. Six groups (3 species) and 41 groups (6 species) were sighted during late summer and midsummer, respectively (Tables 4.9 and 4.10).

Combining ship and aerial surveys, 17 cetacean species were sighted in the EPA slope (Table 4.14). Fifteen species were sighted during summer and 14 species in winter. Only three species

were sighted during late summer when there was very little survey effort. Nine species were

sighted in three or more seasons. In general, cetaceans were found throughout the study area

during summer, Winter and spring. However, few cetaceans were sighted in a small area in the extreme northern part of the study area during both summer and winter (Figures 4.6-4.8).

The most commonly sighted species (from aerial and ship surveys combined) in the EPA slope

were bottlenose dolphins (91 and 27 sightings, respectively), pantropical spotted dolphins (53

and 37 sightings), Risso's dolphins (31 and 11 sightings) and dwarflpygmy sperni whales (20

and 6 sightings) (Tables 4.4 and 4.9). The most abundant species from both aerial and ship

surveys were the pantropical spotted and spinner dolphins. The abundance estimates of

pantropical spotted dolphins were 13,649 (0.26) and 7,432 (0.40) from aerial and ship surveys, respectively (Tables 4.5 and 4.10). The abundance estimates of spinner dolphins were 8,670

(0.48) and 5,319 (0.75) from aerial and ship surveys, respectively. Other species with abundance

156
estimates over 1,000 from aerial or ship surveys were bottlenose dolphins, Atlantic spotted

dolphins, Risso's dolphins, striped dolphins, and clymene dolphins.

The summer and winter aerial survey abundance estimates of dwarf/pygmy sperm whales,

bottlenose dolphins, Risso's dolphins, and pantropical spotted dolphins varied by a factor of two or

more, but the difference was only significant for dwarf/pygmy sperm whales (p<0.10) (Table 4.6).

The overall abundance of cetaceans combined from aerial surveys was 36,517 (0.16). The summer

abundance, 35,291 (0.19), was similar to the winter abundance, 38,039 (0.25). The overall

abundance by survey ranged from 25,317 (0.31) for summer 1997 to 45,327 (0.24) for summer

1996 (Table 4.7).

The combined effort from the 1996 and 1997 spring ship surveys of the oceanic northern Gulf was

8,596 km (Table 4.8). This effort produced 310 cetacean group sightings (Table 4.9). Nineteen

cetacean species were identified in oceanic northern Gulf waters during GulfCet II, including: 1)

dwarf sperm whales and pygmy sperm whales which were each identified twice, 2) one

Blainville's beaked whale, and 3) Fraser's dolphins which were sighted only during EPA slope

ship surveys.

The most common species sighted were pantropical spotted dolphins (84 sightings), Risso's

dolphins (36 sightings), sperm whales (33 sightings), bottlenose dolphins (26 sightings) and

dwarf/pygmy sperm whales (23 sightings) (Table 4.9). By a factor of four, pantropical spotted

dolphins were the most abundant species in the study area with an estimate of 46,625 (0.24)

animals. This was followed by spinner dolphins with an estimate of 11,251(0.66) and clymene

dolphins with an estimate of 10,093 (0.40) animals (Table 4.11). Estimates for striped dolphins,

bottlenose dolphins, Risso's dolphins, short-finned pilot whales and melon-headed whales

ranged from 1,000 to 5,000. The abundance of all other species was less than 1,000 each.

Cetaceans were sighted throughout the study area, but fewer were sighted in the western Gulf

(Figure 4.8). The sighting rate of all cetacean groups (groups/transect km) west of 93.0°W was

about half of the rate east of 93.0°W.

The abundance estimates from the GulfCet II surveys of the oceanic northern Gulf were very

similar to those reported by Hansen et al. (1995) for the same area from surveys conducted from

1991-1994 (i.e., GulfCet I) (Table 4.13). However, the GulfCet II estimates were not as precise because of the smaller effort. The 1991-1994 abundance estimates were based on over 22,000

km of effort compared to 8,596 km for the GulfCet II estimates. The only significant difference

in abundance was for the Atlantic spotted dolphin (p<0. 10). Otherwise, the same species were

sighted and the abundances of the more commonly sighted species were similar. These include

pantropical spotted dolphins, Risso's dolphins, bottlenose dolphins, sperm whales and

dwarf/pygmy sperm whales. In general, only those species for which sightings were uncommon

had abundances that varied by more than a factor of two.

The combined effort from the 1996 and 1997 spring ship surveys of the GulfCet I study area was

3,596 km (Table 4.8). Fourteen cetacean species were sighted in 166 sightings (Table 4.9). The

most common species sighted were pantropical spotted dolphins (41 sightings), Risso's dolphins

(23 sightings), sperm whales (22 sightings), dwarf/pygmy sperm whales (14 sightings) and

157
bottlenose dolphins (11 sightings). Pantropical spotted dolphins were the most abundant species

in the study area (N== 16,141, CV =0.37) followed by clymene dolphins (6,557, 0.70), shortfinned pilot whales (2,253, 0.58) and bottlenose dolphins (2,158, 0.76) (Table 4.12). Estimates

for striped dolphins, Risso's dolphins and melon-headed whales ranged from 1,000 to 2,000. The abundance of all other species was less than 1,000 each.

The abundance estimates for commonly sighted species from the GulfCet II surveys of the

GulfCet I study area were generally similar to those reported by Hansen et al. (1996) for the

same area from spring surveys conducted from 1991-1994 (Table 4.13). These species include

sperm whales, melon-headed whales, bottlenose dolphins, pantropical spotted dolphins and

striped dolphins. The estimate from the GulfCet II surveys for short-finned pilot whales was

about 10 times larger (but not significant, p>0.1 0), and that of Atlantic spotted dolphins was

about four times smaller (p<0.10) than those reported by Hansen et al. (1996). The GulfCet II

estimates for dwarf/pygmy sperm whales and Risso's dolphins were about two times larger but

not significantly different (p>O.lO). Because they were based on less effort, the Gulfcet II

estimates were not as precise as those by Hansen et al. (1996).

Seven species sighted during GulfCet I were not sighted in the GulfCet I study area during

GulfCet II. These include Bryde's whales, pygmy killer whales, killer whales, false killer

whales, rough-toothed dolphins and Fraser's dolphins. Sightings of these species were

uncommon during GulfCet I, where the effort totaled 13,507 km. Effort during GulfCet II was

about 29% of that during GulfCet I, which would reduce the chance ofsighting uncommon

species.

4.4.2 Species Accounts

To quickly access abundance and distribution information for each species from ship and aerial

surveys of the EPA shelf and slope study areas and ship surveys of the oceanic northern Gulf,

summaries for each species are provided below. Each summary includes: 1) the number of

on-effort sightings (Tables 4.4 and 4.9); 2) estimates of overall mean group-size (Tables 4.5,

4.10 and 4.11); 3) the range of group-sizes based on all surveys; 4) estimates of overall

abundance (Tables 4.5, 4.10 and 4.11); 5) seasonal abundance estimates from aerial surveys for

those species sighted 15 or more total times (Table 4.6); and 6) a summary of distribution based

on maps from all aerial and ship surveys (Figures 4.9-4.34). For each mean group-size and

abundance estimate, the coefficient of variation (CV) of the estimate is given in parentheses.

Biyde's whales were sighted three times during ship surveys and two times during aerial

surveys. Groupsizes averaged 2.3 (0.56) and 4.0 (0.75) animals from ship and aerial surveys of

the EPA slope, respectively, and 2.0 (one sighting) animals from ship surveys of the oceanic

northern Gulf. Group-sizes ranged from 1 to 7 animals. Abundance estimates were 29 (1.05) and

25 (1.06) from ship and aerial surveys of the EPA slope, respectively, and 22 (1.07) for the

oceanic northern Gulf. All Bryde's whale sightings occurred in the EPA slope near the 100 m

isobath (Figures 4.9 and 4.10). Bryde's whales were sighted in each of the three survey seasons.

Sperm whales were sighted 60 times during ship surveys and 10 times during aerial surveys.

Group-sizes averaged 3.0 (0.28) and 1.5 (0.18) animals from ship and aerial surveys of the EPA

158
slope, respectively, and 1.4 (0.15) animals from ship surveys of the oceanic northern Gulf.

Group-sizes ranged from 1 to 6 animals. Abundance estimates were 57 (0.57) and 37 (0.42)

from ship and aerial surveys of the EPA slope, respectively, and 387 (0.45) for the oceanic

northern Gulf. Almost all sperm whale sightings were in the north-central and northeastern Gulf

(Figures 4.11 and 4.12). There was a concentration of sightings near the Mississippi River delta.

Sperm whale sightings in the EPA slope occurred primarily near the 1,000 m isobath in the

northern half of the study area. Sperm whales were sighted in each survey season in the EPA

slope and oceanic northern Gulf.

Dwarf/pygmy sperm whales were sighted 33 times during ship surveys and 20 times during

aerial surveys. During ship surveys, dwarf sperm whales and pygmy sperm whales were each

identified twice. Group-sizes averaged 1.0 (0.00) and 1.8 (0.19) animals from ship and aerial

surveys of the EPA slope, respectively, and 2.2 (0.17) animals from ship surveys of the oceanic

northern Gulf. Group-sizes ranged from 1 to 6 animals. Abundance estimates were 66 (0.74) and

188 (0.31) from ship and aerial surveys of the EPA slope, respectively, and 733 (0.53) for the

oceanic northern Gulf. Seasonal abundance in the EPA slope was 36 (0.68) in winter and 311

(0.33) for summer. Dwarf/pygmy sperm whales were widely distributed in the oceanic northern

Gulf including the EPA slope (Figures 4.13 and 4.14). One sighting occurred in the EPA shelf.

Dwarf/pygmy sperm whales were sighted during each survey season in the oceanic northern Gulf

and the EPA slope.

Cuvier' s beaked whales were sighted four times during ship surveys and three times during aerial

surveys. Group-sizes averaged 2.0 (0.50) animals from aerial surveys of the EPA slope and 2.8

(0.33) animals from ship surveys of the oceanic northern Gulf. Group-sizes ranged from 1 to 4

animals. Abundance estimates were zero and 22 (0.83) from ship and aerial surveys of the EPA

slope, respectively, and 159 (0.69) for the oceanic northern Gulf. Cuvier's beaked whales were

sighted on the lower continental slope near the 2,000 m isobath and beyond (Figures 4.15 and

4.16). At these depths, sightings were widely distributed in the oceanic northern Gulf and the

EPA slope. Cuvierts beaked whales were sighted during each survey season in the oceanic

northern Gulf and in summer in the EPA slope.

Beaked whales of the genus Mesoplodon were sighted eight times during ship surveys and five

times during aerial surveys. One of the eight ship sightings of beaked whales from the ship was

determined to be a Blainville's beaked whale. Group-sizes averaged 2.2 (0.22) animals from

aerial surveys of the EPA slope and 1.1(0.13) animals from ship surveys of the oceanic northern

Gulf. Group-sizes ranged from 1 to 4 animals. Abundance estimates were zero and 59 (0.51)

from ship and aerial surveys of the EPA slope, respectively, and 150 (0.50) for the oceanic

northern Gulf. In general, Mesoplodon spp. were broadly distributed in waters greater than 1,000

m deep in the oceanic northern Gulf (Figures 4.17 and 4.18). All EPA sightings were in the

southern part of the study area. Mesoplodon spp. were sighted in each season surveyed in the

oceanic northern Gulf and in summer and winter in the EPA slope.

One melon-headed whale group of 125 was sighted during the study. It occurred in the

northwestern Gulf during a spring survey of the oceanic northern Gulf (Figures 4.20). The

abundance for the oceanic northern Gulf was estimated to be 1,734 (1.07).

159
Bottlenose dolphins were sighted 92 times during ship surveys and 152 times during aerial

surveys. In the EPA shelf, group-sizes averaged 8.7 (0.13) and 7.3 (0.19) animals from ship and

aerial surveys, respectively. Group-sizes ranged from 1 to 90 animals. Abundance estimates

were 1,056 (0.33) and 1,824 (0.25) from ship and aerial surveys of the EPA shelf, respectively.

Seasonal abundance in the EPA shelf was 1,119 (0.30) in winter and 3,281 (0.37) in summer.

In the EPA slope, bottlenose dolphin group-sizes averaged 9.6 (0.43) and 9.9 (0.16) animals

from ship and aerial surveys, respectively, and 13.1 (0.37) animals from ship surveys of the

oceanic northern Gulf. Group-sizes ranged from ito 172 animals. Abundance estimates were

1,025 (0.56) and 3,959 (0.22) from ship and aerial surveys of the EPA slope, respectively, and

3,040 (0.55) for the oceanic northern Gulf. Seasonal abundance in the EPA slope was 7,291

(0.35) in winter and 2,980 (0.27) in summer.

Bottlenose dolphins were sighted throughout the EPA shelf and were widely distributed on the

upper EPA slope (1001,000 m) (Figures 4.21 and 4.22). West of 88°W, bottlenose dolphins

were widely distributed but occurred much closer to 100 m isobath. Bottlenose dolphins were

sighted during each survey season in the oceanic northern Gulf, EPA slope and EPA shelf.

Risso's dolphins were sighted 47 times during ship surveys and 31 times during aerial surveys.

Group-sizes averaged 9.9 (0.20) and 8.8 (0.23) animals from ship and aerial surveys of the EPA

slope, respectively, and 8.4 (0.14) animals from ship surveys of the oceanic northern Gulf.

Group-sizes ranged from ito 57 animals. Abundance estimates were 679 (0.67) and 1,317

(0.32) from ship and aerial surveys of the EPA slope, respectively, and 3,040 (0.30) for the

oceanic northern Gulf. Seasonal abundance in the EPA slope was 2,007 (0.39) in winter and 761

(0.51) in summer. Risso's dolphins were widely distributed along the mid-slope in the EPA

slope (Figures 4.23 and 4.24). In the oceanic northern Gulf, no groups were sighted in the

extreme western Gulf. Elsewhere, they were widely distributed, but concentrations occurred on

the upper slope near the Mississippi River delta. Risso's dolphins were sighted during each

survey season in both the oceanic northern Gulf and EPA Slope.

Atlantic spotted dolphins were sighted 32 times during ship surveys and 24 times during aerial

surveys. In the EPA shelf, group-sizes averaged 18.9 (0.23) and 31.8 (0.25) animals from ship

and aerial surveys, respectively. Group-sizes ranged from 3 to 75 animals. Abundance estimates

were 1,827 (0.46) and 1,096 (0.50) from ship and aerial surveys of the EPA shelf, respectively.

In the EPA slope, Atlantic spotted dolphin group-sizes averaged 23.1 (0.17) and 24.8 (0.43)

animals from ship and aerial surveys, respectively, and 13.6 (0.11) animals from ship surveys of

the oceanic northern Gulf. Group-sizes ranged from 1 to 85 animals. Abundance estimates were

1,055 (0.66) and 1,800 (0.43) from ship and aerial surveys of the EPA slope, respectively, and

528 (0.69) for the oceanic northern Gulf. Seasonal abundance in the EPA Slope was 1,758

(0.55) in winter and 1,126 (0.42) for summer.

Atlantic spotted dolphins were widely distributed on the EPA shelf, but fewer groups were

sighted in the western portion of the study area (Figures 4.25 and 4.26). They were widely

distributed in both the EPA slope and oceanic northern Gulf, but occurred only in the shelf-edge

161
region near the 100 m isobath. Atlantic spotted dolphins were sighted during each survey season

in the oceanic northern Gulf, EPA shelf, and EPA slope.

Pantropical spotted dolphins were sighted 129 times during ship surveys and 53 times during

aerial surveys. Group-sizes averaged 50.4 (0.24) and 67.4 (0.13) animals from ship and aerial

surveys of the EPA slope, respectively, and 45.4 (0.13) animals from ship surveys of the oceanic

northern Gulf. Group-sizes ranged from 5 to 650 animals. Abundance estimates were 7,432

(0.40) and 13,649 (0.26) from ship and aerial surveys of the EPA slope, respectively, and 46,625

(0.24) for the oceanic northern Gulf. Seasonal abundance in the EPA slope was 8,226 (0.36) in

winter and 18,020 (0.29) in summer. Pantropical spotted dolphins were sighted well away from

the shelf edge, but otherwise throughout all waters searched in the oceanic northern Gulf and

EPA slope (Figures 4.27 and 4.28). Pantropical spotted dolphins were sighted during each

survey season in both the oceanic northern Gulf and EPA slope.

Striped dolphins were sighted 13 times during ship surveys and eight times during aerial surveys.

Group-sizes averaged 25.4 (0.17) and 66.7 (0.26) animals from ship and aerial surveys of the

EPA slope, respectively, and 79.0 (0.11) animals from ship surveys of the oceanic northern Gulf

Group-sizes ranged from 2 to 160 animals. Abundance estimates were 416 (0.96) and 2,198

(0.50) from ship and aerial surveys of the EPA slope, respectively, and 4,381 (0.55) for the

oceanic northern Gulf. Striped dolphins were sighted away from the shelf edge and were widely

distributed in the EPA slope and in eastern oceanic Gulf none were sighted in the extreme

western Gulf (Figures 4.29 and 4.30). Striped dolphins were sighted during each survey season

in the oceanic northern Gulf and EPA slope.

Spinner dolphins were sighted 17 times during ship surveys and seven times during aerial

surveys. Group-sizes averaged 216.4 (0.51) and 263.1 (0.28) animals from ship and aerial

surveys of the EPA slope, respectively, and 101.4 (0.46) animals from ship surveys of the

oceanic northern Gulf Group-sizes ranged from 15 to 750 animals. Abundance estimates were

5,319 (0.75) and 8,670 (0.48) from ship and aerial surveys of the EPA slope, respectively, and

11,251(0.66) for the oceanic northern Gulf. Spinner dolphins were sighted away from the shelf

edge and were widely distributed in the EPA slope and in northeastern oceanic Gulf. None were

sighted west of the Mississippi River (Figures 4.31 and 4.32). Spinner dolphins were sighted

during each survey season in the oceanic northern Gulf and EPA slope.

Clymene dolphins were sighted 11 times during ship surveys and five times during aerial

surveys. Group-sizes averaged 97.4 (0.22) animals from aerial surveys of the EPA slope and

90.1 (0.18) animals from ship surveys of the oceanic northern Gulf. Group-sizes ranged from 4

to 150 animals. Abundance estimates were zero and 2,292 (0.52) from ship and aerial surveys of

the EPA slope, respectively, and 10,093 (0.40) for the oceanic northern Gulf. Clymene dolphins

were sighted well away from the shelf edge and were widely distributed in western oceanic Gulf

during spring and in the northeastern Gulf during summer and winter (Figures 4.33 and 4.34).

Clymene dolphins were sighted during each survey season in the oceanic northern Gulf, and in

summer and winter in the EPA slope.

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4.4.3 Distribution, Density, and Abundance of Sea Turtles

Three species of sea turtles were sighted in the EPA shelf area: 85 loggerheads, four leatherbacks

and three Kemp's ridleys. Ten unidentified chelonid sea turtles were also recorded. Overall and

seasonal abundances of loggerheads were estimated. Because of small sample sizes, only the

overall abundance of leatherbacks and Kemp's ridleys were estimated (Table 4.16).

The overall abundance of loggerhead sea turtles in the EPA shelfwas 503 (0.20). Loggerhead

sea turtle abundances were similar during summer and winter with 480 (0.30) and 524 (0.23),

respectively (Table 4.16). Loggerhead sea turtles were sighted throughout the EPA Shelf during

both summer and winter (Figure 4.36). Estimates of abundance for leatherback and Kemp's

ridley sea turtles were 24 (0.48) and 12 (1.14) turtles, respectively. There were an estimated 42

(0.45) unidentified chelonids (These were probably loggerheads). The four sightings of

leatherback sea turtles were widely distributed (Figure 4.35).

In the EPA slope, two species of sea turtles were sighted: 28 leatherbacks and 27 loggerheads.

Six unidentified chelonid sea turtles were also recorded. Overall and seasonal abundances of

leatherbacks and loggerheads were estimated (Table 4.16). The overall abundance estimate of

leatherback sea turtles in EPA slope was 168 (0.23) turtles. Leatherback sea turtles were about

2.5 times more abundant in summer than winter with estimates of 230 (0.58) and 90 (0.48),

respectively (Table 4.16). However, the difference was not significant (p>0. 10). Leatherback

sea turtles were sighted throughout the EPA slope. However, the majority of the summer

sightings occurred in the northeastern part of the study area, whereas the winter sightings were

concentrated to the southeast (Figure 4.35).

The overall abundance estimate of loggerheads was 141 (0.27). Loggerhead sea turtles were

about 12 times more abundant in winter than summer with estimates of 286 (0.27) and 24 (0.77),

respectively. The difference was significant (p<0.10). Loggerhead sea turtles were sighted

throughout the EPA slope during winter. Many of the sightings occurred near the 100 m isobath

(Figure 4.36).

4.5 Discussion

The results from this study of cetacean abundance and distribution are similar to those from

previous studies of the northern Gulf of Mexico (Fritts et al. 1983, Blaylock and Hoggard 1994,

Mullin et al. 1994c, Hansen et al. 1995, Hansen et al. 1996, Jefferson 1996, Mills and

Rademacher 1996, Mullin and Hansen, in press). Bottlenose dolphins and Atlantic spotted

dolphins inhabit the continental shelf and shelf-edge region almost exclusively. The oceanic

cetacean community in the northern Gulf is composed of at least 19 species that usually inhabit

deep, tropical waters. With exception of the clymene dolphin, which is endemic to warm

Atlantic basin waters, these deep-water species have distributions that are pantropical (e.g.,

melon-headed whale, spinner dolphin) or broader (e.g., killer whale, sperm whale) (Jefferson et

al. 1993).

Compared to other parts of the world, maximum group-sizes in the northern Gulf were generally

small, and groups were almost exclusively made up of a single species. During this study, the

163
largest group sighted was 750 spinner dolphins (27.5°N, 85.0°W). In the eastern tropical Pacific,

the northwestern Atlantic, and the Pacific adjacent to California, many of the same species occur

in groups that commonly exceed 1,000 animals and routinely occur in multi-cetacean species

groups (Leatherwood and Reeves 1983, CeTAP 1982, Barlow 1995, Forney et al. 1995).

4.5.1. Temporal flistribution

Most of the cetacean species that are routinely sighted in the northern Gulf of Mexico apparently

occur throughout the year. Including the results of this study, there are sighting records from

three or more seasons of at least 16 cetacean species (Mullin et al. 1994c, Hansen et al. 1995,

1996, O'Sullivan and Mullin 1997). The seasonal abundance of several species in slope waters

(e.g., dwarf/pygmy sperm whale, Risso's dolphin and pantropical spotted dolphins) may vary at

least regionally. However, the seasons of a species' maximum or minimum abundance are not

consistent among studies.

Hansen et al. (1996) estimated cetacean abundances for all four seasons for the GulfCet I study

area. They reported maximum abundances of dwarf/pygmy sperm whales in summer and spring.

Abundance estimates of Risso's dolphins in the fall were about 20% of those in the other three

seasons, and the minimum abundance for pantropical spotted dolphins occurred during fall. In

the EPA slope where surveys were only conducted during winter and summer, dwarf/pygmy

sperm whales were estimated to be more than eight times as abundant in summer than winter

(<0.10) (Table 4.6). While in the EPA slope the differences were not significant (p>0.10), the

winter and summer abundances of Risso's dolphins and pantropical spotted dolphins had

opposite patterns; Risso's dolphins were most abundant in winter and pantropical spotted

dolphins in summer

Synoptic surveys of the oceanic northern Gulf have been conducted only during the spring

(Hansen et al. 1995). No synoptic surveys have been conducted in the southern Gulf. Therefore,

it cannot be determined whether a decline in a species' abundance in one region is matched by an

increase in another. The overall abundance of cetaceans in the EPA slope was similar for

summer and winter, but inter-survey abundances were more variable (Table 4.7). Hansen et al.

(1996) reported a marked decline in the abundance of cetaceans during fall in the GulfCet I study

area, with the other three seasons remaining similar.

The abundance of bottlenose dolphins in the EPA shelf was about three times larger in summer

than winter (p<0.10) (Table 4.6). Scott et al. (1989) reported similar or greater seasonal

variability in the abundance of bottlenose dolphins for small regions of the continental shelf in

the northern Gulf, but the overall abundances for the entire continental shelf were seasonally

similar.

4.5.2 Spatial Distribution

Except for the sperm whale, commonly sighted species in the EPA slope were widely distributed

in the study area, albeit in many cases, at different depth ranges. Within the study area, there

was no evidence of seasonal shifts in the distribution of all cetaceans in general or of each

species. However, sightings of many species were too few in at least one season to speculate

164
about seasonal differences in their spatial distributions. While there was no evidence of seasonal

shifts in the locations of sightings within the study area, seasonal abundance patterns indicate

that many individuals may move completely out of the study area during different seasons.

Results from the GulfCet I study area were similar (Hansen et al. 1996, Davis et al. 1998).

Cetaceans were sighted throughout the oceanic northern Gulf (Figure 4.8). However, sighting

rates of cetacean groups were about 80% larger east of 90.0°W compared to west. With a few

exceptions noted below, the distributional results for the oceanic northern Gulf for most species

were similar to previous studies (Mullin et al. 1994a and 1994b, Hansen et al. 1995, Hansen et al.

1996, O'Sullivan and Mullin 1997).

Bottlenose dolphins and Atlantic spotted dolphins were most commonly sighted in association

with the continental shelf edge throughout the northern Gulf, generally near the 100 m isobath.

With few exceptions, Bryde's whales were found along a narrow corridor near the 100 m isobath

in the northeastern Gulf. Spinner dolphins were found primarily in the north-central and

northeastern Gulf. Short-finned pilot whales and melon-headed whales were found almost

exclusively in the north-central and northwestern Gulf. Pantropical spotted dolphins and striped

dolphins occurred throughout the northern Gulf, but sightings of both species were rare in the

extreme northwestern Gulf. Killer whales ranged throughout the northern Gulf, but most

sightings occurred in a broad, but distinct region southwest of the Mississippi River delta.

Dwarf'pygrny sperm whales were found throughout the northern Gulf. These two species are

very cryptic, and apparent clumps in their distribution may result because sightings only occur

during excellent survey conditions. Sperm whales were also found throughout the northern Gulf,

but there was usually an aggregation of sightings along the 1,000 m isobath near the Mississippi

River delta. Risso's dolphins were concentrated in areas along the upper continental slope, but

sightings were made throughout the northern Gulf (Baumgartner 1997, Davis et al. 1998).

False killer whales, pygmy killer whales and Fraser's dolphins were uncommon but widely

distributed. Mesoplodon spp. and Cuvier's beaked whale were widely distributed in deep water

(usually >1,000 rn). In previous studies, nearly all sightings of rough-toothed dolphins and

clyrnene dolphins were west of the Mississippi River delta, but during GulfCet II both species

were sighted east of the delta. Also, bofflenose dolphins were found much further from the

100 m isobath in the northeastern Gulf where, compared to the rest of the northern Gulf, the

continental slope is less precipitous to a depth of about 1,000 m. In contrast to previous studies,

no striped dolphins or sperm whales were sighted in the northwestern Gulf during GulfCet II.

4.S.3 Cetacean Abundance

The oceanography of the northern Gulf is very dynamic, and there are distinct differences

between the northwestern and northeastern Gulf (see Chapter 2). The cetacean distribution plots

indicate that there are potential differences in species occurrence between the two regions.

However, the plots may not reflect actual differences in abundance because effort was not

uniformly distributed, and the plots are of groups that vary greatly in size. Density estimates

from the two regions provide a more meaningful comparison. The densities of cetacean species

from the GulfCet I study area from spring ship surveys reported by Hansen et al. (1995) are

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Table 4.17. Comparison of density estimates (D = animats/100 km2) of cetaceans from the GulfCe

continental slope study area (EPA slope) as assessed by ship and aerial surveys. (Aersignificantly different from the Ship Survey 91-94 estimates, p<O. 10). 1- GulfCet I, Hansen et al. (1996); 2 - GulfCet II, this study Species NW Slope Ship Survey 9 1-94' Ship Surveys 96972 D(CV) D(CV) 

Ship Survey

D(CV) 

Bryde's whale 0.003 (0.85)

0 0.041 (1.05) 

Sperm whale 0.160 (0.30) 0.149 (0.56) 0.08 1 (0.57) 

Dwarf/pygmy sperm whale 0.122 (0.35) 0.299 (0.64) 0.094 (0.74) Cuvier's beaked whale 0.015 (0.43) 0.051 (0.90)

0 

Melon-headed whale 1.663 (0.38) 1.040 (0.97)

0

Pygmykillerwhale 0.037 (0.68)

0 0 

Killer whale 0.073 (0.48) 0 0 Falsekillerwhale 0.011 (0.67) 0 0.411 (0.97) Short-finnedpilotwhale 0.150(0.51) 1.457(0.58)

0 

Rough-tootheddolphin 0.143 (0.39) 0 0.023 (0.96) Fraser's dolphin 0.067 (1.24) 0 1.337 (1.01)Bottlenosedolphin 2.158 (0.30) 1.396 (0.76) 1.455 (0.56) Risso's dolphin 0.520 (0.27) 1.166 (0.38) 0.960 (0.67) Atlantic spotted dolphin 0.905 (0.42) 0.211(0.77) 1.498 (0.66) Pantropical spotted dolphin 6.596 (0.23) 10.439 (0.37) 10.547 (0.40) Striped dolphin 1.872 (0.54) 1.110 (0.76) 0.591 (0.96) Spinner dolphin 0.850 (0.64)

0 7.548 (0.75) 

Clymene dolphin 1.893 (0.38) 4.241 (0.70)

0 
hawksbill is usually associated with reefs or similar habitats in tropical and subtropical waters and is thought to be rare in the northeastern Gulf of Mexico. The green turtle, a herbivore, is

restricted to warm shallow waters with sea grass beds (National Research Council 1990). Both

loggerhead and Kemp's ridley sea turtles are known to occur on the continental shelf throughout

the northern Gulf of Mexico. Studies indicate that adult loggerheads and Kemp's ridley are

benthic carnivores (Dodd 1988). The leatherback, which feeds primarily on jellyfish, is the most oceanic of the sea turtles. However, its distribution is not entirely oceanic, and it is commonly

found in continental shelf waters along the Atlantic coast of the United States (Hoffman and

Fritts 1982). Leatherbacks also occur on the continental shelf in the northern Gulf of Mexico

(Fritts et al. 1983, Lohoefener et al. 1988, 1990).

The overall density of loggerhead sea turtles in the EPA shelf (4.08 turtles/l00 km2) was 20

times that of the EPA slope. The majority of loggerheadsin the EPA slope (19/2 1) were sighted during winter. While many winter sightings were near the 100 m isobath, as in GulfCet I, there

were sightings of loggerheads over very deep waters (i.e., >1000 m). It is not clear why adult loggerheads would occur in oceanic waters unless they were traveling between foraging sites on

distant and disjunct areas of the continental shelf or seeking warmer waters during winter. The density of loggerheads in the GulfCet I study area was 0.048 turtles/i 00 km2 (Hansen et al.

1996), about 25% of that for the EPA slope. The abundance of sea turtles in the Gulf of Mexico

appears to increase dramatically east of Mobile Bay (NIvIFS unpublished data). Lohoefener et al. (1990) found a maximum density of 0.50 turtles/100 km2 in continental shelf waters off

Louisiana west of the Mississippi River.

Based on a summary of several studies, Hansen et al. (1996) concluded that in the northwestern

Gulf the primary habitat of the leatherback sea turtle was oceanic (>200 m). In contrast, the

overall densities of leatherback sea turtles in the EPA shelf and slope were similar, with

estimates of 0.19 turtles/100 kin2 (0.48) and 0.24 turtles/100 km2 (0.23), respectively. Seasonal differences in leatherback densities were not noted for the Gulftet I study area (Hansen et al. 1996). In the EPA slope, the summer density of leatherbacks was about twice that of winter, but

the difference was not significant (p>0. 10).

The overall density estimate of leatherbacks during GulfCet 1(0.18 turtles/100 km2 [0.19]) was

similar to GulfCet II values. Comparatively few leatherbacks were sighted during fall SEFSC

surveys of the entire northern Gulf of Mexico continental shelf (NMFS, SEFSC unpublished data). However, the majority of the sightings occurred just north of DeSoto Canyon in the vicinity of the EPA shelf. The nearly disjunct summer and winter distributions of leatherbacks

indicates that specific areas may be important to this species either seasonally or for short periods of time. During the GulfCet I surveys, leatherback sightings were concentrated,

indicating that their distribution patterns were irruptive in nature.

In summary, the GulfCet II aerial surveys provided the first assessment of sea turtle abundance

and distribution over a large area of the oceanic northeastern Gulf of Mexico. Three sea turtle

species occurred in the EPA study areas: loggerhead, Kemp's ridley, and leatherback sea turtles.

The significance of the oceanic Gulf to loggerheads is not clear. However, the leatherback sea

turtle, an endangered species, occurred in significant numbers inhabited the study areas during

both summer and winter.

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4.5.5 Bias

Without correction, both aerial and ship surveys should result in negatively biased abundance

estimates (i.e., estimates are smaller than the actual number of animals). In theory, because the

aircraft moves about 10 times faster than the ship, ship surveys should be less biased if availability bias is the primary source of bias (i.e., cetaceans below the surface, see below).

While seasonal differences in abundance could certainly be a confounding factor, aerial survey

abundances were larger than those from ship surveys for commonly sighted species in the EPA

(Table 4.17). This could result from aerial observer's ability to see below the surface into the

relatively clear slope waters and sight animals that would missedby shipboard observers. While no specific analysis was conducted, aerial surveys were probably conducted in better sighting

conditions overall than ship surveys because researchers can be more selective of the survey

conditions. Also, in less than optimal conditions (Beaufort sea state ?3), cetaceans are probably

seen more easily from aircraft than ships. Results from the EPA shelfwere mixed when

comparing aerial and ship abundance estimates. The abundance of bottlenose dolphins was

larger from aircraft surveys, but the abundance for Atlantic spotted dolphins was larger from ship

surveys. Despite differences in possible bias between aerial and shipboard surveys, only the abundance estimates for bottlenose dolphins were significantly different (p<0.l 0) between the

two methods.

Both the aerial and ship abundance surveys were designed to meet the assumptions of linetransect theory (Buckland et al. 1993). Meeting the central assumption, that objects on the

transect line are detected with certainty (g(0) = 1) is problematic with cetacean and sea turtle

surveys. Because this assumption is violated in most cases, the abundances reported here are

most certainly negatively biased (g(0) < 1). There are two types of bias: "availability bias" and

"perception bias" (Marsh and Sinclair 1989). Cetaceans and sea turtles dive and can remain out

of view during the time the survey platform is in the area. As a result, some groups on the

transect line are missed because they are not "available" to be seen. Other groups on the transect

line are available to be seen but are missed (not perceived) by observers.

The degree of availability bias is species and platform dependent(e.g., aircraft vs. ship, see below). Because different cetacean species feed at different depths in the water column, they

have different dive cycles. For example, sperm whales have very long average dive times,

whereas dive time is probably much shorter for all dolphin species. The probability that a sperm

whale will be at the surface in the area that is being searchedis much less than for most dolphins

Therefore, negative bias is greater for the sperm whale. Unless the dives of all members of a

group are synchronous, group-size and group diving behavior is also a factor that affects

availability bias. The larger the group, the higher the probability that at least one animal from

the group will be on the surface. The degree of diving synchrony and the length of the dive cycle also could bias the assumption that all members of a group are counted.

The magnitude of perception bias is dependent on cetacean group-size, behavior and sighting

conditions (e.g., Beaufort sea state). Large groups and/or active groups are less likely to be missed than small groups, resting groups or species whose behavior does not produce

pronounced cues (e.g., blows, splashes). For example, abundance estimates for pantropical

spotted dolphins are probably less negatively biased because they occur in large active groups

169
least 16 cetacean species inhabited continental slope waters of the northeastern Gulf Bottlenose

dolphins were most common in terms of the number of sightings, but pantropical spotted

dolphins and spinner dolphins occurred in much larger groups, and were the most abundant

species. Comparisons of summer and winter abundance of commonly sighted species from slope

waters indicated that dwarf/pygmy sperm whales and pantropical spotted dolphins may be more

abundant during summer, whereas Risso's dolphins may be more abundant during winter.

Loggerhead and leatherback sea turtles occurred in slope waters. Leatherbacks were widely

distributed in slope waters and may have been more abundant during summer; loggerheads were

much more abundant during winter. At least 17 cetacean species occurred in the oceanic

northern Gulf, and pantropical spotted dolphins was the most abundant species. GulfCet II

abundance estimates for the oceanic northern Gulf were similar to previous estimates based on

1991-1994 GulfCet I surveys.

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Chapter 5

An Acoustic Survey of Cetaceans in the Northern Gulf of Mexico

Jeffrey C. Norris, William E. Evans and Shannon Rankin

5.1 Introduction

The primary purpose of the acoustic survey was to describe the distribution and habitats of

cetaceans in the northeastern Gulf of Mexico based on species-specific vocalizations detected by

a towed hydrophone array. In addition, this survey recorded man-made noise that could impact

cetaceans. This information might prove useful in the management of future oil and gas

development in the Gulf.

Visual and acoustic surveys of cetaceans in the northern Gulf were conducted during GulfCet I

(Norris et al. 1996). Sperm whale density estimates were based on aerial visual, shipboard visual

and shipboard acoustic surveys. The shipboard visual and shipboard acoustic density estimates

(2.02 vs. 2.04 animals per 1000 1cm2) were not significantly different using the criteria of nonoverlap of 95% CI. Likewise, abundance estimates for sperm whales using acoustic and

shipboard visual data were essentially identical: 316 and 313 animals, respectively. The acoustic

survey estimated the presence of more dolphins in the study area than the other survey

techniques. The most meaningful comparison is group density, since it avoids the estimation of

group size. Using acoustic methods, the estimated group density was 8.08 groups/1000 km2

compared to 4.39 groups/1000 km2 for the shipboard visual survey and 4.95 groups/bOO km2 for

the aerial survey. The dolphin abundance estimates from the GulfCet I study were 1.98 times

greater (36,760) than the minimum shipboard visual survey estimates (18,584) (Norris et al.

1996).

Other acoustic surveys, some using fixed arrays and others using towed systems, have been

performed. Bowhead whales (Balaena mysticetus) were surveyed as they migrated past a linear

hydrophone array that was fixed to the shore-fast ice at Pt. Barrow, Alaska (Clark et al. 1986,

Clark and Ellison 1988). Other studies used concurrent visual and acoustic survey methods (Zeh

et al. 1988). There have been two attempts to develop acoustic survey techniques to count and

study the behavior of sperm whales. Investigators used multiple hydrophones to determine three

dimensional locations of vocalizing sperm whales in the Caribbean Sea (Watkins and Schevill

1 977a) and the North Atlantic (Leaper et al. 1992). Leaper et al. used paired arrays to determine

sperm whale locations, but determining the position of one array relative to the other proved

difficult. Goold (1996) and Goold and Fish (1998) conducted an acoustic survey of common

dolphins (Deiphinus deiphis) off the coast of Wales to determine if seismic exploration affected

their distribution. They observed localized disturbances to animals within a distance of about 1

km from high source levels. They also showed that pulses from a 2,120 cubic inch air gun array

have appreciable energy to at least 15 kHz. Jay Barlow (NMFS, personal comm.) recently

completed an extensive acoustic survey in the eastern tropical Pacific.

Acoustic surveys significantly out-perform ship-based visual surveys (in terms of encounters per

transect length) where the two methods have been directly compared (Thomas et al. 1986).

173
During GulfCet I, encounter rates were 22% higher for the acoustic survey than the concurrent

visual survey. The acoustic survey was on effort 95% of the available time for the Guiftet

cruises: total acoustic effort was 12,219 km. Poor weather can prevent visual sightings, whereas

acoustic detection is rarely degraded by weather. Similarly, acoustic surveys can continue at

night whereas visual surveys must stop. During GulfCet I, 65% of dolphin contacts were at night

even though acoustic survey effort was evenly divided between night and day. In addition,

acoustic surveys create a permanent record in the form of tape recordings that document

encounters and permit future analysis. By making tape recordings of sounds heard during the

survey, future researchers can verify survey results, identify and count vocalizations (and

possibly individuals), and describe biases. These recordings also allow comparisons of the

present acoustic environment in the northern Gulf with recordings made during future surveys.

Fourteen deiphinid species are found in the northern Gulf of Mexico (Table 1.2). Relative to our

ability to identify their vocalizations, these 14 species can be divided into three groups according

to the size of their recorded whistle repertoire:

species with adequate recordings or with sufficiently distinctive vocalizations, such as the

spenn whale, killer whale, and pantropical spotted dolphin.

common species for which there are few or no recordings, such as the Risso's dolphin

rare species for which there are few or no recordings, for example, Fraser's dolphin

During GulfCet I (Norris et al. 1996), recordings were made day and night and during all weather

conditions. Because there was significant acoustic effort when it was impossible to visually

confirm the identity of the animal, there remains a need to develop procedures to identify the

source of cetacean signals in the absence of visual identification.

Characterizing the acoustic features of marine mammal signals has been attempted by a number

of investigators. Fristrup and Watkins (1992) used a large set of parameters to characterize a

variety of vocalizations from pimiipeds, whales and dolphins. Their goal was an automated,

machine-based identification system. Steiner (1981) and Wang (1993) used a series of acoustical

parameters to differentiate dolphin signals.

The difficulty in identifying an acoustic contact varies with each species. Some species are

easily identified; killer whales and sperm whales, for example, have distinctive vocalizations.

Other species are less easily identified. In the Gulf of Mexico, there are several closely related

dolphin species (e.g., five stenellid species) with vocalizations that sound similar (to us). There

are also species whose vocalizations are poorly known. For example, during GulfCet I, we made

the first recordings of Fraser!s dolphin (Leatherwood et al. 1993). Most of the effort described

below has focused on describing the acoustic patterns and creating a classification system for the

more commonly recorded cetaceans encountered in the GulfCet study area.

Many signals are used in the marine geophysical survey industry, including those used for

bottom and sub-bottom profiling and to position towed hydrophone arrays. The physical

characteristics of these seismic signals vary, especially the amplitude and, therefore, the distance

they may affect animals. Use of intense, low frequency seismic sounds for oil exploration has

been increasing in offshore waters (MMC 1998). Typical sub-bottom profiling signals (hereafter

174
called seismic exploration signals) may have source intensities greater than 220 dB re 1 pPa at

im, with the primary energy occurring below 500 Hz (Richardson et al. 1995). Air guns, water

guns, sparkers and boomers produce these signals; each type generates different frequencies and

amplitude levels (Richardson et al. 1995). The frequencies are generally lower than those used in

odontocete vocalizations. However, the high intensity and regular presence of seismic

exploration in the northern Gulf of Mexico has raised concerns regarding its potential impact on

cetaceans.

5.2 Study Area

Acoustic surveys were conducted in the Eastern Planning Area (EPA continental shelf and slope)

and adjoining offshore waters (Figure 1.2) during the late summer 1996 (Gyre96GO6, October

10-29) and mid-summer 1997 (Gyre97GO8, August 4-22) Gyre cruises. During these surveys,

we made hydrophone recordings in different hydrographic features that might influence cetacean

distribution, including the MOM area, cyclones, the confluence zone between the cyclone and

anticyclone, and the periphery of the anticyclone. The location of hydrographic featureswas

determined from near real-time (updated every three days) satellite data of sea surface height

immediately prior to and during each cruise and later confirmed using shipboard CTD and XBT

measurements.

5.3 Methods

5.3.1 Equipment

The new hydrophone array used in GulfCet II (Figure 5.1) was similar to the one used in Gulftet

I, having multiple hydrophones variably spaced along a cable. The new array is spectrally flat

(i.e., no peaks in sensitivity) from 6 Hz to 18 kHz, with approximately 183 dB re ivolt per iPa

sensitivity at 7.2 kHz. This made it more sensitive, permitting a greater detection range for

acoustic contacts. The new array was also thinner and could be towed at faster speeds. The eight

hydrophones are wired in series, so that each can be independently recorded. This permits

broadband recordings that are relatively omnidirectional. However, the omnidirectionality

increases the detection of ship and other low frequency noise that requires low frequency

filtering. Access to the signal from individual hydrophones was used to localize a sound source.

The array was designed, by variably spacing the hydrophones, to have optimal reception

characteristics for sperm whale vocalizations.

Three audio tape recorders were used during the late summer 1996 cruise: two eight-channel

analog recorders (a Racal V-Store and a TEAC) and a two-channel cassette recorder.

Researchers from TAMUG used the Racal and cassette recorders. Scientists from the Monterey

Bay Aquarium Research Institute (MBART) used the TEAC to localize recorded signals,

particularly sperm whale contacts. A 3 3/4 ips recording speed with a 12.5 kHz bandwidth was

used on the Racal. While at sea, acoustic signals were processed on a GulfCoast 486

microcomputer utilizing SIGNALTM software. This software contained a subroutine that enabled

real time spectrograms to be generated. The signals were also auditorally monitored with either

speakers or headphones. In the acoustics laboratory at TAMUG, both SIGNAL and

CANARYTM software were used for spectral analysis. During the mid-summer 1997 cruise,

175
Tow Vessel t / I. 30m Deck Cable 

10.8 2.4 16.1 14.2 20.8 

Hydrophone separation (m) 79.2m Figure 5.1. The configuration of the linear hydrophone array, including the tow vessel, tow c

the array. The placement and separation of the hydrophones is also indicated. Hydrophone Hydrophones Hydrophone Hydrophone 

#1 #2#3 #4 #5 
another recording system was used. Ouput from the array was split so that signals below 2 kllz

were recorded on the eight channel Racal, and the higher frequency signals were recorded on

either a two-channel cassette recorder or an eight-channel digital recorder (Tascam DA-88).

5.3.2 Analysis

Cataloging acoustic contacts was the first step in processing data gathered during a cruise. While

at sea, written records were made of the tape number, elapsed time on the tape recorder, realtime, date and signal source (when known) of recorded sounds. On return to TAM1JG, acoustic

signals were reviewed, cataloged and, when possible, identified. This could be easily done for

some sounds when the animal had been seen or when the signals were species specific, such as

for sperm whales. Many of the unidentified signals were dolphin whistles. To identify an

unknown sound, it was statistically compared to a database of signal parameters from known

sources (see below for details). In addition to the analysis of cetacean vocalizations, recorded

man-made signals were analyzed to assess their potential impact on cetacean distribution and

behavior.

Sperm whale signals were analyzed for bearing and range using RainBow Click software written

by Dr. Douglas Gillespie. Signals from two channels were analyzed three ways (Figure 5.2).

The waveforms from the left and right channels were displayed, showing the time-of-arrival

difference. The time difference was calculated and a bearing to the source was computed. Each

pulse was charted as a single point in the bearing/time window. Multiple pulses at the same

bearing produced a bearing line. Different bearing lines indicated that multiple whales were

present. Since multiple animals could be on a single bearing, the pulse spectra was also

compared. A reference pulse permitted comparison of pulses. The RainBow Click system was

used to determine the number of vocalizing animals and the range to the whale(s). The bearing

to the whale at two points a known distance apart defined a triangle, from which range to the

initial contact could be calculated.

5.3.3 Characterization of Dolphin Whistles

Acoustical analyses were conducted on both identified and unidentified contacts. While the

recordings from identified sources were catalogued, copies of all high quality whistles and burst

pulses were made on a computer. This first group of recordings was ordered by species. The

second group of recordings comprised unidentified sources. Signals were analyzed with

CANARY' 1.2 bioacoustics software developed by Cornell University (Ithaca, New York).

Signals were digitized at a 44.1 kHz sampling rate. Spectrograms were made using a 1,024

point, fast fourier transform resulting in a 5.805 ms frame length with a frequency resolution of

43 Hz. The filter bandwidth was 699.4 Hz. Signals were conditioned using a Hamming

window. Following procedures described by Wang (1993) and Steiner (1981), a series of

parameters were defined for each whistle: the frequency and time of the beginning and end of the

signal; maximum and minimum frequency; center time; frequency change; duration; and peak

time and peak frequency (i.e., the time and frequency of maximum amplitude in the signal). The

following signal characteristics were defined based on the appearance of the spectrogram:

number of inflections; whether the beginning and end frequency swept up, down, or was

constant; number of contour breaks; number of steps (abrupt frequency changes); and signal type

(e.g. upsweep, downsweep, and so on). Acoustic data were then exported to a spreadsheet from

177
Rinnb ow Clfrlc - C rtkt r2 0 74rev-O.rIk 1L i!J 4::]'. 1 tuing, Tiiu dhpLiy fil# tcri Ienhfchun t,1ay a4 Wrj k'w Ed UTC - 2333:334 Sect$on - 00:00:3420 s 0 1' C OO4Y45s WhaJr 1 (lIck 10 Sound Card -22 k-tz 1O?4bip Enrg 21iW9 1/2thirtIon

5Ii

Figurc 5.2. Irnagc of thc Rainbow screcns. Thc lop scrcn isthc Bcari!1Timc Display, that sho

Icaiing Iincs a* 94. 126. ai-d 152 dcgrccs. Thc bttcini left scrccn shows thc click wathe left and right chairneIs Ihe Ixttom right screen shows the spectra of a single cii
which the final parameters were calculated. Time parameters included start and end time,

duration, center time as a percentage of overall signal duration, and peak time as a percentage of

overall signal duration. Frequency parameters were beginning and end frequency and their

bandwidth, high and low frequency and their bandwidth, and peak frequency.

Discriminant function analysis (DFA) was used to derive classification rules from the identified

signals so that, given the same parameters from an unknown species, classification could be

made. DFA is based on the relationship between membership in some class (for example, species

x, y, or z) and a series of variables describing common features contained by members of all

classes (for example, frequency variables of the vocalizations). DFA was performed using

Statistica (Statsoft, Tulsa, OK). The central premise was that each species' calls would be

sufficiently distinctive to be correctly classified 75% of the time or better. Relatively simple

acoustic parameters were used to classify these signals assuming there were acoustical correlates

to gross morphometric differences between species. Based on our experience with both marine

and terrestrial animals, larger animals produce lower frequency signals than smaller ones.

According to this hypothesis, bottlenose dolphins should produce lower frequency whistles than

the smaller Stenelki species. In addition, it was assumed that sympatric species of similar sizes

should encode their signals with acoustic differences that permit differentiation between species.

The final identification was made by combining information on the context of the call, the lexicon

used in the contact, and iterations of the DFA on a series of different calls from a single contact. A

Shannon-Weiner diversity index was used to describe whistle diversity for all calls by each species.

This index is a measure of the distribution of observations among categories, in this case the

distribution of various vocalizations by species. If a species repeatedly used only a single

vocalization, there would be no diversity in its usage and the Shannon-Weiner diversity index

would equal zero. On the other hand, if a species had multiple whistles, but used each one only

once, then the diversity would be highest (i.e., equal one). More typically, animals use multiple

whistles to varying degrees and the diversity index for each species varies between zero and one.

5.3.4 Effects of Seismic Exploration Signals on Cetaceans

Line transect data from both GulfCet I and GulfCet II cruises were analyzed to determine the

potential, largescale effects of seismic exploration signals on marine mammal distributions.

"On-effort" status for this aspect of the GulfCet study was defined as occurring when both visual

and acoustic teams were "on effort. Effort lines were separated on average by 75 km, and no

attempt was made to either approach or avoid seismic exploration vessels. Sounds from seismic

exploration were recorded using the Racal V-Store tape recorder. Visual survey results were

used for cetacean distribution. Additional information regarding the study areas and visual and

acoustical data acquisition can be obtained from Norris et al. (1996).

Seismic exploration signals were recorded, digitized and then analyzed using CANARY digital

signal processing software. Relative signal intensity, signal duration, and maximum, minimum

and peak frequencies were determined for signals using the full frequency bandwidth of the

program. The relative intensity of seismic signals was estimated from the signal-to-noise ratio

(signal intensity in decibels above ambient). The signal-to-noise ratio was calculated by

averaging the intensity of a 120 ms segment of recording, using a 180 dB bandwidth from 20 to

179
200 Hz, from both the background (ambient) and the seismic pulses. Signahtonoise

measurements were divided into three categories: low, 0 dB (no seismic signals); moderate,

0-12 dB; and high, >12 dB above ambient.

Hydrographic data were analyzed for potential interdependencies between hydrographic features,

seismic exploration and presence of cetaceans. For GulfCet I, hydrographic features were

defined by sea surface height measured by satellite altimetry. For GulfCet II, major

hydrographic features were determined by shipboard CTD and XBT data used to compute

dynamic sea surface height. Five hydrographic regions were defined: cyclone, anticyclone,

periphery of the eddy, confluence zone, and "other" areas outside these defined features.

Effort was divided into 42-minute blocks for statistical analysis. Sighting rates (in sightings per

hour) were determined for three cetacean categories: all species, sperm whales and pantropical

spotted dolphins. A nonparametric Kruskall-Wallis test was used to determine the difference in

sighting rate for each combination of cetacean category, sound level category and hydro graphic

feature. For additional information on the effects of seismic exploration pulses on cetacean

distribution, see Rankin (1999).

5.4 Results

5.4.1 Survey Effort

There was a total of 5,228 km of acoustic survey effort, with 2,784 km during the 1996 late

summer cruise and 2,444 km during the 1997 mid-summer cruise. During the late summer

cruise, effort was fairly uniform throughout the study area north of 27°N, with several lines of

effort as far south as 25°N (Figure 5.3). Acoustic effort occurred in waters as shallow as 50 m

off of the Florida panhandle and Alabama. To the south and east, waters deeper than 3,000 m

were sampled. Poor weather significantly affected both acoustic and visual sampling during this

cruise, particularly at the beginning. In contrast, during the mid-summer cruise there was no

effort south of 26°N (Figure 5.4), but increased effort to the east along the Florida escarpment to

sample the various hydrographic features. There was relatively less acoustic effort at night due to

mid-water net tows for biological sampling (see Chapter 3).

5.4.2 Cetacean Contacts

We had a total of 73 dolphin and 20 sperm whale contacts during the two GulfCet II cruises

(Table 5.1). There were 1.5 times as many dolphin contacts during the mid-summer cruise

compared to the late summer cruise. There were 11 and 9 sperm whale contacts during the late

summer and mid-summer cruises, respectively. We had 13 and 16 unidentified dolphin contacts,

respectively, during the two cruises. Sperm whales were the most commonly encountered

identified cetacean during the late summer cruise, while pantropical spotted dolphins were the

most commonly encountered species during the mid-summer cruise.

180
24 - I ' I I 24

-90 -89 -88 -87 -86 -85 -4

Figure 5.3. Acoustic effort transects (bold lines) conducted during late summer

1996 cruise. Dashed lines represent isobaths. Solid contour lines

represent sea surface dynamic height in 5 cm intervals. The cyclone

is between 88 and 100 cm, the confluence between 100 and 122 cm,

and the anti-cyclone between 125 and 150 cm.

181
30

29

27-

26-

25-

-90

lOOm

28-

3000 m

-89 -88 -87 -86 -85 -84

120 160

----. 140

.__

182

0

29

28

27

-26

25

24 I I H 24 -90 -89 -88 -87 -86 -85 -84

Figure 5.4. Acoustic effort transects (bold lines) completed during the midsummer 1997 cruise. Dashed lines represent isobaths. Solid

contour lines represent sea surface dynamic height in 5 cm intervals.

The cyclone is between 92 and 105 cm, the confluence between

105 and 129 cm, and the anticyclone between 130 and 176 cm.
Table 5.1 Numbers of cetacean acoustic contacts by cruise and species.

183

Species i 996 Late Summer 1997 Mid- Combined Gyre Gyre Cruise summer Gyre Cruises

Cruise

Rough-toothed dolphin 1 1 2

Bottlenose dolphin 3 2 5

Pantropical spotted dolphin 6 20 26

Striped dolphin 2 1 3

Clymene dolphin i 0 1

Spinner dolphin o 2 2

Atlantic spotted dolphin 2 1 3

Unidentified Stenella 1 1 2

Unidentified dolphins 13 16 29

Total dolphins 29 44 73

Sperm whales ii 9 20

Total Cetaceans 40 53 93
5.4.3 Cetacean Distribution

5.4.3.1 Sperm Whales

The distribution of sperm whales in the central and eastern Gulf did not appear to be random

during the two cruises. Rather, they appeared to be linearly related, on an approximate northsouth axis during the late summer 1996 cruise (Figure 5.5), and east-west during the midsummer 1997 cruise (Figure 5.6). Of the 11 sperm whale contacts during the late summer cruise,

nine were west of 87° W. Whales were detected in the same area near the MOM as had been

observed in all GulfCet I cruises. This area now contains a number of deep-water oil platforms

that were not present at the beginning of GulfCet I. Sperm whales were found approximately 35

km farther south than on previous cruises, although there is no evidence that this was in response

to increased oil and gas activity in the area. The findings, however, show that sperm whales still

maintain a presence in offshore waters south of the MOM. No animals, however, were detected

immediately east of the delta, even though there was significant survey effort. Nearly half (five of 11) of the contacts were entirely or partially in waters deeper than 2,000 m. This is in contrast

to the GulfCet I results (when effort was limited to water depths less than 2,000 m) where sperm

whales occurred along the mid-to-lower slope (ca. 1,200 m ocean depth). During the midsummer cruise, sperm whale contacts were limited to a nat-row band running along the 28°

parallel between 89° and 86°W longitude. This was the case even though effort was basically north-south. Six of the nine contacts were in waters deeper than 2,000 m. Sperm whale contacts

during the late summer cruise averaged 26 km in length (n=1 1, c = 34, range = 1-114 km), while

for the mid-summer cruise they were significantly shorter (Mann-Whitney U, p<O.00), averaging

5 7 km (n=9, 7.3, range = 0.5-21.6 km).

5.4.3.2 Dolphins

Dolphins were detected throughout the study area during both cruises. Dolphin distributions

during the late summer cruise were concentrated in either relatively shallow waters or within a

single deep-water area (Figure 5.7). The shallow contacts, almost all in waters less than 200 m,

were distributed along the continental shelf of the EPA. Almost one-third of all dolphin contacts were in deep water south of 27°N. During the mid-summer cruise, dolphins were much more

uniformly distributed in pelagic waters, and there were few contacts in the shallow waters of the

eastern EPA (Figure 5.8). Even in pelagic waters, there were "holes" in their distribution, with

no contacts in an approximately 1° square area centered at approximately 27.5°N and 88° W.

The number and distribution of pantropical spotted dolphin contacts were markedly different

between years. In late summer 1996, three of four contacts were in very southern waters, among

the most southern of any contacts during either cruise (Figure 5.9). In contrast, during midsummer 1997, most of the 20 contacts were in a narrow strip between the 1,000-2,000 m depth

contours running east of 88° W longitude south of the Big Bend area of Florida.

The remaining identified dolphin contacts were a mixture of shallow-water species (i.e.,

bottlenose and Atlantic spotted dolphins) found along the northern edge of the study area, plus

several contacts from pelagic species distributed throughout the study area (Figure 5.10).

184
Figure 5.5. Location of acoustic contacts from length are represented by crosses, averaged 26 km in length. sperm whales during the 1996 late summer cruise. C

whereas continuous contacts longer than 5 nm are rep
24 I -92 -90 -88 -86 -84 

Figure 5.6. Location of acoustic contacts from sperm whales during the 1997 mid-summer cru

rim in length are represented by crosses, whereas continuous contacts longer than Contacts averaged less than 6 km in length, significantly less than in 1996.
I H I -92 -90 -88 -86 -84 

Figure 5.7. Location of acoustic contacts from all dolphins during the 1996 late summe
I I I -92 -90 -88 -86 -84 

Figure 5.8. Location of acoustic contacts from all dolphins during 1997 mid-summe
I H -92 -90 -88 -86 -84 

Figure 5.9. Location of acoustic contacts from pantropical spotted dolphins during the 1996 lat

and 1997 mid-summer cruise (dots). The northern most 1996 contact is actually tw
-92 -90 -88 -86 -84 

Figure 5.10. Location of acoustic contacts: bottlenose dolphins during the 1996 late sununer cr

the 1997 mid-sunijrer cruise (closed dots); striped dolphins during 1996 (open squcircle); clymene dolphins during 1996 (open triangle); spinner dolphins during 199spotted dolphins during 1996 (open diamond) and 1997 (closed square); and rough

(dotted square) and 1997 (closed triangle).
In late summer, the distribution of unidentified dolphins was divided between waters to the north and animals in shallow a disjunct set of contacts far to the south (Figure 5.11). The more numerous contacts in late summer were distributed throughout the deeper waters north of 27°N.

5.4.4 Ecological Correlates

During both cruises, cetacean distributions appeared to be influenced by the presence of a cyclone to the north, paired with an anticyclone to the south. While the feature ages were similar in both years, their relative location and orientation were different. In 1996, the cyclonewas centered south-southeast of the MOM and to the northwest of the anticyclone. In 1997, the cyclone was southeast of the MOM and northeast of the anticyclone, near the DeSoto Canyon. The cyclone was larger in 1996 than in 1997.

5.4.4.1 Sperm Whales

Sperm whale contacts were more dispersed and were significantly longer in length and duration in late summer 1996 compared to mid-summer 1997. With few exceptions, sperm whale contacts in late summer were on the western periphery of the cyclone, including contacts south of the MOM (Figure 5.12). One contact began on the northern periphery of the cyclone and continued into the center. Another contact ran across the confluence toward the center of the anticyclone. The dispersed distribution in 1996 contrasted with that in 1997, when sperm whale contacts were either inside or on the periphery of the cyclone (Figure 5.13). They were also farther east than the year before, probably because of the more easterly location of the cyclone. Most sperm whale contacts during mid-summer were accompanied by dolphins, often pantropical spotted dolphins. With one exception, there were no sperm whale contacts in the anticyclorie.

5.4.4.2 Dolphins

The distribution of dolphin acoustic contacts during the 1996 late summer cruise was disjunct, with approximately half in shallowwater along the shelf break (Figure 5.14). This area is largely unaffected by cyclones. The other half of the contacts occurred along the southern edge of the cyclone or in the confluence zone. Dolphin distribution in 1997, particularly in the north and west, appears to have been influenced by fresh water from the MOM area that was transported off the shelf by the cyclone (Figure 5.15, see Chapter 2 for details). Animals were distributed along a curve circling the upwelling cyclone and following the geostrophic flow between the eddies. There were fewer shallow water contacts during 1997.

The few pantropical spotted dolphin contacts during the late sunmier cruise were located in the confluence zone (Figure 5.16). During the mid-summer cruise, most pantropical spotted dolphin contacts were in or on the periphery ofthe cyclone or in the confluence; only two contacts occurred in the anticyclone (Figure 5.17).

During the late summer cruise, unidentified dolphins were either distributed along the shelf break (probably bottlenose or Atlantic spotted dolphins) or in the cyclone (Figure 5.18). During the mid-summer cruise, most of the unidentified dolphin contacts occurred along the periphery of the

191
30- 28- 26- 24-92 -90 -88 -86 -84 -82 Figure 5.11. Location of acoustic contacts from unidentified dolphins during the late summer 1996 cruise (crosses) and the 1997 mid-summer cruise (dots).
30 28 26 - 24 1 

-92 -90 -88 -86 -84 

Figure 5.12. Distribution of sperm whales relative to hydrographic and bathygraphic data during 

Solid contour lines represent sea surface dynamic height in 5 cm intervals. The cyccm, the confluence between 100 and 122 cm, and the anti-cyclone between 125 an

indicate the location of CTD and XBT stations along the cruise track.
2430 28 26 -92 Figure 5.13. -9k, -öö -86 -84 Distribution of sperm whales relative to hydrographic and bathygraphic data during t

Solid contour tines represent sea surface dynamic height in 5 cm intervals. The cyclon

the confluence between 105 and 129 cm, and the anticyclone between 130 and 176 cmlocation of CTD and XBT stations along the cruise track. in 
-92 -90 -88 -86 -84 

Figure 5.14. Distribution of all do'phin acoustic contacts relative to hydrographic and bathygr

sunljner cruise. Solid contour lines represent Dynamic Sea Surface Height (SSH

dyn cm correspond to a cyclonic, cold core eddy, and areas where SSH 125 dycyclonic, warm core eddy. Small crosses indicate location of CTD and XBT stat
2430 28 - 26Figure 5.15. Distribution -92 -90 -88 -86 -84 

of all dolphin acoustic contacts relative to hydrographic and bathyg

summer cruise. Solid contour lines represent Dynamic Sea Surface Height (SSH

dyn cm corresponds to a cyclonic, cold core eddy, and areas where SSH 130 dy

warm core eddy. Small crosses indicate location of CTD and XBT samples statio
30 28 26 24 -92 -90 -88 -86 -84 

Figure 5.16. Distribution ofpantropical spotted dolphins relative to hydrographic and ba

the 1996 late summer cruise. Solid contour lines represent Dynamic Sea Su

Areas where SSH 100 dyn cm correspond to a cyc1onic cold core eddy, adyn cm correspond to an anti-cyclonic, warm core eddy. Small crosses indi

XBT samples along the cruise track. Most of the contacts were in the conve
28 26 2430 -92 -90 -88 -86 -84 Figure 5.17. Distribution 

of pantropical spotted dolphin acoustic contacts relative the 1997 mid-summer to hydrograph

cruise. Solid contour lines is between 92 and 105 represent sea surface dynamic he

cm, the confluence between 105 and 129 cm, and the anticy

Small crosses indicate the 

focation of CTD and XBT stations afong the cruise track

associated with the periphery of the cyclone, as were the sperm whales during that 
2430 28 26Figure 5.18. Distribution of all unidentified dolphin acoustic contacts relative to hydrographic a

-92 dyn cm correspond to a cyclonic eddy, and areas where SSH> 125 dyn cm correspo

Small crosses indicate location of CTD and XBT stations along the cruise track. 1996 late summer cruise. Solid contour lines represent Dynamic Sea Surface Heigh

-90 -88 -86 -84 
cyclone or in the confluence,

River Were entrained in

including the northern edge where waters from the Mississippi the confluence zone (Figure 5.19). Two contacts occurred in the anticyclone.

5.4.5 Whistle Characterization and Identification

A total of 1,117 vocalizations from nine species were analyzed (Table 5.2). These nine species represent over 75% of the total estimated dolphin population in the northern Gulf of Mexico (Table 4.11). Three species alone, bottlenose dolphin, clymene dolphin, and pantropical spotted dolphin, represent over 70% of all the dolphins in the study area (Table 4.11).

The whistles were classified into six whistle types based on spectrograms. The most common was a concave whistle, and the constant frequency whistle was the least common (Table 5.2). There was inter-specific variability in whistle usage. This is best seen in the diversity indices for the nine species. The pantropical spotted dolphin had the most diverse whistle usage, indicating that it used all six whistle types and didn't emphasize one particular call. The Atlantic spotted dolphin had the least diverse whistle lexicon. Half of the lexicon of calls for both Atlantic spotted and striped dolphins were concave whistles. Additionally, Atlantic spotted dolphins did not use convex whistles, while Fraser's dolphin did not use constant frequency whistles. The five species in the genus Stenella used upsweep whistles three times as often as non-stenellids. Overall, the most commonly used vocalizations were upsweeps and concave whistles, each representing 25% of the overall lexicon. This difference in usage was one clue to species identification. Additionally, the acoustic characteristics of the whistles were highly variable and could be used for identification (Table 5.3). The parsing of this variability using discriminant function analysis permitted grouping by species. The final factor used for identification was ocean depth at the time of the recording. Two species, bottlenose and Atlantic spotted dolphin, were found primarily in shallow waters, with all other dolphin species in deeper waters. There was considerable inter-specific overlap in the mean values for the acoustic parameters for all identified dolphins (Table 5.3). This inter-specific overlap prevented satisfactory classification of the combined data set. The inability to classify these signals was expected, because initial comparisons were made between all whistles for all recorded species. There was greater intra- specific variability between different whistle types (e.g. pantropical spotted dolphin upsweeps versus downsweeps) than inter-specific variability within whistle types (e.g. pantropical spotted dolphin sine whistles vs. clymene dolphin sine whistles). By combining information about the lexicon, its physical context and the DFA characterizations, a contact identification decision tree was constructed (Figure 5.20).

We analyzed 60 whistles from 11 unidentified dolphin contacts that were recorded during the two surmner cruises. The same DFA used on the identified contacts was applied to the unidentified signals, and the decision tree was used to identify four contacts. Unidentified contacts that were not whistles (i.e. dolphin pulses) could not be identified with this procedure. A scatter plot from the DFA of sine whistles from all species shows that two species, bottlenose dolphin and false killer whale, are separated from all of the other dolphin whistles (Figure 5.21). Sine whistles were the most common signal used by these two species. If the sine whistle was identified as coming from either a false killer whale or bottlenose dolphin, then the ocean depth

200
30 28 26 - 24 -92 -90 60 40 -88 120 -86 -84 

Figure 5.19. Distribution of unidentified dolphin acoustic contacts relative to hydrographic an

1997 mid-sununer cruise. Solid contour lines represent Dynamic Sea Surface He

<105 dyn cm correspond 

to a cyclonic, cold core edy, and areas where SSH 13

cyctonic, warm-core rings. Small crosses indicate location of CTD and XBT stat
Table 5.2. Number of whistles types by species and the Shannon-Weiner Index for whistle dive

Species Constant Freq. Upsweeps Whistle Type Downsweeps Concave Convex Si

Pantropical spotted dolphin Atlantic spotted dolphin 2 61 24 43 27 3Clymene dolphin 5 7 1 21 0 

Striped dolphin 1 29 3 48 16 3Spinner dolphin 2 25 9 58 8 1

Bottlenose dolphin 36 89 24 24 22 3Rough-toothed dolphin 2 6 ii 24 6 3Fraser's dolphin 15 34 6 16 3 3 False killer whale 0 1 2 11 5 6 

Total 32 23 42 35 23 73 

95 275 122 280 110 23
Table 5.3 Means for acoustic parameters for all identified dolphins from all cruises. Duration is 

parameters are in Hz. Species Dur. Beg. Freq. End Freq. Acoustic Parameters Beg-End High Low Freq. Freq. Freq. High-L

Freq

Pantropical spotted dolphin Atlantic spotted dolphin Clyinene dolphin Striped dolphin Spinner dolphin Bottlenose dolphin Rough-toothed dolphin Frasers dolphin False killer whale All species 0.669 0.390 0.409 0.473 0.743 0.611 0.377 0.703 0.591 0.585 10036 10464 10766 9845 9389 8836 6945 10735 7138 9099 12012 12076 12058 10346 12216 7975 8177 10367 6910 10252 4300 2528 2790 2220 4709 2902 1554 2253 1492 3027 14416 12698 13621 11635 13741 10879 8486 12960 8428 11913 7739 6866 9245 7824 7708 5738 6377 8684 5706 7240 6677 5832 4376 3811 6033 5141 2109 4276 2722 4673 
All Other Whistles Concave Beginning -End DFA Convex DFA Concave All Whistle Durations Pantropical Spotted Dolphins Striped Dolphins DFA Down Sweeps Clymene Dolphins Striped Dolphins DFA Up Sweeps All Other Whistles 

W

Atlan

Do

Rough-Toothed Frase?s Dolphin Dolphin Striped Dolphins Spinner Dolphins W

Figure 5.20. Decision tree for identifying unknown dolphin contacts. The factors used in th

1) discriminant function analyses of acoustic parameters from specificwhistle 

whistles) from identified signals; 2) the ocean depth at the contact location; ansuch as mean whistle duration. The identity of a dolphin contact can be made 

decision rules to multiple whistles within a single contact.
Root 1 Figure 5.21. Scatter plot of discriminant function analysis of sine whistles from bottlenose d

whales (squares) and all other dolphins (circles). All other dolphins are reliably other species, which can then be separated according to the ocean depth at the c
at the contact location could distinguish the shallow water bottlenose dolphin from the deep water false killer whale. This same procedure was used for other whistle types

With a sufficiently large or distinctive repertoire within a contact, identification can be confidently made. For example, one contact was identified as coming from rough-toothed dolphins using the following reasoning. The contact was recorded in water depths of 1,500 m. This eliminates bottlenose and Atlantic spotted dolphins as likely sources. All five vocalizations were a specific type of upsweep whistle. Rough-toothed dolphins primarily (88%) use this call, whereas no other species uses it more than 34% of the time. Lastly, the sound of the signal, its spectrogram and the DFA analysis suggested it was from rough-toothed dolphins Figure 5.22 illustrates the DFA scatterplot for the rough-toothed dolphin whistles, the five whistles from the unidentified contact, and all other dolphin whistles. The whistles from the unidentified contact are well matched to those from the rough-toothed dolphin. Similar analyses identified previously unknown contacts as being from pantropical spotted dolphins, Atlantic spotted dolphins and unidentified Stenella species.

5.4.6 Effects of Seismic Exploration on Cetaceans

In the nearly 17,000 km of acoustic monitoring in the Gulf of Mexico during GulfCet I and II, many forms of man-made sounds were recorded. These sounds could adversely affect animals by physically injuring their hearing, causing them to leave the area, or masking their own

communication and food-finding signals (Richardson et al. 1995). The form of the signal, therefore, has an impact on whether it will affect an animal. A loud, pulsed explosion may deafen an animal, whereas a continuous signal (e.g. vessel noise) may result in the animal not hearing conspecifics calling, or interfere with reception of their own signals. Seismic signals are often described as being strictly low frequency signals, with energies only below 1 kHz. However, we recorded loud, pulsed signals associated with seismic exploration with center frequencies at 2.5 kHz, with little energy below 1 kHz (Figure 5.23). These signals are similar in frequency to sperm whale clicks, and are probably audible to the whales. Additionally, we recorded other signals, centered at 3.5 kHz that are used to locate the multiple hydrophone arrays relative to each other and the sound source (Figure 5.24). During the late summer 1996 cruise, we recorded a long series of these pulses (Figure 5.25) that were very similar to sperm whale signals.

Subsets of the acoustic and visual data were examined to determine if seismic signals affect cetacean distribution. Over 514 hours of"on effort" recordings covering approximately 6,819 km of survey effort were used. A total of 108 hours, or 21% of the total time, contained sounds associated with seismic exploration (Figure 5.26a). The percent time with seismic exploration

sounds was 10.4% for GulfCet I (Figure 5.26b) and 34.1% (Figure 5.26c) for GulfCet II. The final cruise of GulfCet II had the greatest percentage of seismic exploration sounds, with pulses present 49.8% of the time. These sounds were not uniformly distributed, but were concentrated in the more shallow areas of the study area, the upper and mid-continental slope, and away from the deeper waters further to the south.

Signal to noise ratios were described to further understand the average distribution of seismic exploration signals. Measurements of 52 seismic exploration signals were analyzed for signal

206
-5 -4 -3 -2 -1 0 

Root I Figure 5.22. Scatter plot of the discriminant function analysis of upsweep whistles from ro

the unidentified contact (triangles) and all other dolphins (circles) used to iden
0 I r I 1.0Sperm Whale Pulses Sei

'V I Sperm Whale Pulses I Se

2.0 Seconds 3.0 

Figure 5.23. Spectrogram (top) and waveform (bottom) illustrating sperm whale pulses and t

spectrogram displays frequency (y-axis) as a function of time (x-axis) with increamplitude. The waveform displays amplitude over time. Eight sperm whale pulsfrequency at approximately 2.5 kHz. The first seismic pulse is also centered at 21 kHz. The second pulse is centered at 900 Hz. The final pulse, centered at less t

'V 
Sperm WhalesGulfUet survey vessel towing single hydro- phone array Pin

Seismic Sou

Pingers 7&8 

Figure 5.24. Schematic diagram, not to scale, of the seismic noise sources and receivers. The geo

pulses from three sources: pingers for the acoustic positioning system; one high freqpro ifier; and a low frequency seismic profiler. For the acoustic positioning system, tsix arrays, with a seventh pinger on a seismic profiling source, and the eighth pinger figure, there are sperm whales producing pulses. All of these signals are detected by 

the GulfUet survey vessel. In actuality, the geophysical survey vessel is 16 km from geophysical arrays are probably five km long, while the GuliCet hydrophone array is 
Array Locator Pulses Seismic Pulses 

10 

2 4 6. & Time (Seconas) Figure 5.25. Spectrogram of three types of signals used in the seismic exploration industry. A 

frequency (y-axis) as a function of time (x-axis), with relative darkness indicatinlocator pulses occur prior to the seismic pulses. The locator pulses are centered ais centered at 2.5 kHz, with little energy below I kHz and above 5 kElz. The seco

at 900 Hz, and its echo is centered below 100 Hz. Six seconds after the echo, the 
Effort with seismic signals present: 21% Effort wit

A. GULFCET I & II seismic si

present: 7

Effort with seismic signals present: 10% Effort with seismic signals present: 34% B. GULFCET I Effort without seismic signals present: 90% 

Figure 5.26. Percentage of effort with and without the presence of seismic exploration signa

(northern Gulf), (B) for seven GulfCet I cruises (western and central Gulf), and 

(eastern and central Gulf).
characteristics. The overall average signal to noise ratio was 8.4 dB re 1 j.iPa, that is, on average

a seismic exploration signal was 8.4 dB above the ambient noise recorded by the array as it was

towed behind the survey vessel. The maximum signal to noise was 13.1 dB and a minimum of

4.3 dB. With regard to the spectral characteristics, the overall average maximum frequencies were 613.5 Hz , while the minimum was 26.7 Hz. The highest frequencies recorded from these

signals were 2,426 Hz. The overall average peak frequency was 81.7 Hz, ranging from

35- 107 Hz.

Vocalizations of many cetaceans, as well as shrimp, were concurrently recorded with seismic

exploration survey pulses. These included sperm whales, striped dolphins, Fraser's dolphin,

pantropical spotted dolphins, clymene dolphins, spinner dolphins and rough-toothed dolphins In

addition, there were many unidentified cetacean vocalizations,

There was no significant difference between the overall sighting rate and the sound level when

tested for each of the hydrographic regions. Distributions of two species, with sufficient number of sightings, were additionally examined. The sperm whale sighting rate did not differ

significantly between the different sound levels for any of the hydrographic features. The

sighting rate of the paritropical spotted dolphin was found to be significantly higher for the >12

dB areas within the hydrographic feature defined as "other" (x2 = 10.26, p=.005). The sighting

frequencies for the remaining hydrographic features did not differ significantly by sound level.

5.5 Discussion

5.5.1 Comparing Visual and Acoustic Surveys

There are a number of appropriate comparisons between the visual and acoustic survey methods used during Gulftet II. Contact rate is the number of contacts per unit effort distance. Acoustic

and visual contact rate can be compared for area and species group. For all dolphin species

(including unidentified species) along the continental slope, the visual survey had 66 contacts in 2,572 km of effort or 0.026 contacts/km. This compares to 17 contacts in 2,172 km or 0.0078

contacts/kilometer for acoustic effort. The visual survey contact rate was three times greater than

the acoustic rate. In the oceanic northern Gulf, the visual survey had 97 contacts for all dolphins in 8,595 km of effort or 0.0113 contacts/kin. This was similar to the acoustic survey, where there were 28 contacts in 2,812 km of effort or 0.0100 contacts! km. For sperm whales throughout the

study area, the visual survey had 29 contacts in 11,167 km of effort or 0.0026 contacts/km. The acoustic survey had 20 contacts in 4,984 km of effort or 0.0040 contacts/kilometer, which is 1.54

times greater than the visual rate. Easy recognition of vocalizations and greater detection range best explain the higher acoustic contact rate for sperm whales. Additionally, their long dive

times reduce the likelihood that all animals will be seen. In some cases, acoustic contacts lasted for many hours, which couldn't occur for the visual survey. The smaller number of dolphin

contacts for the acoustic survey is difficult to explain, but few animals were contacted during the

late summer 1996 cruise. Neither visual nor acoustic surveys had many contacts during that

cruise, which represented approximately half ofthe acoustic effort but less than a quarter of the visual effort.

212
There was general agreement on locations of sperm whales between acoustic and visual methods. During the late suimner cruise, contacts by both methods were largely limited to the western side

of the study area (Figures 6.3 and 5.12). However, animals were detected acoustically south of

27°N where none were visually detected. This probably resulted from the poor weather conditions that limited visual effort. During the mid-summer cruise, agreement between the two

survey methods was also strong; most sperm whale contacts occurred around the cyclonic eddy

(Figures 6.9 and 5.13).

In comparing dolphin contacts, there appeared to be important differences between visual and

acoustic survey methods. During the late summer cruise, there were fewer acoustic than visual

contacts on the continental shelf, but more acoustic than visual contacts at the confluence of the

cyclone and anticyclone (Figures 6.2 and 5.14). There were also more acoustic contacts on the

eastern slope of the study area. However, both methods showed dolphins farther south in the margin of the anticyclone. The agreement in contact location between methods was better in

mid-summer 1997, with most dolphin contacts in or near the cyclonic eddy or at the confluence between the cyclone and anticyclone (Figures 6.8 and 5.15). In both cases, there were many

contacts on the eastern edge of the study area, reflecting the strong eastern geostrophic flow

created by the confluence zone (see Figure 2.09 and 2.11) and stronger shelf upwelling than in

late summer 1996. In a similar fashion, there were a number of dolphin contacts to the west of

the cyclone, due perhaps to fresh water from the MOM area that was entrained in the confluence

zone and transported over the slope (Figure 2.11). There were five visual contacts farther south

in the anticyclone that were not detected by the acoustic survey (Figures 6.8 and 5.15).

5.5.2 The Effects of Seismic Exploration on Cetaceans

The results show that the cetacean sighting rate did not change significantly between areas of

low, moderate, or high intensity seismic exploration sounds. As described in the methods, these

sound level categories were relative (i.e., signal-to-noise ratio) to the noise level at the hydrophone array. A major source of ambient noise during our recordings was the ship while

traveling at nine knots. If a dolphin were to hear a seismic signal at a similar distance, the signaltonoise ratio would be considerably higher than for our recordings because of the noise produced by the survey vessel towing our hydrophone array.

There were almost no instances where our survey vessel was within 5 km of a seismic vessel.

All of the seismic exploration signals examined in this study appeared to come from distant vessels because they were relatively low intensity and low frequency in character (Rankin 1999). If we had been closer to the sound source, the signals would have been higher in intensity and

contained higher frequencies. If seismic signals affect dolphins, it is probably due to the higher

frequencies which they hear best.

Our analysis of the potential effects of seismic signals on cetaceans was unable to detect smallscale (i.e., < 100 km) changes in distribution. Finer resolution studies, similar to those conducted

by Goold (1996) in the Irish Sea, and in closer proximity to the sound source are needed to

determine the local impact of these signals. Three aspects of the cetacean community are

relevant to the potential effects of seismic exploration in the Gulf: a large and diverse dolphin

community, a large and stable sperm whale population, and very few baleen whales. Dolphins,

213
which are the largest component of the cetacean community, are least Sensitive to signals below

1 kHz (Richardson et al. 1995). It is therefore likely that their own hearing thresholds limit the

impact of seismic signals. This would be true for nearly all encounters in this study, as the

signals were from distant sources. The species likely to be most sensitive to low frequencies,

Bryde's whale, regularly occurs in small numbers along the shelfbreak in the eastern Gulf.

There appears to be site fidelity among some sperm whales in an area of intense oil industry

activity near the MOM. Nevertheless, sperm whales continue to use this area of heavy seismic

exploration. It is unclear whether the site fidelity is a consequence of low sensitivity to seismic

noise or a high motivation to remain in the area. Townsend (1935) noted the presence of sperm

whales in the north-central Gulf of Mexico as far back as the mid-1800's. This indicates that

sperni whales have historically occupied this area and have been exposed to noise for many

years. Their continued presence would appear to indicate either tolerance or insensitivity to this noise.

In explaining the possible responses of cetaceans to seismic signals, it is necessary to understand

both how and why cetaceans might react. While the auditory thresholds of a few dolphin species

are known, little is known about either sperm whale or baleen whale hearing. Because of their

low-frequency vocalizations and auditory morphology, it is thought that baleen whales have

good low frequency hearing. We suggest that sperm whales also have good low frequency hearing

because of their large size and use of relatively low frequency pulsing. Both dolphins and sperm

whales produce high amplitude, transient pulses. Like most animals, they have mechanisms to

protect their hearing against loud sounds. If the rise time of a seismic pulse is slower than their

own signals, it is likely that cetaceans can protect their hearing against loud pulses. There would

still be a limit beyond which their hearing could still be damaged by sufficiently loud transients.

5.5.3 Sources of Variance in Acoustic Surveying

The sources of acoustic survey variability can be broadly divided into natural variation and

sampling biases. Natural variability is a result of the distribution ofthe animals and the physical conditions in which they are found. Regardless of whether the distribution is random, clumped

or correlated with some natural variable (e.g., food distribution), there will be variation in the

number of contacts encountered per track line. There will also be variation in the detection

distance, as a function of ambient noise, and variability in the transmission characteristics of the

water mass. Additionally, there will be variation in the acoustic signal used to identify the

presence of the animal. Signals will vary by frequency, duration, amplitude, the animals'

behavioral state, and their relative distance from the hydrophones. Variation in these acoustic

variables will affect the survey results to the degree that the hydrophone has the bandwidth and

sensitivity to detect the signals. The ability to determine the temporal structure of the signal may

also be affected by the transmission medium. For example, multiple transmission channels, as a

result of temperature and salinity variations with depth, can produce multi-paths so that pulse

configurations are altered.

Marsh and Sinclair (1989) described sources of bias in strip transect surveys. They suggested a

method for describing these biases that has many parallels to problems that might occur with

acoustic surveys. The authors described two general forms of bias: perception bias and

availability bias.

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Perception bias occurs where animals are present but not detected. In the acoustic domain, this

occurs when an animal makes a sound that is not heard. Given the idiosyncrasies of each

person's perceptual capabilities, perception bias can be considered observer bias. This bias may

be due to a number of factors including the observer's lack of experience or knowledge of the

pertinent sounds, or to inattention due to fatigue or lack of motivation.

Availability bias in the acoustic domain occurs when a sound is made but is undetectable to the

observer. This may be due to masking by loud noise (i.e. the S/N ratio of the signal is below the detection threshold) or otherwise being imperceptible. The parallel condition in the visual domain occurs when an animal is unavailable to be seen because the water is turbid.

Variability can be reduced using two approaches: acoustic methods and data stratification.

Acoustic methods improve the signal-to-noise ratio, which enhances sound detection. Data can

be stratified by either noise conditions and/or by species. The signal-to-noise ratio can be

improved by increasing the signal strength and using band pass filters. A 6 dB increase in

signal4o-noise will result in a doubling of the detection distance and a four-fold increase in the

overall strip width. This will increase the detection of acoustic contacts, which will decrease the

variance and result in a more precise abundance estimate. We used 24 dB/octave filters to

increase signal-to-noise.

Data stratification reduces variance by grouping data based on a similar parameter, such as signal amplitude, species or noise. Stratification requires a case-specific analysis of the pertinent data.

For example, in visual surveys, data is stratified according to similar detection characteristics of

the various species; those that are similarly visible have the same detection function. Likewise,

acoustic data could be stratified according to the type of vocalization (e.g., whistles, echolocation

pulses, sperm whale pulses). Stratification by noise level is based on the fact that, given equal

signal amplitudes, noise limits our detection distance. Classif'ing data according to ambient noise conditions reduces the overall variance of the realized strip width.

5.6 Conclusions

The northern Gulf of Mexico has a large and diverse cetacean community, containing at least 19

species. The diversity of this community is comparable to the northeastern Atlantic and the

eastern tropical Pacific (Ham et al. 1985, Wade and Gerrodette 1993). The acoustic survey had 73 dolphin and 20 sperm whale contacts during two cruises, with 29 dolphin and 11 sperm whale contacts during the late summer 1996 cruise and 44 dolphin and 9 sperm whale contacts during

the mid-summer 1997 cruise. Recordings were made for sperm whales, pantropical spotted

dolphins, clymene dolphins, spinner dolphins, striped dolphins, Atlantic spotted dolphins, false killer whales, bottlenose dolphins, rough-toothed dolphins and Fraser's dolphins. No recordings were made for any baleen or beaked whale.

The distribution of deep-water cetaceans appeared to be influenced by the cyclone and

anticyclone pair in the eastern Gulf. Sperm whales in the Gulf ofMexico were more closely

associated with the cyclone and the MOM and less with the confluence zones than some of the

pelagic dolphin species. Almost half of the dolphin contacts in 1996 were in relatively shallow

waters north of the influence of the cyclone and anticyclone. The more pelagic contacts were

215
found considerably to the south in the confluence zone. In 1997, dolphins appeared to have three

association patterns relative to the existing hydrographic features. Some northern contacts

appeared to be influenced by the entrainment of Mississippi River outflow as it was brought seaward by the confluence zone; others were associated with the center of the cyclone; and

finally others occurred within the confluence zone, in particular with the strong western current produced by the geostrophic flow. There were only two dolphins and a single sperm whale

contact inside or near the northern periphery of the anticyclone. This distribution pattern

suggests that anticyclones are not preferred areas for cetaceans in the Gulf of Mexico.

The whistles of nine dolphin species were characterized based on the analysis of 1,117 signals.

There were species-specific patterns to whistle usage and acoustic structure. Based on these patterns, some unknown contacts were identified as coming from pantropical spotted dolphins, Atlantic spotted dolphins, rough-toothed dolphins and false killer whales.

A diversity of anthropogenic signals were recorded during nearly 17,000 km of acoustic effort during GulfCet I and II. Many of these were low frequency seismic exploration signals, but higher frequency seismic signals were also recorded. Bryde's and sperm whales can probably detect these signals. Cetaceans were exposed to a significant and increasing amount of noise,

particularly off the MOM and in the eastern Gulf. Seismic exploration signals were detected

during 21% of recordings, ranging from a low of 10% during the earlier GulfCet I project to

more than half during the last GulfCet II cruise. There was no significant difference in the

cetacean sighting frequency for low, medium, and high noise zones when controlling for differences in hydrographic conditions.

216
Chapter 6

Cetacean Habitat in the Northern Gulf of Mexico

Randall W. Davis, Joel G. Ortega-Ortiz, Christine A. Ribic, William E. Evans,

Douglas C. Biggs, Patrick H. Ressler, John H. Wormuth, Robert R. Leben,

Keith D. Mullin and Bernd Wilrsig

6.1 Introduction

There are many factors that influence the spatial and temporal distribution ofcetaceans. These

factors include physiographic and hydrographic characteristics, prey distribution, breeding and

calving areas, and predation. Anthropogenic factors such as ship activity, oil and gas production, and seismic exploration (Richardson et al. 1995) may also influence whale and dolphin distribution. Previous studies (including the GulfCet I program) have demonstrated correlations

of cetacean distribution with physiographic features such as ocean depth and seafloor slope

(Evans 1975, Hui 1979, 1985, Selzer and Payne 1988, Sutcliffe and Brodie 1977, Payne et al. 1986, Whitehead et al. 1992, Jaquet and Whitehead 1996, Baumgartner 1997, Davis et al. 1998). Some studies have also demonstrated correlations between cetacean distribution and

hydrographic characteristics that may secondarily affect prey availability (Reilly 1990, Reilly

and Thayer 1990, Waring et al. 1993, Reilly and Fielder 1994, Jaquet et al. 1996, Jaquet and

Whitehead 1996, Croll et al. 1998, Fiedler et al. 1998, Tynan 1997, Tynan 1998, Goold 1998, Griffin 1999). However, it was not possible during GulfCet Ito correlate cetacean distribution

with hydrographic variables in the northern Gulf of Mexico (Davis and Fargion 1996, Davis et

al., 1998). This may have resulted from: (1) the small number of cetacean sightings with

simultaneous hydrographic data, (2) the dynamic hydrography of the Gulf with periodic intrusion

of the Loop Current and the formation of anticyclones (warm-core eddies) and cyclones (cold- core eddies) that move across the northern Gulf and (3) factors other than hydrographic features

that influence the wide-ranging movements of cetaceans. Physiographic and hydrographic

features alone may not fully explain cetacean distribution. Instead, the distribution of cetaceans

may be better explained by prey availability, which is secondarily influenced by hydrographic

features.

An objective of the GulfCet II program was to characterize cetacean habitat in the eastern Gulf

of Mexico (Figure 1.4). Correlation of environmental features with sighting data may improve our understanding of cetacean ecology and indicate which, ifany, physical and biological oceanographic variables influence cetacean distribution. During two of the four GulfCet II cruises, we conducted visual cetacean surveys and simultaneously collected data on the marine

environment arid zooplankton biomass. Cetaceanhabitat associations were statistically analyzed

for six physical and biological oceanographic variables. In addition, we retrospectively analyzed

satellite remote sensing data for sea surface height (SSH) anomaly and shipboard cetacean

sightings for the GulfCet I study area (Figure 1.2). These data were combined with the GulfCet II data to examine the relationship between cetacean distribution and hydrographic features for

the entire oceanic northern Gulf (Figure 1.5). A similar analysis of seabird-habitat associations is presented in Chapter 8.

217
6.2 Methods

6.2.1 Cruise Data

To determine the relationship between cetacean distribution and habitat characteristics, two dedicated cruises aboard the R'V Gyre were conducted in the eastern Gulf of Mexico in late summer of 1996 (Gyre96GO6) and mid-summer of 1997 (Gyre97GO8) (Figure 1.4 and Table 6.1). Simultaneous data were collected on cetacean distribution, hydrography and zooplankton biomass. In addition, the Gyre received near real-time plots from the Colorado Center for Astrodynamic Research of SSH anomaly that identified hydrographic features such as

anticyclones and cyclones. This enabled us to direct the ship's course into features that may have directly influenced the distribution of cetaceans or their prey.

In addition to the dedicated cruises described above, we used cetacean sighting data from 10 cruises in the north-central and western Gulf of Mexico collected from 1992-1994 during the GulfCet I Program (Davis and Fargion 1996, Davis et al. 1998) (Table 6.1). These data were

combined with sighting data from all four cruises conducted in the northern Gulf during GulfCet II. The GulfCet I data were collected from a 154,621 kin2 area located between the 100 to

2,000 m isobaths west of 87.5°W (Figure 1.2). The GulfCet II data were collected from the

oceanic northern Gulf of Mexico, an area of 396,960 km2 located within the U.S. Economic

Exclusive Zone (EEZ) in waters deeper than 100 m (Figure 1.5). This area included the GulfCet I study area, Eastern Planning Area and the focal study area for cetacean habitat surveys (Figures 1.2 and 1.4). Data from the GulfCet I study have been analyzed previously for cetacean-habitat

associations using bathymetric and shipboard hydrographic data (Davis and Fargion 1996, Davis et al. 1998). For the current analysis, we retrospectively determined SSH anomaly from satellite

remote sensing data for GulfCet I cetacean sightings and combined these data with comparable GulfCet II data for the northern Gulf. This large data set from 14 cruises was analyzed for correlations between cetacean distribution, ocean depth and hydrographic features.

6.2.2 Cetacean Sightings

During cruises, line transect data were collected by two teams of three observers during daylight hours, weather permitting (i.e., no rain, Beaufort sea state <4) using standard vessel survey data collection methods for cetaceans (Buckland et al. 1993) (see Chapter 4 for details). Each team had at least two members experienced in shipboard cetacean observation and identification

techniques. Two observers searched for cetaceans using 25x binoculars mounted on the ship's

flying bridge. The third observer maintained a search of the area near the ship using unaided eye or 7x hand-held binoculars, and recorded data. The observers rotated through each of these three

stations every 3 0-40 minutes, and each team alternated two-hour watches while there was

daylight. The vessel speed was usually 15 kmlhr but varied with sea conditions. Sighting effort

was interrupted to approach a group of animals to confirm identification, or due to weather conditions. On-effort transect segments located within a range of 5 km from each other were considered as the same segment.

218
Table 6.1. Cruises used in the habitat-association analysis.

6.2.3 Hydrography and Satellite Remote Sensing of Sea Surface Height Anomaly

Hydrographic data collection during the late and mid-summer Gyre cruises is described

in Chapter 2. Briefly, dynamic sea surface height anomaly (DHA), mixed layer depth

(MLD) and 15°C depth were determined from CTD and XBT data. The Gyre was equipped with a through-the-hull pumped seawater system. Sea surface temperature

(SST), conductivity (salinity) and fluorescence (a measure of chlorophyll concentration) were measured with a Sea-Bird Electronics Corporation in-line sensor and logged every

two minutes with the ship's Serial ASCII Interface Loop (SAIL) system.

Hydrographic features for GulfCet I and II shipboard surveys were characterized using

coincident altimetry from the TOPEXJPOSEIDON and ERS-1&2 satellites. Daily fields of SSH

topography on a 1/4 degree grid were estimated by adding the height anomaly fields, interpolated

from the available along-track altimeter data, to a model mean surface. To quantify the

relationship between sightings and the remotely sensed physical environment, the SSH anomaly

was computed for each sighting location from the gridded height fields.

Ocean depths for cetacean sightings were determined from the Global Relief Data

(National Geophysical Data Center, NOAA) that has a resolution of 5 mm. Ocean depth

gradient was calculated for each point in the bathymetric database to produce a seafloor slope in units of meters per km.

219

Vessel Survey No. Season Project Start date End date

NIMFS Oregon II 199 Spring 92 GulfCet I 4/22/92 6/7/92

NMFS Oregon II 203 Winter 93 Guiftet I 1/6/93 2/12/93

NIVJFS Oregon II 204 Spring 93 GulfCet I 5/4/93 5/28/93

TAMU Longhorn 1 Spring 92 Gulfcet I 4/14/92 4/30/92

TAMIU Pelican 2 Summer 92 GulfCet I 8/10/92 8/24/92

TAMU Pelican 3 Fall 92 GulfCet I 11/8/92 11/21/92

TMs'IU Pelican 4 Winter 93 GulfCet I 2/12/93 2/25/93

TAMU Pelican 5 Spring 93 GulfCet I 5/24/93 6/4/93

TAMEJ Pelican 6 Summer 93 GulfCet I 8/27/93 9/7/93

TAIvJ1U Pelican 7 Winter 93 GulfCet I 12/4/93 12/14/93

NMFS Oregon II 220 Spring 96 & GulfCet II 4/16/96 6/9/96

Early Summer

NMFS Oregon II 225 Spring 97 & GulfCet II 4/16/97 6/10/97

Early Summer

TAMIU Gyre Gyre96GO6 Late Summer 96 GulfCet II 10/11/96 10/19/96

TAMU Gyre Gyre97GO8 Summer 97 GulfCet II 8/6/97 8/20/97
6.2.4 Predicted Mean Biomass (PMB) of Zooplankton in the MLD

A 153 kHz narrow-band Rowe Deines Instruments (RIM) Acoustic Doppler Current Profiler

(ADCP) mounted on the Gyre was used to collect acoustic backscatter (Sw) data during the late

summer (Gyre96GO6) and mid-summer (Gyre97GO8) cruises in the eastern Gulf of Mexico (see Chapter 3 for details). Acoustic data were used to make predictions about zooplankton and

micronekton biomass in the water colunm. Biomass predictions were based on empirical

calibration of the acoustic signal from the ADCP with samples of zooplankton and micronekton

collected with a Multiple Opening and Closing Net and Enviromnental Sensing System

(MOCNESS; 1 m2 mouth area and 333 im mesh) (Flagg and Smith 1989, Zhou et al. 1994,

Ashjian et al. 1994, Ressler et al. 1998, Zimmerman and Biggs, 1999). For each MOCNESS

tow, the mean S measured at a given depth during the tow was matched with the measured

displacement volume biomass from the corresponding depth-discrete net sample. Linear

regression of the logio of MOCNESS displacement volume biomass (cc.m3) as a function of

mean S, (in dB re m1 4m(1) was used as a first-order empirical model of predicted mean biomass

(PMB; cc-m3). PMB values (based on S, data averaged every 300 seconds) had a horizontal

resolution of 0.3-1.5 km, depending on ship speed, and a vertical resolution of 4 m. Depth

discrete PIVIB estimates were vertically integrated from 10-50 m to give a summary PMB value

(cc.m2) within the MLD. MOCNESS samples were sorted for paralarval cephalopods and

myctophids, and the numbers per square meter ofsea surface was calculated. Cephalopod

paralarvae were used as an indirect assessment of adult cephalopods (which are an important

cetacean prey item) because the latter evade capture nets.

6.2.5 Statistical Methods

6.2.5.1 Cetacean Categories

Cetaceans were subdivided into five groups based on large body size and deep diving ability

(sperm whales, Category 2), intermediate body size and dietary preference for squid (squideaters, Category 3), phylogenetic relationship and occurrence in deep water (> 200 m) habitats

(oceanic stenellids, Category 4), or preference for shallow water (< 200 m) habitats (bottlenose

and Atlantic spotted dolphin Category 5). However, some species belong to more than one

category. For example, sperm whales and the oceanic stenellids (pantropical spotted dolphin, spinner dolphin, clyrnene dolphin and striped dolphin) are also squid-eaters (Pen-in et al. 1973, Clarke 1996, Ridgway and Harrison 1994). Nevertheless, grouping was necessary to reduce the number of species and increase the sample size for modeling with physiographic and hydrographic variables. Cetacean sightings were grouped as follows:

Cetaceans (all species identified)

Sperm whales

Squideaters (except for sperm whales). Dwarf and pygmy sperm whales, false killer

whales, melon-headed whales, pilot whales, pygmy killer whales, Risso's dolphins,

rough-toothed dolphins and all the members of the Family Ziphiidae

Oceanic stenellids. Oceanic dolphins from the genus Stenella: clymene dolphins,

pantropical spotted dolphins, spinner dolphins and striped dolphins but not Atlantic

spotted dolphins

220
5. Hottlenose dolphins and Atlantic spotted dolphins

6.2.5.2 GulfCet II Data for the Eastern Gulf of Mexico

For the late and mid-summer Gyre cruises, where multiple environmental variables were

collected, the presence of cetacean categories was modeled with logistic regression for each

cruise (Hosmer and Lemeshow 1989). The environmental variables used were bottom depth

(m), bottom depth gradient (m.km1), sea surface salinity (SSS, psu), dynamic SSH anomaly

(DHA, dyn cm), surface chlorophyll concentration (jig.L1), and PMB (cc.m2). Sea surface

temperature was not used in the analysis because of the almost uniformly wami surface water in

the Gulf during the sununer. Step-wise logistic regression was run using S-plus 4.5 software

(Mathsoft 1997). Only deepwater (depth >800 m) stations that had values for all environmental

variables were used in the analysis. As a result, not all species or species categories had

sufficiently large sample sizes for each cruise.

6.2.5.3 GulfCet I and II Data for the Oceanic Northern Gulf of Mexico

For the 14 cruises in the oceanic northern Gulf of Mexico, the presence of cetacean species or

species groups was analyzed with a Chi-square test for independence. Freeman-Tukey deviates

were calculated to determine the significant Chi-square contribution in each class. The

environmental variables used in the analysis were ocean depth and hydrographic features. Oneffort transects were divided into 18.5 km-long (10 nautical miles) segments. If a segment of oneffort transect were less than 9.25 km, it was added to the previous segment. If it were greater

than 9.25 km, it was treated as a separate segment. A total of 2,351 transect segments were

obtained from 14 cruises. The mean segment length was 16.6 km ± 5.61 S.D. Cetacean

categories were matched to their respective transect segment.

The oceanic northern Gulf was divided into four ocean depth categories: continental shelf(< 200

m), upper continental slope (200-1,000 m), lower continental slope (1,000-2,000 m), and abyssal

region (> 2,000 m). The study area was also divided into four hydrographic features based on

DHA for GulfCet II data and SSH anomaly for GulfCet I data. The four features were: (1)

anticyclone; (2) cyclone; (3) confluence; and (4) "other". Criteria to delineate hydrographic

features varied for each cruise. For GulfCet I Pelican and Longhorn cruises in the north-central

arid western Gulf (Table 6.1), SSH anomaly? 10 cm was considered an anticyclone; SSH

anomaly -10 cm was considered a cyclone. SSH anomaly < 10 and> -10 cm was classified as

"confluence" or "other" depending on its location relative to a cyclone-anticyclone pair. For the

GulfCet I Oregon II cruises, hydrographic features were determined using SSH anomaly as well

as depth of the 15°C isotherm obtained from CTD and XBT casts. This was necessary because

some of these surveys were conducted in the northeastern Gulf where SSH anomaly was not as

accurate as it is in the north-central and western Gulf. This reflects the fact that the Loop

Current is temporally persistent but spatially variable in the location of its northern perimeter,

which makes it difficult to establish a spatially accurate, model mean surface for the evaluation

of SSH anomaly data from the northeastern Gulf. For bottlenose dolphins and Atlantic spotted

dolphins (Category 5), the Chi-square test was run for the cyclone and "other" hydrographic

features only, since anticyclones and confluence areas do not exist on the continental shelf where

these species most often occur (Davis et al. 1998).

221
Hydrographic features for GulfCet II Gyre cruises were determined using DHA. For the 1996

late summer cruise, a cyclone was classified as any area with DHA 0 to -12 cm. An area with

DHA 25 to 50 cm was classified as an anticyclone. An area of confluence (0 to 22 cm) occurred

between cyclone-anticyclone pairs (from 26.2-26.6°N and 86.5-87.5°W). For the 1997 midsummer Gyre cruise, a cyclone was defined as 0 to -13 cm and an anticyclone as 25 to 71 cm. A

confluence area (0 to 24 cm) was observed from 26.9-27.9°N and 86.3-87.8°W. For the GulfCet

II Oregon II cruises, we used SSH anomaly as described in the paragraph above to delineate the

four hydrographic features because only partial CTD and XBT data were available.

6.3 Results

6.3.1 Hydrographic Features

Satellite remote sensing showed a broad area of temporally persistent but spatially variable

cyclonic circulation in the northeastern Gulf throughout 1996 and 1997 (Figures 2.29 and 2.32,

see Chapter 2 for details). The MLD and 15°C depth were shallower in the cyclone than in the

confluence or anticyclone during late summer 1996 and mid-summer 1997 (Tables 2.3 and 2.4).

The Deep Chlorophyll Maximum (DCM) was located at or just above the depth at which nitrate

was first detectable; this averaged 50 m below the surface in the cyclone and 100 m in the

anticyclone. The cyclones were also distinguishable by sea surface temperature, salinity and

chlorophyll. In late summer of 1996, surface waters of the cyclone averaged 1°C cooler and

1 psu less saline than surface waters of the anticyclone. Surface chlorophyll was lower than

0.05 jigL' in the warm, nutrient-depleted surface waters of the anticyclone, while within the

cyclone and over the rest of the continental margin, surface chlorophyll concentrations ranged

0.05-0.3 jigL1. The highest surface chlorophyll concentrations were encountered near the

continental shelf in areas of greatest freshwater input from the Mississippi River. In midsummer of 1997, surface temperatures over most of the continental margin were 30-31°C, while

surface temperatures in the cyclone ranged from 29-30°C. Freshwater from the Mississippi

River was entrained into the flow confluence between cyclone-anticyclone pair (Figure 2.33) and

reached much farther offshore than in 1996. Surface chlorophyll concentrations in this river

water reached 2.0 jtgL1. In comparison, surface chlorophyll levels in the interior of the cyclone

ranged 0.05-0.15 j.igL1, while in interior of the anticyclone, surface chlorophyll concentrations

were extremely low (<0.05 j.tg.L1).

6.3.2 Predicted Mean Biomass (PMB) of Zooplankton

PMB during daytime was low throughout the study area, with values ranging from 0.2-1.2 ccm2

during late summer 1996 and 0.7-2.2 ccm2 during mid-summer 1997 (Figures 3.14, 3.1819 and

3.24-25). It was lowest in the anticyclone (0.2-0.7 ccm2 and 0.7-1.2 ccm2 for late suminer

1996 and mid-summer 1997, respectively) and in patches at the western and eastern edges of the

cyclone during both cruises. PMB was highest (as high as 1.2 ccm2 and 2.2 ccm2 for late

summer 1996 and mid-summer 1997, respectively) near the MOM along the continental slope, in

the center of the cyclones, and throughout the confluence between the cyclone-anticyclone. At

night, PMIB was higher than during daytime (maximum = 3.2 ccm2 during both cruises) due to

diel vertical migration of zooplankton and micronekton (Figures 3.14, 3.18, 3.20, 3.24 and 3.26),

Nevertheless, PMB was still highest in the center of the cyclones, in areas along the continental

222
slope and in the confluence zone; PMB was lowest in the anticyclones. Generally, PMB was

higher during mid-summer 1997 than during late summer 1996.

Higher zooplankton and micronekton biomass may indicate richer concentrations of cetacean

prey, since the number of cephalopod paralarvae per m2 found in the MOCNESS samples was

significantly correlated with integrated zooplankton and micronekton biomass in those samples

(r2 = 0.73, p <0.001; see Chapter 3 for details). The five most abundant families of cephalopod

paralarvae collected during GulfCet II were Enoploteuthidae, Ommastrephidae, Pyroteuthidae,

Cranchiidae, and Onychoteuthidae. Previous studies have shown that each of these families is

preyed on by cetaceans (Clarke 1996).

The number of trawis was too small to provide statistical information on the abundance of

myctophids in different hydrographic features. The anticyclonic trawl contained the least

number of myctophids per m2 of sea surface (0.15 myctophidsm2), while both the confluence

and cyclonic trawis contained much greater numbers (0.59 myctophidsm2 and 0.81

myctophidsm2, respectively) (Figure 3.12, see Chapter 3 for details). The anticyclonic trawl

was also the least diverse, with 8 genera represented, while the confluence trawl had 14 genera

and the cyclonic trawl had 13 genera. Each of the genera found in the anticyclonic trawl was

found in the confluence and cyclonic trawis, but the relative proportions of each genera changed

for each trawl. (Figure 3.13).

6.3.3 GulfCet II Data: Eastern Gulf of Mexico (Logistic Regression Analysis)

Late summer 1996 Cruise

Visual survey effort, hydrographic features and bathymetry for the late summer Gyre cruise are

illustrated in Figure 6.1. "On-effort" time covered all four hydrographic features. The presence

of all cetaceans (Category 1) was related to depth, depth gradient, and chlorophyll, although the

proportion of variance explained (0.127) was relatively low. Specifically, cetaceans were more

likely to be present in areas where depth was shallower, depth gradient was less steep and

chlorophyll concentrations were higher (Figure 6.2). Cetaceans were more likely to be seen in

areas with a mean depth of 1,754 m (SE = 195, n = 23), a mean depth gradient of 13.2 mlkm (SE

= 2.2) and a mean chlorophyll concentration of 0.144 gL' (SE 0.010). Cetaceans were less

likely to be seen in areas with a mean depth of 2,170 m (SE = 84, ii = 97), mean depth gradient

of 16.6 mlkm (SE 1.8) and a mean chlorophyll concentration of 0.125 .igL' (SE = 0.004).

Note that only deepwater (depth >800 m) sightings were used in the analysis, although a number

of shallow water sightings are also shown in Figure 6.2.

The presence of sperm whales (Category 2) was related to ocean depth, depth gradient, sea

surface salinity, DHA and PMH (proportion variance explained = 0.350). Sperm whales were

more likely to be seen in areas where depth was shallower, depth gradient was less steep, DHA

was negative arid PMB was higher (Figure 6.3). Specifically, sperm whales were more likely to

be seen in areas with a mean depth of 1,581 m (SE = 227, n = 10), mean depth gradient of 12.8

mlkm (SE = 0.5), DHA of -2.86 cm (SE = 1.79) and mean PIVIIB of 0.86 ccm2 (SE = 0.08). The

largest number of sightings occurred south of the MOM and extended southeast into the cyclone.

Sperm whales were less likely to be seen in areas with a mean depth of 2,170 m (SE 84, n =

223
-90 -89 -88 -87 -86 -85 -84

-30

-90

Longitude West

Figure 6.1. Visual survey effort (very bold lines) for the late sunmier 1996 cruise. Thin

dashed lines represent isobaths. Bold solid (positive) and bold dashed

(negative) lines are sea surface dynamic height anomaly (DHA, cm) relative

to the 100 cm mean. Contour intervals are 5 cm. The cyclone is between 0

and -12 cm, the confluence between 0 and 22 cm, and the anticyclone

between 25 and 50 cm.

224
Longitude West

Figure 62. Cetacean (Category 1, all identified species) sightings (+) during the late

summer 1996 cruise. Thin dashed lines represent isobaths. Bold solid

(positive) and bold dashed (negative) lines are sea surface dynamic height

anomaly (DHA, cm) relative to the 100 cm mean. Contour intervals are 5

cm. The cyclone is between 0 and -12 cm, the confluence between 0 and 22

cm, and the anticyclone between 25 and 50 cm.

225
226

-9 -88 -86 -84

-90 -89 -88 -87 -86 -85

Longitude West

Figure 6.3. Sperm whale (Category 2) sightings (+) during the late summer 1996 cruise.

Thin dashed lines represent isobaths. Bold solid (positive) and bold dashed

(negative) lines are sea surface dynamic height anomaly (DHA, cm) relative

to the 100 cm mean. Contour intervals are 5 cm. The cyclone is between 0

and -12 cm, the confluence between 0 and 22 cm, and the anticyclone

between 25 and 50 cm.

-84
97), mean depth gradient of 16.6 rn/km (SE = 1.7), mean D}IA of 4.16 cm (SE - 1.37) and mean

PMB of 0.73 ccm2 (SE = 0.02).

There were not enough sightings of squid eaters (Category 3) to analyze. The two sightings

beyond the shelf break occurred along the lower slope and over abyssal depths, respectively, in a

cyclone (Figure 6.4). There were seven sightings of Stenella (Category 4), but these sightings

were not statistically explained by any of the environmental variables considered in the models.

All were along the lower slope or over abyssal depths (Figure 6.5). Four sightings were within a

cyclone, two in an area of confluence, and one in an anticyclone.

The presence of bottlenose dolphins and Atlantic spotted dolphins (Category 5) was not

analyzed for these variables since some of them (e.g., cyclones and anticyclones) are not

present on the continental shelf. All of the sightings were on the continental shelf or along the

shelf break (Figure 6.6).

Mid-summer 1997 Cruise

Visual survey effort, hydrographic features and bathymetry for the mid-summer Gyre cruise are

illustrated in Figure 6.7. "On-effort" time covered all four hydrographic features. The presence

of all cetaceans (Category 1) was related to ocean depth, although the variance explained was

extremely low (0.018). The presence of cetaceans may have been associated with shallower

depths (mean = 1,863 m, SE = 124, n = 59) compared to areas where cetaceans were not seen

(mean depth 2,335 m, SE = 75, n = 83) (Figure 6.8). Sightings occurred in all four

bydrographic features, although most sightings occurred in the cyclone, around the periphery of

the cyclone and in confluence zone. Again, only deepwater (depth >800 m) sightings were used

in the analysis although a number of shallow water sightings are also shown in Figure 6.8.

The presence of sperm whales (Category 2) was related to depth gradient. The proportion

variance explained by the depth gradient model was 0.099. The presence of sperm whales may

have been associated with steeper depth gradients (mean = 44.5 mlkrn, SE = 12.1, n 12)

compared to areas where sperm whales were not seen (mean = 16.3 mlkm, SE = 2.6, n = 83)

(Figure 6.9). This was probably due to a cluster of sightings during this cruise along the Florida

escarpment, an area of steep depth gradient. Most of the sightings occurred along the lower

slope and over abyssal depths less than 3,000 m in the cyclone or confluence zone; one sighting

occurred in the anticyclone.

The presence of squid eaters (Category 3) was related to sea surface salinity, although the

proportion of variance explained was extremely low (0.029). The presence of squid eaters may

have been associated with higher salinity (mean = 35.653, SE = 0.437, n 6) compared to areas

where squid eaters were not seen (mean salinity = 33.992, SE 0.249, n 83) (Figure 6.10).

Most sightings occurred along the lower slope and over abyssal depths on the periphery of the

cyclone or in the confluence zone; one sighting occurred in the anticyclone.

The presence of oceanic stenellids (Category 4) was related to DHA, but the proportion of

variance explained was only 0.096. The presence of oceanic stenellids was associated with areas

of lower DHA (mean = 0.82 cm, SE = 2.62, n = 27) compared to areas where stenellids were not

227
Longitude West

Figure 6.4. Squid-eater (Category 3, see text for list of species) sightings (+) during the

late summer 1996 cruise. Thin dashed lines represent isobaths. Bold solid

(positive) arid bold dashed (negative) lines are sea surface dynamic height

anomaly (DHA, cm) relative to the 100 cm mean. Contour intervals are 5

cm. The cyclone is between 0 and -12 cm, the confluence between 0 and 22

cm, and the anticyclone between 25 and 50 cm.

228

-87 -86 -85 -84

-90 -89 -88 -87 -86 -85 -84

-90 -89 -88
-90 -8 -87 -86 - Longitude West

Figure 6.5. Stenellid (Category 4, see text for list of species) sightings (+) during the

late summer 1996 cruise. Thin dashed linesrepresent isobaths. Bold solid

(positive) and bold dashed (negative) lines are sea surface dynamic height

anomaly (DHA, cm) relative to the 100 cm mean. Contour intervals are 5

cm. The cyclone is between 0 and -12 cm, the confluence between 0 and 22

cm, and the anticyclone between 25 and 50 cm.

229

-90 -89 -88 -87 -86 -85 -84
-90

-89 -88 -87

Longitude West

Figure 6.6. Bottlenose dolphin arid Atlantic spotted dolphin (Category 5) sightings (+)

during the late summer 1996 cruise. Thin dashed lines represent isobaths.

Bold solid (positive) and bold dashed (negative) lines are sea surface

dynamic height anomaly (DHA, cm) relative to the 100 cm mean. Contour

intervals are 5 cm. The cyclone is between 0 and -12 cm, the confluence

between 0 and 22 cm, and the anticyclone between 25 and 50 cm.

230
25-

24

-90 -89 -88 -87 -86 -85 -84

0

U i-j # % ,;._-' '' \

'

'S

p

,.-,' \

'1

60

0

.1 ;':\ L-''"

40

20

231

-84

24

-90 -89 -88 -87 -86 -85

Longitude West

Figure 6.7. Visual survey effort (very bold lines) for the mid-sunmier 1997 cruise. Thin

dashed lines represent isobaths. Bold solid (positive) and bold dashed

(negative) lines are sea surface dynamic height anomaly (DHA, cm) relative

to the 105 cm mean. Contour intervals are 5 cm. The cyclone is between 0

and -13 cm, the confluence between 0 and 24 cm, and the anticyclone

between 25 and 71 cm.

30-

29- -29

-28

27

lOOm

28-

- 20

27-

3000 m

26- -26

-25
-89 -88 -87 -86 -85

Longitude West

Figure 6.8. Cetacean (Category 1, all identified species) sightings (+) during the midsummer 1997 cruise. Thin dashed lines represent isobaths. Bold solid

(positive) and bold dashed (negative) lines are sea surface dynamic height

anomaly (DHA, cm) relative to the 105 cm mean. Contour intervals are 5

cm. The cyclone is between 0 and -13 cm, the confluence between 0 and 24

cm, and the anticyclone between 25 and 71 cm.

232

-90 -9 -88 -7 -86 -84
26-

25-

-90

lOOm

- QQ1)

-89

20

i40 z

ti)

60

3000m

-88

2O0P24-

%%' 'ii+'

0

-90 -89 -88 -87 -86 -85

Longitude West

Figure 6.9. Sperm whale (Category 2) sightings (+) during the mid-summer 1997 cruise.

Thin dashed lines represent isobaths. Bold solid (positive) and bold dashed (negative) lines are sea surface dynamic height anomaly (DHA, cm) relative to the 105 cm mean. Contour intervals are 5 cm. The cyclone is between 0

and -13 cm, the confluence between 0 and 24 cm, and the anticyclone between 25 and 71 cm.

233

-87

40

20

-86 -85 -84

-84

29

28

-27

-26

-25

-24

0

29-

28-
-90

-89 -88 -87 -86 -85 -84

I

I

I -89 -88 -87 -86 -85

Longitude West

Figure 6.10. Squid-eater (Category 3, see text for list of species) sightings (+) during the

mid-summer 1997 cruise. Thin dashed lines represent isobaths. Bold solid

(positive) and bold dashed (negative) lines are sea surface dynamic height

anomaly (DHA, cm) relative to the 105 cm mean. Contour intervals are 5

cm. The cyclone is between 0 and -13 cm, the confluence between 0 and 24

cm, and the anticyclone between 25 and 71 cm.

234

-84
seen (mean DHA = 16.34 cm (SE = 2.6, n 83) (Figure 6.11). Oceanic stenellids were seen

from the upper slope to abyssal depths; most occurred in the cyclone, on the periphery of the

cyclone or in the confluence zone.

Again, the presence of bottlenose dolphins and Atlantic spotted dolphins (Category 5) was not

analyzed for this group of variables. All sightings occurred on the continental shelfor along

the shelf break (Figure 6.12).

6.3.4 GulfCet I and!! Data: Oceanic Northern Gulf ofMexico (Chi-square Analysis)

Cetaceans (Category 1) in the oceanic northern Gulfwere sighted differentially with respect to ocean depth (Table 6.2). Cetaceans were concentrated along the upper continental slope (200- 1,000 m) and were sighted less frequently than expected over abyssal regions (>2,000 m). Sperm whales (Category 2) occurred more frequently along the lower slope (1,000-2,000 m)

(Table 6.3), and the squid-eaters (Category 3) were concentrated along the upper slope (Table 6.4). However, commonly sighted species such as Risso's dolphin heavily influenced the

preferred ocean depth range for squid-eaters and disguised the fact that some less frequently

sighted species (e.g., Mesoplodon spp., Clarke 1996) occurred over the lower slope and abyssal

areas (Figure 4.18). Oceanic stenellids (Category 4) occurred more frequently over the lower slope and in abyssal areas (Table 6.5). Bottlenose dolphins and Atlantic spotted dolphins

(Category 5) occurred most frequently on the continental shelf (<200 m) and along the upper slope; they had a lower than expected occurrence along the lower slope and in abyssal areas

(Table 6.6).

Cetaceans were sighted differentially with respect to hydrographic features (Table 6.7).

Cetaceans in general (Category 1) and oceanic stenellids in particular (Category 4) were

concentrated in cyclones and were less frequently observed in anticyclones (Tables 6.7 and 6.10). Similarly, the squid-eaters (Category 3) occurred less often than expected in anticyclones

(Table 6.9). Sperm whales (Category 2) were infrequently seen near anticyclones when these

nutrient-poor features were surveyed during GulfCet II. However, when sperm whale sightings

from GulfCet I and II were combined, sperm whales in the oceanic northern Gulf were sighted in

proportion to the survey effort expended in the four hydrographic features (Table 6.8). Bottlenose dolphins and Atlantic spotted dolphins (Category 5) occurred mostly in features other

than cyclones, anticyclones and confluence areas, probably because they were concentrated

along the continental shelf and upper slope where these features do not normally occur (Table

6.11).

6.4 Discussion

To characterize cetacean habitat, we used an integrated approach that included the analysis of

hydrographic and bathymetric features. In addition to physical features, we measured

zooplankton and micronekton biomass derived from both net arid acoustic sampling to indicate

the amount of potential food available for higher trophic level foragers. Although the diets of most cetaceans in the Gulf are poorly known, we hypothesized that hydrographic regimes in the

study area have different levels of potential prey that influence cetacean distribution. We further

hypothesized that these food stocks would be locally concentrated in nutrient-rich areas offshore

235
I I

I -89 -88 -87 -86 -85

Longitude West

Figure 6.11. Stenellid (Category 4, see text for list of species) sightings (+) during the

mid-summer 1997 cruise. Thin dashed lines represent isobaths. Bold solid

(positive) aiid bold dashed (negative) lines are sea surface dynamic height

anomaly (DHA, cm) relative to the 105 cm mean. Contour intervals are 5

cm. The cyclone is between 0 and -13 cm, the confluence between 0 and 24

cm, and the anticyclone between 25 and 71 cm.

236

-90 -88 -86 -85 -84
-89 -88 -87 -86 -85 -84

-89 -88 -87 -86 -85

Longitude West

Figure 6.12. Bottlenose dolphin and Atlantic spotted dolphin (Category 5) sightings (+)

during the mid-suinnier 1997 cruise. Thin dashed lines represent isobaths.

Bold solid (positive) and bold dashed (negative) lines are sea surface

dynamic height anomaly (DHA, cm) relative to the 105 cm mean. Contour

intervals are 5 cm. The cyclone is between 0 and -13 cm, the confluence

between 0 and 24 cm, and the anticyclone between 25 and 71 cm.

237
Table 6.2. Contingency table of presence/absence of cetaceans (Category 1) in the on-effort tran

ocean depth region. The depth range, number of effort segments analyzed for presencexpected number of segments with presence or absence of cetaceans (F), X2 contribu

Freeman-Tukey deviates in each class are reported. Freeman-Tukey deviates with ab

significant for the X2 (Chi-squared test of independence) result. Class Depth range (m) Presence of cetaceans n E (E - 

E 

Continental shelf <200 Absent 251 257.3 0.16 

Present 115 108.6 0.37 

Upper slope 200-1,000 Absent 439 483.0 4.01 

Present 248 203.9 9.51 

Lower slope 1,000-2,000 Absent 485 466.1 0.76 

Present 178 196.8 1.80 

Abyssal region >2,000 Absent 478 446.4 2.33 

Present 157 188.5 5.27 

TOTAL 2351 2351 24.21 
Table 6.3. Contingency table of presence/absence of sperm whales (Category 2) in the on-effort 

to ocean depth region. The depth range, number of effort segments analyzed for preswhales (n), expected number ofsegments with presence or absence of cetaceans (K), and Freeman-Tukey deviates in each class are reported. Freeman-Tukey deviates witsignificant for the X2 (Chi-squared test ofindependence) result. Class Depth range (m) Presence of cetaceans

n

E (E

- 

E 

Continental shelf <200 Absent 365 352.4 0.45 

Present 1 13.5 11.62 

TJpperslope 200 1,000 Absent 657 661.5 0.03 

Present 30 25.1 0.82 

Lower slope 1,000-2, 000 Absent 631 638.4 0.09 

Present 32 24.5 2.27 

Abyssalregion >2,000 Absent 611 611.5 0.00 

Present 24 23.5 0.01 

TOTAL 2351 2351 15.29 
Table 6.4. Contingency table of presence/absence of squid-eaters (Category 2) in the on-effort t

to ocean depth region. The depth range, number of effort segments analyzed for pres

(n), expected number of segments with presence or absence of cetaceans (E), X2 conFreeman-Tukey deviates in each class are reported. Freeman-Tukey deviates with absignificant for the X2 (Chi-squared test of independence) result. Class Depth range (m) Presence of cetaceans n E (E - 

E 

Continental shelf <200 Absent 362 338.7 1.59 

Present 4 27.2 19.83 

Upper slope 200 - 1, 000 Absent 611 635.8 0.97 

Present 76 51.1 12.09 

Lower slope 1,000-2,000 Absent 609 613.6 0.04 

Present 54 49.3 0.44 

Abyssal region >2,000 Absent 594 587.7 0.07 

Present 41 47.2 0.83 

TOTAL 2351 2351 35.86 
Table 6.5. Contingency table of presence/absence of oceanic stenellids (Category 2) in the on-e

respect to ocean depth region. The depth range, number of effort segments analyzed oceanic stenellids (n), expected number ofsegments with presence or absence of ceta

- nJ2E1), and Freeman-Tukey deviates in each class are reported. Freeman-Tukey de

are significant for the X2 (Chi-squared test of independence) result. Class Depthrange(m) Presence of cetaceans

n

E (E

- n)2 

E 

Continental shelf <200 Absent 364 330.5 3.39 

Present

2 35.4 31.61 

Upper slope 200 - 1, 000 Absent 620 620.3

0 

Present 67 66.6

0 

Lower slope 1,000-2,000 Absent 581 598.7 0.52 

Present 82 64.3 4.87 

Abyssal region >2,000 Absent 558 573.4 0.41 

Present 77 61.5 3.86 

TOTAL 2351 2351 44.66 
Table 6.6. Contingency table of presence/absence of bottlenose dolphins andAtlantic spotted do

effort transect segments with respect to ocean depth region. The depth range, numbefor presence or absence of bottlenose dolphins and Atlantic spotted dolphins (n), exp

presence or absence of cetaceans (E), X2 contribution ([E - nj2E'), and Freeman-Tukreported. Freeman-Tukey deviates with absolute value >1.5 are significant for the X2 independence) result. Class Depth range (m) Presence of cetaceans n E (E_n)2 

E 

Continental shelf <200 Absent 270 338.7 13.96 

Present 96 27.2 173.52 

Upper slope 200-1, 000 Absent 612 635.8 0.90 

Present 75 51.1 11.13 

Lower slope 1,000-2,000 Absent 659 613.6 3.35 

Present 4 49.3 41.58 

Abyssal region >2,000 Absent 635 587.7 3.80 

Present 0 47.2 47.27 

TOTAL 2351 2351 295.51 
Table 6.7. Contingency table of presence/absence of cetaceans (Category 1) in the oneffort transect segments with respect to hydrographic features. The number of effort segments with presence or absence of cetaceans çn). expected number of

segments with presence or absence of cetaceans (E), X contribution ([E -

n]2E), and Freeman-Tukey deviates in each feature class are reported.

Freeman-Tukey deviates with absolute value >1.5 are significant for the X2

(Chi-squared test of independence) result.

Table 6.8. Contingency table of presence/absence ofsperm whales (Category 2) in the oneffort transect segments with respect to hydrographic features. The number of

effort segments with presence or absence of sperm whales (n), expected

number of segments with presence or absence of cetaceans (K), X2

contribution ([E - n]2E), and Freeman-Tukey deviates in each feature class

are reported. Freeman-Tukey deviates with absolute value >1.5 are significant for the X2 (Chi-squared test of independence) result.

243

Feature class Presence of

cetaceans

n E

(E-n)2 Freeman-Tukey

E Deviate

Anticyclone Absent 348 350.1 0.01 -0.10

Present 18 15.8 0.29 0.57

Confluence Absent 53 52.6 0 0.09

Present 2 2.3 0.06 -0.10

Cyclone Absent 298 300.4 0.02 -0.12

Present 16 13.6 0.42 0.68

Other Absent 1200 1195.8 0.01 0.13

Present 50 54.1 0.32 -0.54

TOTAL 1985 1985 1.13 (df= 3, p=0.7671)

Feature class Presence of

cetaceans

n E

(En)2 Freeman-Tukey

E Deviate

Anticyclone Absent 288 258.5 3.37 1.80

Present 78 107.5 8.09 -3.04

Confluence Absent 37 38.8 0.09 -0.26

Present 18 16.1 0.21 0.50

Cyclone Absent 196 221.7 3.00 -1.77

Present 118 92.2 7.21 2.54

Other Absent 881 882.8 0 -0.05

Present 369 367.1 0.01 0.11

TOTAL 1985 1985 21.98 (df= 3, p<O.O01)
Table 6.9. Contingency table of presence/absence ofsquid-eaters (Category 3) in the oneffort transect segments with respect to hydrographic features. The number of

effort segments with presence or absence of squid-eaters (n), expected number

of segments with presence or absence of cetaceans (K), X2 contribution ({En]2ff'), and Freeman-Tukey deviates in each feature class are reported.

Freeman-Tukey deviates with absolute value >1.5 are significant for the X2

(Chi-squared test of independence) result.

Table 6.10. Contingency table of presence/absence ofoceanic stenellids (Category 4) in

the on-effort transect segments with respect to hydrographic features. The

number of effort segments with presence or absence of oceanic stenellids (n),

expected number of segments with presence or absence of cetaceans (H), X2

contribution ([E - n]2K1), and Freeman-Tukey deviates in each feature class

are reported. Freeman-Tukey deviates with absolute value >1.5 are significant

for the X2 (Chi-squared test of independence) result.

Feature class Presence of

cetaceans

n E

(E-n)2 Freeman-Tukey

E Deviate

Anticyclone Absent 336 324.3 0.26 0.66

Present 30 41.6 2.02 -1.90

Confluence Absent 46 48.7 0.25 -0.36

Present 9 6.2 1.96 1.06

Cyclone Absent 254 278.2 2.00 -1.47

Present 60 35.7 15.53 3.56

Other Absent 1123 1107.6 0.43 0.47

Present 127 142.3 3.36 -1.30

TOTAL 1985 1985 25.81 (df= 3, p<O.0001)

244

Feature class Presence of

cetaceans

n E

(E-n)2 Freeman-Tukey

E Deviate

Anticyclone Absent 343 334.4 0.22 0.48

Present 23 31.5 2.31 -1.58

Confluence Absent 50 50.2 0 0

Present 5 4.7 0.01 0.22

Cyclone Absent 285 286.9 0.01 -0.10

Present 29 27.0 0.14 0.41

Other Absent 1136 1142.3 0.03 -0.18

Present 114 107.6 0.37 0.62

TOTAL 1985 1985 3.09 (df= 3, p =0.3764)
Table 6.11. Contingency table ofpresence/absence of bottlenose dolphins and Atlantic

spotted dolphins (Category 5) in the on-effort transect segments with respect to hydrographic features. The number ofeffort segments with presence or absence of bottlenose dolphins and Atlantic spotted dolphins (n), expected number of segments with presence or absence of cetaceans (E), X2 contribution ([E - n]2K1), and Freeman-Tukey deviates in each feature class are reported. FreemauTukey deviates with absolute value >1.5 are significant for the X2

(Chi-squared test of independence) result.

Presence of Feature class n

(E - Freeman-Tukey E cetaceans E Deviate

Absent 191 180.6 0.59 0.78

Present 55 65.3 1.64 -1.30

Absent 55 65.3 1.64 -1.30

Present 34 23.6 4.54 1.97

TOTAL 335 335 8.41 (df= 1, p<O.0037)

245

Cyclone

Other
from the Mississippi River, within cyclonic, cold-core eddies, and along the high-shear edges of anticyclones (see Chapter 3 for additional detail). Although the results support these two hypotheses, the variance explained inmany of the models was low. This may indicate seasonal,

inter-annual and geographic variability in cetacean-habitat association.

Combining the results from the logistic regression and Chi-square analyses, cetaceans in general were concentrated along the continental slope in areas of cyclonic circulation where chlorophyll was elevated. They were less likely to occur over water deeper than 2,000 m and in

anticyclones. Sperm whales tended to occur along the lower slope and, in some areas, in

cyclonic eddies (i.e., low DHA) with elevated PMB. Squid-eaters occurred more frequently

along the upper slope in areas outside of anticyclones. Oceanic stenellids occurred more often

over the lower slope and abyssal regions in areas of cyclonic or confluence circulation. Finally, bottlenose dolphins and Atlantic spotted dolphins were seen most frequently on the continental shelf or along the upper slope, but outside of deepwater hydrographic features such as cyclones and anticyclones.

The physical forcing functions for ocean circulation in the north-central and eastern Gulf of

Mexico are river discharge, wind stress, and the Loop Current (LC) (see Chapter 2). The major

river system influencing this region is the Mississippi-Atchafalaya River, with most of the outflow transported west along the coast (Cochrane and Kelly 1986). Flow on the continental

shelf is largely wind-driven, with buoyancy forcing effects from low-density river water. Beyond the shelf-break, the LC largely determines mesoscale circulation. Once or twice

annually, the LC sheds anticyclones that migrate westward and spawn cyclones. The high

concentration of nutrients in cyclones stimulates new biological production in the near-surface mixed layer (Biggs et al. 1988, Biggs 1992). In contrast, the LC and anticyclones are depleted of nutrients in the photic zone and have low biological productivity. In addition, frontal zones at

the periphery of eddies can entrain low salinity, high-chlorophyll shelf water and transport it off

the shelf (Biggs and MUller-Karger 1994). Frontal zones may also be created during periods of northern extension of the LC.

Recent studies have used acoustic techniques to assess zooplankton and micronekton biomass as

a direct or indirect index of food resources for cetaceans (Croll et al. 1998, Macaulay et al. 1995, Beardsley et al. 1996, Fiedler et al. 1998). Although there was a pronounced diel fluctuation in the vertical migration of sound-scattering organisms during this study, integrated PMI3 was

always greater in cyclones than in anticyclones (see Chapter 3). These former areas have a

shallower MLD and lower dynamic SSH due to doming of cold, deep water. Our results show

that cyclones and confluence zones are areas of locally concentrated zooplankton and

micronekton, including cephalopod paralarvae and myctophids, that we suggest result from

nutrient-rich water and locally high primary production in the mixed layer (Biggs et al. 1988, Chapter 3). Cetaceans were concentrated in thesemesoscale features. This is illustrated in color

composite plots for cetaceans (Category 1) and sperm whales (Category 2) that show the distribution of sightings, bathymetry, DHA and nighttime PMB for the late and mid-summer cruises (Figures 6.13-6.16). In the plots for sperm whales, we have included both sightings and

acoustic contacts (see Chapter 5), most of which occurred in the cyclones, confluence zones or

the MOM area where PMB was highest. A comparison of Figures 6.15 and 6.16 shows that, while sperm whales were seen in the MOM area in late summer 1996, more of them were seen

246
24

-90 -9 -87 -86

Longit Lide rCt

247

PMB ft2 0.7 1.2 1.7 2.2 2.7 3.2 3.7 42 ccni

Fieu!e 6.13. CLa4xan iCawgorv 1 all idenütied specie)ightins () durr Lh laLe

urnrner I)6 cruisc. Thin dashed lines represent iobaths. I3old soUd

OiLi%re) ilnd hnld dashei (flcaative! lines are sea surface dyraniic heghi

uiumdy (DHA, cm) rcIaLivC to thc (JO cm !nean, Conicur inrrva1s ai 5 cm.

The cc1onc ic buwcn I) and -17 cm. the cont'liience between 0 and 22 enu.

und th uni icy clone xtwcen 25 and 50 cm. The color conUIuN ind,cate

riithttme prcdi<tcd mean liomass i'MB ccin) in the interval bSo rn.

24

-
-90

30

29-

Iuain

Ti]

28

26

25 -

-89

20

c?.

+

--

24

-87

41j

+ i

_\ I - :::_

24- --:

-90 -89 - -Xi

Longrnde West

21)

PMB ft2 0.7 1.2 1.7 2.2 2.7 3.2

tLfl (\I 4. Cetacean (Cateory 1, all idcT-ili tItd spcc.ics sihtirgs durtng Thc midsummer 1)97 cruise. Thin Jhcd Hncs rcprscnt isobaths. 8old solid

(positjve and ho'd dasicd gaivc kncs arc sea urfacc dynamic licjht

anomaly JMtA. cn) ntht0c lo thc 105 cm mcan. Contour intervaLs are 5

cm, 1 Eie cycl4rIc Es bewccn 0 and -13 cm. the co ucncc bctvcen J and 24

cm, and the anticyclone b.:wcm 25 and 11 cm. 1 he c1or coinour mthcate

nighttime predictrd mean hiornass (PMI3, cern'2) in th iiucrva. 10-50 in.

29

-2

-27

25

24
-c'o -89 -87

:9

25

20

-9() -89 -88 -T -86 -85 -84

Longitude \\rt

PMB (U 0.7 1.2 1.7 2.2 2.7 3.2 3.7 4.2. ecrn

Figure 6.15. Sperm whale (Category 2) sihting (-) und acciuLic LOn(aCL. (very bc1d

lines and dots durin2 the latc suiucier L996 cruic. Th dasheLi iue

represent isohaths. Hold soLid Ipoi1ive and bo'd iashed (neaL%e lines are

sea suthicc dynamic hciht inon:aly (l)IL;. crn rrlatie to the l0) cm nie4n.

Cniour inIna1s are 5 cm. Ie cyclone is h1.%veen U and - 2 Liii. the

conllueiwc berci and 22 cm. d1Tientcclone hetecn 25 arid 5(crn.

The color cornou indk-ate in ghui rtie predicted mean hornass (PM. cc.m)

in mh inteival 10-50 m,

249

l C'

-86 -

27

k4
-90

30

29-

26

25

lUlL r. : ;cwi

21

-')) -$9

20

- -87 -86 -85 -84

30

60

P 1 B 0.2 Ci. 7 .2 1 7

2I)

r\i -' lI,\

40

20

- 87 -86

I ,onjtude \1r..t

--29

-28

2.7 3.2 3.7 4.2 ccm

6.16. Sperm whidc (Ca,tcgory 2: ightiugs (-i-) ard aco..stic coittacis vety hold

bns rnd clots) dining the mid-summer 14 7 crui5e. 'FbLn dashed lines

ieprescut. iobuth. toJd solid positivc) and hold dashed (negative lines

ac uL.rtcc d'r'nLffliC height ar.omaty(L)1li. enu relative to the 105cm

mn Cniourintcrrals arc cm. The cyclone is between 0 and -13 cm.

the conñuencc between 0 and 24 erm and the anticy clone hetceri i i.nd 71

ciii. The color contours indicate nightime pdicted mean hkirnass (PMft

ecm2) in the iterva1 10-50 m.

-26
100-200 km due east over the DeSoto Canyon in mid-summer 1997. In 1996, the MOM area

was strongly cyclonic, but the next year the cyclone was centered farther east and sperm whale

occurrence shifted similarly. Since cyclones in the northern Gulf are dynamic and usually

associated with westward moving cyclone-anticyclone pairs, cetacean distribution will be dynamic. However, with near real-time satellite remote sensing of SSH anomaly, these features can be tracked and used to predict where cetaceans may be concentrated.

As mentioned in Chapter 3, 28 cephalopod families are eaten by cetaceans, and cephalopods are

the main food constituent of 28 odontocetes (Clarke 1996). Of these cephalopod families, the

ommastrephids, onychoteuthids, cranchiids and enoploteuthids are numerically important in the diets of sperm whales, members of the Family Ziphiidae, dwarf and pygmy sperm whales, false killer whales, melon-headed whales, pilot whales, Risso's dolphin, pantropical spotted dolphins, spinner dolphins and striped dolphins (Clarke 1996, Pen-in et al. 1973, Wolff 1982). The

paralarvae of all of these cephalopod families were found in Gulftet II trawl samples, although

the family Enoploteuthidae was by far the most abundant. Although the number of trawis in this

study was too small to detect a difference in squid paralarvae abundance in the different

hydrographic features, a statistically significant relationship existed between integrated

zooplankton biomass and integrated cephalopod pa.ralarvae (a major component of cetacean prey) numbers (see Chapter 3 for details). This implies that higher zooplankton and micronekton

biomass indicate richer concentrations of squid in the cyclones and confluence regions than in

the anticyclones. In addition, the abundance and diversity of myctophids, a second important cetacean prey group, appear to be greater in the cyclones and confluence regions than in the anticyclones, although the sample size was too small to analyze statistically. These results

suggest that the amount of prey for cetaceans (and seabirds) may be consistently greater in the

cyclone and confluence areas (as opposed to anticyclones) making them preferential areas for

foraging.

In addition to the presence of hydrographic features, ocean depth influences cetacean distribution

in the northern Gulf. In earlier studies (Davis and Fargion 1996, Baumgartner 1997, Davis et al.

1998, Mullin et al. 1994c), the distribution of cetaceans in the north-central and western Gulf of

Mexico in waters deeper than 100 m (i.e., the GulfCet I study area, Figure 1.2) was differentiated

most clearly with ocean depth. Atlantic spotted dolphins were consistently found in the

shallowest water on the continental shelf and along the shelf break. Bottlenose dolphins were

found most commonly along the upper slope in water significantly deeper than that for Atlantic

spotted dolphins All the other species and species categories were found over deeper ocean

depths; these were Risso's dolphins, short-finned pilot whales, pygmy/dwarfsperm whales, rough-toothed dolphins, spinner dolphins, sperm whales, striped dolphins, Mesoplodon spp.,

pantropical spotted dolphins, clymene dolphins and unidentified beaked whales (Ziphiidae). Risso's dolphins and shortfinned pilot whales occurred along the upper slope and, as a subgroup, were significantly different from striped dolphins, Mesoplodon spp., pantropical spotted

dolphins, clymene dolphins, and unidentified beaked whales, which occurred in the deepest

water. Pygmy/dwarf sperm whales, rough-toothed dolphins, spinner dolphins, and sperm whales occurred at intermediate depths between these two subgroups and overlapped them. In terms of cetacean distribution based on ocean depth, our results for the combined GulfCet I and II data for

the northern oceanic Gulf are consistent with these earlier results.

251
The enhanced presence of cetaceans along the slope instead of abyssal areas of the northern Gulf

probably results from the collision of mesoscale eddies with the continental margin which

enhances primary and secondary productivity, especially along the upper continental slope. Skipjack (Katsuwonus pelamis), blackfln tuna (Thunnas atlanticus), swordfish (Xzphias gladius), and blue marlin (Makaira indica) have been reported by fisherman to be locally abundant in

these areas (Roffer's Ocean Fishing Forecasting Service, pers. corn.). The presence of large, apex-predators such as tuna, bilifish and cetaceans indicates reliable food resources along the

continental slope.

In the north-central Gulf, an additional factor affecting cetacean distribution may be the narrow

continental shelf south of the Mississippi River delta. Low salinity, nutrient-rich water may occur over the continental slope near the MOM or be entrained within the confluence of a

cyclone-anticyclone eddy pair and transported beyond the continental slope. This creates a deep- water environment with locally enhanced primary and secondary productivity and may explain

the presence of a resident population of endangered sperm whales within 50 km of the Mississippi River delta (Townsend 1935, Berzin 1972). Previous studies have shown that sperm

whales in the north-central Gulf occur along the mid-to-lower slope (Collum and Fritts 1985, Davis et al. 1998). The results from the late and mid-summer cruises are in agreement with

previous studies and show sperm whales inhabiting areas along the lower slope where DHA is

low (i.e., within cyclones or confluence zones) and PMB is higher (Figures 6.15 and 6.16).

Similar associations with cyclones and higher PMB were obtained for the squid-eaters and

oceanic stenellids, although squid-eaters occurred along the upper slope and oceanic stenellids

along the lower slope and in water greater than 2,000 m deep (Jennings 1982). Along the

continental slope of Nova Scotia and the northeastern United States, sperm whales have been

observed over similar ocean depths (200-1,500 m; Whitehead et al. 1992.) and in areas of

increased productivity along frontal systems (Waring et al. 1993, Griffin 1999). In the South Pacific, sperm whales occur in areas with high secondary productivity and steep underwater

topography (Jaquet and Whitehead 1996, Jaquet et al. 1996). However, it should be noted that

the association of sperm whales with low DHA and elevated PMB was statistically significant

only for the late summer cruise, although most of the sperm whale sightings during the midsummer cruise also occurred in the cyclone and confluence zone. Sperm whale sightings for the

entire oceanic northern Gulf (GulfCet I and II sightings combined) showed no significant

relationship with hydrographic features. The inconsistent results for sperm whales indicate

important problems with this type of analysis that may result from: 1) the difficulty in obtaining

large numbers of sightings with simultaneous environmental data for some species, 2) combining

sightings collected over large geographic and temporal scales in which important but subtle

factors may be obscured, 3) the low resolution (>60 nm) of many environmental variables and 4)

the lack of data on cetacean behavior (e.g., diving, social and sexual) and at-sea movements. We

should also remember that the relationships between physical and biological processes are subtle

and complex, and that factors other than hydrographic features and potential prey abundance

may also influence the distribution of sperm whales and other cetaceans (Jaquet and Whitehead 1996, Jaquet et al. 1996). For example, the seasonal movements of sperm whales and other

cetaceans may be affected by reproductive and migratory behavior, for which we currently have

little information.

252
Unlike the other cetaceans, the distribution of bottlenose dolphins and Atlantic spotted dolphins was not correlated with the cyclones that occur in deeper waters beyond the shelf break. The preference of these two species for the shallow waters of the continental shelf and upper slope generally precludes them from feeding around cyclones and areas of confluence. The same

appears to be true of Bryde's whale, which have been sighted in the northeastern Gulf in water

100 m deep (Davis et aL 1998, see also Chapter 4) and along the shelf break (Mullin et al. 1 994c). We have little information on the environmental variables that influence the distribution

of these species or their prey because hydrographic surveys have concentrated on deeper waters beyond the continental shelf.

6.5 Conclusions

Cetaceans in the northeastern and oceanic northern Gulf ofMexico were concentrated along the continental slope in or near cyclones. These eddies are mesoscale features with locally

concentrated zooplankton and micronekton stocks that appear to develop in response to increased

nutrient-rich water and primary production in the mixed layer. The exceptions were bottlenose dolphins, Atlantic spotted dolphins and possibly Bryde's whale that typically occur on the

continental shelf or along the shelf break outside of major influences of eddies. Low salinity, nutrient-rich water from the Mississippi River, which may also contribute to enhanced primary

and secondary productivity in the north-central Gulf, may explain the presence of a resident

population of endangered sperm whales south of the delta. However, since cyclones in the northern Gulf are dynamic, cetacean distribution will undoubtedly change in response to the movement of prey associated with these hydrographic features.

253
Chapter 7

Behavior and Site Fidelity of Sperm Whales in the Northern Gulf of Mexico

Bernd Wursig, David W. Weller, Spencer K. Lynn, Joel G. Ortega-Ortiz,

Andrew J. Schiro, Troy D. Sparks and William E. Evans

7.1 Background and Historical Overview in the Gulf

Sperm whales are the most common large whale in the northern Gulf of Mexico (Schmidly 1981, Mullin et al. 1994c, Davis and Fargion 1996, Jefferson 1996, Jefferson and Schiro 1997, Wiirsig

et al. in press). The regular occurrence of sperm whales in the Gulf is known from whaling

records of the mid-1700s to the early 1900s, and more recent aerial and ship-based census work

(Townsend 1935, Collum and Fritts 1985, Mullin et al. 1994c, Jefferson 1996, Jefferson and

Schiro 1997). Population estimates indicate a minimum of 300-530 sperm whales are likely to

inhabit the northern Gulf (Davis and Fargion 1996, Jefferson 1996, Waring et al. 1997).

Sperm whales generally inhabit deep waters near oceanic islands, continental shelves and

submarine ridges (Clarke 1956, Berzin 1972, Rice 1989). A similar pattern appears in the northern Gulf of Mexico, with a majority of sperm whale sightings in water depths from 104- 2,742 in, with a mean bottom depth of 1,228 m (s.d. ± 617 m) (Collum and Fritts 1985). More

recent studies found that sperm whales in the north-central Gulf of Mexico preferentially occur in water depths from 900-1,200 m, with most sightings along the 900 m isobath (Mullin et al. 1994c, Davis and Fargion 1996).

While general information regarding the occurrence and distribution of sperm whales in the Gulf

has become increasingly available (Mullin et al. 1 994c, Davis and Fargion 1996, Jefferson 1996), little is known about details of habitat use and behavior. For example, it is currently unknown if

individuals or groups of sperm whales are resident in the Gulf year-round, are transient to the

region, or undertake migrations to and from other oceanic basins such as the Atlantic or

Caribbean.

Within the framework of Texas A&M University's GulfCetI & II Programs, the first preliminary

information concerning occurrence patterns, habitat use, site fidelity, surface behaviors and some aspects of individual codas of sperm whales in the northern Gulf of Mexico has now been

gathered. Results from this research are presented here.

7.2 Methods

The locations of sperm whale sightings during the late summer and mid-summer Gyre cruises

(Gyre96GO6 and Gyre97GO8, respectively) were overlaid a posteriori with oceanographic

characteristics that show the features of anticyclone, cyclone, confluence between the two, and

"other" (See Chapter 6 for details of methods).

The GulfCet I & II Programs conducted two surveys and devoted significant effort to behavioral

observations and photo-identification ofsperm whales. The first sampling period was between

20-28 August 1994. The second sampling period was more limited in duration, and occurred on

255
I I 98 96 94 92 Longitude West 90 

Figure 7.1. Map of the focal study areas: (A) denotes the GulfCet I TIO Cruise 8 study are

Gyre Cruise 96G06 study area.
ship-based observers described the surface behavior ofsperm whale groups generally within 2 km of the survey vessel. Data were collected mainly by use of 25 x 150 pedestal-mounted

binoculars; however, smaller hand-held binoculars, video cameras and 35-mm cameras were also used. Following a pre-defined behavioral ethograrn, group size, behavior, and respiration variables were recorded.

Three behavioral states were identified: (1) rafting -- whales essentially motionless at the surface with no observable behavior other than breathing; (2) travel -- whales moving in a discernible direction as indicated by forward movement; and (3) social -- whales in tight fotmation and in

regular body contact with each other. The behavior of sperm whale groups was monitored over the course of each encounter and any changes were noted. Group size estimates represented a consensus of all trained observers involved in collecting data for a particular focal group.

The orientation of whales relative to the survey vessel was systematically monitored. In this procedure, observers noted whale orientation (relative to the ship) as determined by the point on a clock face in which whales would "see" the survey ship. For example, if the ship was directly abeam of a whale's left side, its view ofthe ship was 9 o'clock. Each hour on the clock face corresponded to 30° of arc. The clock face technique was easy and quick to use in the field but limited orientation estimates to 300 intervals. The distribution of whale orientations at the time of initial sighting was compared against a random (uniform) circular distribution with a chi-square test. The true orientation of whales (at time of first sighting) relative to North was determined and compared to swell direction with Moore's test for paired angular distance (Zar 1996). Whales with indiscernible or variable orientations were not included in the analysis.

Sperm whales displayed two dive types during our observations: fluke-up and slip-under dives. Fluke-up dives were presumably associated with deep vertical dives, and were characterized by the whale raising its caudal peduncle and flukes into the air prior to submergence. Slip-under dives consisted of a submergence without showing any portion of the flukes (or other behavioral dive indicator) being observed, and were presumably shallow horizontal dives. Frequency data were used to examine the percentage of each dive type as a function of daylight period.

Respiratory information was obtained from individual whales during focal observation sessions by recording the rate and timing of their blows. A Mann-Whitney U non-parametric analysis of variance was used to compare the amount of time between successive blows (inter-blow interval) as a function of surfacings which ended in either fluke-up or slip-under dives. Interpretation of

these data warrant caution, however, as blow intervals may not have been entirely independent; that is, the same whale may have contributed to more than one observation session.

Group size estimates from the two GuliCet behavior study periods, different portions of the northern Gulf, and different observation platforms (aerial vs. ship-based) were compared using a Mann-Whitney U analysis and t-tests (Zar 1996). Analyses of sperm whale surface behavior were exploratory in nature. While significant alpha levels were taken to be p <0.05, no corrections were made for experiment-wise error rate due to the increased alpha associated with multiple statistical tests.

An important part of sperm whale behavior is their vocal repertoire. Worthington and Schevill (1957) first described the characteristic "hammering" click produced by sperm whales (Figure

258
7.2). Unlike most other odontocetes, which can produce whistles and clicks, sperm whales are

known to produce only the latter (Backus and Schevill 1966). Four main types of clicks have been categorized: usual clicks, slow clicks, creaks, and codas (Worthington and Schevill 1957, Backus and Schevill 1966, Watkins and Schevill 1977b, Weilgart and Whitehead 1988, Moore et al. 1993). Basic information on the coda patterns produced by sperm whales in the northern Gulf

of Mexico is provided in this report for future comparisons within and outside of the Gulf.

During July and August 1995, a collaborative research effort was conducted between the

National Geographic Society (NGS) and Texas A&M University (TAMIJ). The research

objective of the NGS study was to attach an integrated video, audio, and dive-depth recorder,

called "Crittercam," (Marshall 1998) to sperm whales in the north-central Gulf of Mexico, and

thereby provide a unique opportunity for TAMIJ researchers to learn more of whale vocal and

surface behavior. While not part of the GulfCet Program, findings from this research provide

insight into the behavior ofsperm whales in the Gulf, and are therefore included here. During

the study, the research team was stationed at British Petroleum's MC-109-A, an oil production

platform standing in approximately 300 m of water just east of the Mississippi River Canyon.

Trips from the oil platform to regions identified as having high densities of sperm whales (Mullin

et aL 1 994c, Davis and Fargion 1996) were made on a daily basis, weather permitting. A 10-rn

sailing vessel and 5.4m inflatable boat were used to find, tag and track sperm whale groups.

7.3 Results

7.3.1 Distribution

During the late summer and mid-summer cruises, sperm whales showed an apparent affinity for cyclonic (cold-core) eddies, confluence zones and the area south of the mouth of the Mississippi

River (MOM), with only one whale group (during mid-summer) within an anticyclonic, or warm

core eddy (See Chapter 6 for details). The line transect effort was 1613 km for late sumnier and

1695 km for mid-summer.

7.3.2 Group Size

Most sperm whale observations reported here were likely sub-groupings of larger schools. Mean

sub-group size as determined from both the large and small vessels ranged from two to three

individuals. No differences in group size were detected between the two GulfCet focal study

periods (p = 0.76) or between aerial and ship-based surveys (p = 0.30). Similarly, no differences

were found in group size by hour of the day (p = 0.10) during focal studies, or in the northern

Gulf overall (p 0.80). However, larger groups of sperm whales in the Gulf have been observed. For example, on 24 August 1994, when short-finned pilot whales and sperm whales were seen

together during an apparent agonistic interaction (Weller et al. 1996), six sperm whales were

initially sighted. After three hours, other sperm whales had joined and the total group size at the

surface reached 12 animals.

7.3.3 Individual Recognition

A total of 37 individual whales were identified during 1994 and 1996. Initial sighting locations were mostly in waters from 700-1,100 m. Inter-sighting distances were calculated for within day

259
4020 - -20 - -40 0.0 2.0 II I 

4.0 

Time (msec) Figure 7.2. Waveform of a single sperm whale click.
periods (n = 9), between days (n 4), and between years (n = 4) (Table 7.1). Four of the 37 whales identified were resighted on multiple days intra-annually, and four individuals were

resighted inter-annually. The distance between resightings ranged from: 2.39-9.92 km on the

same day: 17.30-24.31 km between days and 36.58-46.16 km inter-annually.

7.3.4 Photogrammetry

Photogrammetric work included calculation of the body sizes for five individuals. Body sizes

ranged from 6.6-10.4 m (Table 7.2.), and were within typical size ranges for females and young

(Best 1979). Due to the present incompleteness of testing accuracy and repeatability of

measurements, further interpretation of these data warrant caution at this time.

7.3.5 Behavioral State Time Budgets

Rafting was the most frequently observed behavioral state, and accounted for 57.7% of behavior

(Figure 7.3). Peaks in frequency of rafting and travel varied over the course of the day. In

contrast, socializing was observed at a relatively constant but low level throughout the day. Interpretation of these data must be done with caution, as survey effort across days was not equal.

7.3.6 Orientation of Whales

When whales were first sighted, their orientation relative to the ship was non-random (p =

0.001). A histogram of orientation angles indicated that whales generally viewed the survey

vessel at angles to their sides or behind, and less commonly in front of them (Figure 7.4.)

Angular differences between the true orientation of whales and the direction from which swells

were coming are shown in Figure 7.5. Of all observations, 52.3% (n = 56) found whale

orientations to be between 0° and 90°, while 47.7% (n = 51) were between 90° and 180°. The

similar frequency of angular differences between 0-90° and 90-180° suggested that sperm whales may not have been orienting in a particular direction relative to the prevailing swell direction,

and the statistical null hypothesis that whale orientation and swell direction were the same was

rejected (p = 0.002).

7.3.7 Frequency of Dive Types

The proportion of fluke-up dives appeared to be higher than slip-under and unknown dives between 08:00 and 11:00 hr than at other time (Figure 7.6). In contrast, slip-under dives were

proportionally higher than fluke-up dives between 12:00 and 13:00 hrs, but this pattern was again

reversed starting at 14:00 hr.

7.3.8 Blow Intervals

Differences in mean blow intervals for fluke-up (mean 13.4 sec, s.d. ± 5.38, n 1018) and slip- under (mean = 156 sec, s.d. ± 7.77, n 224) dive sequences were detected (p = 0.0004) (Figure 7.7 and Table 7.3). Intervals between blows were significantly longer for sequences culminating

in slip-under dives than for those ending in fluke-up dives.

261
Table 7.1. Sperm whale sighting locations, dates and distances between resightings for photographically identified individuals.

262

Whale

ID

Date Lat. Long.

°W

Distance

(kin)

Whale

ID

Date Lat. Long.

°W

Distance

(kin) 001 08/23/94 28.7388 88.7525 016 08/28/94 28.5687 88.9020 001

001

08/23/94

08/25/94

28.7122

28.7308

88.6553

88.8680

9.92

20.83

016

017

10/20/96

08/25/94

28.9150

28.6568

88.9250

88.8427

38.55

002 08/22/94 28.7080 88.8550 018 08/22/94 28.7048 88.8367 003 08/23/94 28.7120 88.7157 019 08/28/94 28.5610 88.9078 003 08/23/94 28.7125 88.6557 5.85 020 08/23/94 28.7277 88.7568 004 08/23/94 28.7120 88.7157 021 10/20/96 28.9540 88.9150 004 08/23/94 28.7125 88.6557 5.85 022 10/28/96 28.0112 89.1528 005 08/24/94 28.7405 88.8012 023 10/28/96 28.0291 89.1286 005 08/28/94 28.5388 88.8977 24.31 024 10/28/96 28.0462 89.1244

005 10/20/96 28.9540 88.9150 46.16 025 10/20/96 28.6610 89.0040

006 08/23/94 28.7163 88.7382 026 10/20/96 28.6420 88.9910

006 08/28/94 28.5743 88.9032 22.54 026 10/20/96 28.6250 89.0060 2.39 006 08/28/94 28.5447 88.8980 3.33 027 10/20/96 28.6250 89.0060

007 08/23/94 28.6973 88.6348 028 10/20/96 28.6420 88.9910

008

009

08/28/94

08/28/94

28.5673

28.5447

88.9023

88.8980

028

029

10/20/96

08/23/94

28.6250

28.7363

89.0060

88.6610

2.39

009

010

10/20/96

08/23/94

28.9370

28.7130

88.9190

88.6917

43.64 029

030

10/20/96

10/20/96

28.6070

28.8640

89.0060

88.9770

36.58

010 08/25/94 28.7308 88.8680 17.30 031 10/20/96 28.8910 88.9430

011 08/28/94 28.5743 88.9032 032 08/26/96 28.7853 88.7542

012 08/25/94 28.7100 88.8605 033 10/20/96 28.9150 88.9250

013 08/24/94 28.6828 88.8615 034 10/28/96 28.0746 89.1713

014 08/23/94 28.7015 88.8058 035 10/28/96 28.0462 89.1244 014 08/23/94 28.7087 88.7808 2.56 036 08/23/94 28.7358 88.6575

015 08/23/94 28.7015 88.8058 037 10/20/96 28.6610 89.0040

015 08/23/94 28.7087 88.7808 2.56

015 08/23/94 28.7163 88.7382 4.24
Table 7.2. Size estimates for GUM sperm whales using Gordon (1990) photograminetry techniques. Size (m) 10.4 9.4 6.6 10.3 8.3 Summary statistics for the five whales measured by photogrammetry. Count Mm. Max. Mean SEM SD Variance Range Size Size Whale 1 Whale 2 V/hale 3 Whale 4 Whale 5 Table 7.3. Descriptive statistics for categories of inter-blow intervals (IBI). Units are in seconds. Category

N Mi IBI Max. IBI Mean SEM SD Skewness Kurtosis (sec) (sec) (sec) Fluke-up 1018

2 56 13.4 0.17 ±5.38 2.08 9.54 Slip-under 224 1 50 15.6 0.53 ±7.77 1.19 2.37

5 6.6 10.4 9.0 0.70 ±1.58 2.51 3.8
o Raft (n=64)

Social (o=10)

Travel (n=37)

Daylight flour

Figure 7.3. Frequencies of Raft, Social, and Travel behavior as a function of daylight hour.

264

7 8 9 10 11 12 3 14 15 16 17 18 19
30

25

20

15

10

5

0

Number of Observations (n.151)

Clock Face Orientation to Survey Vessel

Figure 7.4. Whale orientation relative to survey vessel. For example, if the whale viewed

the vessel directly to its left side, the view would be 9 o'clock.

265

1 2 3 4 5 6 7 8 9 10 12

0

I'
1614 -

12 -

0 F'

'0

ii

N nfl

N F1J1n

266

fl19J1.

C on o on o on C on cc on '0 '0 In cc on cc eq eq eq eq '0 0 tI CC

Angular difference(degrees)

Figure 7.5. Frequency distribution of angular differences between whale orientation and

swell direction. For example, orientations of 180° indicate whales swimming in

the same direction as prevailing swell.

Number of Obsei-vatjons (n=107)

eq eq r9hflfl r n

o on - cc cn 0 In C In 0 on cc 'n C on cc IN N men 'r It onol '0 'ON N cc
12

10 -

i:i Fluke-up (n=51)

Slip-under and Unknown (n=36)

267

N N

N

8 9 10 11 12 13 14 15 16 17 18 19

Daylight Hour

Figure 7.6. Frequency distribution of S1ipunder and Unknown and Fluke-up dives as a

function of daylight hour.
20

15 -

Fluke-Up (n=1018)

Slip-Under (n=21 9)

13.4

15.6

Fluke-Up Slip-Under

Dive Type

Figure 7.7. Mean inter-blow interval for Fluke-up and Slip-under dives. Error bars represent standard error of the mean.

268
7.3.9 Crittercam

A total of five distinct coda types was recorded from approximately six whales during two

Crittercani sessions. Session one lasted 5.3 mm and recorded vocal behavior from four whales.

A four-click pattern accounted for 50.0% of the codas recorded, while seven-click and eight-click

patterns accounted for 29.0% and 14.0%, respectively. In addition, six-click and nine-click

codas accounted for the remaining 7 0%; however, these two coda patterns were viewed as

variations of patterns previously identified (i.e., either adding or deleting one click from a coda).

The four-click coda pattern was shared (e.g. more than one animal vocalizing with that coda at

the same time) 10.0% of the recording time. An ANOVA followed by Fisher's post hoc

comparisons test of the four-click inter-click-interval (ICI) indicated a significant difference

between all ICI combinations (p = 0.0001).

Crittercam session three was approximately 9 0 mm in duration and recorded two whales that

dove to a depth of approximately 600 m. Upon tagging, the whale carrying the Crittercam began

a steep dive and started producing a click train vocalization. Approximately 5 mm into the dive,

this whale started to produce an eight-click coda pattern. At this time, a second whale began

vocalizing, producing a four-click coda pattern. The eight-click pattern represented 74.0% of the

codas while the four-click pattern represented 24.0% of the total coda vocalizations during the

session.

7.4 Discussion

7.4.1 Distribution

The distribution of sperm whales is treated with habitat descriptions of all cetaceans in the

habitat association chapter of this report. For the presently-evaluated cruises (Gyre96GO6 and

Gyre97GO8), there was a tendency for sperm whales to occur more frequently at the edges of,

and closely associated with, cyclonic activity. However, for the overall sperm whaleoceanographic comparisons of 14 previous cruises, including the two mentioned here, there is a

statistically significant trend showing whales to associate preferentially near the edges of anticyclonic activity. The apparent discrepancy may be as simple as the whales occurring at one

or the other edge of the so-called confluence between anticyclonic and cyclonic features. This

confluence is an active oceanographic area with potentially greater productivity due to upwelling

at the point of divergence, particularly if the divergence originates near the continental shelf

(Biggs and MUller-Karger 1994).

7.4.2 Group Size

The mean group size reported here closely follows previous results from the Gulf of Mexico.

Mullin et al. (1 994c) found that the average sperm whale group size was 2.1 animals

(range = 1-1 1). The data of Collum and Fritts (1985) showed that sperm whale group size

averaged 3.5 animals (range 1-14). More recent aerial surveys documented a mean group size

of 2.6, while shipboard visual surveys during the same period reported a mean group size of 2.0

(range = 1-12) (Davis and Fargion 1996). Mullin et al. (1994c) proposed that the small group

sizes for the Gulf of Mexico might correspond to the whale "clusters" described by Whitehead

269
and Arnbom (1987). Whitehead and Arnbom (1987) defined a cluster as whales swimming in a

coordinated manner within 100 m of one another. Whitehead (1989) noted that mixed groups in

the Galapagos formed one to four clusters while at the surface, with an average of 1.7 animals

and a mean inter-cluster distance of 213 m. On average, social clusters formed once per day and had a range of 4-20 animals (Whitehead 1989). The average cluster size for sperm whales in Sri

Lanka was 3.1 (Gordon 1987).

Davis and Fargion (1996) point out that if a cluster contains on average 1.7 animals arid groups

contain roughly 20 animals, then various members of a group may surface in 10-12 clusters over

a period of 3 0-90 mm. While this latter conclusion was not quantitatively substantiated by the

work reported here, it was the behavioral observers' impression that once a group of sperm

whales was detected, it was common to then begin sighting additional groups in the same general

vicinity. Thus, the larger group of 12 animals that interacted with short-finned pilot whales on

24 August 1994 (Weller et al. 1996), probably represented most, if not all, subgroups of an entire

school of sperm whales.

7.4.3 Individual Recognition

The occurrence patterns and movements of individually identified sperm whales reported here

indicate that at least some whales maintain a level of intra- and inter-annual fidelity to the MOM

region. These findings, when combined with historic whaling records (Townsend 1935) and

results from ship and aerial surveys (Davis and Fargion 1996), suggest that the Mississippi River

outflow, the narrow continental shelf and the close proximity of deep water to the MOM area

plays an important role in the ecology of sperm whales in the northern Gulf of Mexico.

Schmidly and Shane (1978) suggested that a resident population of sperm whales might inhabit

the Gulf. Preliminary findings presented here appear to support this hypothesis; however,

additional data are needed to confirm year-round residency. The social structure and movement

patterns of sperm whales in the Gulf are currently unknown. Based upon long-term studies of

sperm whales off the Galapagos Islands (Whitehead and Arnbom 1987), we suggest the

following scenarios for whales in the Gulf of Mexico: 1) female-based groups remain in the Gulf

year-round and males "roam" into the area on a sporadic and occasional basis; 2) female-based

groups immigrate to and emigrate from the Gulf to mix with other geographic populations; 3)

males and females remain in the Gulf year-round. The last of these hypotheses is unlikely as the

lack of adult male sightings during our studies remains a gap in our current data.

Only by continued behavioral studies in combination with other techniques such as genetic

sampling and satellite tracking can questions regarding population size, structure, and movement

patterns be better addressed. In light of the current expansion of oil and gas development

activities into the offshore regions of the Gulf, more detailed information on both the basic

behavioral ecology and potential industry-related disturbance are critical for deep water

cetaceans such as the sperm whale.

7.4.4 Photogrammetry

The social organization of sperm whales in the Gulf of Mexico is presently unknown. World

wide, however, sperm whales are categorized into three main group types: mixed, bachelor and

270
solitary males (Caidwell et al. 1966, Best 1979, Whitehead and Ambom 1987). Mixed groups consist of mature females with their dependent calves and sexually immature animals of both sexes (Best 1979). Body size estimates, obtained from the photogranmietric research presented here, fall within the typical range for mixed groups consisting of females, immature males and calves (Best 1979). Strong sexual dimorphism occurs in sperm whales (Leatherwood and Reeves 1983), with males growing to 16 m, while mature females generally achieve less than 11 m in length. No apparent observations of unusually large whales (as related to others within a group) were recorded during the Gu!fCet study, and this may suggest that large adult males are

infrequent visitors to the northern Gulf.

7.4.5 Behavior, Dive Type and Respiration Information

The most frequently observed behavioral state during focal observation sessions was rafting. If

rafting is indicative of both a physiological refractory period following a prolonged dive and, at the same time, a physiological preparatory period for a future dive, then the increase of this

particular behavior may be related to periods of greater foraging. The trend for rafting to occur throughout the day may be suggestive of regular daylight foraging. However, rafting may alternately be indicative of rest or surface time unrelated to post-dive recovery. The trend for

fluke-up dives to be more frequent at 8:00, 11:00, 15:00 and 16:00 hrs overlaps in time with an apparent increase of rafting behavior. Therefore, we suggest that rafting may indeed reflect periods of increased foraging.

The inter-blow interval for surface times followed by slip-under dives is significantly longer than the same parameter for surface bouts ending with fluke-up dives. A more rapid breathing

sequence may, as is the case during fluke-up dives, suggest post-dive recovery and pre-dive preparation, and may help to explain the apparent association between fluke-up dives and rafting behavior. While interpretation of these data are preliminary and cannot be used conclusively, the hypotheses presented here may provide some information on the foraging ecology of this cetacean in the northern Gulf of Mexico.

7.4.6 Orientation of Whales

The angle at which whales viewed the ship when first detected by observers was not randomly distributed. Instead, the whales were directed away from or sideways relative to the ship when they were first seen. Although the ship tended not to approach whales head-on, initial orientations were expected to be uniformly random, and to change as the ship approached for behavioral observations. Two explanations for this behavior are possible: (1) whales may have reacted to the ship and reoriented before they were detected or (2) factors other than the ship

(e.g., a physical or biological gradient, or environmental noise) may have been influencing the observed non-random orientation. One such environmental gradient could be swell direction. In rough seas, for instance, sperm whales may orient into or away from oncoming swells. The

ambiguous findings regarding whale orientation with respect to swell direction reported here warrant more careful data collection. Improvements could be made in estimating both the

rwhales view" of the ship and the swell direction. For example, the estimated swell direction

often changed by 20° or more between successive observer evaluations over as little as one hour.

It seems doubtful that actual swell direction was so variable.

271
Overall, we tend to favor the explanation that whales were oriented away from the vessel when

first seen, possibly to reduce their perception of vessel noise. However, more data need to be

gathered before this possibility can be accepted (or rejected) with certainty.

7.4.7 Crittercam

Codas are thought to occur mostly at the surface and in social contexts. They are rhythmic patterns of clicks (Watkins and Schevill 1 977b, Whitehead and Weilgart 1990) usually consisting

of 3-40 clicks with an average of 10 clicks/coda. Each coda may be repeated from 2-60 times

(Watkins and Schevill 1977a, Watkins and Schevill 1977b). Watkins et al. (1985) proposed two

types of codas: identity codas, and general use or shared codas. Identity codas are patterns that are unique to an individual for at least several hours. General use codas are vocalizations that are

shared among groups of whales (Watkins and Schevill 1977b). Since codas were often heard at

the surface and in conjunction with social interactions in the southeast Caribbean, Watkins and

Schevill (1977b) believed that codas have a communicative function (Watkins 1980, Whitehead

and Weilgart 1991). Mullins et al. (1988) noted that codas were only heard when more than one

whale was present off the Scotian Shelf. There also appear to be geographical differences in

spacing and composition of codas between the Galapagos (Weilgart 1990) and the southeast Caribbean (Moore et al. 1993).

Crittercam sessions of the present study revealed the use of codas for at least two individuals at a depth of 600 m. This finding provides new information on the apparent use of codas at depth. Future research employing these techniques may help to further elucidate the role of the coda. Vocal comparisons between the Gulf of Mexico and other regions may prove useful in determining stock structure and geographic isolation of sperm whale populations.

7.5 Couclusions

Behavioral observations showed variable, die! patterns in dive type, surface activity and

respiratory parameters. We suggest that these trends may be related to foraging and food

capture. The group composition and group size ofsperm whales in the Gulf of Mexico resemble estimates for other populations. Body size measurements indicate that female-based groups are common, while the absence of sightings of large males remains a gap in our developing understanding of these whales. Previous studies have shown that sperm whales in the north- central Gulf occur along the mid-to-lower slope (Collum and Fritts 1985, Davis et al. 1998). The

results from the late and mid-summer cruises are in agreement with previous studies and show

sperm whales inhabiting areas along the lower slope within cyclones or confluence zones (see Chapter 6 for details). Results from this study indicate that the behavior of sperm whales in the northern Gulf of Mexico resembles that of populations in other geographic locations. The survey data (Chapter 4) and apparent inter- and intra-year site fidelity of numerous photo-identified

individuals in the MOM area suggests that the north-central Gulf is essential habitat for a

breeding population of sperm whales. However, critical uncertainties exist in our understanding of population structure, behavior and seasonal movements relative to mesoscale hydrographic

features.

As well, there exist critical uncertainties in potential behavioral changes, vocal reactions, and overall habitat use changes of sperm whales relative to survey and industrial activities. It is

272
hoped that future studies of sperm whale behavior allow for observational or experimental work

on reaction to especially industrial seismic surveys, whose loud sounds travel long distances and

therefore have the potential to affect sperm whales in short and long term.

273
Chapter 8

Seabird Ecology

Nancy A. Hess and Christine A. Ribic

8.1 Introduction

As part of the GulfCet II program, three seabird surveys were conducted during cruises in the

northern Gulf of Mexico. These surveys occurred in different seasons. From 17 April to 9 June

1996, surveys were conducted aboard NOAA ship RIV Oregon II(NOAA Cruise 220). The

northern slope and oceanic Gulf (Figure 4.4), and northeast Gulf shelf and slope waters (Figure

4.2) were surveyed during this spring and early summer cruise. From 10 to 29 October 1996

(late summer) and 5 to 21 August 1997 (mid-summer), surveys were conducted from the RV

Gyre. These cruises covered the central pelagic and northeastern continental shelf and slope

regions of the Gulf (Figure 4.3). The spring Oregon II cruise was conducted at the same time of

year as previous Gulf of Mexico surveys conducted by the National Marine Fisheries Service

(Davis and Fargion 1996). The late summer survey was the first large-scale, shipboard seabird

survey to take place in the Gulf of Mexico during the month of October, a time of seabird

migration in this area. Previous late summer and autumn shipboard surveys occurred in late

August to early September and November (Davis and Fargion 1996). Fritts and Reynolds (1981)

conducted aerial surveys along coastal Texas and Florida areas during August and November.

Southeast Texas, Louisiana and southwest Florida coastal waters were surveyed (via aircraft) by

Fritts et al. (1983) every other month starting in February for a total of six surveys. However,

little is known about seabird associations with physical features of the Gulf. Ribic et al. (1997a)

identified possible associations with offshore eddies and indicated the need for further work on

seabird/oceanic feature association to understand the distribution of seabirds in the Gulf.

8.2 Methods

8.2.1 Cruise Logistics

The spring cruise (Oregon II cruise 220) spanned 44 days and surveyed 6,401 km (see Chapter 4

for details; Mullin and Roden 1996). The cruise consisted of three legs targeting different

regions of the Gulf. Two of the legs consisted of transects spaced evenly across the oceanic Gulf

and continental shelf. The third leg, in early summer, focused on the northeastern continental

slope and shelf water. During the late summer survey (Gyre96GO6), there were 16 days of

effort, covering approximately 2,015 km of the Gulf. This cruise surveyed two regions: an eddy

pair in the north-central Gulf that contained a cyclonic eddy and an anticyclonic Loop Current

eddy (LCE), and a northeastern region over the continental shelf and slope, in the Minerals

Management Service's Eastern Planning Area (EPA). The mid-summer cruise (Gyre97GO8) had

17 days of effort, resulting in approximately 2,500 km surveyed track line in the Gulf. The area

surveyed covered an eddy pair in the north-central Gulf and the EPA. The Oregon II (spring

cruise) and Gyre (mid-summer and late summer cruises) seabird surveys were conducted using

different methodologies.

275
8.2.2 Spring Cruise Survey Methods

The spring visual survey recorded seabirds opportunistically and coincident with marine

mammal sightings from the Oregon II. Two teams of three people alternated observation duties.

When birds were seen during the survey, they were identified to the lowest taxonomic level

possible, (dependent on the skill of the observer). Two of the observers used 25 x 150 "big eye"

binoculars. The third observer recorded data and used hand-held binoculars or naked eye in this

survey. Surveys were conducted during daylight hours in Beaufort Sea State 5 or less.

Only seabirds identified to the family/subfamily group were included in the analysis. Entries of

"unidentified seabird" were not included in the total number of birds. Percent of total seabirds

by group was the first index of relative abundance calculated. The second level of analysis

addressed group composition. Within each group a breakdown by species (e.g., pomarine jaeger,

parasitic jaeger, and long-tailed jaeger) and unidentified (e.g., jaeger spp.) was tallied. The data

were compared to data from other NMFS cruises that occurred at the same time of the year using

the same survey methods (Davis and Fargion 1996). The spring surveys of 1992, 1993, 1994

also used the methodology of opportunistically recording seabirds and other birds spotted during

marine mammal surveys. Percentages of total birds in each group were compared between the

different surveys.

8.2.3 Mid-summer and Late Summer Cruise Survey Methods

During the surveys from the Gyre, observations occurred by a three-person bird survey team

during daylight hours. Observers rotated so that two observers surveyed simultaneously.

Observers used hand-held 8x or 1 Ox binoculars. Observations occurred from the flying bridge of

the Gyre from sunrise to sunset, except during rain, Beaufort Sea State 6 or greater, and when the

ship was stopped at hydrographic stations or for sperm whale observations. The Gyre seabird

surveys consisted of continuous strip transects using the method of Tasker et al. (1984). The late

summer survey used 300-m wide transects; the mid-summer seabird survey used 300 m and

600 m strip transects. The survey area was measured off one side of the ship, sweeping from the

bow to 90° perpendicular to the ship. The side of the ship (port or starboard) containing the

survey area was selected based on lowest amount of glare from the sea surface. A range finder

as described by Heinemann (1981) was used to determine the limits of the 300 m and 600 m

bands. Sightings of birds outside of the strip transect survey area were also recorded.

Only seabirds identified at least to the family/subfamily level were included in analysis. The

total number of birds seen while "on effort," regardless of distance from ship, was used to assess

presence and absence and relative numbers of species seen on the continental shelf compared to

deeper water. Seabirds were tallied according to whether they were present on the continental

shelf(< 200 m) or in deeper water (> 200 m depth). The total number of birds, seen at any

distance from the ship, divided by the number of effort hours was used to compare the late

summer and mid-summer cruise data to GulfCet I survey data (Davis and Fargion 1996).

Species abundance in the 300-rn strip transect was categorized by hydrographic environment to

assess species distributions, as well as species diversity, richness and evenness. The numbers of

birds seen within the strip transect for both the late summer and mid-summer cruises were

designated as occurring within one of six hydrographic environments as defined in Chapter 2.

276
The hydrographic environments were: the cyclone, Loop Current eddy (anticyclone), confluence,

mouth of the Mississippi River (MOM), and other margin. The other margin was defmed as

regions with bottom depth greater than 200 m not directly associated with the eddy system. The

sixth hydrographic environment was the continental shelf which was defined as regions in water

less than 200 m deep.

Due to a relationship between kilometers of survey effort and species seen, diversity indices

were calculated only for hydrographic environments with at least 200 km survey effort, the point

where the relationship between species sighted and effort leveled off. The standardized Shannon

Index for diversity was calculated and expressed as a ratio of observed diversity to maximum

diversity possible for each hydrographic environment (or region) using:

-Z(p1)log(p1)/log(S)

where i = 1, 2,..., S, the number of species seen in the region and p1 equals the proportion of the

th species out of the total number of birds (Magurran 1988). Standardization allows direct

comparison of diversity between regions where different numbers of species were seen.

Standardized diversity indices fall between zero and one. The Simpson Index was used as a

measure of species dominance, or the probability that the next species seen is the same as the

previous. It was calculated as:

E(p)2

where i 1, 2,..., S, the number of species seen in the region and p1 is the proportion of the th

species out of the total number of birds (Magurran 1988). This would indicate species

prevalence in a hydrographic environment. A high Simpson Index value (close to one)

suggested that one species was most abundant and most frequently seen.

To conduct habitat analyses, the continuous strip transect was split into approximately equallength strip segments, with a target length of ca. 10 nautical miles to match the scale used in the

marine mammal analysis. Ship turns, stops for hydrographic stations and marine mammal

observations, and Beaufort Sea State> 5 were excluded from subsequent analysis. To match the

marine manmial habitat analysis, we excluded transects with water depth less than 200 m, as

well as one transect from the late summer survey with incomplete environmental data. For the

late summer survey, a 300-rn transect band width was used with a transect mean length of 7.63

nautical miles (SD = 2.02, n = 73). For the mid-summer survey, a 600-rn transect band width

was used with a mean transect length of 7.10 nautical miles (SD = 1.82, n = 103). For each

transect, the presence or absence of seabirds was tallied for the most abundant species seen

during the cruise. Analysis was conducted only for the species seen in at least 10% of the

transects.

To test a species' affinity for a particular environmental feature, presence or absence was

tabulated by hydrographic environment. Transects were assigned to one of the five hydrographic

environments, excluding the continental shelf, by matching the transect midpoint to the nearest

XBT or CDT station and categorizing the transect to the same environment as the closest station.

A G-test of independence (Fienberg 1980) was conducted to determine ifa significant

277
relationship existed between the bydrographic environments and species presence. Standardized

residuals (Fienberg 1980) were calculated to determine which hydrographic environments were

associated with the presence of individual species. Significance was assessed at alpha = 0.05.

Trends were assessed at alpha = 0.10. Analyses were conducted using SYSTAT7.0 (SPSS

1997).

Several oceanographic variables were calculated to analyze the relationship of seabirds to the

marine environment. Sea surface temperature and salinity were calculated for each transect by

averaging values at the transect start and finish times from the Gyre SAIL system data (see

Chapter 2 for details). Sea surface height and geostrophic flow were calculated by averaging the

transect start and finish point values using remote sensing data processed at the University of

Colorado Center for Astrodynamics Research (CCAR). Bottom-depth values for each transect

were obtained from the ETOPO-5 database. A program written for the marine mammal data

analysis (J. Ortega, TAMUG) was used to calculate bottom-depth gradient. Transect chlorophyll

values were calculated by the same method as sea surface temperature and salinity using SAIL

system data. Predicted mean biomass (PMB) was estimated by matching the midpoint of each

transect to the closest position with predicted mean biomass integrated over a depth interval of

10-50 m (see Chapter 3).

A high degree of correlation occurred among the environmental variables. To decrease the

amount of correlation between the predictor variables and to test competing hypotheses, four

models were developed:

Bathymetry, using bottom depth and slope of bottom depth as predictor variables of species

presence or absence in transects;

Surface physical propei-ties, using sea surface salinity and temperature as predictor variables;

Sea surface height, using sea-surface height and magnitude of geostrophic flow velocity as

predictor variables; and

Standing plankton stock, using PMB and surface chlorophyll as predictor variables

Generalized additive models, a nonparanietric smoothing technique used in fishery science

(Swartzman et al. 1995, Welch et al. 1995), were used. This technique analyzes nonmonotonic

nonlinear relationships that are difficult to model with standard regression techniques. Analyses

were done in S+4.5 (Statistical Sciences 1998). The "fit" of the different models, based on the

proportion of variance explained, was compared. To illustrate model results (and infer what

environments may be preferred), predictions for the best-fitting model were graphically

presented.

8.2.4 Bias

Observation methods, bird behavior in response to the ship, and bird movement can result in

biased estimates of both species composition and species numbers in a survey. The two different

survey methods employed during the spring and summer cruises produced results that should not

278
be used in direct comparison with each other. During the spring cruise, observers recorded

seabirds opportunistically with marine mammal sightings using both handheld and 25x

binoculars. As a result, sightings may have been biased toward more visible flocks of birds

(Tasker et al. 1984).

Researchers have examined some of the sources of bias for transect survey methods of the type

used during the summer cruises. Broni et al. (1985) found that the side of the ship chosen for observations did not affect survey results. The range finder we used to estimate distance was

assessed by Briggs et al. (1985) to be accurate within 10%. The skill, experience and fatigue of

an observer can affect survey results (Van der Meer and Camphuysen 1996). Therefore, two

observers worked together during all survey effort. To minimize fatigue, a half-hour break was

taken after surveying for one hour.

Since seabirds may have been attracted to the ship, precautions were taken to prevent doublecounting the same bird. During the summer cruises, seabirds circling the ship were counted

once. Additionally, observation effort made while the ship was turning or when ship speed was

less than 4 knots was excluded from analysis to minimize the chance of seabirds being counted multiple times.

Seabird behavior can affect survey results. Seabirds in-flight may have a higher inclusion

probability in a transect than stationary birds. Spear and Ainley (1997) found that seabird flight

speed and direction varied with wind speed and direction. Ifa seabird is flying faster due to

favorable wind conditions, it is more likely to be seen than a bird sitting on the water's surface.

This bias varies by species, and information on flight speeds is needed to adjust the counts.

These speeds are not available for all species in the Gulf of Mexico. Therefore, this chapter

investigates only the presence and absence of species in transects rather than using seabird

density.

8.3 Results

Latin names are listed in Table 8.1 for all species sighted on the three cruises. Common and

Latin names were taken from American Ornithologists' Union (1998).

8.3.1 OveEall Abundance

During the spring cruise, 5,918 seabirds were recorded during 334.8 effort-hours covering 6,401

km. Twenty-two seabird species were represented by eight groups (Table 8.2). Terns were the

most abundant seabirds seen during the cruise; almost 70% of the total seabirds were terns. The

next most abundant group of seabirds was storm-petrels (16.7% of the total seabirds), followed

by gulls (7.4% of total birds). Shearwaters (3.0%) and jaegers (2.1%) were seen in lesser

numbers. Sulids (gannets and boobies), frigatebirds, phalaropes, and tropicbirds combined

amounted to just over 1% of the total seabirds.

Over one-third of the tems counted were not identified to the species level (Table 8.2).

Unidentified terns comprised 28.1% of the total birds counted, and 40% of the total number of

terns. The most abundant tern species was the sooty tern, accounting for one-fifth of the terns

seen. Bridledlsooty terns, black terns and sandwich terns each comprised approximately one279
Table 8.1. Common and Latin names of seabirds seen during the spring, mid-summer and late

summer cruises.

Order Procellariiformes

Family Procellariidae

Audubon's shearwater (Puffinus iherminieri)

Cory's shearwater (Calonectris diomedea)

Greater shearwater (Puffinus gravis)

Manx shearwater (Puffinuspuffinus)

Sooty shearwater (Puffinus griseus)

Family Hydrobatidae

Band-rumped storm-petrel (Oceanodroma castro)

Leach's storm-petrel (Oceanodroma leucorhoa)

Wilson's storm-petrel (Ocean ites oceanicus)

Order Pelecaniformes

Family Phaethontidae

Red-billed tropicbird (Phaethon aethereus)

White-tailed tropicbird (Phaethon lepturus)

Family Sulidae

Masked booby (Sula dactylatra)

Northern gannet (Morus bassanus)

Family Fregatidae

Magnificent frigatebird (Fregata magnfIcens)

Order Charadriiformes

Family Laridae

Subfamily Stercorariinae

Long-tailed jaeger (Stercorarius longicaudus)

Parasitic jaeger (Stercorarius parasiticus)

Pomarine jaeger (Stercorarius pomarinus)

Subfamily Larinae

Herring gull (Larus argentatus)

Laughing gull (Larus atricilla)

Subfamily Sterninae

Arctic tern (Sterna paradisaea)

Black tern (Chlidonias niger)

Bridled tern (Sterna anaethetus)

Brown noddy (Anous stolidus)

Common tern (Sterna hirundo)

Least tern (Sterna antillarum)

Royal tern (Sterna maxima)

Sandwich tern (Sterna sandvicensis)

Sooty tern (Sterna fuscata)

280
Table 8.2. Number and percent of seabird species seen during the spring cruise.

(continued)

281

Number Percent of total

Shearwaters

Audubon's shearwater 164 2.8

Cory's shearwater 1 0.0 *

Unidentified shearwater 15 0.2

Total shearwaters 180 3.0

Storm-petrels

Band-rumped storm-petrel 52 0.9

Wilson's storm-petrel 17 0.3

LeacWs storm-petrel 2 0.0 *

Unidentified storm-petrel 916 15.5

Total storm-petrels 987 16.7

Tropicbirds

White-tailed tropicbird 1 0.0 *

Unidentified tropicbird 2 0.0 *

Total tropicbirds 3 0.0 *

Sulids

Northern gannet 37 0.6

Masked booby 2 0.0 *

Total sulids 39 0.7

Frigatebirds

Magnificent frigatebird 7 0.1

Jaegers

Pomarinejaeger 85 1.4

Parasitic jaeger 9 0.2

Long-tailed jaeger 1 0.0 *

Unidentified jaeger 27 0.5

Total jaegers 122 2.1

Gulls

Laughing gull 428 7.3

Herring gull 1 0.0 *

Unidentified gull 8 0.1

Total gulls 437 7.4

Terns

Sooty tern 834 14.1

BridledlSooty tern 533 9.0
Table 8.2. Continued.

* less than 0.1% of total seabirds seen.

Table 8.3. Percentages of taxonomic seabird groups for the spring cruises.

1992 1993 1994 1996

Terns 66.2 72.9 64.0 69.9

Storm-petrels 17.1 12.7 22.8 16.7

Gulls 3.3 4.0 7.8 7.4

Shearwaters 0.8 2.7 2.4 3.0

Jaegers 9.2 6.2 1.8 2.1

Sulids 3.0 1.1 0.3 0.7

Frigatebirds 0.4 0.3 0.8 0.1

Tropicbirds 0.0 0.1 0.1 0.1

Number Percent of total

Sandwich tern 519 8.8

Black tern 494 8.3

Bridled tern 63 1.1

Royal tern 10 0.2

Common tern 7 0.1

Least tern 2 0.0 *

Brown noddy 1 0.0 *

Unidentified tern 1,664 28.1

Total terns 4,127 69.7

Phalaropes 16 0.3

Unidentified phalarope 16 0.3
eighth of terns seen. Bridled terns, royal terns, common terus, least terus and brown noddies

were also identified.

Over 90% of the storm-petrels seen were not identified to the species level. Of the storm-petrel

species identified, band-rumped storm-petrels were the most abundant. Wilson's storm-petrel

and Leach's storm-petrel were also seen during the cruise but accounted for less than 20 birds

during the entire cruise. Most of the gulls seen were identified to species, with the majority of

gulls being laughing gulls. One herring gull was seen, and eight gulls were not identified at the

species level. Shearwaters were most frequently recorded as Audubon's shearwater. One Cory's

shearwater was spotted, and 15 shearwaters were not identified to species level. About threefourths ofj aegers seen were pomarine j aegers. Nine parasitic jaegers and one long-tailed jaeger

were also identified with the remaining jaegers not identified to species level. Sulids were

comprised primarily of northern gannets, and two masked boobies were counted. The seven

frigatebirds counted were magnificent frigatebirds. Three tropicbirds were counted, one of

which was identified to the species level as a white-tailed tropicbird. Sixteen phalaropes were

seen. Together, unidentified terns and storm-petrels represented almost one-half of all seabirds

seen. The most abundant seabirds overall on a species level were sooty terns (14.1%), sandwich

terns (8.8%), black terns (83%) and laughing gulls (7.3%).

The percentages of total seabirds in each group from the 1996 spring cruise were similar to

percentages found in previous spring surveys conducted with the same survey methods (Davis

and Fargion 1996) (Table 8.3). In each of the surveys, terns accounted for between 64.0% and

72.9% of total birds. Storm-petrels were the next most abundant genera in each of the four

surveys, accounting for 12.7-22.8% of the total number of birds. Gulls ranged from 3.3-7.8% of

the total. Shearwaters ranged from being the fourth most abundant group in 1996 (3.0% of the

total birds), to being the sixth most abundant group (0.8%) in 1992. Percentages ofjaegers

fluctuated. During 1992 and 1993, 9.2% and 6.2% of total birds, respectively, were jaegers.

However in 1994 and 1996, there were comparatively fewer jaegers; 1.8% and 2.1% of the total

number of birds, respectively.

Sulids also decreased in relative abundance over the four-year period. Three percent of total

birds in 1992 were gannets or boobies. Tn 1994 and 1996, less than one percent of seabirds were

sulids. In any year, frigatebirds never comprisedmore than one percent of the total number of

birds. Tropicbirds were rare in all four survey years. Only a few hundredths of a percent of total

birds were tropicbirds.

During the late summer cruise, 279 seabirds were counted representing 14 species and eight

family/subfamily groups (Table 8.4). Terns were the most common group, accounting for 46.1%

of total birds. Gulls were the next most abundant group, totaling almost one-quarter of birds

seen. Jaegers were the third most commonly seen birds (13.7% of total birds). Shearwaters

accounted for 8.6% of total birds seen. Together, terns, gulls, jaegers and shearwaters comprised

over 90% of the total. Frigatebirds (4.3%), tropicbirds (1.4%), sulids (1.4%), stormpetrels

(1.1%) and phalaropes (<0.1%) were also seen. Many terns were not identified to species (55

out of 128 terns).

283
Table 8.4. Seabird numbers tallied by water depth seen while on-effort during the

mid-summer and late summer cruises.

284

Late summer

1 2 Shallow Deep Total

Mid-summer

3 4 Shallow Deer Total

Shearwaters

Audubon's shearwater 2 9 11 0 154 154

Cory's shearwater 1 6 7 2 8 10

Greater shearwater 0 0 0 2 1 3

Manx shearwater 0 0 0 0 5 5

Sooty shearwater 0 0 0 0 1 1

Unidentified shearwater 2 4 6 1 20 21

Total shearwaters 5 19 24 5 189 194

Storm-petrels

Band-rumped storm-petrel 0 2 2 1 249 250

Leach's storm-petrel 0 0 0 1 0 1 Wilson's storm-petrel 0 0 0 0 10 10

Unidentified storm-petrel 0 1 1 2 60 62

Total storm-petrels 0 3 3 4 319 323

Tropicbirds

Red-billed tropicbird 0 2 2 0 2 2

Unidentified tropicbird 0 2 2 0 0 0

Total tropicbirds 0 4 4 0 2 2

Sulids

Masked booby 0 4 4 0 4 4

Frigatebirds

Magnificent frigatebird 6 6 12 4 0 4

Unidentified frigatebird 0 0 0 170 4 174

Total friatebirds 6 6 12 174 4 178

Jaegers

Long-tailed jaeger 0 0 0 1 1 2

Parasitic jaeger 0 0 0 0 1 1 Pomarinejaeger 1 24 25 1 13 14

Unidentified jaeger 2 11 13 0 1 1 Total j aegers 3 35 38 2 16 18

Gulls

Laughing gull 44 11 55 21 17 38

Herringgull 5 2 7 0 0 0

Unidentified gull 3 0 3 0 2 2

(continued)
'Shallow water (<200 m depth) along the continental shelf During the late

summer cruise about 16.3% of effort occurred in shallow water.

2 Deep water (>200 m depth) contained regions along the continental slope and deep

water. About 83.7% of effort occurred in deep water.

Shallow water (<200 m depth) along the continental shelf During the mid-summer

cruise about 13.5% of effort occurred in shallow water.

water (>200 m depth) contained regions along the continental slope and deep

water. About 86.5% of effort occurred in deep water.

285

Table 8.4. Continued.

Late summer Mid-summer

Shallow1 Deep2 Total Shallow3 Deep4 Total

Total gulls 52 13 65 21 19 40

Terns

Arctictern 0 0 0 0 2 2

Arctic/Comnn tern 0 2 2 2 1 3

Black tern 8 1 9 852 267 1,119

Bridledtern 1 5 6 0 70 70

Bridled/Sooty tern 0 5 5 6 67 73

Commontern 7 1 8 1 3 4

Leasttern 0 0 0 0 1 1

Royal tern 17 19 36 14 5 19

Sandwich tern 0 0 0 21 3 24

Sootytern 0 7 7 0 111 111

Unidentified tern 43 12 55 77 26 103

Total terns 76 52 128 973 556 1,529

Phalaropes 0 1 1 0 22 22

Unidentified phalarope 0 1 1 0 22 22
Over one-quarter of the total terns seen during the late summer cruise were royal terns (36 birds).

Black tems (nine birds) and common terns (eight birds) were the next most abundant terns.

Sooty terns, bridled terns, bridledlsooty terns and common/arctic terns accounted for the

remaining 20 terns counted. The majority of gulls seen were laughing gulls (84.6%). Seven

herring gulls were seen, and three gulls were not identified to species level. Out of the 38 jaegers

seen, 25 were identified as pomarine jaegers; the rest were not identified. Audubon's

shearwaters comprised almost half of the shearwaters seen (11 out of 24 birds). Seven Cory's

shearwaters were spotted, and the remaining six shearwaters were not identified to the species

level. All 12 frigatebirds were magnificent frigatebirds. Four tropicbirds were seen; two were

red-billed tropicbirds and the other two were not identified to species. All four sulids seen were

masked boobies. Three storm-petrels were seen, two of which were identified as band-rumped

storm-petrels. One was not identified to the species level. Laughing gulls and unidentified terns

each comprised about one-fifth of all seabirds seen during the late summer cruise. Royal terns

were the next most abundant, accounting for 12.9% of the total number of birds. Pomarine

jaegers and unidentified jaegers combined added up to 13.7% of the total birds. Magnificent

frigatebirds and Audubon's shearwaters were the next most abundant species. Those seven

groups totaled three-quarters of the seabirds seen.

The late summer cruise species' numbers tallied by water depth show species affinities for either

shallow or deep water (Table 8.4). Shearwaters were more frequently encountered in deeper

water regardless of species: only five out of the 24 shearwaters were seen on the continental

shelf. Storm-petrels, tropicbirds and boobies were seen exclusively in water greater than 200 m

deep. Half of the frigatebirds seen were detected on the shelf, and the other half in deeper water.

The majority ofjaegers (35 out of 38) were seen farther offshore. Both species of gullswere

seen in greater numbers on the continental shelf. Terns were found both in shallow and deeper

water depending on species. Black terns, common terns and terns not identified to species were

found predominantly on the continental shelf. Royal terns were almost evenly split between the

continental shelf and deeper water. Bridled terns and sooty terns were only seen in deeper water.

Shearwaters, frigatebirds, tropicbirds, jaegers and terns were detected in late summer (October)

at rates between the August-September and November rates found during the GulfCet I cruises

(Table 8.5). Storm-petrels were seen at rates less than August-September and November rates.

Sulids were also seen at a rate less than the late summer and autumn rates from previous work.

Gulls were seen at rates higher than the previously reported late summer and autumn values.

During the mid-summer (August) cruise, over 2,100 seabirds were counted, representing 23

species from eight family/subfamily groups (Table 8.4). Two-thirds of the seabirds seen were

terns, totaling 65.3% of total seabirds counted. Storm-petrels were the next most abundant

group, accounting for 14.8% of total birds. Shearwaters comprised 8.9% of the seabirds counted,

and frigatebirds accounted for 8.1% of total seabirds. Gulls (1.8%), jaegers (<1%), sulids (<1%)

and tropicbirds (<1%) were also seen.

The majority of terns were black terns (73.2%), which accounted for half of all seabirds seen

during the mid-summer cruise (Table 8.4). Sooty terns were the second most abundant tern

species (7.3%). Bridled terns (4.6%) and either bridled or sooty terns (4.8%) were the next most

represented terns. Twenty-four sandwich tenis and 19 royal terns were seen. Two arctic terns,

286
Table 8.5. Cross-seasonal comparison of taxonomic group sightings per effort hour.

Davis and Fargion (1996)

2 This study

287

February1 May-June1 Auust2 Aug-Sent' October2 November'

Shearwaters 0.01 0.06 1.13 1.30 0.17 0.12

Storm-petrels 0.01 0.24 1.84 0.06 0.02 0.03

Frigatebirds 0.00 0.00 1.06 0.20 0.08 0.00

Sulids 0.60 0.02 0.03 0.13 0.03 0.12

Tropicbirds 0.00 0.00 0.01 0.03 0.03 0.00

Jaegers 1.05 0.08 0.11 0.00 0.26 1.08

Gulls 2.23 0.00 0.38 0.04 0.45 0.05

Tems 0.05 0.21 9.50 24.65 0.89 0.04

3.95 0.61 14.06 26.41 1.93 1.44
four common terns, three common or arctic terns and one least tern were also encountered. One

hundred and three terns were not identified to species.

The majority of storm-petrels seen were band-rumped storm-petrels (77.4%). Ten Wilson's

storm-petrels and one Leach's storm-petrel were counted. Sixty-two storm-petrels were not

identified to species. Five different species of shearwaters were encountered during the August

cruise. The majority of shearwaters were Audubon's shearwater (79.4%). Ten Cory's

shearwaters, five Manx shearwaters, three greater shearwaters and one sooty shearwater were

seen. Twenty-one shearwaters were not identified to species. One hundred and seventy-eight

frigatebirds were counted. No other frigatebird is known to be present in the Gulf (Harrison

1996).

All of the gulls identified to species in mid-summer were laughing gulls (95.0%); two gulls were

unidentified. Three-fourths ofthejaegers seen were pomarinejaegers (77.8%). Two long-tailed

jaegers, one parasitic jaeger and one unidentified jaegerwere counted. All four boobies seen

were masked boobies. The two red-billed tropicbirds were seen.

A pattern of segregation by species in the two different depth categories was apparent for the

mid-summer cruise (Table 8.4). Shearwaters were more frequently seen in deeper water

(97.4%). However, the trend was not the same for all shearwaters; Audubon's, Manx and sooty

shearwaters were seen exclusively in deep water. Two of the ten Cory's shearwaters and two of

the three greater shearwaters were seen in shallow water. Although the one Leach's storm-petrel

was seen in shallow water, the majority (98.8%) of storm-petrels were seen in deeper water.

Tropicbirds and boobies were exclusively located in deeper water. Frigatebirds were

predominantly located in shallow water; only four out of 178 frigatebirds were seen where water

depths were greater than 200 m. Jaegers were primarily seen in deeper water. However one

pomarine and one long-tailed jaeger were encountered in shallow water. Gulls, most of which

were laughing gulls, were evenly split between shallow and deep water. Terns were seen in both

shallow and deep water depending on species. Arctic terns, least terns and sooty terns were only

seen in deeper water. Common, bridled and bridledlsooty terns were predominantly seen in

deeper water. The majority of black terns, royal terns, sandwich terns and terns not identified to

species were seen in shallow water.

A comparison of mid-summer sightings per hour with previous findings (Davis and Fargion

1996) indicates some seasonal patterns (Table 8.5). Shearwaters, storm-petrels and frigatebirds

peaked in sighting rates in the mid-summer cruises (Figure 8.1 a). Sulids, jaegers and gulls were

seen more frequently during winter cruises (Figure 8. ib). Terns peaked in abundance during the

mid-summer cruises; note that they were seen at a rate ten times higher than other taxonomic

groups. Some differences in sighting frequencies are evident in two cruises that occurred at

similar times of the year (summer). Storm-petrels, frigatebirds, jaegers and gulls were seen more

frequently in August 1997 compared to August-September 1993. Either interannual variability

or habitat differences could be responsible.

288
a)

2.0

0.0 -

2.5

1.0

0.5

0.0

0

b)

Shearwaters

- - Storm-petrels -

- Frigatebirds

Tems

February May-June August October November

289

--

Sulids

- - - Jaegers - - - Gulls

J

Figure 8.1. Cross-seasonal comparison of sightings per effort hour for

(a) shearwaters, storm-petrels and frigatebirds, (b) sulids, j aegers and

gulls, and (c) terns.

February May-June August October November

c)

February May-June August October November

30

-25

20

15

10
8.3.2 Species and Diversity by Hydrographic Environment

Of the birds seen within the 300 m strip transect during the late summer cruise, some species

were seen in many of the six hydrographic environments, while some species were seen only in

one or two hydrographic environments (Table 8.6). Laughing gulls were present in every region

except for the confluence. Laughing gulls, however, were present in greatest numbers on the

continental shelf. Pomarine jaegers were seen in every environment except for the continental

shelf. Audubon's shearwaters were not seen in either the anticyclone or "other margin." Royal

terns were seen in the MOM and "other margin." Cory's shearwater and masked boobieswere

seen only in the cyclone and in "other margin." Herring gulls and magnificent frigatebirds were

seen in "other margin" and continental shelf however, a herring gull was also present in the

cyclone. Band-rumped stonn-petrel, sooty tern, and red-billed tropicbird were seen inside the

cyclone. A common tern was seen in "other margin."

Species richness or the number of seabird species seen in the 300 m continuous strip transects

during the late summer cruise varied with hydrographic environment (Table 8.7). The most

species (9) were seen in the cyclone. "Other margin" (regions with bottom depth greater than

200 m not directly associated with the eddy system) contained eight species, the next highest

number in any hydrographic environment. The MOM area and continental shelf contained four

species. The confluence and anticyclone contained two species, but these environments were

covered with less effort than others.

During the late summer cruise the cyclone had a greater diversity of species compared to the

"other margin" (Table 8.7). In the "other margin" regions, the majority of sightings were royal

terns, which decreased species diversity. The continental shelf sightings were mainly laughing

gulls, which was reflected in the lower Shannon Index and higher Simpson Index. The low

Simpson Index values of the cyclone and the "other margin" regions indicate neither

environment was dominated by one species.

During the mid-summer cruise, the distribution of species seen within the 300 m strip transect

across the hydrographic environments was not uniform (Table 8.8). Frigatebirds were only seen

on the continental shelf; as was the long-tailed jaeger. Royal terns, sandwich terns and black

tens were seen predominantly on the continental shelf. The two masked boobies seen in the

cyclone. Four of the five Manx shearwaters were also seen in the cyclone. Band-rumped storm

petrels were seen in all of the hydrographic environments except for the MOM area. A large

number of band-rumped storm-petrels were seen in the "other margin" region in addition to the

confluence and cyclone. Laughing gulls were found mainly on the continental shelf, but were

present in every region but the MOM. Black terns, while predominantly found on the

continental shelf, were also found in the confluence, MOM, cyclone and anticyclone. Audubon's

shearwaters were found in the confluence, cyclone, anticyclone and "other margin." Bridled

tens were seen predominantly in the cyclone, confluence and anticyclone. Sooty terns were also

seen in the cyclone, confluence and anticyclone, but were seen in larger numbers in "other

margin" regions.

The "other margin" and cyclone contained the greatest number of species during mid-summer

(Table 8.7). Fewer species were found on the continental shelf, confluence and anticyclone.

290
Table 8.6. Species tallied by hydrographic environment within 300 m strip transec

Hydrographic environment Kilometers of effort MOM164 Other Continental Margin Shelf 359 204 Cyclone Con

320 

Audubon's shearwater 3 0

2 1 

Band-rumped storm-petrel

0

0

0

2 

Commontem

0 1 0 0 

Cory's shearwater

0 1 0 1 

Herring gull

0 1 3 1 

Laughing gull 2 6 Ii

4

Magnificent frigatebird

0 3 1 0

Masked booby

0 1 0 1 

Pomarinejaeger 6 2

0

2

Red-billed tropicbird

0 0 0 1 

Royaltem

7 13

0

0

Sootytem

0 0 0 1 
Table 8.7. Species numbers and diversity for each hydrographic environment for sightin

300 m strip transect. Kilometers of effort refers to the length of effort in each Diversity indices were calculated only for regions with greater than 200 km e

Hydro graphic environment MOM Other Continental Margin Shelf Cyclone ConfluLate summer cruise Kilometers of effort 164 359 204 320 

Number of species seen

4 8 4

9

Total seabirds seen 18 28 17 14 

Scaled Shannon Index 0.77 0.73 0.93 Simpson Index 0.28 0.47 0.15 Mid-summer cruise Kilometers of effort 92 365 220 350 Number of species seen 5 12

9 12 

Total seabirds seen 33 126 391 74 

Scaled Shannon Index 0.56 0.25 0.83 

Simpson Index 0.40 0.79 0.17 
Table 8.8. Species tallied by hydrographic environment within 300 m strip transect for the 

Hydrographic environment Kilometers of effort MOM92 Other Continental Margin Shelf 365 220 Cyclone Confluence A

350 285 

Arctictern

0

0

0

0 1 

Audubon's shearwater 0 8 0 24 28Band-rumped storm-petrel

0 77 1 11 12

Black tern 26 15 346 6 49 Bridled tern

0 1 0 12 6 

Common tern

2 1 0

0

0

Cory's shearwater

0 1 1 0

0

Greater shearwater

0

0 2 1 0

Laughing gull

0 5 12

4

4

Long-tailed jaeger

0

0 1 0 0

Magnificent frigatebird

0

0 3 0

0

Manx shearwater

0

0

0

4 1 

Masked booby

0

0

0

2

0

Pomarine jaeger

0

0

0

4

0 

Red-billed tropicbird

0 1 0 0 0 

Royaltern 2 1 ii 1 0Sandwich tern 2 1 14

0

0

Sootytern

0 13

0

4 8 

Wilson's storm-petrel 1 2 0 1 0
Only five species were encountered in the MOM region. However, MOM covered the least

survey area. Diversity was the greatest in the cyclone followed by the anticyclone and

confluence. The continental shelf had the lowest species diversity resulting from the large

numbers of black terns. The Simpson Index was close to one for the continental shelf, indicating

the predominance of black terns in that region.

8.3.3 Species' Affinities to Hydrographic Eegion

During the late summer cruise, laughing gulls (Figure 8.2) and pomarine jaegers (Figure 8.3)

were the only species seen in at least 10% of the transects. Laughing gulls were seen most

frequently in the "other margin" (four out of 24 transects), in two transects in the cyclone, and in

one of the MOM transects (Table 8.9). There was no detectable difference in presence and

absence of laughing gulls across the different hydrographic environments (G = 3.9, df = 4, p

0.425). Pomarinejaegers were seen in transects across all hydrographic environments except for

the anticyclone. However, they were encountered more frequently than expected in transects in

the MOM area and confluence (G = 10.2, df= 4, p = 0.03 7).

Audubon's shearwaters (Figure 8.4) were seen in half of the transects in the cyclone during the

mid-summer cruise and less frequently in the confluence, anticyclone and other margin (Table

8.9). The presence of Audubon's shearwaters was not independent of the features (G 11.1,

df = 4, p 0.026). Residual analysis indicated more Audubon's shearwaters were encountered

in the cyclone than expected, and less were in the MOM area than expected.

Band-rumped storm-petrels were also found in every deep water environment except for the

MOM. Their distribution pattern was not uniform across the environments (G 9.9, df= 4, p =

0.042). Band-rumped storm-petrels were more likely to be encountered in "other margin"

regions (Figure 8.5).

Black terns (Figure 8.6) were seen in all environments, most notably near the MOM (Table 8.9).

Black tern presence differed across hydrographic environments (G = 29.5, df= 4, p = 0.000).

Specifically, more black terns were encountered near the MOM than expected, and less in the

anticyclone and cyclone.

There was no statistically significant relationship with either bridled (Figure 8.7) or sooty terns

(Figure 8.8) and the hydrographic environments (bridled tern: G 4.7, df= 4, p = 0.324; sooty

tern: G = 5.3, df= 4, p 0.257). Both bridled and sooty terns were seen more frequently in the

"other margin" and confluence (Table 8.9), areas where less saline water was entrained around

the cyclone (see chapter 2). As with band-rumped storm-petrels and Audubon's shearwaters,

neither bridled nor sooty terns were encountered near the MOM.

8.3.4 Seabird Habitat Preferences

During late summer, sea surface temperature arid salinity explained the most variance of

laughing gull presence or absence in transects off of the continental shelf (Table 8.10). The

model predicted laughing gull presence in transects with sea surface temperatures ranging from

26.0°C to 26.5°C (Figure 8.9). The plankton-standing stock model also explained a similar

294
I

295

-90 -88 -86 -84

Longitude

Figure 8.2. Distribution of laughing gulls (late summer cruise). Filled stars represent

transects included in seabird-habitat analysis with laughing gulls present.

Filled circles represent transects included in seabird-habitat analysis with

laughing gulls absent. Empty stars represent transects not included in

seabird-habitat analysis with laughing gulls present. Empty circles represent

transects not included in seabird-habitat analysis with laughing gulls absent.

Negative height (cyclone) is designated by heavy dashed lines; positive

height (confluence and anticyclone) by heavy solid lines.
30 o/ o

-jO

100 in - -- - .lf

iOQrn

,

\:--i-_. , _ % a

0 I ' L\ II

I -' ' I-', __ ,i\ "

0

I [ --S-.l. / 1%

3000m

- 26

2

a20

a

fi 2

20

40

a

296

-90 -88 -86 -84

Longitude

Figure 8.3. Distribution of pomarine jaegers (late summer cruise). Filled stars represent

transects included in seabird-habitat analysis with pomarine jaegers present.

Filled circles represent transects included in seabird-habitat analysis with

pomarine jaegers absent. Empty circles represent transects not included in

seabird-habitat analysis with pomarine jaegers absent. Negative height

(cyclone) is designated by heavy dashed lines; positive height (confluence

and anticyclone) by heavy solid lines.
Table 8.9. Number of transects containing species within hydrographic environments. Hydrographic environment MOM Other Margin Cyclone Confluence Anticy

Late summer cruise total transects Laughing gull Pomarinejaeger 11

14 24 25

6 

4 2 0

1 2

2

Mid-summer cruise total transects

7 29 26 22 

Audubon's shearwater

0 7 13 10

Band-rumped storm-petrel

0 13

7

6 

Blacktern

7 11 2 5 

Bridled tern

0

7 3 4 

Sootytern 0 9 5 5 
Longitude

Figure 8.4. Distribution of Audubon's shearwaters (mid-summer cruise). Filled stars

represent transects included in seabird-habitat analysis with Audubon's

shearwaters present. Filled circles represent transects included in seabirdhabitat analysis with Audubon's shearwaters absent. Empty circles represent

transects not included in seabird-habitat analysis with Audubon's shearwaters

absent. Negative height (cyclone) is designated by heavy dashed lines;

positive height (confluence and anticyclone) by heavy solid lines.

298
Longitude

Figure 8.5. Distribution of band-rumped storm-petrels (mid-summer cruise). Filled stars

represent transects included in seabird-habitat analysis with band-rumped

storm-petrels present. Filled circles represent transects included in seabirdhabitat analysis with band-rumped storm-petrels absent. Empty stars

represent transects not included in seabird-habitat analysis with band-rumped

storm-petrels present. Empty circles represent transects not included in

seabird-habitat analysis with band-rumped storm-petrels absent. Negative height (cyclone) is designated by heavy dashed lines; positive height

(confluence and anticyclone) by heavy solid lines.

299
Longitude

Figure 8.6. Distribution of black terns (mid-summer cruise). Filled stars represent

transects included in seabird-habitat analysis with black terns present. Filled

circles represent transects included in seabird-habitat analysis with black

terns absent. Empty stars represent transects not included in seabird-habitat

analysis with black terns present. Empty circles represent transects not

included in seabird-habitat analysis with black terns absent. Negative height

(cyclone) is designated by heavy dashed lines; positive height (confluence

and anticyclorie) by heavy solid lines.

300

-90 -88 -86 -84
Longitude

Figure 8.7. Distribution of bridled terns (mid-summer cruise). Filled stars represent

transects included in seabird-habitat analysis with bridled terns present.

Filled circles represent transects included in seabird-habitat analysis with

bridled terns absent. Empty circles represent transects not included in

seabird-habitat analysis with bridled terris absent. Negative height (cyclone)

is designated by heavy dashed lines; positive height (confluence and

anticyclone) by heavy solid lines.

301
Longitude

Figure 8.8. Distribution of sooty terns (midsunimer cruise). Filled stars represent

transects included in seabird-habitat analysis with sooty terns present. Filled

circles represent transects included in seabird-habitat analysis with sooty

terns absent. Empty circles represent transects not included in seabird-habitat

analysis with sooty terns absent. Negative height (cyclone) is designated by

heavy dashed lines; positive height (confluence and anticyclone) by heavy

solid lines.

302
Table 8.10. Percent of total variance in seabird presence or absence in transects explained

using generalized additive models.

a Variables used in each of the models are explained in section 8.2.3.

303

a Model

Bathynietry

Surface

Properties

Sea Surface

Height

Plankton

Standing Stock

Late summer cruise

Laughing gull 31.8% 45.5% 24.5% 45.3%

Pomarine jaeger 30.8% 12.1% 38.0% 34.5%

Mid-summer cruise

Audubon's shearwater 13.0% 7.1% 7.4% 13.0%

Band-rumped storm-petrel 6.9% 7.0% 5.6% 18.3%

Black tern 13.5% 41.7% 22.3% 37.4%

Bridled tern 15.9% 10.3% 16.1% 17.1%

Sooty tern 12.2% 18.1% 10.1% 16.4%
1,

, a) Co

q

LC) -

CNJ 0

r

0:, 0

Co 0

N

C

+

+

304

I I I 25.0 25.5 26.0 26.5 27.0 27.5

b) Sea surface temperature (°C)

0.10 0.15 0,20 0.25 0.30

Chlorophyll (mg/rn3)

Figure 8.9. Presence of laughing gulls (late smnmer cruise) predicted by

(a) sea surface properties model and (b) plankton standing

stock model. Plus signs indicate presence in transects (as

predicted by model). Dots represent absence in transects (as

predicted by model).
amount of variance; laughing gulls occurred more frequently in areas with higher chlorophyll and PMB.

Pomarine jaeger presence and absence in transects was best explained by the model

incorporating sea surface height and magnitude ofgeostrophic velocity (Table 8.10). Pomarine

jaegers occurred in areas with a higher geostrophic velocity, but where the sea surface height was

not as large as within the interior of the anticyclone (Figure 8.10). The planktonstanding stock model, using surface chlorophyll and PMB integrated from 10-50 m depth as predictor variables,

also explained a large amount of variance in pomarine jaeger presence and absence. They were predicted to be seen in transects of low chlorophyll and high P1v113.

The presence of Audubon's shearwater during mid-summer was best explained by the

bathymetry model using depth and depth gradient as predictor variables, and the planktonstanding stock model, using surface chlorophyll and PMB as predictor variables (Table 8.10).

However, neither model explained much of the variation. The bathymetry model predicted

Audubon's shearwater in several combinations of bottom depth and slope of bottom depth

(Figure 8.1 la). The presence of Audubon's shearwater was predicted in two clusters in the plankton standing stock model: one transect with high chlorophyll (which was located in the

confluence), and several transects with chlorophyll less than 0.20 mg/rn3 and PMB ranging from

1.4-1.8 cc/m2 (Figure 8.11b).

The planktonstanding stock model best explained band-rumped storm-petrel presence (Table 8.10). This model predicted band-rumped storm-petrel presence in transects with 0.10-0.25 mg/rn3 chlorophyll and PMB (10-102 m) ranging from 1.0-1.4 cc/rn2 (Figure 8.12). Presence

was also predicted in areas of high chlorophyll similar to Audubon's shearwater.

Sea surface temperature and salinity best explained the presence of black terns (Table 8.10). The

model predicted that black terns would be present in areas where salinity was less than 32 psu, and temperature was less than 30.5°C (Figure 8.13). Additionally, black terns were predicted to

be present in transects with high sea surface temperature if salinity was below 28 psu. Black

terns were not predicted to be encountered in waters more saline than 32.5 psu regardless of sea

surface temperature.

The plankton-standing stock model best explained bridled tern presence (Figure 8.14). However, this model predicted the presence of bridled tems in only one transect. It is noteworthy that this

transect was the same confluence transect in which Audubon' shearwaters, band-rumped storm- petrels and black terns were predicted to be present.

Sea surface temperature and salinity best predicted sooty tern presence (Table 8.10).

Specifically, the model predicted sooty tern presence in transects with a temperature between

30.5°C and 3 1.3°C and a salinity of approximately 32 psu (Figure 8.15).

305
c,'J

+

+

+

I

J I -20 0 20

Sea surface height (dyn. cm)

Figure 8.10. Presence ofpomarinejaegers (late summer cruise) predicted

by sea surface height model. Plus signs indicate presence in

transects (as predicted by model). Dots represent absence in

transects (as predicted by model).

306

40
: I

+ +

++

e : +

_ __

I

I

I I 500 1000 1500 2000

Bottom depth (m)

+

.

.1. 1

+

0.6 0.8 0.2 0.4

Chlorophyll (mg/rn3)

Figure 8.11. Presence of Audubon's shearwaters (midsummer cruise)

predicted by (a) bathymetry model and (b) plankton standing

stock model. Plus signs indicate presence in transects (as

predicted by model). Dots represent absence in transects (as

predicted by model).

307

a)

+

2500 3000

r

C)

b)

I::

C)
c'J

q

+

308

0.2 0,4 0.6 0.8

Chlorophyll (mg/rn3)

Figure 8.12. Presence of band-ramped storm-petrels (mid-summer cruise)

predicted by plankton standing stock model. Plus signs

indicate presence in transects (as predicted by model). Dots

represent absence in transects (as predicted by model).
++ +

+

309

29.5 30.0 30.5 31.0 315

Sea surface temperature (°C)

Figure 8.13. Presence of black terns (mid-summer cruise) predicted by sea

surface properties model. Plus signs indicate presence in

transects (as predicted by model). Dots represent absence in

transects (as predicted by model).
LC) c'.1

LC) +

0.6 0.8 0.2

310

0.4

Chlorophyll (mg/rn3)

Figure 8.14. Presence of bridled terns (mid-summer cruise) predicted by

plankton standing stock model. Plus signs indicate presence

in transects (as predicted by model). Dots represent absence

in transects (as predicted by model).
.. S.

+

311

29,5 30.0 30.5 31.0 31.5

Sea surface temperature (°C)

Figure 8.15. Presence of sooty terns (mid-summer cruise) predicted by

sea surface properties model. Plus signs indicate presence in

transects (as predicted by model). Dots represent absence in

transects (as predicted by model).
8.4. Discussion

8.4.1 Overall Abundance

Seabird groups and species present in the Gulf of Mexico varied by season. These observations

match expectations based on previous work detailing life histories of seabirds seen in the Gulf of

Mexico. Fritts and Reynolds (1981) categorized birds as: summer migrant pelagic, summer

resident, wintering marine, or permanent residents. Summer migrant pelagic species were

defined as species that are present in the Gulf in the summer but breed primarily elsewhere.

These include shearwaters, storm-petrels, boobies, tropicbirds, bridled terns and black terns.

Examples of summer residents (seabirds that breed in the Gulf) are sooty terns, least terns and

sandwich terns. Permanent residents are laughing gulls, royal terns and magnificent frigatebirds.

Wintering marine birds are northern gannet, herring gulls and jaegers.

The spring cruise data suggest terns, storm-petrels and gulls were common in the Gulf during

May and June. Jaegers and shearwaters were also present but in lower numbers. Tropicbirds,

sulids and frigatebirds were rarely seen in the Gulf during the spring. The species composition

of the late summer sightings was different from those of the spring cruise, as October is a period

of migration and transition to winter distribution. Two of the three most commonly identified

species (laughing gull and royal tern) were year-round residents of the Gulf. Pomarinej aegers,

Gulf winter migrants, were the third most commonly identified bird. The mid-summer sightings

included a large number of species. Black terris, the most abundant species during the midsummer cruise, were summer migrant pelagic species. The four next most abundant species

during the mid-summer cruise were: band-rumped storm-petrels (summer migrant pelagic),

magnificent frigatebirds (permanent residents), Audubon's shearwaters (summer migrant

pelagic) and sooty terns (summer residents).

Terns, which were the most abundant bird group during the spring cruise, are either year-round

residents, summer residents or summer migrants; their presence during May/June would be

expected. Storm-petrels were also frequently encountered during the spring cruise; this would be

expected since they are summer migrants. Jaegers were not frequently seen in the Gulf during

spring; this may be expected because they are primarily a winter species and may have left the

Gulf region by May/June to return to their breeding grounds. Tropicbirds and sulids were not

commonly encountered during the spring cruise; this may be expected as principally nonbreeding birds would be present in the Gulf during the spring. Three of the four most frequently

seen species (sooty tern, sandwich tern and black tern) were summer residents or summer

migrants. Laughing gulls are year-round residents and were the fourth most frequently seen

species.

Comparison of the late and mid-summer sighting rates to those from other shipboard surveys

reveals a seasonal pattern of shifting species abundance. Although shearwaters, a summer

migrant, are present year-round, they are most abundant during the summer and autumn. Stormpetrels are also present year-round, and previous data suggest a peak abundance during the

spring (Davis and Fargion 1996). However, the mid-summer cruise had a sighting rate of 1.84

storm-petrels/effort hour, higher than the previously recorded spring sighting rate of 0.24 stormpetrels/effort hour.

312
While Fritts et al. (1983) consider magnificent frigatebirds to be a year-round species, our peak

sightings during summer and early autumn suggest that frigatebirds may only be pelagic during

part of the year. The mid-summer sighting rate of frigatebirds was higher than previously

reported values, and the species was seen primarily in coastal waters. However, the frigatebirds

seen during late summer were split evenly between shallow and deep water, suggesting that they may travel farther offshore after the breeding season.

Most tern species in the Gulf are summer residents or summer migrants, which was reflected in

the large spike in tern sighting rates during August (Davis and Fargion 1996). However, in the midsummer cruise, the sighting rate of tems was much lower than previously recorded. This

difference may have resulted from the areas surveyed. Previous cruises focused on the shelf and

slope region of the northern Gulf, whereas the mid-summer survey in this study was more

pelagic. Between the late summer and mid-summer cruises, there was a different pattern of species presence and absence. During mid-summer, black terns were present in large numbers along the northern Gulf coast during migration. Sandwich terns were present only in midsummer, primarily in coastal waters, but were not present in late summer Bridled and sooty

terns were present in increased numbers during mid-summer in deeper water. Royal terns, a year-round resident of the Gulf, were present during both mid- and late summer and seemed to

prefer coastal habitats during mid-summer.

Gull abundance reflected two trends resulting from species with different life histories. While

laughing gulls were present year-round, herring gulls, a wintering marine species, peaked in

abundance in February (Davis and Fargion 1996). Laughing gulls exhibited a different pattern of distribution than other year-round species such as the magnificent frigatebird or royal tern.

During mid-summer, laughing gulls were found evenly between shallow coastal water and

deeper water. In late summer, laughing gulls were found in higher numbers in coastal water.

Unlike the other year-round species, laughing gulls were found in higher proportions in coastal

water during late summer (after the breeding season).

Sulid abundance increased during the winter (mainly northern gannets) and summer (mainly

masked boobies), a trend also noted by Davis and Fargion (1996). The northern gannet being a

wintering marine species could explain the difference in sulid abundance, while the masked

booby is a summer resident in the Gulf.

Davis and Fargion (1996) described pomarinejaegers, considered uncommon in the Gulf by Duncan and Havard (1980), as a wintering marine species. They were seen during the late

summer cruise at lower rates than winter, which may indicate that migrating jaegers were just beginning to arrive in the Gulf at that time. The mid-summer data show that jaegers are present

in the summer at higher levels than previously reported (Davis and Fargion 1996) and may

indicate some non-breeding j aegers remain in the Gulf.

8.4.2 Role of Hydrographic Environments

The results of both late summer and mid-summer cruises indicate the cyclone had the greatest

species diversity of the hydrographic environments. Additionally, the confluence and Loop

Current eddy during mid-summer had a greater species diversity than the continental shelf.

313
Species-specific habitat relationships with hydrographic environments were found for several

species. Pomarine j aegers were more likely to be present near the MOM during late sunimer

Audubon's shearwaters during mid-summer were more likely to be seen inside the cyclone,

while band-rumped storm-petrels were more likely to be present in the "other margin" areas.

Black terns were encountered more frequently near the MOM. An interesting result was that

while the MOM area had higher chlorophyll concentrations, many pelagic species (e.g.,

Audubon's shearwater, band-ramped storm-petrel, bridled tern and sooty tern) were not seen at

all in this area.

8.4.3 Seabird Habitat

A generalized additive model, using indicators of plankton-standing stock (measured by surface

chlorophyll and predicted mean biomass integrated from 10-50 m depth) best predicted (or was

tied with another model) seabird presence for five of the seven species analyzed. A model

incorporating sea-surface properties of temperature and salinity best predicted presence for black

tern, sooty tern and laughing gull (tied with plankton-standing stock). The sea surface height and

bathymetry models best explained pomarine jaeger and Audubon's shearwater (tied with

plankton-standing stock) presence, respectively.

Sea surface salinity and temperature best predicted the presence of black tern and sooty tern.

This was expected for black terns, which have been noted to follow the fresh water plume of the

Mississippi River and coastal waters (Ribic et al. 1997a, Davis and Fargion 1996). Although

sooty tern presence in transects was best explained by the same surface physical properties, the

relationship was not the same as with black terns. Sooty terns were predicted to be present in

salinity of about 32 psu, which is more saline than where black terns were found, but less saline

than average values for the cyclone. This may reflect the affinity of sooty terns for lower salinity

patches of water entrained around the cyclone (see Chapter 2). However, the percent of total

variance explained by the model was low; there are probably other factors not considered that

contribute to the distribution of sooty terns in the Gulf of Mexico. In the eastern tropical Pacific,

sooty tems have been associated with more saline water than other seabird species (Ribic et al.

1 997b).

Two competing models predicted laughing gull presence in late summer plankton-standing

stock and sea-surface properties. Laughing gulls were predicted in transects of increased PMB

and increased concentrations of chlorophyll. The sea-surface properties model predicted

laughing gulls in intermediate ranges of temperature. Ribic et al. (1997a) found laughing gulls in

areas of low salinity and steep thermoclines. These findings are consistent. During late summer,

there was an inverse relationship with salinity and chlorophyll concentration.

In late summer, pomarine jaegers were predicted to occur in areas with higher geostrophic flow

and intermediate sea surface height. This suggests that pomarine jaegers may have been

attracted to regions such as the confluence or the edge of the anticyclone. However, in the

analysis of presence and absence in the hydrographic environments, pomarine jaegers were more

likely to be encountered in the MOM area. This species may not be attracted to one particular

environment during late summer.

314
Audubon's shearwater and band-rumped storm-petrel presence in mid-summer was best

predicted by surface chlorophyll and PIvIB. Both species were predicted to be present at lower

chlorophyll concentrations (generally less than 0.2 mg/rn3). This finding is consistent with the

observation that neither species was present in transects in the MOM, which is where the

majority of high chlorophyll transects occurred.

8.5 Conclusions

The three cruises provided more information on seasonal distribution and abundance of seabirds

in the Gulf of Mexico. The relationship between hydrographic environment and species

diversity was examined. We found the highest species diversity in the cyclone and lowest on the

continental shelf. Several species exhibited affinities to specific hydrographic environments.

Pomarine jaegers and black terns preferred the MOM area. Audubon's shearwaters were more

likely to be present in the cyclone, and barid-rumped storm-petrels in "other margin" areas.

Species' presence was analyzed with models incorporating bottom depth, sea-surface properties,

and plankton standing stocks using generalized additive models. Results suggest that the

presence of laughing gull, Audubon's shearwater, band-rumped storm-petrel and bridled tern

presence was best predicted by indicators of plankton-standing stock, using sea surface

chlorophyll concentrations and PMB as predictor variables. For laughing gull, black tern and

sooty tern, sea surface temperature and salinity best predicted presence. Models incorporating

sea surface height and bathymetry predicted pomarinejaeger arid Audubon's shearwater

presence.

315
Chapter 9

Summary, Conclusions and Recommendations

Randall W. Davis, William E. Evans, Jeffrey C. Norris, Bernd Wiirsig,

Keith D. Mullin, Douglas C. Biggs, John H. Wormuth and Christine A. Ribic

9.1 Objectives of the GulfCet Program

The Gulf of Mexico is a semi-enclosed, partially land-locked, intercontinental sea lying on the

western margin of the Atlantic Ocean. It has been called the American Mediterranean Sea and

has a total area of about 1.5 million square kilometers. As a large marine ecosystem, it has a

unique bathymetry, hydrography and productivity. Cetaceans, sea turtles and seabirds are upper

trophic level predators that play an important role in the pelagic marine ecosystem of the Gulf of

Mexico. However, there was little information about the distribution, abundance and ecology of

these marine vertebrates in the Gulf until surveys began in 1981 (Fritts et al. 1983, Mullin et al.

1991, Mullin et al. 1994c). These early studies eventually lead to the Gulfcet I and II Programs

in the 1 990s, which were based on the most extensive surveys of cetaceans, sea turtles and

seabirds ever conducted in the offshore waters of the northern Gulf ofMexico (Davis and

Fargion 1996, Baumgartner 1997, Davis et al. 1998, this report). The objectives of the GulfCet

II field studies (1996-97) were to: 1) expand the geographical coverage of ship and aerial surveys

that were conducted previously during GulfCet 1(1992-94), which surveyed the north-central

and northwestern Gulf of Mexico, 2) estimate the minimum abundances of cetaceans and sea

turtles in areas surveyed during 1996-97, 3) collect simultaneous hydrographic data and

biological samples during the ship surveys to better define the habitat associations of cetaceans

and seabirds, and 4) collect acoustic data on cetacean sounds and identify and record other

natural and man-made underwater sounds to provide additional insights into cetacean

distribution and behavior, particularly in relation to noise from seismic exploration vessels.

To accomplish these objectives, we used an integrated approach that included visual (aerial and

shipboard) and acoustic (shipboard) surveys of the distribution ofcetaceans, sea turtles and

seabirds and simultaneous hydrographic measurements. We also used near real-time sea surface

altimetry from the TOPEX/POSEIIDON and ERS satellites to determine the location of

hydrographic features (e.g., cyclones, anticyclones and confluence zones) during shipboard

surveys. The sea surface altimetry maps enabled us to adjust the ship's course so that we could

survey and sample hydrographic features that could influence the distribution of cetaceans and

seabirds. Archival satellite sea surface altimetry data also allowed us to retrospectively

determine the location of hydrographic features for analysis with GulfCet I cetacean sightings

collected from 1992-94. In addition to characterizing hydrographic features during GulfCet II,

we measured zooplankton and micronekton biomass derived from both net and acoustic

sampling to indicate the amount of potential food available for higher trophic level foraging by

cetaceans and seabirds. We hypothesized that hydrographic features in the study area had

different levels of potential prey that influence cetacean and seabird distribution. We further

hypothesized that these food stocks would be locally concentrated in nutrient-rich areas offshore

from the Mississippi River, within cyclonic eddies, and along the high-shear edges of cyclonic

eddies.

317
9.2 Cetaceans

Nineteen cetacean species were identified in the oceanic northern Gulf of Mexico (398,960 km2,

Figure 1.5) during GulfCet II surveys. Total abundance estimates ranged from 86,705 (based

shipboard surveys) to 94,182 (based on highest estimate for each species from either shipboard

or aerial surveys) animals (Table 9.1). Pantropical spotted dolphins were the most abundant

species (an estimated 46,625 animals), followed by spinner dolphins (11,251) and clymene

dolphins (10,093). Estimates for bottlenose dolphins, striped dolphins, melon-headed whales,

Atlantic spotted dolphins, Risso's dolphins and short-finned pilot whales ranged from 5,618 to

1,471. Abundances of all other species were less than 1,000. Cetaceans were sighted throughout

the study area, but fewer were sighted in the western Gulf. There are now sighting records

during three or more seasons for at least 16 cetacean species. The seasonal abundance of several

species (e.g., dwarf/pygmy sperm whale, Risso's dolphin, pantropical spotted dolphin) may vary

regionally in continental slope waters. The distribution and abundance estimates of common

cetacean species in both the oceanic northern Gulf (Figure 1.5) and GulfCet I study (Figure 1.2)

areas were generally similar to previous estimates based on surveys conducted from 1992-94

(Mullin et al. 1994c, Davis and Fargion 1996).

Seventeen cetacean species were sighted in the Minerals Management Service's Eastern

Planning Area (EPA, 70,470 kni2, Figure 1.4). The abundance estimate basedon aerial surveys

(which were more extensive than the ship surveys in the EPA) was 38,184 total animals. In

general, cetaceans were found throughout the EPA each season. The most abundant species

were pantropical spotted dolphins (13,649) and spinner dolphins (8,670). Other species with

abundance estimates over 1,000 based on aerial surveys were bottlenose dolphins, Atlantic

spotted dolphins, Risso's dolphins, striped dolphins and clymene dolphins The seasonal

abundance of some species may vary regionally in continental slope waters. For example,

dwarf/pygmy sperm whales were 9-times more abundant in the summer than in the winter.

Cetaceans in the northeastern and oceanic northern Gulf of Mexico were concentrated along the

continental slope in or near cyclones and the confluence of cyclone-anticyclone eddy pairs. Net

tows and acoustic backscafter measurements with an Acoustic Doppler Current Profiler showed

that cyclonic eddies and confluence areas are mesoscale features with locally concentrated

zooplankton and micronekton stocks that appear to develop in response to increased nutrient-rich

water and primary production in the mixed layer. A significant relationship existed between

integrated zooplankton biomass and integrated cephalopod (a major component of cetacean prey)

paralarvae numbers, indicating that higher zooplankton and micronekton biomass may correlate

with higher concentrations of cetacean prey. In the north-central Gulf, an additional factor

affecting cetacean distribution may be the narrow continental shelf south ofthe Mississippi River

delta. Low salinity, nutrient-rich water may occur over the continental slope near the mouth of

the Mississippi River (MOM) or be entrained within the confluence ofa cyclone-anticyclone

eddy pair and transported beyond the continental slope. This creates a deep-water environment

with locally enhanced primary and secondary productivity and may explain the presence of a

resident, breeding population (see Chapter 7) of endangered sperm whales within 50 km of the

Mississippi River delta. Overall, the results suggest that the amount ofprey for cetaceans (and

seabirds) may be consistently greater in the cyclone, confluence areas, and south of the MOM,

making them preferential areas for foraging (Figures 6.13-6.16). Since cyclones in the northern

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Table 9.1 Cetaceans ranked by abundance with mean body mass, species-total biomass (percent

of total biomass shown in parentheses) and minimum estimated food requirements for

the oceanic northern Gulf of Mexico.

Minimum abundance estimates from GulfCet II visual survey data. 2 Data published by Carwardine (1995).

Total biomass was derived by multiplying the species abundance by the mean body mass.

Estimated minimum food requirement (MFR, tonnesyf1) for each species or species category

was calculated as: IVJFR = [(((A x B x C) / (D x B)) / 106) x 365], where A = annual species

abundance in the study area; B = species resting metabolic rate (kcallday,) equal to 70M°75

(M = mass in kg); C 2, the multiple of resting metabolic rate used to estimate the actual field

metabolic rate (Hinga 1979); D = 0.8, the assimilation efficiency (Lockyer 1981); B = 1 kcallg

wet weight, the assumed caloric density of prey (Hinga 1979, Ham et. al. 1985); 106 is the

conversion factor from grams to tonnes (106 g = kg = 1 tonne); arid 365 is the conversion

factor from days to year.

319

Species Abundance1 Mean

adult body

mass (kg)2

Species-total

biomass (kg)3

(% of total)

Mm. food

requirement

(toimes.yf')4

Pantropical spotted dolphin 46,625 90 4,196,250 (17) 87,023

Spinner dolphin 11,251 50 562,550 (2) 13,513

Clymene dolphin 10,093 50 504,650 (2) 12,122

Bottlenose dolphin 5,618 150 842,700 (3) 15,381

Striped dolphin 4,858 90 437,220 (3) 9,067

Melon-headed whale 3,965 160 634,400 (3) 11,394

Atlantic spotted dolphin 3,213 100 321,300 (1) 6,490

Risso's dolphin 3,040 300 912,000 (4) 13,997

Short-finned pilot whale 1,471 2,000 2,942,000 (12) 28,101

Rough-toothed dolphin 852 100 85,200 (<1) 1,721

False killer whale 817 1,000 817,000 (3) 9,280

DwarUpygmy sperm whale 733 225 164,925 (1) 2,720

Sperm whale 530 20,000 10,600,000 (43) 56,935

Pygmykillerwhale 518 110 56,980 (<1) 1,124

Killerwhale 277 3,000 831,000 (3) 7,172

Cuvier's beaked whale 159 1,800 286,200 (1) 2,807

Fraser's dolphin 127 160 20,320 (<1) 365

Bryde's whale 35 12,000 420,000 (2) 2,563 Total 94,182 24,634,695 281,774
Gulf are dynamic and usually associated with westward moving cyclone-anticyclone pairs,

cetacean distribution will be dynamic. However, with near real-time satellite remote sensing of

sea surface altimetry, these features can be tracked and used to predict where pelagic cetaceans

may be concentrated. The exceptions are bottlenose dolphins, Atlantic spotted dolphins and

possibly Bryde's whale that typically occur on the continental shelf or along the shelf break

outside of major influences of eddies. We have little information on the environmental variables

that influence the distribution of these three species or their prey because hydrographic surveys

have concentrated on deeper waters beyond the continental shelf.

Although cetaceans were commonly observed throughout the GulfCet study area during all four

seasons, we could not determine whether animals were in transit or resident in the study area for

extended periods. It is possible that the oceanic northern Gulf encompasses only a portion of the

home range for many of the species observed. For example, a radio-tagged pantropical spotted

dolphin in the eastern tropical Pacific (ETP) moved over 500 km in 27 days (Perrin et al. 1979).

Without additional information on daily movement patterns and feeding behavior, significant

uncertainties remain in our understanding of cetacean association with mesoscale hydrographic

features. The relationships between physical and biological processes are subtle and complex,

and factors other than hydrographic features and potential prey abundance may influence the

distribution of cetaceans. The seasonal movements of cetaceans may be affected by reproductive

and migratory behavior, although we currently have little information on the behavior of pelagic

species. The exception is sperm whales south of the MOM, which appear to reside along the

lower slope throughout the year. We suggest that this area may be essential habitat for sperm

whales in the northern Gulf; although additional information on population stnicture, seasonal

movements and behavior is needed.

The diversity of cetaceans in the study area was comparable to that along the continental slope of

the northeastern United States and in the eastern tropical Pacific (ETP) (Ham et al. 1985, Wade

and Gerrodette 1993). However, the overall density of cetaceans (0.24 km2) in the oceanic

northern Gulf of Mexico was only 29% and 45%, respectively, of the cetacean densities in the

northeastern U.s. (0.80 krrf2) and the ETP (0.53 km2). Dolphins (Family Delphinidae) were the

most common (>98%) cetaceans in all three areas. However, there were more than five times as

many pilot whales along the northeastern U.S. than in the oceanic northern Gulf Notable in

their absence from the Gulf are common dolphins (Deiphinus deiphis). Baleen whales

represented a large percentage (56%) of the total cetacean biomass along the northeastern U.S.

(Ham et al. 1985), but were a negligible percentage (ca. 2%, primarily Bryde's whale) in the

northern Gulf. The lower densities of cetaceans in the Gulf compared to the northeastern U.5.

and the ETP could be related to more oligotrophic conditions and lower concentrations of prey.

The estimated minimum biomass of cetaceans in the oceanic northern Gulf is 24,634,695 tonnes

(Table 9.1). Of this biomass, sperm whales represent 43%. Together, pantropical spotted

dolphins, sperm whales and short-finned pilot whales constitute 72% of the estimated cetacean

biomass. The minimum food requirement for all cetaceans (assuming adult body masses for

each species) in the oceanic northern Gulf is an estimated 281,774 tonnes per year, of which

pantropical spotted dolphins, sperm whales and short-finned pilot whales consume 61% (Table

9.1). Dividing the minimum food requirement by the size of the study area (398,960 km2) gives

a cetacean food consumption rate of 0.71 tonneskm2yf1. This value is about 12% and 46% of

the estimated annual food consumption for cetaceans living along the continental slope in the

320
northeastern U.S. and the ETP, respectively, and it probably reflects the lower primary and

secondary productivity in the Gulf (Ham et al. 1985, Wade and Gerrodette 1993).

To obtain a better understanding of the seasonal and annual distribution, abundance and habitatassociations of cetaceans, a survey of the entire Gulf with simultaneous satellite and

conventional radio tracking and photo-identification of the predominant species (e.g., pantropical

dolphins, sperm whales) is needed. Skin biopsies and skin swabbing (Harlin et al. 1999) for

genetic analysis of population structure and blubber biopsies for assessing diet (based on fatty

acid profiles that can be used to identify prey) should be taken. In addition to tracking the

movements of cetaceans at sea, animal-borne satellite telemeters can record information on

diving behavior. When combined with dietary information from blubber fatty acid profiles,

diving behavior can provide new and much needed information on foraging areas, prey species

and resource partitioning among cetaceans (Evans 1971, Tanaka 1987, Mate 1989, Men-ick et al.

1994, Davis et al. 1996). We believe that the diet of many cetaceans found in the Gulf is

dominated by cephalopods and mesopelagic fishes associated with the vertically migrating

acoustic deep scattering layer (Pen-in et al. 1973, Clarke 1996, Ridgway and Harrison 1994).

However, there is little direct dietary information for pelagic cetaceans in the Gulf. Future

studies should increase acoustic and net sampling of zooplankton and micronekton in different

hydrographic features so that we can develop stronger statistical correlations between cetacean

distribution and their potential prey. Behavioral data are also needed to determine whether

animals use certain areas for specific activities, such as sociaL'sexual behavior, foraging, resting,

or transiting.

9.3 Sea Turtles

GulfCet II aerial surveys provided the first assessment ofsea turtle abundance and distribution

over a large area of the oceanic northeastern Gulf of Mexico. Three sea turtle species were

sighted in the EPA study areas: loggerhead, Kemp's ridley and leatherback sea turtles. The

leatherback and Kemp's ridley sea turtles are listed as endangered and the loggerhead is listed as

threatened.

The overall density of loggerhead sea turtles in the EPA shelfwas 20 times that of the EPA

slope. The seasonal densities of loggerheads in the EPA shelf were similar. However, the

majority of loggerheads in the EPA slope were sighted during winter. While many of the winter

sightings were near the 100 m isobath, there were sightings of loggerheads over very deep water

(i.e., >1000 m). It is not clear why adult loggerheads would occur in oceanic waters unless they

were traveling between foraging sites or migrating to distant nesting areas. Adult loggerhead sea

turtles forage primarily over continental shelves and in estuaries from the subtropics to warm

temperate waters (National Research Council 1990). In the western North Atlantic, nesting

occurs along the Atlantic coast of Florida, but significant nesting extends north to North Carolina

and along the Gulf of Mexico coast of Florida. This region constitutes the second largest

breeding aggregation of loggerhead nesting in the world. Elsewhere in the Gulf ofMexico, only

a few sporadic nests occur in the northern Gulf west of Florida but significant nesting does occur

in disjunct locations along and near the Yucatan Peninsula (Can- et al. 1982, National Research

Council 1990).

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Leatherbacks were sighted throughout the EPA slope and were about 12 times more abundant in

the winter than summer. A small number also occurred in the EPA shelf during summer. The

nearly disjunct summer and winter distributions of leatherbacks indicates that specific areas may be important to this species either seasonally or for short periods of time. Adult leatherback sea

turtles forage from the tropics to subarctic waters (National Research Council 1990). While

generally called a pelagic or oceanic species, in previous studies, leatherbacks have been

routinely found in nearshore waters in the western Atlantic and Gulf of Mexico (CeTAP 1982,

Fritts et al. 1983). In the western North Atlantic, they nest primarily in the southwestern

Caribbean in Costa Rica and in the eastern Caribbean from the Dominican Republic to Trinidad. Low levels of leatherback nesting occur along the Atlantic coast of Florida. In the Gulf of

Mexico, minimal nesting has been reported in the southwestern Gulf along the coast of Mexico

(Carr et al. 1982). Adult leatherbacks feed primarily on jellyfish, often at great depths (e.g., 1,000 m), but also eat other coelenterates that are often concentrated near convergent zones and

fronts (Bjorndal, 1997). Since these features are often dynamic and ephemeral, it may explain the

in-uptive distribution of leatherbacks in the Gulf of Mexico during both GulfCet I and II.

Kemp's ridley sea turtles were sighted only three times in EPA shelf waters, but many were

probably missed due to their small adult size. Kemp's ridleys occur in the Gulf of Mexico and

western North Atlantic. Adults forage throughout the northern Gulf ofMexico in shallow shelf

waters but they concentrate off Louisiana, Mississippi and Alabama. The only significant nesting beach for this species is in western Gulf of Mexico at Rancho Neuvo, Mexico (Can et al. 1982, National Research Council 1990).

9.4 Seabirds

Seabird species present in the Gulf of Mexico varied by season. The species composition of the

sightings during late summer reflected a pattern of migration and transition to a winter

distribution. Two of the three most commonly identified species (laughing gull and royal tern) in

late summer were year-round residents in the Gulf. Pomarine jaegers, a wintering marine species

in the Gulf, were the third most commonly identified species. During mid-summer, the black

tern was the most abundant species, followed by band-rumped stonn-petrels (summer migrant pelagic), frigatebirds (permanent resident), Audubon's shearwaters (summer migrant pelagic) and

sooty terns (summer resident).

Cyclones had the greatest diversity of seabird species, although habitat use varied among

species. Pomarinejaegers were more likely to be present in the MOM area during late summer.

Audubon's shearwaters were more likely to be encountered inside a cyclone, while band-rumped

storm-petrels were more likely to be present in the areas other than cyclones, anticyclones or

confluence zones during midsummer. Black terns were encountered more frequently in the MOM area during mid-summer Generalized additive models incorporating indicators of

plankton standing stock (surface chlorophyll and predicted mean biomass of zooplankton and

micronekton) best-predicted seabird presence for five of the seven species analyzed. Other

predictive models were: sea surface properties of temperature and salinity for black tern, sooty

tern, and laughing gull; sea surface height for pomarinejaeger; and bathyrnetiy for Audubon's

shearwater. Seasonal surveys are needed to better assess community structure and seabirdhabitat associations.

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9.5 Potential Impacts of Oil and Gas Exploration and Production

Eighty-three percent of the crude oil and 99% of the gas production in United States federal

waters occurs in the Gulf of Mexico, primarily along the Texas-Louisiana continental shelf and

slope (Cranswick and Regg 1997). By 2003, oil production in the Gulf is projected to increase 43% (range = 23 to 63%), and gas production is projected to decrease by 9% (range = -23 to 18%) (Melancon and Baud 1999). Production from deepwater fields (depth >305 rn) will account for about 59% of the daily oil production and 27% of the daily gas production in the Gulf. By the early 2000's, exploration wells may be drilled at 3,000-rn depths at the very edge

of the Exclusive Economic Zone, and production systems will be designed for depths of 1,800 rn

and greater (Cranswick and Regg 1997). The long-term forecast for petroleum transportation is

for the total volume to increase into the next century. The projected number of service-vessel

trips for lease sales in the central Gulf alone could exceed 700 trips per day or 260,000 trips

annually (Marine Mammal Commission 1999). The cumulative impact of these and other human

activities on cetaceans in the northern Gulfcannot be predicted with certainty (Marine Mammal

Commission 1998, 1999). However, it can be anticipated that cetaceans along the continental

slope will encounter increasing oil and gas exploration and production activities that include: surface and subsurface construction; FPSO (Floating Production, Storage and Offloading

facilities) activities; waste discharge; service-vessel and aircraft traffic and noise; geophysical

surveying; and oil spills (Cranswick and Regg 1997). One way to assess the long-term effects of

these activities on cetacean distribution and abundance is through a monitoring program of

indicator species (e.g., endangered sperm whales, pantropical spotted dolphins, clymene

dolphins) that commences ahead of the widespread implementation of deepwater exploration and production. Such a monitoring program would involve traditional aerial and shipboard visual

surveys, shipboard acoustic surveys, behavioral observations, and satellite and conventional

radio telemetry studies of the predominant cetacean species that can provide an adequate sample

size for statistical analysis. The shipboard acoustic surveys are particularly useful because they monitor the presence of vocalizing cetaceans as well as ambient noise levels. In addition, these

kinds of data can be archived for later analysis as exploration and production activities develop

and change over long periods of time. However, a monitoring program alone may not be able to

determine whether population changes have resulted from oil and gas exploration and production

activities, other anthropogenic causes (e.g., incidental take in fisheries, commercial shipping

noise and collisions, plastics, river-borne contaminants), or natural changes in the marine environment. An assessment of the potential effects of oil and gas exploration and production

activities must include experimental studies with testable hypotheses.

Sounds associated with offshore oil exploration in the Gulf of Mexico are becoming increasingly

common. The peak intensity of seismic exploration pulses recorded during GulfCet II was found

to be typically below 250 Hz, with an average peak intensity at 82 Hz. The average intensity of

pulses was found to be 8.4 dB above ambient, more than double ambient levels. However, there

was no significant difference in the sighting frequency between the different acoustic levels

examined for GulfCet I and II, contrary to previous findings (Mate et al. 1994) suggesting that

sperm whales may avoid seismic vessel noise. In addition, the observed distribution of cetaceans

did not differ significantly with seismic sound intensity for different hydrographic regions.

However, the present study did not examine smaller-scale behavioral impacts. While the

GulfCet I and II programs offered an opportunity to examine the potential effects of human

323
acoustic perturbation on the large-scale distribution oflocal cetacean populations, they lacked

the ability to control for both the acoustic level and the hydrographic region. To better

understand the potential impact of man-made noise on local cetacean populations, a systematic

study is needed that can control for acoustic variation in different hydrographic features. Ideally, such a study would examine small-scale behavioral changes in addition to distribution.

The Gulf of Mexico is rich in species occurring throughout the food chain that are acoustically

active. It is unfortunate that so little information is available on ambient noise levels, source

levels from fish and dolphins, and especially accurate data on exposure levels as a function of

frequency during geophysical surveys, explosive removal ofplatforms and other noise associated with oil and gas development. The northern Gulf of Mexico is ideal for using acoustic

monitoring to study the potential effects of man-made noise on cetaceans. The sound reception

and production capabilities ofmany families of cetaceans are well known (Richardson et al.

1995). Every group of cetaceans studied has been found to vocalize and to have broad-band

hearing sensitivity. This includes all of the 19 species that regularly occur in the northern Gulf

of Mexico, even though quantitative data on hearing exists for only five of these (i.e., bottlenose

dolphins, false killer whales, killer whales, rough-toothed dolphins, and short-finned pilot

whales). Lack of quantitative data on the responses of marine animals, especially cetaceans, to

man-made noise emphasizes the concerns expressed by the Marine Mammal Commission (1999)

about long-term, adverse effects. Better and precise measurements of the acoustic environment

of cetaceans is needed along with acoustic monitoring. In addition to the comprehensive library

of marine animal sounds that the GulfCet study has already archived, a great deal of data on

ambient noise associated with shipping and oil and gas exploration has been recorded. These data await analysis and interpretation.

There are significant uncertainties in our understanding of short and long-term effects of seismic

and other loud industrial sounds on the behavior and distribution of Gulf cetaceans. Potential

impacts of seismic survey sounds include: 1) masking sounds made by cetaceans for

communication, navigation, sensing their environment and prey capture, 2) causing animals to

abandon or avoid important feeding and breeding areas, or altering migratory routes, 3) affecting

the distribution, density and movements of important prey species, 4) causing physiological or

psychological stress, and 5) causing temporary or permanent hearing loss (Richardson et al.

1995, Marine Mammal Commission 1999). Behavioral monitoring and in situ behavioral

experiments, as have been conducted for gray (Eschrichtius robustus), bowhead (Balaena

mysticetus), and humpback whales in other United States federal waters, are needed to improve

our understanding of these potential impacts. Behavioral work should include assessments of

respiration, diving, speed of travel, group size, and general behavior patterns relative to seismic

survey sounds, their proximity to the animals, and their loudness at certain frequencies. It should

also include, where possible, conventional and satellite radio tagging for large-scale (time and

geography) movement information, and electronic recording or telemetering of depth of dive and heart rate. Observational and experimental work relative to assessments of disturbance need not

be conducted on all species, but should target sperm whales and such potential dolphin "indicator

species" as oceanic bottlenose dolphins, pantropical spotted dolphins, spinner dolphins or

clymene dolphins. The methods for tagging and tracking these animals have been largely

resolved, as have the behavioral data gathering protocols that have been so useful in helping the

MMS in making management-related decisions in other federal waters. Without a monitoring

324
program working in cooperation with the oil industry, it is impossible to make well-reasoned

judgernents on the potential impacts of geophysical survey activities on pelagic cetaceans.

The effects of oiling on cetaceans, sea turtles and seabirds have been reviewed (Clark and

Gregory 1971, Hansen 1981, 1985, Geraci and St. Aubin 1982, 1985, 1988, 1990, Geraci et al.

1983, Clapp et al. 1983, Smith et al. 1983, Fry and Lowenstine 1985, Fry et al. 1985, St. Aubin

et al. 1985, Lutz et al. 1986, Vargo et al. 1986, Butler et al. 1988, Lutz and Lutcavage 1989, Piatt et al. 1990, Hutchinson and Simmouds 1991). Although the immediate effects of oiling on

cetaceans and sea turtles are considered less severe than for seabirds, chronic, sublethal effects

are unknown. This is of special concern for the population of endangered sperm whales residing

southeast of the Mississippi River delta in an area of active oil and gas production (Wthsig

1990). Clapp et al. (1983) suggested that even a small spill could be harmful to seabirds if it

occurred in an area where they aggregate. In the Gulf of Mexico, this could occur if oil were

advected into the confluence region ofa cyclone-anticyclone pair and transported offshore.

9.6 Recommendations for Future Research

In light of the current expansion of oil and gas activities into deep-water regions of the Gulf,

more detailed information on the population biology, ecology, behavior and potential industry- related impacts is needed for cetaceans, sea turtles and seabirds. The following are research

recommendations in approximate order of priority for the northern Gulf of Mexico:

Information is needed on the movements, diving behavior and site fidelity of endangered

sperm whales along the continental slope southeast of the Mississippi River delta. Satellite

telemeters should be attached to sperm whales to examine seasonal movements and diving

behavior in relation to mesoscale hydrographic features and estimates of potential prey

abundance. Skin biopsies and skin swabbing should be conducted to determine how closely

related Gulf sperm whales are to those from the adjacent Atlantic Ocean and Caribbean Sea. Blubber biopsies should be taken to assess diet (based on fatty acid profiles that indicate dietary preferences). Finally, a photo-identification study in this region should be conducted

to assess the site fidelity of individual whales.

Monitoring of cetacean and sea turtle distribution and abundance in the northern oceanic

Gulf during the NMFS ichthyoplankton surveys should continue. These surveys can be

conducted at relatively low cost because they occur simultaneously with ongoing cruises. Research on cetacean habitat-associations using satellite remote sensing and shipboard measurements should continue. Although progress was made during the GulfCet II study, much remains to be learned about cetacean habitat in the western and southern Gulf. Ifshifis in a species' distribution occur, a better understanding of habitat will be needed.

A Guiftet III Program for the southern Gulf of Mexico (south of the U.S. Economic

Exclusive Zone including the Straits of Florida) should be initiated in cooperation with

Mexico (this could be accomplished through U.S. and Mexico university cooperative

research programs already in existence). Research objectives should be similar to those of

GulfCet II (i.e., cetacean, sea turtle and seabird seasonal distribution and abundance surveys

325
and habitat studies). This would complement studies of northern Gulf waters where oil and gas activities already occur.

Conduct controlled experiments on the effects ofseismic sounds on cetaceans. In addition,

researchers should monitor the effects of seismic ship activity on the behavior and

distribution of cetaceans. Potential changes in sperm whale vocalizations during geophysical exploration should be examined. Finally, a retrospective analysis should be conducted of

GulfCet cetacean distribution data relative to seismic vessel location, signal characteristics, and source level. This research will require the cooperation of the geophysical survey

industry. Specifically, we will need information on when and where seismic survey activities

occur arid the types of seismic signals produced. Observers should be placed on seismic vessels to monitor cetacean behavior.

Research on genetic stock structure should be given added emphasis. The Marine Mammal

Protection Act mandates that stocks of cetaceans be protected. Stocks can be defined in a

number of ways, but genetic analyses can provide the clearest indication of divisions.

Questions of interest for each species include: (1) Should cetaceans in the Gulf of Mexico be

considered as separate stocks from the adjacent Atlantic/Caribbean? (2) Is there more than

one Gulf of Mexico stock? (3) Are there seasonal differences in occurrence patterns? Genetic analysis of skin samples collected during vessel surveys both in the Gulf and

adjacent waters could begin to answer these questions. Of particular interest is Biyde's

whale. Bryde's whales have displayed considerable local variation world-wide, and analysis of skin biopsy samples could determine whether the northern Gulf stock is restricted

geographically and genetically isolated from animals in the Atlantic Ocean.

Seasonal seabird surveys should continue to better assess community patterns and understand bow patterns change within a season in response to varied marine environments, such as the

presence of eddies. In the present study, we found that seabirds responded to different conditions created by the outflow of the Mississippi River and hydrographically defined mesoscale features.

Taxonomic analyses of the Multiple Opening/Closing Net and Environmental Sampling

System (MOCNESS) and Isaacs Kidd Midwater Trawl (IKMT) samples collected during the present study should continue. This will allow us to identifr indicator species for the hydrographic features and extend the usefulness of the Acoustic Doppler Current Profiler (ADCP) data. In future studies, we should increase our sampling intensity in the major hydrographic features so that we can develop stronger statistical correlations.

In conclusion, the continental slope in the northern Gulf of Mexico is an area that supports a diverse cetacean community, but one whose density does not equal areas such as the continental

slope in the northeastern United States and the ETP. We now have a better understanding of the

mesoscale features that influence cetacean distribution, and the use of satellite remote sensing of

sea surface altimetry is increasing our ability to predict where pelagic cetaceans may be concentrated. Against the background of growing oil and gas exploration and development, continued research and monitoring are needed to assess the potential impacts of these activities

on pelagic cetaceans, sea turtles and seabirds in the Gulf of Mexico. The GulfCet Program has

326
demonstrated that any future monitoring programs should be long-term, with intensive sampling

effort to detect significant changes in the density and distribution of these upper trophic level marine predators.

327
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