Status of the Eastern Oyster 

(Crassostrea virginica) 

March 5, 2007 
ii

Eastern Oyster Biological Review Team Members

Patrick Banks Louisiana Department of Wildlife and Fisheries 

Mark Berrigan FL Dept. of Agriculture and Consumer Services, Division 

of Aquaculture 

Arpita Choudhury National Ocean Service, CCEHBR 

Leslie Craig NMFS, Southeast Regional Office, NOAA Restoration 

Center 

Dale Diaz Mississippi Department of Marine Fisheries 

Fred Kern National Ocean Service, CCEHBR 

Jamie King NMFS, NOAA Chesapeake Bay Office 

Mike Marshall North Carolina Division of Marine Fisheries 

Lance Robinson Texas Parks and Wildlife Department 

Frank Steimle NMFS, Northeast Fisheries Science Center 

Rich Takacs NMFS, NOAA Restoration Center 

Gary Wikfors NMFS, Northeast Fisheries Science Center 

Team Liaisons 

Kim Damon-Randall NMFS, Northeast Regional Office 

Jennifer Moore NMFS, Southeast Regional Office 

Acknowledgements

The Eastern Oyster Biological Review Team would like to acknowledge the 

contributions of the following people who provided information that assisted in the 

development of this document: Dr. Sean Powers, Dr. Matthew Hare, Dr. Roger Mann, 

Dr. Patrick Gaffney, Dr. Bonnie Brown, Dr. John Kraeuter, Mr. Troy Alphin, Mr. Bill 

Anderson, Mr. Mike Voison, and Dr. Robert Rheault. We would also like to thank the 

peer reviewers: Dr. Jason Curole, Dr. Bruce Barber, and Dr. Phillippe Goulletquer. 

This document should be cited as: 

Eastern Oyster Biological Review Team. 2007. Status review of the eastern oyster (Crassostrea 

virginica). Report to the National Marine Fisheries Service, Northeast Regional Office. 

February 16, 2007. 105 pp. 
iii

Table Of Contents

1 EXECUTIVE SUMMARY .............................................................................................................1 

2 INTRODUCTION ...........................................................................................................................2 

 2.1 General Introduction..................................................................................................2 

 2.2 ESA Background.......................................................................................................3 

 2.3 The Petition ...............................................................................................................4 

3 SPECIES BIOLOGY.......................................................................................................................5 

 3.1 Life History ...............................................................................................................5 

 Morphology..................................................................................................5 

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

 Larval Phase.................................................................................................6 

 Environmental Tolerances ...........................................................................7 

 Growth and Feeding.....................................................................................8 

 Habitat Preference........................................................................................8 

3.2 Ecology and Population Dynamics.............................................................................9 

 Recruitment................................................................................................10 

 Ecological Role..........................................................................................10 

3.3 Distribution ...............................................................................................................12 

 Historic Distribution ..................................................................................12 

 Current Distribution...................................................................................13 

3.4 Historic and Current Abundance ..............................................................................13 

3.5 Population Genetic Structure ....................................................................................15 

 Overview of Genetic Markers....................................................................15 

 Application of Genetic Markers to C. virginica ........................................16 

 Atlantic and Gulf Populations....................................................................17 

Laguna Madre Population..........................................................................21 

Physiological or Morphological Evidence of C. virginica

subspecies ..................................................................................................21 

4 ANALYSIS OF THE ESA’S FIVE FACTORS............................................................................26 

 4.1 Habitat Threats ........................................................................................................26

 Habitat Degradation and Loss....................................................................26 

 Coastal Demographics/Social Changes .....................................................32 

4.2 Overutilization for Commercial, Recreational, Scientific or Educational Purposes33 

 Overview of Harvest..................................................................................33 

 Commercial Utilization by Region ............................................................35 

 Bycatch ......................................................................................................44 

 Recreational Utilization .............................................................................44 

 Summary of Overutilization Assessment...................................................45 

4.3 Predation and Disease .............................................................................................46 

 Predation ....................................................................................................46 

 Infectious Oyster Disease Agents ..............................................................47 
iv

4.4 Regulatory Mechanisms ..........................................................................................49 

 Federal........................................................................................................50 

 State/Local .................................................................................................50 

 International ...............................................................................................52 

4.5 Other Natural and Manmade Impacts .....................................................................52 

 Introductions ..............................................................................................52 

 Harmful Algal Blooms...............................................................................54 

 Hurricanes..................................................................................................56 

5 AQUACULTURE..........................................................................................................................58 

 5.1 Cultivation and Aquaculture of Eastern Oysters.....................................................58 

6 STATUS OF POPULATION ........................................................................................................60 

 6.1 Quantitative Stock Assessments..............................................................................60 

 6.2 Preliminary Stock Assessments...............................................................................61 

 6.3 Survey of Eastern Oyster Status..............................................................................62 

 6.4 Extinction Risk Considerations...............................................................................71 

7 CONSERVATION ACTIONS ......................................................................................................72

 7.1 Past, Current, and Anticipated Actions ...................................................................72 

8 RESEARCH NEEDS ....................................................................................................................76 

 8.1 Gaps in Knowledge/Future Needs...........................................................................76 

9 CONCLUSION..............................................................................................................................76 

LITERATURE CITED ..............................................................................................................................77

APPENDIX I: GLOSSARY OF TERMS ...............................................................................................100 

APPENDIX II: SURVEY........................................................................................................................103 

APPENDIX III: SURVEY TABLE.........................................................................................................105 
v

List of Figures

Figure 1. Eastern oyster (Crassostrea virginica) photo ......................................................................5 

Figure 2. Sampling sites in Canada: Gulf of St. Lawrence (Reeb and Avise 1990), Prince 

Edward Island (Hoover and Gaffney 2005).......................................................................22 

Figure 3. Sampling sites along the Atlantic Coast: Piscataqua River, MA (Hoover & 

Gaffney 2005), Cape Cod, MA (Buroker 1983), Woods Hole, MA (Karl & Avise 

1992; Hare & Avise 1996; Hare and Avise 1998), Narragansett Bay, RI (Reeb & 

Avise 1990), Mullica River, NJ (Hoover & Gaffney 2005), Lewes, DE (Hoover & 

Gaffney 2005), Chesapeake Bay (Small & Chapman 1997; Milbury et al. 2004), 

Wachapreague, VA (Hoover & Gaffney 2005), Pamlico Sound, NC (Small & 

Chapman 1997), Cape Fear, NC ( Hoover & Gaffney 2005), Georgetown, SC 

(Hoover & Gaffney 2005), Charleston, SC (Karl & Avise 1992; MacDonald et al. 

1996; Hare & Avise 1998), Sapelo Island, GA (Reeb & Avise 1990), Cumberland 

Island, GA ( Karl & Avise 1992; Hare & Avise 1996), Skidway Island, GA (Reeb 

& Avise 1996)....................................................................................................................23 

Figure 4. Sampling sites in Florida: Fernandina (Hoover & Gaffney 2005), Jacksonville 

(Reeb & Avise 1990), St. Augustine (Hare & Avise 1996), Oak Hill (Hare & 

Avise 1996), Merritt Island (Hare & Avise 1996), New Smyrna (Karl & Avise 

1992; Hare & Avise 1996), Cape Canaveral (Buroker 1983), Stuart (Karl & Avise 

1992), Indian River (Hoover & Gaffney 2005), Sebastian (Hare & Avise 1996), 

Vero Beach (Hare & Avise 1996), Fort Pierce (Hare & Avise 1996), Jupiter (Hare 

& Avise 1996), West Palm Beach (Reeb & Avise 1990), Boynton Beach (Hare & 

Avise 1996), Palm Beach (Hare & Avise 1996), Deerfield Beach (Hare & Avise 

1996), Pompano Beach (Hare & Avise 1996), Miami (Hare & Avise 1996), Cedar 

Key (Hoover & Gaffney 2005), Panacea (Hedgecock & Okazaki 1984; Karl & 

Avise 1992; MacDonald 1996; Hare & Avise 1998), St. Petersburg (Reeb & 

Avise 1990), Port Charlotte (Karl & Avise 1992). ............................................................24 

Figure 5. Sampling sites in the Gulf of Mexico: Cedar Key, FL (Hoover & Gaffney 2005), 

Panacea, FL (MacDonald 1996), Apalachicola, FL (Hoover & Gaffney 2005), 

Panama City, FL (Reeb & Avise 1990), Biloxi, MS (Groue & Lester 1982), 

Grand Isle, LA (Reeb & Avise 1990; Karl & Avise 1992; Hare & Avise 1996; 

Hare & Avise 1998; Hoover & Gaffney 2005), Galveston, TX (Small & Chapman 

1997), Matagorda, TX (Hedgecock & Okazaki 1984; King et al. 1994), Corpus 

Christi, TX (Buroker 1983), Laguna Madre, TX (Groue & Lester 1982; 

Hedgecock & Okazaki 1984; King et al. 1994), Brownsville, TX (Buroker 1983; 

Reeb & Avise 1990), Tabasco, MX (Hoover & Gaffney 2005), Campeche, MX 

(Hedgecock & Okazaki 1984). ..........................................................................................25 
vi

Figure 6. Landings of C. virginica, showing linear sequence of fishery expansion and 

decline along eastern North America between Maine and Texas (from Kirby 

2004). .................................................................................................................................35 

 

Figure 7. Response to the question: Are oysters present in the estuary? ..........................................64 

Figure 8. Response to the question: Does the estuary support harvest?............................................64 

Figure 9. Response to the question: Based on fisheries dependent analysis, is the population 

stable? ................................................................................................................................65 

Figure 10. Response to the question: Based on fisheries independent analysis, is the 

population stable? ..............................................................................................................65 

Figure 11. Response to the question: Is the population stable without restoration and/or 

enhancement?.....................................................................................................................66 

Figure 12. Response to the question: Is there sufficient recruitment? ................................................66 

Figure 13. Response to the question: What are the rationale supporting restoration and/or 

enhancement activities? .....................................................................................................67 

Figure 14. Response to the question: Is oyster harvest impacted by disease/parasites? .....................67 

Figure 15. Response to the question: Is interstate transfer of oyster seed regulated?.........................68 

Figure 16. Response to the question: Is transfer of oyster seed within state boundaries 

regulated?...........................................................................................................................68 

Figure 17. Response to the question: Is harvest regulation by size?...................................................69 

Figure 18. Response to the question: Is harvest regulation by bag limit?...........................................69 

Figure 19. Response to the question: Are regulations sufficient?.......................................................70 
vii

List of Tables

Table 1. Examples of species exhibiting population genetic structure between Atlantic and 

Gulf populations..................................................................................................................19 

Table 2. Genetic studies of population structure in Gulf and Atlantic populations of 

Crassostrea virginica..........................................................................................................20 

Table 3. Matrix of correlations between responses to survey questions relevant to 

population stability and sustainability. ..............................................................................70 
1 Executive Summary

NOAA’s National Marine Fisheries Service (NMFS) received a petition to list the eastern oyster 

(Crassostrea virginica) as either threatened or endangered under the Endangered Species Act 

(ESA). Following a positive 90-day finding, wherein it was determined that the petition and the 

information in NMFS files contained substantial information indicating that the petitioned action 

may be warranted, a Biological Review Team (BRT) was convened to review the status of 

eastern oysters throughout their range. 

During deliberations, the BRT met three times to analyze and summarize the available 

information on the status of this species. This document is the BRT’s status review for the 

eastern oyster, as guided by the ESA. It presents a summary of published literature and other 

currently available scientific information regarding the biology and status of eastern oysters, as 

well as an assessment of existing regulatory mechanisms and current conservation and research 

efforts that may yield protection for the species. 

As invertebrates, a listing determination for eastern oysters must be based on the species’ status 

throughout “all or a significant portion” of its range. Eastern oysters are widely distributed along 

the East and Gulf coasts of the United States, and their range extends internationally into 

Canadian and Caribbean waters. Several recent genetic studies have been undertaken to better 

understand the population structure of C. virginica, and these studies have found strong patterns 

of differentiation on the basis of different sequencing data. Studies indicate to two separate 

populations, one within the Atlantic region and one within the Gulf of Mexico with an 

intermediate zone between these populations found on the eastern coast of Florida in the general 

area of Cape Canaveral. 

Commercial landings throughout the species’ range along the East Coast have declined to 

approximately two percent of the recorded historic highs. In the Gulf of Mexico, however, 

harvest has generally increased or remained stable in the last several years. Louisiana is now the 

top-producing oyster-state and has contributed an average of 42% of the total U.S. harvest. 

Fishery harvest declines, often cited as cause for alarm, are widely-recognized as unreliable 

indicators of population trends. Landings data are more a metric of fishery success rather than 

species abundance. 

Oysters are considered a keystone species in most estuaries along the Atlantic and Gulf coasts, 

and self-sustaining populations play an essential role in the ecology of these estuaries. There are 

few data available regarding historic and current oyster reef acreage estimates, and available 

fisheries dependent and independent data are limited. In order to gather additional data to assess 

the status of the species, the BRT conducted a telephone survey of state resource managers and 

oyster experts. Respondents were asked to provide the following information for each estuary 

within their region/area: historic and current oyster acreage estimates, harvest rates and 

regulations, the sustainability of oyster populations with and without restoration, recruitment, 

and the primary threats facing oyster populations. Even though oyster harvests are at or near 

record low levels along the majority of the US Atlantic coast, resource managers and 

independent experts surveyed by the BRT indicated that overutilization (overharvesting) is 

currently a minor threat to oyster populations occurring only seven times out of the 286 threats 
2

listed for the 71 estuaries assessed by respondents. Information obtained through the survey also 

strongly suggests that recruitment is sufficient to maintain the viability of eastern oyster 

populations throughout the species’ range except in a portion of the mid-Atlantic (e.g., Long 

Island Sound, Peconic Bay, Hudson-Raritan Estuary). Restoration and enhancement efforts for 

fisheries and conservation are occurring throughout the species’ range, but are more common in 

the north and mid-Atlantic. In estuaries where restoration and enhancement efforts are 

occurring, they are considered necessary to sustain populations in roughly half of the estuaries in 

the mid and south Atlantic regions (presumably, to support commercially viable populations). In 

the North Atlantic (specifically, Connecticut and Rhode Island) and the Gulf of Mexico, 

restoration and enhancement efforts are not necessary to sustain biologically viable populations 

but are considered important to maintaining a fishery and conserving ecosystem services. 

Restoration efforts for oysters are often motivated by interest in reclaiming ecosystem services 

and/or sustaining fisheries, not by a perceived need to protect the species itself. Domestication 

and farming of reproductively-isolated breeds of eastern oysters is expanding to satisfy market 

demand, with the ancillary benefit of moderating harvest pressure on natural populations. 

Eastern oysters display a wide range of survival strategies. They are both colonizers and 

ecosystem engineers and have a high reproductive potential. The species’ ability to adapt to a 

wide range of environmental conditions (e.g. tolerance for low dissolved oxygen and wide 

ranges in salinity and temperature) makes it resilient. Eastern oysters inhabit a naturally-variable 

environment, and evidence suggests that past local extirpations and colonizations have been 

common over geologic time. There are some threats that may be significant at a regional or 

local level. However, while the species encounters many threats throughout its range, none are 

considered to be overwhelmingly dominant or advancing at a rate that would threaten the 

viability of the species throughout its full range. Based on the available information, the BRT 

therefore concluded that the long term persistence of eastern oysters throughout their range is not 

at risk now or in the foreseeable future. 

2 Introduction

2.1 General Introduction

NMFS received a petition from Mr. Wolf-Dieter Busch, Ecosystem Initiatives Advisory Services 

to list eastern oyster as either threatened or endangered under the ESA. Following NMFS’ 

positive 90-day finding, wherein the petition was determined to contain substantial information, 

NMFS convened an eastern oyster BRT to review the status of the species concerned. 

In order to conduct a comprehensive review, the BRT was asked by NMFS to assess the species’ 

status and degree of threat to the species with regard to the factors provided by section 4 of the 

ESA without making a recommendation regarding listing. The BRT was provided a copy of the 

petition and all information submitted as part of the data request that was specified in the Federal 

Register Notice announcing the 90-day finding. The BRT reviewed all this information during 

its consideration and analysis of potential threats to the eastern oyster. This status review 

document is a summary of the information assembled by the BRT and incorporates the best 

scientific and commercial data available. In addition, the BRT summarized current conservation 
3

and research efforts that may yield protection, and drew scientific conclusions about the health of 

eastern oyster resources throughout the species’ range. 

2.2 ESA Background

The purposes of the ESA are to provide a means to conserve the ecosystems upon which 

endangered species and threatened species depend, to provide a program for the conservation of 

endangered and threatened species, and to take appropriate steps to recover a species. The U.S. 

Fish and Wildlife Service (USFWS) and NMFS share responsibility for administering the ESA; 

NMFS is responsible for determining whether marine, estuarine or anadromous species, 

subspecies, or distinct population segments are threatened or endangered under the ESA. To be 

considered for listing under the ESA, a group of organisms must constitute a “species.” 

The ESA provides the following definitions: 

“the term species includes any subspecies of fish or wildlife or 

plants, and any distinct population segment of any species of 

vertebrate fish or wildlife which interbreeds when mature.” 

“endangered species” is defined as “any species which is in danger 

of extinction throughout all or a significant portion of its 

range.” 

“threatened species” is defined as “any species which is likely to 

become an endangered species within the foreseeable future 

throughout all or a significant portion of its range.” 

Additional criteria regarding entities appropriate for listing under the ESA have been set forth. 

First, there is the ability to identify and list distinct populations segments (61 FR 4722) or 

evolutionarily significant units (56 FR 58612) when a population satisfies the criteria of being 

discrete and significant; however, these policies are limited to vertebrates and therefore, are not 

within the scope of this status review. 

The process for determining whether a species (as defined above) should be listed is based upon 

the best available scientific and commercial information. The status is determined from an 

assessment of factors specified in section 4(a)(1) of the ESA including: 

(A) The present or threatened destruction, modification, or curtailment of its 

habitat or range; 

(B) Overutilization for commercial, recreational, scientific, or educational 

purposes; 

(C) Disease or predation; 

(D) Inadequacy of existing regulatory mechanisms; 

(E) Other natural or manmade factors affecting the continued existence of the 

species. 
4

Within this status review report (SRR), the BRT also summarized ongoing protective efforts to 

determine if they abate any risks to eastern oysters. When a species is listed as endangered under 

the ESA, it is afforded complete protection by the ESA, including the development and 

implementation of recovery plans, requirements that Federal agencies use their authorities to 

conserve the species, and prohibitions against certain practices, such as taking individuals of the 

species. Under NMFS policy, when a species is listed as threatened, the prohibitions for take are 

not automatically afforded. These prohibitions must be specifically afforded to a threatened 

species through a special rule (section 4(d) of ESA). Specifically, the prohibitions of section 9 of 

the ESA, in part, make it illegal for any person subject to the jurisdiction of the United States: to 

take (i.e., to harass, harm, pursue, hunt, shoot, wound, kill, trap, capture or collect, or to attempt 

to engage in any such conduct); to import into, or export from, the United States; to ship in 

interstate or foreign commerce in the course of commercial activity; or to sell or offer for sale in 

interstate or foreign commerce any endangered wildlife. To possess, sell, deliver, carry, 

transport, or ship, endangered wildlife that has been taken illegally is also prohibited. However, 

section 10 of the ESA provides NMFS with the authority to grant exemptions to the section 9 

taking prohibitions for scientific research, enhancement, and incidental take permits. The ESA 

provides some exceptions to the prohibitions, without permits, for certain antique articles and 

species held in captivity at the time of the listing. The ESA also provides for possible land 

acquisitions and cooperation with the states. In some instances, species that are not listed under 

the ESA are afforded protection. For example, Section 4(e) of the ESA, entitled “Similarity of 

Appearance Cases,” allows the Secretary (of Commerce or Interior), by regulation of commerce 

or taking, to the extent he deems advisable, to treat any species as an endangered species or 

threatened species even though it is not listed if he finds that: (1) Such species so closely 

resembles a listed species in appearance, that enforcement personnel would have substantial 

difficulty in differentiating between the listed and unlisted species; (2) the effect of this 

substantial difficulty is an additional threat to an endangered or threatened species; and (3) such 

treatment of an unlisted species will substantially facilitate the enforcement and further the 

policy of the ESA. 

2.2 The Petition

On January 11, 2005, NMFS received a petition from Mr. Wolf-Dieter Busch (the petitioner), 

Ecosystem Initiatives Advisory Services, to list eastern oyster (Crassostrea virginica) as 

threatened or endangered under the ESA. After reviewing the information contained in the 

petition and in our files, NMFS determined that there is sufficient information to indicate that the 

petitioned action may be warranted. On May 18, 2005, NMFS published a positive 90-day 

finding in the Federal Register. This initiated this status review process. 

On Wednesday, October 19, 2005, NMFS received a letter from the petitioner dated October 13, 

2005 requesting the recall of the eastern oyster petition. In his letter, the petitioner indicated that 

his request to withdraw the petition was due to the public and industry’s confusion over the 

petition and listing process. He noted the significant concerns of some that the species may be 

listed as endangered; thereby, creating severe restrictions and regulations for this resource. He 

also expressed concern that given the timeline of the review, NMFS may not currently have 

enough information to determine if eastern oyster subspecies exist. He concluded that he hopes 

that NMFS will continue with the review as he considers the status review report to be a 
5

comprehensive resource which will be of great value in focusing restoration activities for this 

resource. 

 

NMFS accepted this request and as a result, ceased the evaluation of the petition. However, a 

considerable amount of effort had been expended by the BRT at the point at which the 

withdrawal of the petition occurred. Also, the completed status review report is the most timely 

and comprehensive resource document for this species. As such, NMFS determined that because 

the report is a useful tool in guiding future management decisions, the BRT should complete the 

status review report. 

3 Species biology

3.1 Life history 

 Morphology

The eastern oyster, Crassostrea virginica, (phylum Mollusca, class Bivalvia, order Ostreoid, 

family Ostreidae) is a monomyarian lamellibranch exhibiting bilateral asymmetry and a 

restricted coelom (Seed 1983). The foot and adductor muscle, present only during larval stages, 

is reabsorbed after metamorphosis resulting in the monomyarian condition (Kennedy 1996, 

Morrison 1996). Valves are asymmetrical with the left valve generally thicker and more deeply 

cupped than the right (Yonge 1960; Galtsoff 1964). When closed there is no gap between the 

two halves. Eastern oysters settle on the left valve leaving the right valve always on top. 

Shell shape and thickness is variable and differs depending on the environment in which the 

oyster grows. Umbones are curved and point toward the posterior, and shells are thicker when 

growing on hard substrates. In silty environments or on reefs, umbones grow generally straight, 

but shells are more fragile than those growing on hard substrates. Solitary oysters found on hard 

substrates are usually rounded with radial ridges and foliated processes while those growing on 

soft substrates and reefs are more slender with few ridges (Stanely and Sellers 1986). 

The interior of the shell has a prominent purple-pigmented adductor muscle scar located close to 

the dorsal end of the valve (Figure 1). The purple pigmentation of the adductor muscle scar 

differentiates the eastern oyster from similar species. 

Robert Howells, TPWD 
6

 Figure 1. Eastern oyster, Crassostrea virginica (photo credit: Texas Parks and Wildlife 

Department) 

Oysters exhibit great morphological plasticity as adults, but quite static morphology and 

behaviors as larvae. The former is due to tremendous environmental variability and lack of 

selection on adult form, and the latter is due to evolutionary forces constraining the single motile 

life stage upon which the species is dependent for long-term persistence in a highly variable and 

ever changing estuarine environment (Carriker 1996). 

Reproduction

Eastern oysters are protandric, individuals first mature as males then typically change to female 

later in life, and there is also evidence suggesting that the process is reversible later in life 

(Thompson et al. 1996). The factors determining sex are varied and complex. Oysters may also 

change sex annually in response to environmental, nutritional and/or physiological stresses 

(Tranter 1958 cited by Thompson et al. 1996; Bahr and Hillman 1967; Davis and Hillman 1971; 

Ford et al. 1990). Other studies suggest that sex determination may be influenced by the sex and 

proximity of nearby oysters (Needler 1932; Burkenroad 1931; Smith 1949; and Menzel 1951 all 

cited by Thompson et al. 1996). In Canada, few males changed sex when in close proximity to 

females (Needler 1932 as cited in Thompson et al. 1996). Burkenroad (1931) found a higher 

ratio of females to males in >4 cm oysters growing unattached and an equal distribution for 

smaller oysters growing in clumps in Louisiana. In South Carolina, sex ratios were skewed 

toward more males when growing in aggregate as opposed to those growing singly (Smith 1949 

as cited in Thompson et al. 1996). Experimental studies in Chesapeake Bay, Maryland showed 

that approximately one-third of single sex oysters had changed to the opposite sex when held in 

trays over the winter (Kennedy 1983). Thompson et al. (1996) noted that sex reversal usually 

occurred between spawning seasons when the gonad was undifferentiated. 

Fecundity is difficult to determine in oysters due to a prolonged spawning period with 

intermittent spawning and redevelopment throughout the year and gonadal tissue that is diffuse 

and integrated into surrounding tissue (Thompson et al. 1996). However, estimates range from 2 

to 115 million eggs per female, depending on size and geographic location (Galtsoff 1930, 1964; 

Davis and Chanley 1956; Cox 1988; Cox and Mann 1992; all cited in Thompson et al. 1996). 

Spawning is initiated by a combination of factors including water temperature, salinity and 

physiochemical interactions (Galtsoff 1964; and Loosanoff 1953 cited by Berrigan et al. 1991; 

Hayes and Menzel 1981; Hofstetter 1977, 1983). Spawning is seasonal (summer) throughout the 

mid to northern Atlantic portions of the species’ range. In southern waters spawning occurs in 

all but the coldest months (Berrigan et al. 1991). Conditions generally required for spawning 

include water temperatures at or above 20° C and salinity higher than 10 practical salinity units 

(psu). When these conditions persist, spawning can continue year-round (Breuer 1962). 

Larval Phase

After fertilization, oysters develop through several free-swimming larval stages before attaching 

to a hard substrate and becoming sessile. The rate of development through these stages is highly 

temperature dependent (Shumway 1996). The mechanisms for larval dispersal and recruitment 
7

are still unclear (Epifanio 1988). Larval retention is generally explained by “passive” transport 

induced by physical factors, by an “active” process involving larval swimming, or by a 

combination of both (Dekshenieks et al. 1996). The first larval stage (trochophore) is formed 

four to six hours following fertilization and lasts approximately one to two days. The 

trochophore larva does not feed, but subsequent larval stages (veliger) are planktotrophic, 

feeding on small plants and animals (Kennedy 1996). Veliger stages, lasting up to two months 

(Hopkins 1931), include several morphological changes to the larvae resulting in a fully 

developed larva possessing a well-developed foot. The foot is used for locomotion when seeking 

a place to attach after settling on appropriate substrate and is reabsorbed upon final 

metamorphosis into an attached oyster. 

As oyster larvae become competent to settle they must locate a suitable substrate upon which to 

attach. Larvae may exhibit exploratory behavior in locating a suitable substrate upon which to 

settle (Burke 1983 as cited in Kennedy 1996). Both environmental and internal cues are used in 

determining when and where veliger larvae will settle (Kennedy 1996). Settlement is a 

behavioral response that can be repeated or reversed and is followed by metamorphosis, which 

results in morphological changes and is permanent (Kennedy 1996). There is evidence that 

suggests metamorphosis is initiated (triggered) by salinity and by chemicals given off by live 

oysters and bio-films on other suitable substrates (Hidu and Haskin 1971, Keck et al. 1971; 

Kennedy 1996). Larvae appear to exhibit negative phototaxis, a preference which suggests that 

habitats with complex interstitial spaces may provide better habitat for settlement. The process 

of settlement, metamorphosis, and attachment normally occurs two to three weeks after hatching, 

but can be delayed for up to a month or longer depending on environmental conditions 

(Stallworthy 1979 as cited by Kennedy 1996; Kennedy 1996). 

Environmental Tolerances 

Temperature, salinity and food availability greatly influence oyster growth, and therefore, rates 

vary seasonally at a given latitude with maximum growth occurring during the summer and fall. 

The minimum temperature reported for growth of oyster larvae was 17.5o C (Hofstetter 1977). 

Eastern oysters have been reported to survive freezing temperatures in shallow-water habitats 

and after being exposed to temperatures in excess of 45o C in intertidal areas (Galtsoff 1964; 

Shumway 1996). However, exposures to temperatures above approximately 35o C will adversely 

affect pumping rate and thereby feeding (Loosanoff 1958; and Galtsoff 1928 as cited by 

Shumway 1996). Growth of oysters in the higher latitudinal regions stops or slows during winter 

(Loosanoff and Nomejko 1949). Oysters are capable of growth throughout the year in the Gulf 

region but optimum temperatures range from 20 to 30o C (Stanley and Sellers 1986). 

Oysters can tolerate salinities from 0 to 42 psu, but the optimum range is 14 to 28 psu (Quast et 

al. 1988; Shumway 1996). A minimum salinity of 10 psu is required for growth with little 

growth occurring at salinities less than 5 psu (Shumway 1996). Viable, reproducing populations 

of oysters have been found to persist off the mouth of the Atchafalaya River in Louisiana where 

salinities sometimes are less than 5 psu for several months (pers. comm. Banks 2006). 

Mortalities usually only occur when water temperatures exceed 30o C in the summer (pers. 

comm. Banks 2006). There is some evidence that suggests the effects of high water temperature 

are exacerbated by low salinity events resulting from heavy rainfall freshets (Shumway 1996).
8

Growth and Feeding

Growth rate is highly dependent on temperature and food supply (Kennedy 1996). Oysters 

undergo rapid growth (reaching 10 mm month-1) during the first six months of life but slow 

throughout the rest of their life (Quast et al. 1988) reaching approximately 15 cm in five or six 

years (Hofstetter 1962; Berrigan et al. 1991). Harvest size (76-90 mm) is reached in the Gulf of 

Mexico 18-24 months after setting (Hofstetter 1977; Berrigan et al. 1991) whereas oysters from 

Long Island Sound take 4-5 years to reach a similar size (Shumway 1996). Shell growth is not 

uniform between the two valves with the left valve growing faster than the right (Carriker 1996). 

Oysters continue to grow throughout their life though the rate diminishes with age (Carriker 

1996). Maximum size can be up to 20.6 cm (Boothbay Harbor, Maine; Galtsoff 1964) to 35.5 

cm in the northeast (Damariscotta River, Maine; Ingersoll 1881 as cited in Carriker 1996). In 

the Gulf of Mexico, eastern oysters have been found to live 25-30 years and reach sizes to 30 cm 

(Martin 1987). These large individuals are usually associated with undisturbed bottoms where 

commercial fishing is prohibited and (Carriker 1996). 

Oysters are filter feeders, feeding primarily on phytoplankton and suspended detritus (Langdon 

and Newell 1996). Food items range in size from 1-30 μ (Mackie 1969; Quast et al. 1988; 

Newell and Langdon 1996). Clearance or filtration rates have been reported to range from 1.5-

10.0 L h-1

g

-1 dry tissue weight (Stanley and Sellers 1986; Newell and Langdon 1996). 

Crassostrea virginica are capable of adjusting feeding rates depending on the size, type and 

composition of the available food source (Baldwin 1995, Baldwin and Newell 1995a, 1995b as 

cited in Kennedy 1996). 

Langdon and Newell (1996) note there is no evidence to suggest that oyster larvae are food 

limited in the wild while other studies have shown that oysters consuming low protein food 

sources had better growth rates than those exposed to high protein food sources (Flaak and 

Epifanio 1978; Utting 1986; both cited by Langdon and Newell 1996). As sessile, non-motile 

organisms, oysters must rely on food-laden water being moved past the oyster in order to extract 

food from the water column. Water flow across oyster reefs has been shown to influence growth 

rate (Newell and Langdon 1996). Excessive water flow causes food particles to move through 

the area before they can be extracted from the water column (Newell and Langdon 1996); too 

slow and there are not enough food particles available to support growth (Grizzle et al. 1992 as 

cited by Newell and Langdon 1996). Optimal growth is a function of multiple interactions of 

environmental factors and food availability (Loosanoff and Nomejko 1949, Shumway 1996). 

 Habitat preference

While oysters are capable of surviving in a wide range of habitat conditions, the preferred habitat 

(general range) conditions in areas where eastern oysters are common, based largely on 

Shumway (1996) and Hargis and Haven (1999) and others as noted, are considered to be: 

Depth- 0.6-2.0 m (range 0-11m) in Canadian waters (Jenkins et al. 1997); mostly between 

0.6-5.0 m in Mid-Atlantic States waters (MacKenzie 1996) although oysters occur 

commonly inter-tidally south of Maryland (Burrell 1997) and in deeper waters in some 
9

areas, e.g., to 8 m in Chesapeake Bay (MacKenzie 1997a); 0.0-4.0 m in Gulf of Mexico 

(MacKenzie and Wakida-Kusunoki 1997; Dugas et al. 1997). 

Salinity-larvae (10-27.5 ppt; 17.5 ppt optimum for Long Island Sound stock (Calabrese 

and Davis 1970)), adults (normally ~5-40 ppt), 

Temperature- larvae (optimum ~20.0-32.5oC) (Calabrese and Davis 1970); adults 

optimum temperatures range from 20 to 30oC (Stanley and Sellers 1986) , survival under 

extremes from -2 to 36 oC, and to 49oC for short periods of time) 

Substrate- larvae (clean hard or shell substrate), adults (various substrates, including mud, 

that support their growing or accumulative community weight) (Jenkins et al. 1997) 

Geomorphology- sheltered drowned river valleys and bar-built lagoonal estuaries 

(MacKenzie and Wakida-Kusunoki 1997) 

pH- larvae, normally 6.75-8.75 (Calabrese and Davis 1966) 

Tidal range- 0.5 m (in restricted lagoons or upper estuaries) to 2.7 m (Gulf of St. 

Lawrence; Jenkins et al. 1997) 

DO- ~20-100% saturation 

Hydrographic circulation- such as to cause oyster larvae to remain near existing reefs but 

with enough exchange to maintain a good food supply and near neutral silt balance on the 

oyster reefs/beds (Lenihan 1999). Sensitivity analyses of some bivalve populations 

suggest that their population stability and growth rates are more sensitive to changes in 

larval survival and recruitment than they are to adult survivorship or fecundity 

(Brousseau 2005), except perhaps when disease (parasite) infection rates are high. 

 

3.2 Ecology and Population Dynamics

The eastern oyster is a remarkably resilient species in the dynamic physical environment of 

intertidal and near-shore estuarine ecosystems. This resilience can be attributed, at least 

partially, to an unusual combination of characteristics. The oyster is both a “colonizer” and an 

“ecosystem engineer.” Species that are colonizers tend to be highly fecund with wide 

distribution of offspring; they are often the first organisms to occupy new niches opened by 

changes in the physical environment. Ecosystem engineers modify the physical environment to 

make it more suitable for long-term survival; they create their own niches. The reef-building 

capabilities of oysters qualify them as ecosystem engineers; accumulation of shell ensures a 

substrate for future generations to occupy and also accomplishes genetic mixing between 

generations – older oysters can mate with younger – thereby conserving genetic diversity. This 

combination of colonizing and engineering capabilities has allowed the oyster to persist in the 

highly-dynamic environment of coastal seas and estuaries, re-colonizing areas impacted by 

storms and other physical displacements and moving with the rise and fall of sea level (Dame 

1993, 1996). 
10

Recruitment

While the mechanisms of dispersal and recruitment are still unclear (Epifanio 1988), there 

remains considerable spatial and temporal variability in both seasonal and local recruitment 

patterns. Nutritional and reproductive stress has been implicated in explaining some of this 

variability (Nelson 1905, Helm et al. 1973, both cited in Kennedy 1996). Estuaries subjected to 

high tidal-flushing activities tend to have low, but consistent recruitment intensities while those 

with low freshwater inflows and sluggish circulation allow for extended residence-time for 

larvae and higher but irregular recruitment (Kennedy 1996). Larval retention is generally 

explained by ‘passive’ transport induced by physical factors, by an ‘active’ process involving 

larval swimming, or by a combination of both (Dekshenieks et al. 1996). Differential 

recruitment patterns on a microhabitat scale have also been identified though the causes are yet 

undetermined (Kennedy 1996). Mortality rates are more sensitive to changes in larval survival 

and recruitment then they are to adult survivorship or fecundity (Brousseau 2005), except 

perhaps when disease-parasite infection rates are high. 

Ecological Role 

Oysters are an ecological keystone species in most estuaries along the Atlantic and Gulf coasts. 

Oyster populations contribute to the integrity and functionality of estuarine ecosystems. Self 

sustaining oyster populations form reefs that 1) contribute to trophic dynamics by promoting 

species diversity; 2) provide structural integrity that supports community stability, enhances 

habitat values and affects water circulation and flow patterns; and 3) perform ecological services 

which improve water quality and recycle nutrients. 

Water Filtration 

Clearance rates (volume of water totally cleared of suspended particles per unit time) for adult 

oysters have been reported as high as 10 L h-1g

-1 dry tissue weight (Jordan 1987 as cited in 

Newell and Langdon 1996). Pseudofeces, physiologically defined as consisting of particles that 

have been trapped (filtered by gill cilia), combined with mucous, transported toward the mouth, 

but rejected by the palps prior to being ingested (Barber 2006), are a significant source of 

sediment on reefs and in areas of little water movement can quickly smother live oysters (Lund 

1957). Due to the high clearance rates of eastern oysters they are being evaluated as a possible 

bioremediation tool to reduce contaminant loading in marsh-estuarine systems (Breitburg et al. 

2000). 

Reef Habitat Creation 

Oyster reefs provide valuable refuge, trophic support and complex structure for a variety of 

juvenile and adult finfish and are considered essential fish habitat for managed and unmanaged 

species (Coen et al. 1999, SAFMC 1998, GSMFC 2004). Oyster reefs may function similarly to 

submerged aquatic vegetation in regions where seagrasses are not abundant. For example, in 

some Texas bay systems spotted sea trout and red drum utilize oyster reefs as foraging areas 

while seagrasses are used in a similar manner in other ecosystems (Holt and Ingall 2000). 
11

Bahr and Lanier (1981) documented over 40 macrofaunal species or groups that live on oyster 

reefs while the total number of species in an oyster community have been identified in excess of 

300 (Wells 1961). Motile arthropods such as crabs (Xanthidae), snapping shrimps, isopods and 

amphipods, polychaetes (e.g. Nereidae, Syllidae), gastropods such as the oyster drill (Stramonita 

haemastoma, Urosalpinx cinerea) as well as sessile invertebrates such as mussels, chitons, 

limpets, barnacles, anemones, bryozoans, hydroids, and sponges may be found in oyster reef 

habitat. Suspension and deposit feeding activities provide trophic support for higher consumer 

levels by converting detritus to animal biomass and to primary producers through mineralization 

of carbon and release of nutrients such as nitrogen and phosphorous (GSMFC 2004). 

Oyster reef configurations are highly variable in both shape and size, ranging from small mounds 

or patch reefs to long, wide ridges that may extend for miles. In soft sediment environments 

oyster reefs can serve to reduce erosional processes as constructed reefs in Louisiana showed the 

potential to reduce adjacent shoreline erosion at low-energy locations (Piazza et al 2005). Reefs 

may ultimately divide bays and change circulation patterns (Diener 1975) thus altering local 

environments and the associated flora and fauna (Britton and Morton 1989). High density oyster 

communities occur in areas where water currents are high enough to supply food to many 

individuals yet low enough to limit turbidity from re-suspending sediments (Britton and Morton 

1989). 

In the northern part of the range, winter freezing limits intertidal survival; therefore, oysters are 

limited to subtidal environments. Growth rates are limited by temperature, and predators tend to 

have a large impact on survival. Thus, oysters are found in subtidal beds at modest densities that 

mitigate predation, but still permit effective spawning. Recruitment tends to be periodic 

(Kennedy 1996). In these environments, natural oyster populations tend to include several 

generations in long-lived, low-relief beds. As many of these beds have been harvested to a point 

where extracting more oysters from them is no longer cost-effective, most oyster production in 

the northeast is based upon aquaculture (MacKenzie 1997a). 

In the mid-Atlantic region, high-relief oyster “reefs” once kept oysters in the upper levels of the 

water column where phytoplankton densities are high and hypoxia is rare (Lenihan and Peterson 

1998). However, remaining reefs are mere footprints of the original structures that were 

removed by decades of harvest (Rothschild et al. 1994; McCormick-Ray 1998; Haven and 

Whitcomb 1983; Hargis and Haven 1999; Jackson et al. 2001). In these ecosystems, parasitic 

diseases often limit survival, and growth may be temperature or food-limited. 

Toward the southeast Atlantic and into the Gulf of Mexico, oysters are rapid colonizers and 

occur in abundance in the intertidal zone. However, many northern Gulf of Mexico populations 

are located subtidally. Growth rates, among intertidal populations are limited by feeding, but 

reproduction, fecundity, and recruitment may be extremely high. Oyster also occur in abundance 

on subtidal reefs where pests and predators (shell-boring sponges, rays) may negatively affect 

survival, but warmer winter temperatures and the year-round growing season act to enhance 

survival. 
12

3.3 Distribution 

Historic Distribution

According to Hargis (1999) and Steimle (2005), during the last ice age, sea levels were about 

100 m lower than present times and most of the proto-estuaries as well as now nonexistent 

estuaries were located further offshore on the Atlantic and Gulf of Mexico continental shelf. 

Where environmental conditions allowed, the eastern oyster inhabited these proto-estuaries. As 

sea level rose these estuaries and the oysters moved inshore and came to occupy the submerged 

river valleys and coastal areas recognized as today’s estuaries, or new estuaries were created via 

barrier beach-lagoon formation. 

In the Northeast, where the glaciers covered the land from Staten Island NY to Cape Cod, 

Massachusetts, and perhaps to Georges Bank, the adjacent dry continental shelf was like an artic 

tundra and probably any ice free estuaries were inhospitable to marginal for the oyster, except at 

the very outer edge of the continental shelf due to the influence of the Gulf Stream. At that time, 

Georges Bank was an island and possibly connected to Cape Cod. Other areas that are now 

submerged shoals in the Gulf of Maine and off Nova Scotia were also islands and potential 

oyster habitat. As glaciers retreated and melted, the sea level rose as did coastal water 

temperatures. About 8000 years before present (YBP) there is evidence that there were oysters 

living on the shelf on the inwardly moving proto-estuaries (Merrill et al. 1965). About this time 

they might have entered the current Chesapeake Estuary System (Hargis 1999) and slightly later 

the Hudson-Raritan system as evidenced from radiocarbon dating of oyster shell collected on the 

mid-continental shelf and within the estuaries. 

In the Hudson River, there is evidence to suggest oyster populations established themselves 

initially about 7000 YBP in the Tappan Zee area. Oyster populations may have retreated down 

stream for a time with climate and rain fall cycles. Newly uncovered fossil oyster reefs in the 

downstream area were also radiocarbon dated to this time, a dating that seems connected with the 

first American shell middens in the area. The radio-carbon dated shell deposition in the middens 

had gaps of several thousand years suggesting a partial loss of the resource in the downstream 

area. 

When each Gulf and Atlantic estuary stabilized to near its current form oyster distribution might 

have varied. For example, Long Island Sound was a post-glacial, fresh water lake for a while 

until possibly a combination of sea level rise and fresh water rise breached the glacial moraine 

dams at both ends, so oysters could not enter that estuary until after the breach-conversion. 

There are data to suggest that temperatures rose quickly and that about 7000-8000 YBP sea level 

may have been near or slightly higher than present, providing an opportunity for the rapid 

northerly expansion of the species. However, there were also mini-ice ages, the most recent 

lasted until about the 18th century, where estuarine temperature might have declined to marginal 

levels for oysters in the northern portion of the species’ range. Thus, the pre-colonial historic 

distribution of the eastern oyster within many Atlantic-Gulf estuaries may have been variable at 

time scales of decades to thousands of years, most likely responding to climate and water flow 

changes and occasional catastrophic events, such as major hurricanes. 
13

Current Distribution

The eastern oyster occurs naturally in a great diversity of habitats along the western Atlantic 

Ocean from the Canadian Maritime Provinces to the Gulf of Mexico, Panama and the Caribbean 

Islands (Carlton and Mann 1996; Abbott 1974; MacKenzie 1997a; Jenkins et al., 1997; FAO 

1978). Crassostrea virginica has also been described from Panama, Venezuela, Brazil and 

Argentina along the Caribbean Sea and the western Atlantic Ocean in Central and South America 

(Wallace 2001). Carriker and Gaffney (1996) report eastern oysters are distributed in the 

western Atlantic from Brazil northward through the Caribbean, and Gulf of Mexico to the St. 

Lawrence River estuary in eastern Canada, a range of some 8,000 km. Harry (1985) suggested 

that names of all populations of this species, such as C. brasiliana, C. floridensis, C. guyensis, C. 

lacerata, C. rhizophorea, and others should be replaced by C. virginica. However, Gaffney 

(Pers. Comm. 2005) now reports that the southern distribution of C. virginica can only be 

verified genetically to the northern Yucatan Peninsula of the Gulf of Mexico at present, and other 

genetically distinct Crassostrea species might occur in the Caribbean. 

Eastern oysters have been transplanted outside of the species natural range. Ruesink et al. (2005) 

listed many transplanted C. virginica populations that have appeared to have survived to present 

in the areas to which they were transplanted or continue in mariculture operations. According to 

Ruesink et al. (2005), surviving, out-of-range C. virginica transplantations (with source in 

parenthesis) are found in: western Canada (North American east coast, since 1883); western US 

(US east coast since 1860s); western Mexico (unknown); Hawaii (unknown, since 1860s); Fiji 

(from Hawaii, 1970); Tonga (US west coast, 1973); Japan (“USA”, 1968); Mauritius-Indian 

Ocean (US west coast, 1972); and possibly England (North American east coast, since 1870s). 

3.4 Historic and Current Abundance

At the time of European colonization and the beginning of the localized heavy exploitation of the 

oyster for diverse purposes, oysters were reported from almost all estuaries along the Atlantic 

and Gulf coast. However, information on specific abundances within estuaries were often vague 

(Ingersoll 1881), especially beyond United States borders and in areas with low human 

population density and areas without active fisheries. 

One confounding factor in understanding the full pre-European colonization distribution and 

abundance of the eastern oyster is that only shell remains are often available as evidence. 

Historic shell relics of the presence and distribution of the putative “eastern” oyster are 

apparently unreliable to define species distributions within the morphologically plastic and 

environmentally adaptive Crassostrea genus. This is especially problematic in the Gulf of 

Mexico and Caribbean (Gaffney Pers. Comm. 2005), which lead to the confusion over the 

species distribution described above. 

Another confounding factor in understanding the recent, i.e., last ~150 yr, historic abundance of 

oysters is the lack of reliable quantitative survey data in many areas. Some current students of 

the species and fishery strongly contest earlier published estimates of abundances and local 

distributions (Kraeuter Pers. Comm. 2005). Kraeuter suspects that these estimates or area 

coverage of specific oyster beds or reefs might be overestimates by 80-90% in some areas. 
14

These early surveys were focused on harvestable beds and often included natural, cultured or 

transplanted beds which may not have been well differentiated. Since the 1830s, the 

transplanting and movement of oysters for better growth has confounded understanding the 

natural abundance and local distribution of the species in many estuaries. 

Abundance of the eastern oyster is known to have varied or declined in many estuaries in which 

it was previously known to be abundant. In some estuaries, abundance has declined due to one 

or more of the threats discussed below. Some populations have declined to the degree that they 

are defined as “ecologically extinct” no longer acting as a keystone species and providing 

ecosystem services to the estuarine ecosystem (e.g., the Hudson-Raritan Estuary). However, 

even in these locations, with effort, oysters can be found. The oyster can be found as isolated 

individuals or clusters even in unlikely urbanized places, such as the Hackensack River, Arthur 

Kill, Harlem River, East River and the Bronx River (Steimle 2005). However, these isolated 

survivors may currently exist at the thinnest of margins even though habitat quality has 

measurably improved and is currently suitable for good growth, as evidenced by oyster culturist 

results in this estuary complex. 

The persistence of oysters in isolated areas at low abundance for perhaps decades, is not 

uncommon. Some local populations are now too widely dispersed to support enough successful 

spawning-fertilization and recruitment for natural repopulation (Pers. Comm. Luckenbach 2005). 

The low abundance situation of the Hudson-Raritan area may exist in other urbanized estuaries 

where oyster population surveys have not been done for decades. Some shellfish surveys were 

conducted without proper oyster sampling gear and focus because the oyster was not considered 

part of a useful or manageable fishery resource any more. Also, local management agencies may 

not want the fact that oysters still exist in areas to be generally known to avoid potential public 

health consequences because of bacterially contaminated water. 

The notable decline of the oyster abundance distributions from estimated historic abundance 

distribution levels seems to be most prevalent in the more urbanized northeast, e.g., Chesapeake 

Bay, the Hudson-Raritan Estuary, southern Long Island NY, and some New England estuaries. 

However, most of the data to document this decline comes from fishery-dependent sources, 

which is somewhat controlled by socio-economic, not ecological, factors (MacKenzie 1996). 

This information base may not present an accurate picture of the abundance and status of oyster 

populations in many areas. The oyster distribution abundances south of Chesapeake Bay seem 

relatively stable, despite occasional major disturbances, such as hurricanes, based upon 

numerous southern Atlantic/Gulf Coast state reports presented during and after the congressional 

hearing on this issue (Marsh 2004; Perret 2005). 

Biological (non-fishery resource focused) surveys have not yet been conducted with appropriate 

sampling gear and at appropriate levels of intensity according to a statistically reliable survey 

designs in areas where oysters previously existed or might still exist at reduced levels. However, 

methods and gear are available to do so (Chai 1992; Jordan et al. 2002). Due to the lack of 

surveys, we may not have an adequate understanding if the oyster has been extirpated from any 

ecologically significant areas. 
15

3.5 Population Genetic Structure 

Overview of Genetic Markers

There is a significant amount of literature on the use of genetic data to delineate taxa at various 

levels such as species, subspecies, and populations. Genetic data are also commonly used to 

understand a taxon’s history in relation to geography, in a field of study known as 

phylogeography. As genetic techniques and markers continue to be perfected and additional 

ones are developed, these genetic tools are increasingly being applied to practical issues 

associated with the conservation of biological diversity. This section provides a basic overview 

of some of the most common types of genetic markers used today, the forces that operate on the 

genome to determine patterns of genetic differentiation, the general classes of data obtained from 

genetic markers, and the nature of inferences that can be drawn from these data. 

Several different types of commonly used genetic markers are: 

• Allozymes are products of alternative alleles at a locus (site on the DNA sequence) 

encoding for a specific enzyme. Because allozymes are functional proteins, they can be 

subject to selection. 

• Nuclear DNA (nDNA) is located in the cell nucleus. It is inherited from both parents 

with recombination. 

• Mitochondrial DNA (mtDNA) is located in the cell mitochondria. It is inherited as a 

single, non-recombinant genetic unit from the mother. Because of its clonal inheritance, 

mtDNA sequences can be used to trace maternal lineages across generations. 

• Microsatellites are tandem repeats of 2-10 base pair nucleotide sequences. These 

sequences are often highly variable in the numbers of repeats they contain, and they are 

usually non-encoding. 

• Restriction fragment length polymorphism (RFLP) is a method that examines genetic 

variation in lengths of DNA, either mitochondrial or nuclear, that result when the DNA is 

cleaved by restriction enzymes. 

• Denaturing Gradient Gel Electrophoresis (DGGE) and Single-Strand Conformational 

Polymorphism (SSCP) are methods that detect variations in genetic sequences. 

• Single Nucleotide Polymorphism (SNP) is a type of DNA sequence variation in which a 

single nucleotide (A, T, C, G) at one sequence position is altered. 

The forces of genetic drift, gene flow, mutation, selection, and geographic history continuously 

act upon genomes. The effect of these forces on a particular genetic marker must be carefully 

considered in the interpretation of genetic data. 

• Genetic drift is a passive process whereby allele frequencies change over time due to 

chance sampling events in the population. Populations with a large effective population 

size (Ne) exhibit slower rates of genetic drift than populations with small Ne. Also, 

mtDNA drifts more rapidly than nDNA because the effective population size of mtDNA 

is smaller by virtue of its maternal-only inheritance. 

• Gene flow is essentially “effective” migration (m), and is dependent upon the number of 

successful (i.e., reproducing) migrants moving between populations each generation. 

Gene flow is estimated as Nem. Genetic differentiation of populations is highly sensitive 

to gene flow. 
16

• Mutation occurs at different rates in different portions of the genome. At one end of the 

spectrum, genetic markers with extremely high mutation rates can become saturated with 

change upon change over time. At the other extreme, parts of the genome with extremely 

low mutation rates are evolutionarily conservative and show little to no differences 

among even distantly related taxa. 

• Selection can act directly on a particular genetic marker, or on genetically linked loci. A 

strategy for ruling out the effects of selection upon a particular genetic marker is to 

examine multiple, independent (unlinked) loci. 

• Geographic history involves the changes that result in the formation of geographic 

barriers or connections among populations. 

The type of information provided by a genetic marker depends upon whether it provides 

frequency data or identity (sequence) data. Frequency data are characteristic of populations, not 

individuals. For example, individuals have alleles and populations have allele frequencies. 

Inferences from frequency data require several assumptions, including selective neutrality of the 

marker and equilibrium between mutation, gene flow, and drift. Frequency data are obtained 

from allozymes, microsatellites, and RFLPs. Identity or sequence data result from mtDNA or 

nDNA sequences, and are characteristic of individuals. Sequence data can be used to infer 

phylogenetic relationships among sequences, resulting in gene genealogies or “family trees.” 

Also, sequence data can often provide insight into population structure not available from 

frequency data. For example, sequence data are often better than frequency data for teasing out 

the effects of geographic history. 

The interpretation of genetic data is a rich field of study that continues to advance rapidly and 

cannot be summarized simply. As more genetic markers are developed and examined, our 

ability to unravel the origins and relationships among taxa increases in power and resolution. 

Along with these advances come challenges, one of which is to understand what it means when 

genetic markers applied to the same populations yield conflicting results. Such conflict does not 

imply that the genetic markers are flawed or unreliable. Nor does it mean that one of them is 

“right” and the other is “wrong.” What it means is that they each have their own interpretation, 

each marker has been uniquely influenced by past experiences (i.e., the forces of genetic drift, 

gene flow, mutation, selection, and history), and some interpretations are just more detailed than 

others. As a rule, a finding of genetic differentiation is a strong conclusion, even if the reason 

for the differentiation is unclear. In contrast, a lack of differentiation is an ambiguous result 

which could be due to any one of several quite distinct factors (e.g., balancing selection, 

continued gene flow, no gene flow but insufficient time since separation for differentiation to 

occur). 

Application of Genetic Markers to C. virginica

Based on the previous section, it is understandable that some populations that appear identical or 

homogenous on the basis of one genetic marker may later be found to be sharply differentiated 

by the application of another genetic marker. This is the case for C. virginica which has an 

extended pelagic larval stage that spans several weeks, and therefore, dispersal along the eastern 

coast was initially assumed to be extensive. In addition to a long pelagic stage, movement of 

oysters within their range and beyond was a common practice from as far back as the 1800’s. At 

that time, transfers were made from the waters of the Chesapeake Bay to New England and 
17

eventually from the Carolinas to the Chesapeake once local populations diminished. In the 

1960’s, stocks from the Gulf of Mexico were brought into the Chesapeake to replenish local 

populations that were declining due to MSX (Carlton & Mann 1996). With such high levels of 

human-mediated movement, it seemed likely that high levels of gene flow would be exhibited all 

along the range of C. virginica. 

As expected from the assumption of high gene flow, populations of C. virginica were initially 

found to be homogenous in allozyme frequencies across a large portion of the species range. An 

early allozyme study by Buroker (1983) provided evidence of a uniform population from Cape 

Cod to Corpus Christi using 32 allozyme loci, which exhibited genetic similarities among 

populations estimated to be 99% (Figure 3,4, & 5). However, allozymes can be less variable 

than other types of genetic markers (e.g., DNA sequence data) because allozymes represent 

variations in the amino acid sequence comprising a protein. Variation at the amino acid 

sequence level may mask underlying genetic variability at the DNA sequence level. Proteins 

usually have important biochemical functions so they could be strongly constrained by natural 

selection, acting to preserve protein functionality. Furthermore, a later re-analysis of Buroker’s 

allozyme dataset by Cunningham and Collins (1994) revealed genetic structure not detected in 

the original analysis. 

With time, more genetic studies were undertaken to better understand the population structure of 

C. virginica. These subsequent studies found strong patterns of genetic subdivision, with 

congruence among several different genetic markers indicating two separate populations: an 

Atlantic population and a Gulf of Mexico population with an intermediate zone between these 

populations located on the eastern coast of Florida. 

Atlantic and Gulf Populations

RFLP studies of the entire mitochondrial DNA (mtDNA) of oysters from the Gulf of St. 

Lawrence to Brownsville, TX (Figure 2,4, & 5) by Reeb and Avise (1990) found two distinct 

(Atlantic and Gulf) genetic groups (genetic divergence level p = 0.026). West Palm Beach, 

Florida appeared to be a transition zone between the two populations with haplotypes from both 

groups found at intermediate frequencies. Based on the data, Reeb and Avise (1990) estimated 

that separation of the two populations occurred 1.2 million years ago. RFLP studies were also 

undertaken by Karl and Avise (1992) on single copy nuclear DNA (scnDNA) and found a 

similar genetic pattern with two distinct populations (Gulf and Atlantic) and an intermediate 

zone on the eastern coast of Florida (Stuart, FL- about 40 miles north of West Palm Beach) 

(Figure 3, 4, & 5). 

Discrepancies between mtDNA RFLP data (Reeb & Avise 1990) and allozyme data (Buroker 

1983) could be due to directional selection in the mitochondrial haplotypes observed or the 

smaller effective population size for mtDNA (as they are maternally inherited) which creates a 

faster rate of genetic differentiation (Karl and Avise 1992). Either hypothesis could be true if it 

were not for the nuclear RFLP data of Karl and Avise (1992) also running counter to the 

allozyme data. More likely balanced selection of the allozyme loci assayed or slower rates of 

evolution at the protein level may explain these discrepancies (Karl and Avise 1992). 

Subsequent reanalysis of Buroker’s data by Cunningham and Collins (1994) found geographic 

structure within the allozyme dataset. However, that structure shows a peninsular Florida group 
18

clustering with an Atlantic group, which is incongruent with geographic population boundaries 

found in nuclear and mitochondrial RFLP studies. 

Small and Chapman (1997) also looked at mtDNA to determine population structure of C. 

virginica but focused exclusively on RFLP analysis of the amplified portion of the mitochondrial 

16s ribosomal gene. No population structure was found between oysters from Pamlico Sound, 

NC; Chesapeake Bay; and Galveston Bay, TX (Figure 3 & 5) which may be indicative of slower 

evolution rates at the 16s locus opposed to the rest of the C. virginica mitochondrial genome. 

Milbury et al. (2004; Figure 3) also looked at the mitochondrial 16s ribosomal gene but used 

Single Nucleotide Polymorphism (SNP) analysis to successfully differentiate between Gulf 

oysters planted in the Chesapeake Bay from local Bay oysters. The discrepancy between the two 

16s studies may be due to a higher level of conservation around certain restriction enzyme sites 

on this particular locus opposed to SNPs which may not be as conserved. Wakefield and 

Gaffney (1996) found enough sequence variation at the 16s locus by denaturing gradient gel 

electrophoresis (DGGE) and direct sequencing to reveal three haplotypes that corresponded to 

the Gulf of Mexico, the South Atlantic, and the North Atlantic (Figure 3, 4, & 5). 

Hare and Avise (1996) amplified the mitochondrial restriction site polymorphisms (RSP) found 

in Reeb and Avise (1990) for BstNI and found a pronounced genetic cline along the eastern coast 

of Florida. Amplification of RSPs from the scnDNA loci from Karl and Avise (1992) also found 

a similar genetic pattern with the frequency of all Atlantic alleles (nuclear as well as 

mitochondrial) dropping 50-75% over the distance of 20 km between Oak Hill and Merritt Island 

(near the vicinity of Cape Canaveral) (Figure 3, 4, & 5). Hoover and Gaffney (2005) amplified 

four nonanonymous nuclear loci from individual samples from Prince Edward Island to Tabasco, 

Mexico (Figure 2, 4, & 5) for RFLP analysis in order to determine population structure. 

Multilocus genetic analysis indicated a similar population structure as that found by Reeb and 

Avise (1990). 

Although these studies indicate an Atlantic/Gulf population structure, other studies have agreed 

with Buroker’s conclusion of a panmictic population. MacDonald et al. (1996) found a lack of 

genetic structure among six anonymous nuclear DNA loci from oysters in Panacea, FL and 

Charleston, SC (Figure 3, 4, & 5). These results could be considered questionable as only two 

populations were used and several markers exhibited deviation from Hardy-Weinberg 

equilibrium. In 1998, Hare and Avise looked at oysters from Massachusetts to Louisiana (Figure 

3, 4, & 5) and found no population structure at three nuclear loci (CV-23, CV-myc, and CV-32 

(also used by and Karl and Avise, 1992)). Heterozygotes were identified by Single-Strand 

Conformational Polymorphism (SSCP) and subsequently sequenced and analyzed. Although 

Hare and Avise (1998) and Karl and Avise (1992) used the same locus (CV-32) and similar 

sampling locations, the two studies came to different conclusions by different techniques. On the 

other hand, Hare and Avise (1996) used a modified version of CV-32 (CV-32.4) and came to a 

similar conclusion as Karl and Avise (1992) of two separate Atlantic and Gulf populations. 

At this time the available genetic data on Crassostrea virginica indicate the existence of separate 

Atlantic and Gulf populations with a transition zone along the eastern coast of Florida (see Table 

1). Although scientists have stopped short of defining these populations as subspecies in the 

published literature, the data suggest that C. virginica may be in the process of incipient 
19

speciation between Atlantic and Gulf populations. 

Crassostrea virginica is not the only western Atlantic species with a notable genetic transition 

from the temperate Atlantic to subtropical Gulf regions. Similar genetic patterns of population 

subdivision between Atlantic and Gulf populations can be found in a wide variety of coastal and 

marine species (Avise 1992; 2000). Some examples are listed in Table 2. 

Table 1. Examples of species exhibiting population genetic structure between Atlantic and Gulf 

populations. 

Species Type Reference

Atlantic croaker Finfish Lankford et al. 1999 

Red drum Finfish Seyoum et al. 1999 

Hermit crab Crustacean Young et al. 2002 

Southern flounder Finfish Blandon et al. 2001 

King mackerel Finfish Gold et al. 1997 

Snapping shrimp Crustacean McClure and Greenbaum 1999 

Tilefish Finfish Katz et al. 1983 

Black sea bass Finfish Bowen and Avise 1990 

Toadfish Finfish Avise et al. 1987 

Horseshoe crab Crustacean Saunders et al. 1986 

Seaside sparrow Bird Avise and Nelson 1989 
20

Table 2. Genetic studies of population structure in Gulf and Atlantic populations of Crassostrea 

virginica. 

Reference Genetic Marker Population 

Structure 

Comments 

Buroker 1983 allozymes 

(32 loci) 

None Allozymes can 

mask underlying 

genetic variability, 

and are often subject 

to natural selection. 

Reeb & Avise 

1990 

mtDNA RFLP Atlantic-Gulf Transition zone near 

West Palm Beach, 

FL. 

Karl & Avise 

1992 

scnDNA RFLP Atlantic-Gulf Transition zone near 

Stuart, FL. 

Cunningham & 

Collins 1994 

reanalysis of Buroker 

(1983) allozyme data 

Atlantic/FloridaGulf 

McDonald et al. 

1996 

nDNA RFLP 

(6 anonymous loci) 

None Deviations from 

H-W equilibrium. 

Only 2 sample sites. 

Wakefield & 

Gaffney 1996 

mtDNA DGGE and 

direct sequencing 

(16s ribosomal locus) 

3 haplotypes: 

Gulf, 

South Atlantic, 

North Atlantic 

Hare & Avise 

1996 

mtDNA RSP 

scnDNA RSP 

Pronounced 

genetic transition 

zone along eastern 

coast of FL. 

Major change in 

both nuclear and 

mitochondrial allele 

frequencies around 

Cape Canaveral, FL.

Small & 

Chapman 1997 

mtDNA RFLP 

(16s ribosomal locus) 

None Possible slower rate 

of evolution at 16s 

mtDNA locus. 

Only 3 sample sites. 

Hare & Avise 

1998 

nDNA SSCP and direct 

sequencing 

(3 loci) 

Populations not 

reciprocally 

monophyletic 

between Atlantic 

and Gulf 

Milbury et al. 

2004 

mtDNA SNP 

(16s ribosomal locus) 

Discerned 

transplanted Gulf 

oysters vs. local 

Chesapeake Bay 

oysters 

Hoover & 

Gaffney 2005 

nDNA RFLP 

(4 non-anonymous loci) 

Atlantic-Gulf Transition zones in 

eastern and 

northwest FL. 
21

 Laguna Madre Population

A genetically distinct population was found in the Laguna Madre area of Texas by different 

studies that have included samples from this general area. Groue and Lester (1982) looked at 

genetic and morphological variation among Eastern oysters in the Gulf of Mexico (Biloxi Bay, 

Mississippi; West Bay, Texas (near Galveston); Drum Bay, Texas; Aransas Bay, Texas (near 

Corpus Christi); lower Laguna Madre, Texas) (Figure 5) and found allozyme frequency 

differences between the lower Laguna Madre and the rest of the sampling sites at the PGI, 4MU, 

and GOT loci. The morphological data, on the other hand, found Laguna Madre and Aransas 

Bay to be similar while all other combinations of sampling sites to be significantly different 

when comparing height, length, weight, and length/height index. Morphological variation was 

believed to be due to environmental influences rather than genetic influences. 

In 1983 Buroker found significantly different allele frequencies at several allozyme loci (Lap-2, 

Mdh-1, and Pgi) between populations at Brownsville and Corpus Christi, TX. Hedgecock and 

Okazaki (1984) also found a significant difference in allele frequencies at several allozyme loci 

(Lap-1, Lap-2, Mdh-1, Pgi, Tpi, Xdh, and complete divergence at Aat-2) between samples from 

Campeche, Mexico and Turkey Bayou, FL (near Panacea, FL) (Figure 4&5). King et al. (1994) 

looked at 16 enzyme systems and two structural proteins in nine oyster populations along the 

Gulf coast of Texas from East Matagorda Bay to South Bay (Figure 5) and found the Laguna 

Madre area to be genetically distinct. An intermediate zone was also found between reefs in 

Northern Corpus Christi Bay and the Upper Laguna Madre (an area of 26 km). Genetic 

differentiation of the Laguna Madre eastern oyster population may be due to adaptation to 

hypersaline conditions (up to 35 ppt) created by low levels of precipitation and lack of river 

inflow as well as selection or genetic drift due to isolation from oyster populations further north 

(King et al. 1994). 

Physiological or Morphological Evidence of C. virginica Subspecies

Species and subspecies are often recognized by morphological differences between populations. 

Due to extreme morphological plasticity, Crassostrea virginica has not yet been examined with 

the goal of identifying such differences. Although in 1951, Loosanoff and Nomejko recognized 

the existence of physiological races along the latitudinal range of C. virginica. Since that time 

most physiological differences have been found to be related to differences in environmental 

conditions. Whether additional physiological or morphological studies would be informative is 

questionable, as any differences between Gulf and Atlantic populations are more likely to be due 

to local environmental conditions rather than genetic differences (Gaffney 1996). 
22

Figure 2. Sampling sites in Canada: Gulf of St. Lawrence (Reeb & Avise 1990), Prince 

Edward Island (Hoover & Gaffney 2005). 
23

Figure 3. Sampling sites along the Atlantic Coast: Piscataqua River, MA (Hoover & 

Gaffney 2005), Cape Cod, MA (Buroker 1983), Woods Hole, MA (Karl & Avise 1992; 

Hare & Avise 1996; Hare and Avise 1998), Narragansett Bay, RI (Reeb & Avise 1990), 

Mullica River, NJ (Hoover & Gaffney 2005), Lewes, DE (Hoover & Gaffney 2005), 

Chesapeake Bay (Small & Chapman 1997; Milbury et al. 2004), Wachapreague, VA 

(Hoover & Gaffney 2005), Pamlico Sound, NC (Small & Chapman 1997), Cape Fear, 

NC ( Hoover & Gaffney 2005), Georgetown, SC (Hoover & Gaffney 2005), Charleston, 

SC (Karl & Avise 1992; MacDonald et al. 1996; Hare & Avise 1998), Sapelo Island, GA 

(Reeb & Avise 1990), Cumberland Island, GA ( Karl & Avise 1992; Hare & Avise 

1996), Skidway Island, GA (Reeb & Avise 1996). 
24

Figure 4. Sampling sites in Florida: Fernandina (Hoover & Gaffney 2005), Jacksonville 

(Reeb & Avise 1990), St. Augustine (Hare & Avise 1996), Oak Hill (Hare & Avise 

1996), Merritt Island (Hare & Avise 1996), New Smyrna (Karl & Avise 1992; Hare & 

Avise 1996), Cape Canaveral (Buroker 1983), Stuart (Karl & Avise 1992), Indian River 

(Hoover & Gaffney 2005), Sebastian (Hare & Avise 1996), Vero Beach (Hare & Avise 

1996), Fort Pierce (Hare & Avise 1996), Jupiter (Hare & Avise 1996), West Palm Beach 

(Reeb & Avise 1990), Boynton Beach (Hare & Avise 1996), Palm Beach (Hare & Avise 

1996), Deerfield Beach (Hare & Avise 1996), Pompano Beach (Hare & Avise 1996), 

Miami (Hare & Avise 1996), Cedar Key (Hoover & Gaffney 2005), Panacea (Hedgecock 

& Okazaki 1984; Karl & Avise 1992; MacDonald et al. 1996; Hare & Avise 1998), St. 

Petersburg (Reeb & Avise 1990), Port Charlotte (Karl & Avise 1992). 
25

Figure 5. Sampling sites in the Gulf of Mexico: Cedar Key, FL (Hoover & Gaffney 

2005), Panacea, FL (MacDonald et al. 1996), Apalachicola, FL (Hoover & Gaffney 

2005), Panama City, FL (Reeb & Avise 1990), Biloxi, MS (Groue & Lester 1982), Grand 

Isle, LA (Reeb & Avise 1990; Karl & Avise 1992; Hare & Avise 1996; Hare & Avise 

1996, 1998; Hoover & Gaffney 2005), Galveston, TX (Small & Chapman 1997), 

Matagorda, TX (Hedgecock & Okazaki 1984; King et al. 1994), Corpus Christi, TX 

(Buroker 1983), Laguna Madre, TX (Groue & Lester 1982; Hedgecock & Okazaki 1984; 

King et al. 1994), Brownsville, TX (Buroker 1983; Reeb & Avise 1990), Tabasco, MX 

(Hoover & Gaffney 2005), Campeche, MX (Hedgecock & Okazaki 1984). 
26

4 Analysis of the ESA’s Five Factors

4.1 Habitat Threats

Habitat degradation/loss

Most environmental or habitat alteration problems affecting fishery resources and their habitats 

are related to socio-economic issues, including many situations that are sometimes classified as 

“natural”, and inadequate managerial attention to these issues in certain areas, or regionally, may 

be considered an overriding threat to oyster and estuarine health. It is interesting that many early 

students of the eastern oyster had little to say about the habitat of oysters and its management, 

although they discussed environmental factors affecting the species (Galtsoff 1964; Coen et al. 

1999). 

The majority of the contaminant or habitat degradation effects noted below are usually localized 

and may not affect regional oyster metapopulations, except perhaps in some major urbanized 

estuaries. Cumulative impacts of several stressors on oysters are difficult to assess, not only at 

the metapopulation level, but also at local population levels because of variability in spawning 

success and planktonic larvae dispersal patterns, and temporal and spatial variability of some of 

the threats. Many or most of these threats, where recognized as more than de minimus, are 

currently under some management effort, but many threats listed below are interrelated with 

other threats (Mann 2000). 

Below is a list and brief summary of most known causes of oyster habitat degradation or loss, not 

in any priority order: 

Fishery-caused 

Source factorsPast unwise harvesting and use practices on public and leased beds, including neglect 

(Rothschild et al. 1994; Hargis and Haven 1999; MacKenzie 1996; Dugas et al. 1997; Lenihan 

and Peterson 1998). 

 Effects- 

-Early oyster dredging in some areas reduced the vertical reef structure and later culture methods 

kept the beds low to facilitate efficient harvesting- this reduced some of the ecological benefits 

of the reef structure to the oyster and associated species (Rothschild et al. 1994; Hargis and 

Haven 1999). 

-Non-replacement of shucked-shell cultch reduced the hard surface and reef structure that 

benefits sustainable oyster populations (MacKenzie 1996; MacKenzie and Wakida-Kusunoki 

1997). 

-Loss of a functionally interconnected system of oyster reefs (functional habitat fragmentation) 

by depleting or transplanting some smaller oyster reefs/beds to enhance others, or to clear 
27

navigational hazards for vessels, has weakened or eliminated the interdependence of spawning 

oyster populations on a system of functionally connected reefs that capitalize upon variable 

environmental conditions during the spawning season (Eggleston 1999; Whitlatch and Osman 

1999). A system of functionally connected reefs can also help control sediment resuspension 

(Rothschild et al. 1994; Mann 2000), another threat. 

-Harvesting natural seed beds without re-shelling reduced the function of these beds (Rothschild 

et al. 1994). 

-Mariculture or fisheries for other species in or near the oyster reefs can compete for habitat 

space and other critical resources (Burrell 1997), or contribute inhibitory waste products 

(biological oxygen demand) (Serve et al. 1999). 

-Closure of some oyster beds because of contamination can place excessive harvesting pressure 

upon remaining open beds and the habitat they provide. 

Changes in freshwater, nutrients, organic material, silt, toxic substances, etc. runoff inputs 

 Source factorsPoint and non-point inputs from human activities within watersheds and water bodies degrade 

water and sediment quality (Galtsoff 1964; Dugas et al. 1997; NRC 2000; Williamson and 

Morrisey 2000), and inadequate enforcement of total maximum daily loads (TMDLs) by 

responsible federal and states agencies in certain estuaries that support oyster populations is 

considered a major threat in these estuaries. 

 Effects- 

-Eutrophication, stimulated by inadequately controlled nutrient inputs, supports excessive 

phytoplankton biomass blooms, which contribute to the development of hypoxic/anoxic 

conditions (see next). Excessive eutrophication may have a negative effect in some estuaries, 

while light to moderate inputs of nutrients may enhance primary and oyster productivity where 

biological potential is limited (Kirby and Miller 2005). 

-Eutrophication can support increased incidences of toxic or harmful algal blooms, “red or brown 

tides” that kill or inhibit oyster survival or growth at all stages of their life cycle (Loosanoff 

1964), and dense inhibitory macro-algae (e.g., sea lettuce, Ulva) coverage can smother shallow 

oyster beds (Galtsoff 1964). 

-Eutrophic populations of plankton can increase the abundance of planktivores, such as 

coelenterates and ctenophores, which can prey upon oyster larvae (MacKenzie 1977). 

-Eutrophication can cause changes in phytoplankton community composition, e.g., increase the 

abundance of species that are too small or large to be effectively filtered and retained (non-toxic 

picoplankton blooms) and reduce the abundance of taxa most useful as food to oysters 

(Loosanoff 1964). 
28

-Hypoxia/anoxia has become more common in the deeper parts of many estuaries and is linked 

to eutrophication (Hagy et al. 2004) (Funderburk et al. 1991; Dugas et al. 1997; Lenihan and 

Petersen 1998). Even short periods of hypoxia/anoxia can be stressful or lethal to oysters, 

especially spat or seed, because they will not have the energy reserves to remain closed for too 

long. 

-Waste disposal, including treated and untreated sewage and cooling waters affect the water 

quality where oyster were or are found and involve eutrophication and toxic chemicals 

(MacKenzie 1996; Dugas et al. 1997). 

-Silt, from a variety of sources including upstream land use (Ulanowicz and Tuttle 1992), can 

cover and “smother” shell, beds and other oyster-suitable substrate inhibiting oyster abundance 

(Galtsoff 1964; MacKenzie 1996). This “muck” can also cause a concurrent shift of the benthic 

community from filter-feeding to deposit-feeding species/mode, which in turn, can contribute to 

the siltation problem by creating silt retaining beds, i.e., macro-infaunal tube fields, or recycling 

buried silt to the surface as erodable feces (Serve et al. 1999). Oyster eggs and larvae are most 

sensitive to suspended sediment (Davis and Hidu 1969). 

- A variety of toxic waste inputs (chemicals, oil, etc...) critically affect oyster larvae and later 

life stage health and survival (Galtsoff 1964; Calabrese et al. 1973; Funderburk et al. 1991; 

Roesijadi 1996; Capuzzo 1996; Dugas et. al. 1997; MacKenzie and Wakida-Kusunoki 1997; 

Dauer et al. 2000; Wintermyer and Cooper 2003), including possibly acting as endocrine 

disruptors (Nice et al. 2003). 

-Increases in shellfish bed closures because of human disease risks cause a loss of maintenance 

attention to beds within those closures, allowing silt to accumulate or destructive alternate uses 

of the beds to happen, e.g., dredged for shell for non-mariculture purposes (Stiles 1911; 

MacKenzie 1996, 1997; Dugas et al. 1997). 

-Synergistic effects of multiple factors, which are not individually harmful, can become harmful 

by accumulatively exceeding physiological stress thresholds of some life stages of the oyster 

(Galtsoff 1964; Shumway 1996). 

Coastal development 

 Source factorsIncreased use of coastal areas for residential, commercial, and recreational purposes 

(urbanization) alters the character of the land and its watersheds, and affects down-stream or 

adjacent estuaries that have supported oyster populations. United States coastal counties 

comprise 17% of land area, but are inhabited by 53% of the population; by the year 2008, the 

coastal county population is expected to increase by approximately 7 million (Crossett et al. 

2004). Urbanization competes with the property fishermen need to oversee, maintain, and use 

oyster stocks. This issue is almost ubiquitous in all coastal areas, or could be, and its potential 

multiple effects on oysters and their habitat can vary greatly among areas, even though specific 

cause and effect relations are poorly documented. 
29

 Effects- 

- A variety of coastal development factors cause the reduction in submerged aquatic vegetation 

(SAV) beds and other habitats (wetlands and ecologically serviceable watershed habitats) that 

may be linked to oyster populations via total ecosystem health considerations. 

-Changes in runoff inputs noted above, including more impervious surfaces and storm drains that 

pour excessive or contaminated waters directly into estuaries, can alter the hydrography and 

quality of aquatic habitats used by oysters (Burrell 1997; Dugas et al. 1997). There may be a 

complex relationship between temperature and salinity and their affects on oyster spat set, 

survival and growth that may reflect genetic or environmental adaptations of parent stock. 

Subtle changes in the mix of temperature and salinity, even to a long evolved estuarine species, 

may be important especially if a local stock has adapted to a specific natural cycle. 

-Use of dredged or shucked shell stock for land-based construction purposes, e.g., lime, 

driveways, roadbeds, etc. depletes the supplies of clean cultch that sustain oyster populations 

(MacKenzie 1997b). 

-Loss of shore line vegetated buffer zones, e.g., marsh grass and mangroves, can increase runoff 

and erosion, as well as bring lawn/garden chemicals in closer direct contact with the estuary and 

its oysters. 

-Increased recreational boating traffic (Burrell 1997) increases turbidity, shoreline erosion, and 

damages SAV beds as well as creates a demand for the removal or dredging of oyster shoals to 

reduce navigation hazards (Kelty and Bliven 2003). 

-Increased local atmospheric pollution, such as acid rain and associated nitrogen inputs, from 

coastal development’s “air sheds”, (e.g. vehicle exhaust, emissions from new pulp mills, or other 

industries involved in coastal development) can be deposited onto estuarine water bodies 

(Cooper and Brush 1991; Paerl and Whithall 1999). 

-Increased demands on natural resources, such as underground or upstream water supplies, can 

alter the hydrographic character of the estuary by diminishing freshwater inputs that help control 

certain predators, parasites, and diseases (Burrell 1997). 

-Loss of shoreline forests (MacKenzie 1997b), besides directly altering watershed hydrogeodynamics, can alter wind exposure in smaller estuarine tributaries that can affect the winddriven component of intertidal and subtidal oyster larvae dispersal processes. 

-Because coastal and regional development usually requires increases in electrical power 

supplies, power generating plants are built that use estuarine waters for cooling. Entrainment of 

oyster larvae into the cooling waters of power plants in some estuarine areas can reduce larval 

stock density from local areas and impact spat recruitment. Super heated cooling water may 

impair un-entrained oyster larvae near the outfall, but some warmer effluent from power plants 

in northern waters may alternately enhance larval recruitment if they recycle >20oC, chemically 
30

untreated, cooling water back into areas containing spawning oysters, if ambient temperatures 

are notably below this level during the extended oyster spawning season and phytoplankton food 

sources are not degraded in the effluent fields. 

- In some areas, the recent installation of treated wood pilings, bulkheads, or docks, can 

introduce metals, such as chromium, copper, and arsenic (CCA), at levels which can be 

temporarily toxic to oysters (Weis et al. 1995). If this use occurs to support boating, then other 

toxic metals can also be released from antifouling paints, such as tributyl tin, used on these boats. 

Habitat user conflicts

 Source factorsEstuaries are increasingly being used by people. This has put the maintenance or restoration of 

oyster reefs or beds in conflict with other existing or proposed uses. 

 Effects- 

- Dredging channels and harbors can increase salinity, tidal amplitude, suspended 

solids/turbidity, or change currents or flow patterns. Dredged sediments may also be deposited 

directly on reefs. These changes can adversely affect oysters (MacKenzie 1996; Dugas et al. 

1997). 

-Channel dredging to accommodate nearshore access to docks on newly developed coastal land 

also can change the hydrographic characteristics of parts of estuaries. This may influence the 

settlement density of oyster spat. The use of these new channels by vessels can increase shoreline 

erosion and also suspended solids burdens (Dugas et al. 1997). 

-Navigation of larger vessels, such as tugs and barges in nearshore environments, may impact 

reefs by covering settlement surfaces with sediment or by direct contact from the vessel hull or 

propeller. 

-Shell mining for non-mariculture purposes depletes potential shell stock for oyster reef/bed 

sustainability and fossil shell that can be recycled as cultch, and leave large holes on the bottom 

of estuaries (MacKenzie 1996). 

-Freshwater dams and diversions to support coastal and other land development reduces or 

minimizes natural seasonal variability in fresh water inputs alter the habitat for oysters. 

-Construction of breakwaters or other non-porous man-made structures within an estuary or 

filling in shorelines with roadways, bulkheads and dikes can alter hydro-geophysical processes, 

e.g., restrict tidal flows, involving salinity and oyster larval distributions, and silt deposition 

budgets in some areas. 

-Agricultural practices, including soil and chemical-application management, still affect some 

estuaries presently or recently containing oyster populations. 
31

-Deforestation and lumbering in some areas has altered the hydro-geophysical processes within 

watersheds supplying down-stream estuaries and their oyster populations (MacKenzie 1997b). 

-Impacts of other fisheries (e.g., clam dredging, shrimp trawling, etc.) may include the resuspension and mobilization of silt if these towed gear are used near oyster beds, and if this silt 

reaches the oyster beds, it can be excessive and damaging to the persistence of the oyster 

populations on those beds (Dugas et al. 1997). 

 

Natural environmental factors 

 Source factorsThis involves any number of natural environmental, biological and climatic factors that are 

primarily beyond the control of man, but when they interact with oyster populations on an eventor gradual trend-basis can influence habitat suitability for oysters, at least temporarily, (e.g., 

climatic change or variation, severe weather events, change in competitive species distributions). 

 Effects- 

-Increases in temperature, e.g., via global warming, can change the distributions of oysters, their 

predators, competitors and associated diseases especially at extreme distribution or tolerance 

limits (See Section 4.3). 

-Climate change, most likely, will affect all coastal ecosystems in the future. Oyster populations 

are, therefore, expected to be subject to fluctuations and perturbations as a result of climate 

change and sea level rise. 

-Changes in the distribution of competitive species (non-exotics), often associated with changes 

in environmental conditions, or reflecting natural pulses of high abundance of the competing 

species for unknown reasons can negatively affect oyster reefs/beds (MacKenzie 1981; White 

and Wilson 1996), e.g., the hooked mussel, Ischadium recurvum. 

- Unintentional introductions of exotic species can alter or compete for the same or similar 

suitable habitat {e.g., smothering tunicates (e.g., Didemnum sp.)} or other ecological resources; 

or they may act as predators, (e.g., veined rapa whelk, Rapana venosa) (Mann and Hardy 2003). 

This may be a greater problem in the future, if introductions of alien estuarine species into areas 

where the oyster occur continue at the present or accelerated rate. 

-Severe weather events, such as hurricanes, can cover oyster beds with silt, wash oysters off 

beds, or impose extended “freshettes” on oyster beds to the degree that they are significantly 

impaired and need restoration (Berrigan 1988, Dugas et al. 1997; Perret et al. 1999;) or they 

become un-economical to restore and are abandoned, e.g. Narragansett Bay RI (MacKenzie 

1996, 1997a) (See Section 4.5). 

-The natural filling and closure of some lagoon inlets, often after a major storm, if not reopened 

can cause lagoons to become hyper/hypo-saline for oysters depending upon the dynamics of 

freshwater inputs to the lagoon (Dugas et al. 1997; MacKenzie and Wakida-Kusunoki 1997). 
32

-Land subsidence, especially along the northern Gulf of Mexico coast, and associated coastal 

erosion can alter habitat suitable for oysters. 

Coastal demographics/social changes

A developing and perhaps over-riding threat to oyster populations and their habitat which 

overlaps several or perhaps all ESA factors is the impact of the destabilization of local fishery 

dependent communities because of gradually or opportunistically changing demographics and 

competition for the resources that these communities depend upon. Dyer and Leard (1994) 

discuss how stable oyster fishery communities in Florida and Louisiana have sustained oyster 

populations and productivity at less annual variance and greater productivity through a 

community ownership concept. They did so better than other Gulf States, who do not have this 

same relationship. The relationship of a coastal community to its oyster resources can be 

extremely important to the sustainability of at least local oyster populations. 

Many communities that rely upon the harvesting of oysters (and other seafood) have developed a 

“folk management” approach based upon decades of trial and error experience passed down 

through generations. They have often developed a useful relationship with regulatory agencies, 

as well. This experience combined with a sense of local community ownership of the resources, 

including leasing, has done well to sustain oyster populations and fisheries for well over a 

century in Florida and Louisiana. More variable and less productive oyster fisheries exist in 

other states without this strong generational community association, according to Dyer and Leard 

(1994). 

At present, there may be new, developing demographic and sociological forces that threaten 

these oyster stewardship communities that can break down the generational continuity in 

participating in the labor intensive and fiscally limited fishery. These forces can be competing 

for resources on land and in the water that these communities (and the sustained oyster 

populations and the habitat they create and support) may depend upon. These forces can be 

enhanced in response to a catastrophic event(s), such as severe hurricane damage, or to gradual 

shifts in social demographics and in the value of the shore to a different demographic group. 

In the past, local oyster dependent communities were able to endure occasional hurricane 

damage as part of the environmental variability they had to accept, in the same way farmers 

accept/adapt to weather. However, today there may be less acceptance of this situation as there 

are other alternate and/or rewarding employment opportunities for people in or outside of these 

communities. Many may not want to continue to invest in the fishery or the fishery dependent 

community after suffering severe damage. The decline of a generational investment in oystering 

and the resource can lead to loss of productive oyster beds and leased grounds maintenance and 

less interest in the long-term sustainability of the fishery. Thus, the oyster populations and 

habitat can suffer as the coastal community gradually turns away from its traditional fishery 

dependence. This may result in resources that support the fishery (e.g., shoreline land and 

dockage) being converted to non-fishery uses such as residential or commercial development. 

This development can exacerbate coastal environmental problems or threats, such as changes in 
33

water flow and increases in siltation and pollution, which are considered threats to oysters (as 

discussed above). 

This change in coastal use can also be accelerated by the current retirement trend in the 

“boomer” generation who are relatively wealthy and are moving to warmer climates and cheaper 

cost-of-living areas, such as in many areas of the south. Many such retirees seek water front 

property and places for their recreational boats, and perhaps better navigation channels for these 

vessels. Storm-damaged fishery communities that are losing generational participation can be 

vulnerable to being sold in tempting real estate deals that reduce the infrastructure needed to 

sustain managed oyster fisheries. 

Many local oyster fishery communities will be faced with the issue of continuing to support their 

heritage fisheries and the type of community that developed around these fisheries. Also, there 

is the potential they will have to accept higher tax rates due to the increase in expensive water 

front residences and expanded service businesses that will follow this influx of outsider home 

owners. This can also result in community stress and destabilization as well as a shift in the 

political attention given to traditional oyster fisheries and the environmental conditions they 

require. Although the oyster population in many areas will survive this change, they may be 

weakened or extirpated in local situations, if stewardship declines and environmental protections 

are not adequate. 

This demographic/social change threat to oysters may be presently greatest in the Gulf, but it has 

occurred and continues in the south and mid-Atlantic regions, as well. 

Many of the habitat threats listed above have likely abated to some degree in many areas, except 

perhaps those of coastal development, through environmental management efforts. The effects 

of some threats may be immeasurable. These threats, combined with the natural variability in 

oyster recruitment in many estuaries, will make it difficult or impossible to attribute specific 

cause and effect relationships of any current diminished oyster distributions with the many 

threats or stressors man has induced, except in controlled experimental situations. 

4.2 Overutilization for Commercial, Recreational, Scientific or Educational Purposes

 Overview of Harvest 

Eastern oysters have been harvested as a food source and their shells have been used as 

construction materials and agricultural products for thousands of years. Warnings that segments 

of the Atlantic coast oyster resource were being over utilized or overfished began early during 

the peak oyster producing years and extended through the 20th century (Winslow 1885, 1889; 

Brooks 1891; Stevenson 1894; Haven et al. 1978; Hargis and Haven 1988). Observation of 

commercial catches provided the primary information on which the overfishing predictions were 

based and included reduction in size class composition, catch per unit effort (CPUE) decreases, 

and spawning stock biomass concerns raised due to local depletion of stocks by intensive harvest 

and later by reduced spatfall (Hargis and Haven 1995). Other authors have shown that excessive 

harvest of oysters on the Atlantic coast followed a pattern that began near northeast population 

centers and moved progressively down the coast. As oyster landings diminished in one area, 
34

oyster fishermen moved southerly to the next oyster area harvesting with more efficient gear than 

locals, taking seed to replenish stocks in their home areas, and allowing local fishermen to learn 

harvest techniques with the new gear. Landing declines continued despite later attempts to 

regulate the harvest (Figure 6) (Kirby 2004; Hargis and Haven 1995; Chestnut 1951). Recent 

investigations in Delaware Bay and the Maryland portion of Chesapeake Bay indicate the 

primary cause of more recent (mid 1990s) declines in oyster abundance was parasite-based 

mortality (Fegley et al. 1996; Homer et al. 1996). 

Consumer demand fueled the efforts to maximize early oyster harvests. Oyster canning 

technology and railroad development during the mid 1800s opened markets for eastern oysters as 

far west as St. Louis and the increased harvests reduced oyster prices lower than those for beef, 

poultry, and fish (MacKenzie 1996). Oysters became a regular part of the American diet during 

oyster season. New Yorkers averaged two meals of oysters per week and consumed 500,000 

bushels of oyster per season in the early 1900s. The people of New Orleans consumed 750,000 

bushels of oysters per year during the same time period (MacKenzie 1996). Establishments 

specializing in serving oysters such as cellars, saloons, parlors, bars and lunchrooms were 

common (Ingersoll 1881). Lipton and Kirkley (1994) found that demand for oysters dramatically 

declined during 1984-1994 as a result of health/nutrition, product safety, water pollution, and 

adulterated product concerns. The apparent decline in demand for oysters occurred while 

seafood consumption in general became an increasingly important part of the American diet as a 

trend toward more healthful eating habits (Lipton and Kirkley 1994). 

Oysters are primarily harvested for their meats but shells left over after shucking and empty 

shells taken as bycatch in fishing operations were used historically for landfill, road building, 

construction, lime production, and poultry grit (Hargis and Haven 1999). The U.S. Departments 

of Commerce and Interior gathered statistics on production of oyster shell byproducts until 1945 

(Hargis and Haven 1999). Since the primary oyster habitat is composed of natural oyster shell 

deposits, harvesting affects not only existing oyster populations but also critical oyster habitat 

because of the removal of shells. In fact, the health of the oyster resource is so closely linked to 

the habitat created by the deposition of their shells that many investigators have used the 

condition of this shell habitat as an indicator of oyster population health (Haven and Whitcomb 

1983; Rothschild et al. 1994; McCormick-Ray 1998; Lenihan and Peterson 1998). Several 

reports cite the loss of oyster habitat due to fishing and coastal development activities as an 

important factor in the decline of oyster populations (Berrigan et al. 1991; Rothschild et al. 1994; 

Hargis and Haven 1999; NCDENR 2001). The use of oyster shell for purposes other than reef 

restoration has been well documented; however, measures to regulate those uses have not. Most 

states now have cull-in-place regulations and programs to purchase and replace oyster shells 

harvested on public bottoms. Provisions to ensure use of oyster shell for habitat, such as banning 

their use in septic fields and disposal in landfills, are often obscure local ordinances. Oyster shell 

is still widely used in landscaping and in dietary supplements. 
35

Figure 6. Landings of C. virginica that show a linear sequence of fishery expansion and 

decline along eastern North America between Maine and Texas (from Kirby 2004). 

Commercial Utilization by Region 

Traditional assessments of oyster resources rely almost exclusively on information from fisheries 

dependent data. Use of landings data to assess abundance is confounded by the quality of 

reporting, various reporting mechanisms, landings and dockside values, and commercial markets. 
36

Fisheries independent data are obtained through surveys designed to directly assess the status of 

a population, independent of the fishery. 

 

Unless otherwise noted, the information presented in this section is summarized in MacKenzie 

1997b. The discussion below focuses on the use of dredges in the fishery, the importation of 

seed oysters, cultch planting activities and other management measures as indicators that the 

market demand for oysters exceeded the ability of the resource to produce them, or 

overutilization. Other mitigating factors such as substantial impacts due to disease, storms and 

market conditions are also included. Oyster abundance assessments were used if available. 

Unless otherwise noted, the information presented in this section is summarized from NOAA 

Technical Report NMFS 127: The History, Present Condition, and Future of the Molluscan 

Fisheries of North and Central America and Europe by MacKenzie et al. (1997). 

Northeast – Maine through Long Island Sound

This was the first area to be heavily exploited for its oyster resources. By the late 1700s oyster 

catches were declining and states passed laws limiting harvest. Dredges were introduced in 

some areas in the early 1800s, and catches declined even more rapidly despite strong demand. 

Seed oysters importation began around 1820 and industry production peaked in the early 1900s. 

Oyster production was limited due to low demand after peak landings in 1910 until 1938 and 

again from 1988 to 1996. Oyster production was impacted by major hurricanes in 1938 and 

1950 that covered many beds with sediment and destroyed harvest vessels. From around 1950 to 

the mid-1980s, inadequate supplies limited oyster production (MacKenzie 1997b). The majority 

of the oysters in the region currently are produced in Connecticut. Production there was greatly 

increased by massive private and government cultch plantings during 1988-1991 (MacKenzie 

1997b). However, NMFS landings data indicate that production has declined steadily since 

1999. 

Maine – The Damariscotta River beds were destroyed by coastal land development and wild 

oyster harvest ended there around 1840; however, small oyster beds were present in several 

locations along the rest of the coast into the 1960s. Primary production of eastern oysters in 

Maine is through aquaculture. Wild oysters harvested from the Piscataqua River are depurated 

before reaching markets (Wallace 1997). 

Massachusetts - Native oysters were considered depleted by the early 1800s in Massachusetts. 

Leaseholders began importing seed shortly thereafter. In the early 1900s Connecticut growers 

obtained leases in Wellfleet Harbor to grow out seed from Long Island Sound. The industry 

declined after the hurricane of 1938. Subsequent oyster production in Massachusetts waters, 

mainly from aquaculture, is difficult to summarize because management and reporting are at the 

municipal level. 

Rhode Island - Attempts to develop the oyster industry in Narragansett Bay began in 1822 with 

the initiation of a bottom lease program. Seed oysters were imported from Chesapeake and Great 

South bays and resulted in landings of 660,000 bushels by 1878. After 1880, Connecticut 

growers took over oystering in Narragansett Bay as they did in Wellfleet Harbor. The 1938 

hurricane and a lack of seed from Connecticut caused a sharp decline in landings. Few oysters 

were planted or harvested from 1960 until 1996 when spat sets on public beds planted with 
37

cultch material in 1993/94 began to produce (MacKenzie 1997b). Oyster landings rose to about 

45,000 bushels in 1997 then slowly declined to an average of around 7,500 bushels for the period 

of 2001-2004 (NMFS Landings Data). 

Connecticut - Connecticut has the most intensive oyster farming operations in the region. A 

large public seedbed of about 3000 acres supplied seed oyster needs in the early years and 

continued to produce until around 1945. Importation of seed oysters began in the 1830s and 

reached 750,000 bushels in the 1850s. Leaseholders began planting shells on their bottomland to 

increase local seed supplies around 1900 and planted up to 3 million bushels of shells per year. 

New sanitation laws caused declines in demand and prices around 1906. The 1938 hurricane 

destroyed many beds as well as another storm in 1950. Starfish, drills and sedimentation 

destroyed most seed until the 1970s when new harvesting and predator control methods were 

developed. Production further increased when the State and Tallmadge Brothers Oyster 

Company vastly increased cultch planting on the seedbeds. Millions of bushels of shells were 

planted from 1987 through 1991. Seed production and landings responded until 1997 when the 

parasites Haplosporidium nelsoni (MSX) and Perkinsus marinus (DERMO) became active 

(MacKenzie 1997b). 

New York - Great South Bay was the major oyster producing area in New York until the 1938 

storm. Oyster growing operations involved transplanting natural seed from low salinity sites in 

the eastern sound to western high salinity areas. After 1870, the local seed supply was 

supplemented by imports from Connecticut. Poor market demand plagued the fishery from 1910 

until 1938. The 1938 storm opened inlets, which increased salinity levels throughout the bay, 

allowing oyster drills to destroy all the seed. Few seed were planted during the 1940s and 1950s. 

Around 1960, MSX became active in the bay and little planting or harvesting was done until 

hatchery production began about ten years later. During the 1980s and 1990s, about 90% of 

New York’s oyster production came from hatchery-reared seed. Some wild harvest production 

was reported in the early 1990s (MacKenzie 1997b). There was an oyster fishery in the Hudson 

River estuary from the 1600s to the 1800s with seed still being harvested in the 1950s (Pers. 

Comm. Steimle 2005). 

Raritan and Barnegat Bays 

Growers planting seed produced most of the oysters in Raritan Bay. Seed was available from 

local sources until demand caused seed to be imported from Chesapeake Bay beginning around 

1820. In 1915, pollution began to curtail harvests when western Raritan Bay was closed. Other 

areas were closed in the 1920s and by 1930 oyster harvesting ceased in the area (MacKenzie 

1997b). 

In 1880, the seed oyster beds of Barnegat Bay were already suffering from over harvesting. 

Seedbeds in other nearby areas were also being depleted by 1892. Further overharvesting and 

increased salinity caused by changes in Beach Haven Inlet during a major 1919 storm continued 

to reduce the fishery. Additional increased salinity and the resultant increase in predation 

occurred in 1925 when the opening of Point Pleasant Canal introduced salt water into the head of 

the bay. Landings in the bay have been insignificant since 1950. A small fishery has existed in 

Great Bay since around 1950; however, MSX and P. marinus mortality caused most growers to 

leave the fishery (Ford 1997) (See Section 4.3). 
38

Delaware Bay

The early Delaware Bay oyster fishery was conducted using tongs and the oysters were taken for 

direct marketing or shipped to Connecticut and Massachusetts as seed for growout. During the 

1800s, dredging was introduced to the bay to meet increasing demands for out-of-state seed. By 

1843 written descriptions already existed of dredges dragging shells from the tops of reefs and 

increasing the size of beds by scattering the shells. At the time these circumstances were thought 

to be good for the seedbeds. 

 

Overharvesting of seedbeds and predation caused some areas to cease production by1900. A law 

to protect the seed beds required that seed be harvested by sail powered vessels only. This 

requirement was removed during WWII, increasing deterioration of the seedbeds. Unexplained 

seed oyster mortalities in the 1940s and 1950 put the beds at an all time low. Delaware River 

flow was examined and revealed low flow allowed oyster drills to move upriver in high salinity 

to the seedbeds. Higher river flow returned in 1968 as did seed supplies and in 1972 a 

tremendous set restored seedbeds that had not produced for 50 years. This set and others over the 

next 12 years sustained the industry through 1985. MSX mortalities began in 1957 and 

continued through the early 1970s. Modified growout strategies and better care of the seed beds 

allowed a modest recovery until 1985. MSX returned in 1985 and caused closure of the seed 

beds until 1990. P. marinus related mortality also became a problem in 1990. The seedbeds 

were closed 6 years between 1987 and 1997 due to disease and poor recruitment. Seed beds 

returned to production twice after serious depletion, 1972 and 1990-91 (Ford 1997). However, 

as noted above, seed supplies are currently low due to five years of poor recruitment and as a 

result, oyster harvest in the bay is severely restricted. 

Laws allowing leasing of bottoms for oyster growout were enacted in 1856 in New Jersey and 

1873 in Delaware. Shell was returned to the seedbeds for several years beginning around 1900. 

Good sets resulted and leased acreage increased from 12,000 acres in 1900 to 30,000 acres in 

1914. From 1880-1930, Delaware Bay harvested 1-2 million bushels annually. From 1930-1957 

landings declined to 1 million bushels per year. Failure to return shells to the beds, drought 

induced drill predation, the Depression and shellfish sanitation concerns were cited as possible 

reasons for the decline (Ford 1997). 

An 1875 law required all oyster shell caught during harvest to be landed and deposited for road 

repair. Continual removal of cultch material over the next 25 years hastened the deterioration of 

public beds; a condition stressed in all reports of the period. New Jersey officials purchased 

shell and returned it to the seedbeds for several years beginning around 1900. Legislation 

requiring seed planters to return 60% of the shell from all oysters harvested was passed in 1946 

but was repealed by 1979. The state planted small amounts of shell afterwards. 

Chesapeake Bay seed were imported at least since 1829 because Delaware Bay seed could not 

meet the market demand. Seed shipments were stopped around 1900 because Virginia officials 

were alarmed at the drain on their resource. In 1950, seed imports restarted but were stopped in 

1957 to stop the spread of MSX. 
39

Chesapeake Bay

Huge amounts of seed oysters were transported from Chesapeake Bay to northern states for 

direct sale and planting during most of the 1800s. In 1879, 2.18 million bushels were used for 

this purpose (Ingersoll 1881). After tremendous oyster landings between 1870 and 1895 of up to 

20 million bushels per year, landings dropped steadily until the1930s. The declining landings 

over 35 years probably indicate declining supply. The excessive harvest removed surface shells 

from oyster reefs developed over centuries, reducing recruitment. Other thin-based beds were 

potentially totally removed. Market conditions (including concerns for shellfish safety) also 

contributed to reduced landings later in the period. Production from1930 to 1955 was relatively 

consistent as management practices and shucking house shell plantings were adequate. 

Production fell again in the Fifties and did not recover in Virginia (MacKenzie 1997b). 

Maryland began fossil shell planting in 1960 with 5-6 million bushels planted in lower state 

waters with histories of good sets. Skip jacks were hired to move seed up the bay after set. 

Maryland landings rose to more than 2 million bushels during the mid 1960s through the early 

1980s. MSX returned to the Maryland portion of the Bay in 1981 along with the invasion of P. 

marinus. Landings subsequently fell to levels of 100,000 to 200,000 bushels per year 

(MacKenzie 1997b). 

Virginia oyster landings fell steadily from 7-8 million bushels per year in the 1880s to the period 

from 1935 to 1955 when production ranged between 2.5 –3.7 million bushels per year. The main 

cause of the decline seems to have been a great reduction in the supply of oysters on the public 

grounds even though hand and patent tongs were the only gear allowed on public beds in deep 

water areas. The wild populations could not withstand the steady harvest pressure with 

concurrent siltation, shell fouling and inadequate shell replacement (MacKenzie 1997b). Cultch 

planting on public grounds began in 1928 with 160,000 bushels. From 1 to 3.5 million bushels 

were planted per year in the 1960s and 1970s but cultch plantings were made primarily in 

support of seed oysters for lease production. Virginia leaseholders used shucked shell to improve 

their planting grounds and also created thousands of acres of suitable oyster habitat. 

From 1859 to 1959, more than 200 million bushels of seed were taken from the James River, 

VA. Transplanting of James River seed oysters by tong fishermen reduced rock height by 1.5 

meters (58.5 in) over a 90-year period indicating impact to the habitat. Due to declining harvests 

in other areas, the James River was used for direct harvest of market oysters following the 

1986/87 season and market oystering was concentrated there through the 1990s. The state 

restricts harvest in the James River to conserve the remaining stock and many fishermen have 

left the fishery (MacKenzie 1997b). Recent landings for the state have been below 20,000 

bushels per year (NMFS data). 

Beginning in 1959 MSX killed most oysters greater than 50mm (2 in.) in high salinity (>15ppt) 

areas in Virginia. During the1980s, annual mortality ranged from 50-70%. MSX was also the 

likely cause of a huge drop in setting densities since the early 1960s due to reductions in 

spawning stock (MacKenzie 1997b). A soup oyster fishery utilizing 1.5-2 inch (38-50mm) 

oysters began in 1957 in the James River. Soup oysters could be marketed directly with no 

further growout and avoided some of the disease mortality. Also, there was no market for seed 
40

due to MSX after 1960. The soup oyster fishery ended in 1976 when Kepone was found in James 

River. 

South Atlantic - North Carolina through the east coast of Florida 

North Carolina subtidal oyster landings accounted for most of the south Atlantic harvest until the 

1890s when canneries were established in South Carolina and Georgia. South of Pamlico Sound 

oyster resources are primarily intertidal. P. marinus mortality became a significant factor 

affecting oysters in Pamlico Sound in 1991. It is only a problem in more southern areas during 

extreme drought conditions. All of the states in the region have summer season harvest closures 

except Georgia. Oyster production is at an all time low due to pollution closures, lack of 

markets, disease in subtidal oysters, habitat impacts and labor shortages. Closures to harvesting 

due to pollution removed 24% of the available harvest area by 1990. Limited relay of polluted 

oysters to public bottom and leases for depuration is used to reduce the effect of the closures 

(Burrell 1997). 

North Carolina - The North Carolina oyster resource was not fully exploited until 1889 when 

fishermen from northern states began harvesting in Pamlico Sound with dredges and mechanical 

tongs (Chestnut 1951). The state attempted to reduce the effects of dredging by limiting the area 

where the gear could be used in 1897. However, as catches declined the area open for dredging 

increased. Oyster landings peaked in 1902 and seed oysters from North Carolina were also being 

shipped to Chesapeake and Delaware bays during this time period (Chestnut 1951). Since 1955 

dredging area has decreased (NCDENR 2001). Cultch planting to increase oyster harvests began 

about 1915 in North Carolina but consistent operation of the program was not achieved until 

1947 (Marshall et al. 1999). A dredge weight limit was also enacted in 1947 along with a daily 

catch limit. Oyster landings continued a general decline and fell below 200,000 bushels per year 

in 1962. Catches were averaging just over 100,000 bushels per year in 1989 when the state 

began to experience mortality from P. marinus infections brought on by a severe drought 

(NCDENR 2001). Landings fell to around 40,000 bushels per year in 1993 and have not 

recovered. Since 1993 more than 95% of North Carolina’s oyster landings have come from 

intertidal beds in the southern part of the state (NCDENR 2001). Harvest from these intertidal 

oyster resources has remained steady in part due to the greater setting intensity and faster growth 

of these stocks. CPUE data is monitored to assess sustainability of the stocks. The oyster season 

has closed early in recent years due to low supplies of harvestable oysters (NCDENR 2001). 

South Carolina - South Carolina’s oyster landings are generally higher than those in North 

Carolina but they have declined similarly. Factors causing the decline are believed to be impacts 

from industrial and residential development, channelization and waterway construction, poor 

markets for intertidal oysters, labor shortages and pollution closures (Maggioni and Burrell 

1982). Over harvesting was not identified as a problem until 1986 when concerns were voiced 

about some of the public grounds. State shellfish grounds were established and management 

measures including rotation schemes and shortened seasons have restored these resources. Oyster 

populations in leased areas are normally more than adequate to meet supplies (Pers. Comm. 

Anderson 2005). 

Georgia and the East Coast of Florida - Pollution closures and development impacts have 

eliminated a large portion of the oyster harvesting areas in Georgia and on the east coast of 
41

Florida (Cowman 1982, Ingle 1982). In 1981, only three harvesters were licensed in Georgia 

and one shucking house was in operation (Cowman 1982). Many of the estuarine systems on 

Florida’s northeast and central coasts support oyster populations, but there is only an 

opportunistic fishery, with only a small percentage of the state’s landings coming from these 

areas (See Section 3.4). 

Gulf of Mexico

Oyster production in the Gulf of Mexico has been highly variable largely as a result of 

fluctuating environmental conditions. Louisiana produces most of its oysters on leases while in 

other Gulf States the grounds are nearly all public. Florida and Alabama only allow tongs for 

oyster harvest on public reefs. Mississippi, Louisiana and Texas allow harvest with dredges. 

Florida, Louisiana, and Texas are the only Gulf States that market oysters year round. Most 

oyster resources in the Gulf are subtidal and exhibit good sets and fast growth. Landings 

gradually increased during the 1960s and 1970s peaking in the early 1980s, declined in the late 

1980s (drought from 1986-1989), and steadily increased after 1993. Confusion regarding the 

potential health risks associated with the consumption of raw oysters has eroded consumer 

confidence, which may affect oyster markets. Loss of habitat is perhaps the most serious and 

chronic problem facing the Gulf oyster industry. Oyster reefs and reef shell have been lost due 

to fishing disturbances, shell removal, and development activities. Cultch planting has been 

important in maintaining productive oyster reefs. Seed and direct harvest areas have been 

shelled. Other activities that conflict with oyster reefs are shrimp trawling, oil and gas facilities, 

channelization, and freshwater diversions. Hurricane tidal surges have buried and scoured oyster 

reefs and changed or removed protective barrier islands (Dugas et al. 1997). 

West Coast of Florida - Harvest from public grounds accounts for 90-95% of Florida oyster 

landings. At least 90% of this harvest comes from Apalachicola Bay (Ingle 1982), which 

contains the state's most commercially valuable oyster reefs. Surveys of oyster populations were 

conducted in Apalachicola Bay as early as 1895 and intermittently until the present, and the 

commercial oyster industry in the city of Apalachicola was first described in 1881 (MacKenzie et 

al. 1997). About 600 acres of submerged lands in Apalachicola Bay are privately-held oyster 

leases, accounting for about 10% of the productive oyster habitat in the bay. Oyster production 

from Florida's Gulf Coast has been variable since the 1960s, ranging from about 1.5 to 6 million 

pounds of meats. Florida's oyster landings generally reflect Gulf-wide production levels and 

trends. Florida oyster landings declined in 1986 due to the impacts of Hurricane Elena on 

Apalachicola Bay in 1985 and poor production extended into 1992 due to drought conditions 

from 1987 through 1989. Oyster abundance increased in 1993 but low demand limited harvests 

(Dugas et al. 1997). The 2002 oyster assessment indicated an abundance of market-sized oysters 

in Apalachicola Bay and suggests that the oyster resource there is underutilized (Arnold and 

Berrigan 2002). Florida oyster managers developed a scale to determine the relative condition of 

oyster resources based on production estimates from oyster bar harvest areas. A production 

estimate of 400 bags per acre indicates a healthy bar capable of sustaining commercial harvest 

while an estimate of 200 bags per acre indicates that harvests should be limited. Oyster 

resources are considered depleted when stocks are below 100 bags per acre (Marsh 2004). Catch 

per unit effort in the oyster fishery dropped from 1986 to 1988, stayed low until 1991, increased 

and remained at relatively high levels until 1997 when it began another decline (Marsh 2004). 
42

Florida began cultch planting on public reefs as early as 1914 and has maintained an oyster 

resource development program since 1949 (Dugas et al. 1997). 

Alabama and Mississippi - Over the long term Alabama and Mississippi combined to produce 

about 12% of the total Gulf oyster landings. The two states suffered dramatic declines in 

production in 1987 that lasted until 1992 (Dugas et al. 1997). Landings in Alabama returned to 

long-term averages after the decline; however, Mississippi’s landings increased to the highest 

levels in 30 years (NMFS Landings Data). 

Louisiana - Oyster production in Louisiana has historically come primarily from leased bottoms 

(Berrigan et al. 1991), although public oyster grounds exhibited sizable increases in production 

during the 1990s and early 2000s (LDWF 2005). Lease acreage expanded from less than 50,000 

acres in 1960 to around 330,000 acres in 1988 (Berrigan et al. 1991). CPUE data do not indicate 

a strong trend and effort remained stable from 1961 to 1986 (Berrigan et al. 1991). The public 

grounds in Louisiana are used as seed areas for the leases and for harvest of market oysters. 

Harvest of market oysters from public grounds has increased since 1992 and exceeded lease 

harvest in 1996 and 2002 (LDWF 2005). Long-term population abundance data indicate the 

Louisiana stock was stable at relatively low levels from 1982 to the early 1990s, increased until 

2001 and declined during 2002 to 2005 (LDWF 2005). The 2005 oyster stock availability of 

2,676,797 barrels (~12 million bushels of seed and market oysters) is the lowest since 1992 but 

still higher than all but one of the 1980s estimates. The Louisiana Wildlife and Fisheries 

Commission uses oyster stock availability data along with recommendations by the Louisiana 

Department of Wildlife and Fisheries – Marine Fisheries Division and the Louisiana Oyster Task 

Force to set the oyster season. Lower stock availability typically results in a shorter season 

(Pers. Comm. Banks 2005). 

 

Texas - The Texas oyster fishery is comprised of two components: the public reef fishery and the 

lease fishery. Leases are found only in Galveston Bay and are utilized as depuration locations 

for oysters transplanted from restricted waters only. Lease harvest comprises 20-25% of the total 

commercial landings in Texas. Long term data indicated a general decline in oyster landings in 

Texas waters from 1956 to 1981, an extremely large increase in 1982 followed by another 

decline in landings until 1987 (Quast et al. 1988). Since 1987, the trend in landings has 

continued to increase to over 5.5 million pounds of meats harvested in 2004 (Campbell and 

Butler In press). Over half of the state’s oyster public reefs are found in Galveston Bay, which 

account for 80% or more of Texas’ annual commercial oyster harvest. 

Fishery independent sampling from 1984-2003 indicates an increasing trend in catch-per-uniteffort (Martinez-Andrade et al. 2005) of market-size oysters (>75 mm) that generally mirrors the 

fisheries dependent landings trend over the same time period (Campbell and Butler In press). 

Catch-per-unit-effort of small (26-75 mm) oysters indicates a slight increasing trend during the 

1984-2003 time period (Martinez-Andrade et al. 2005). 

Canadian Maritimes

In Canada, eastern oysters are restricted to the shallow bays and estuaries of the southwestern 

Gulf of St. Lawrence and the coves of Cape Breton’s Bras d’Or Lakes. Water temperatures in 

this area are warm enough to support species normally found much farther south because the 
43

estuaries have extensive shallow zones and broad intertidal flats, many with brightly colored 

sediments that absorb much radiant energy (Jenkins et al. 1997). The potential for 

overutilization of this resource is great due to extended growout periods of between four and 

seven years to reach market size and intermittent spatfall failures due to local waters not reaching 

spawning temperatures (Lavoie 1982). The early history of eastern oyster landings in the 

Canadian Maritimes is similar to many East Coast states with peak landings occurring around 

1890 followed by a period of declining harvests. Failure to conserve seed oysters and shell, 

disease, and habitat impacts contributed to the decline that bottomed out around 1920. Peak 

oyster landings were around 87,000 bushels in 1890 while the lowest landings were about 7,000 

bushels in 1920 (Jenkins et al. 1997). 

Conservation measures started with a closed oyster harvest season in 1864 and an oyster size 

limit and culling regulations in 1920. All oyster harvesting on public beds is limited to hand 

harvest methods. Malpeque Bay disease had a pronounced effect on oyster stocks in Prince 

Edward Island from 1915 until disease resistant oysters began to allow increased production in 

1940 and in New Brunswick and mainland Nova Scotia between 1954 and 1957 (Jenkins et al. 

1997; DFO 2005a). In 1972, after another period of declining landings, the provincial and 

federal fisheries agencies began a program to rehabilitate the oyster fishery. Rehabilitation 

measures included cultch planting, cleaning natural shell beds fouled with sediment, and 

transplanting oysters for public harvest (Jenkins et al. 1997). Oyster landings responded and 

except for a short period from 1991-1993 have stayed at relatively high levels (Canada 

Department Fisheries and Oceans (DFO) landings data). Approximately 50% of the Canadian 

Atlantic oyster crop comes from lease areas where fishermen primarily utilize relaying of 

polluted wild stock as the means of production (DFO 2005a). The eastern oyster is not among 

the 67 aquatic species protected under Canada’s Species at Risk Act (DFO 2005b). 

Canada’s annual production of approximately 25,000 bushels of eastern oysters is marketed 

primarily in Montreal and Quebec City and very few are exported. Demand exceeds supply 

creating a relatively high price for these high quality shell stock oysters (DFO 2005a). 

East Coast of Mexico

Mexico’s oyster harvest ranks sixth in the world and annually produces around one million 

bushels. Approximately 90% of the harvest is eastern oysters produced along the Gulf of 

Mexico. Economically it is the most important fishery in Mexico and virtually all of the harvest 

is marketed within the country for raw consumption. Eastern oysters grow in large shallow 

lagoons all along the east coast of Mexico in depths that seldom exceed two meters (6.5 feet) 

(MacKenzie and Wakida-Kusunoki 1997). 

Oyster production is based on market demand and is not limited by supply. Fishermen’s 

cooperatives control the harvest and in some areas control the beds where harvest can occur 

(Haro et al. 1982). The federal government imposed rules to protect the oyster resource in 1976. 

All harvesting is done by hand gear or by hand. Each harvest boat or dugout must have a 

harvester and a culler to return shells and small oysters to the beds as they are harvested. Also, 

no more than 20% of the oysters can be marketed in the shell because those shells are rarely 

returned to the beds. If more than 20% is sold in the shell, the cooperative must buy shells from 
44

another area and plant those shells on the beds. Oyster shells cannot be sold for other purposes 

(MacKenzie and Wakida-Kusunoki 1997). 

Mexican lagoons receive regular abundant spatfall and the spawning season runs from March 

through October. Growth rates are high with most oysters reaching a harvestable size of 70-

75mm (2.75-2.95 in.) in eight months. There are high mortalities of eastern oysters in most areas 

at one year of age and 76mm (3 in.) or greater in shell height. Only one survey for P. marinus

has been conducted and infection levels were high in the area surveyed (Burreson et al. 1994). It 

is suspected that P. marinus is prevalent in the other lagoons as well. Impacts from red tides, oil 

pollution and outbreaks of illness from poor sewage treatment near oyster harvesting areas also 

affect availability and demand for oysters (MacKenzie and Wakida-Kusunoki 1997). 

The management of oyster beds is under control of the fishermen’s cooperatives. The harvest 

procedure consists of fishermen harvesting an entire bed over a period of several weeks or 

months. Stored shells are then spread on the bed for cultch and fishermen move on to the next 

bed. Oysters on the planted bed will be ready for harvest in 6-12 months and will be of relatively 

uniform size suited for the prevalent cocktail and half shell trade (MacKenzie and WakidaKusunoki 1997). 

Oyster spawning and growth characteristics, management provisions, and habitat restoration 

activities appear to be effectively protecting Mexico’s oyster populations from overutilization. 

Local observers feel that increasing cultch planting and expanding aquaculture could expand the 

Mexican oyster industry. Also, with better shellfish sanitation and/or depuration, oysters could 

be exported to the United States and other countries (MacKenzie and Wakida-Kusunoki 1997). 

Bycatch 

Bycatch was not discussed in any of the references used in this section. Oysters could be 

harvested in shrimp and crab trawls if these gears were towed over oyster reefs, but their design 

would not allow significant capture. Mechanical methods for clam harvesting will readily take 

oysters, but this type of gear is not normally allowed on oyster rocks because of the damage it 

causes to the habitat. Fishing gear impacts on oyster populations are more appropriately a 

habitat degradation/loss issue (Berrigan et al. 1991; Chestnut 1955). 

Recreational Utilization

There is very little data on recreational oyster harvesting. The 1988 Texas Oyster Fishery 

Management Plan presents data that indicate the CPUE for recreational harvest was 0.3 kg (0.66 

lbs.) of oyster meat per hour from 1983-1986. During the same period the average annual coast 

wide landings were 5,300 kg (11,684 lbs.) of meat (Quast et al.1988). Effort increased each year 

of the survey rising from 19,610 to 23,730 man-hours per year (Quast et al. 1988). Recreational 

oyster fishing activity was relatively low compared to finfish fishermen (Quast et al. 1988). 

Berrigan et al. (1991) concluded that it is likely more people have become involved in 

noncommercial oyster fishing in the past ten years due to variable market supplies and increased 

prices. No recreational harvest data was available. In North Carolina, recreational harvest of 
45

oysters is considered to be significant in some areas and was considered a factor in intertidal 

oyster declines in the 1960s, but there is no data (NCDENR 2001). 

At least seven of the Atlantic and Gulf coast states have license requirements for recreational 

oyster harvesting. Daily catch limits typically range between 100 oysters and two bushels of 

oysters (NCDMF unpublished data). 

Summary of Overutilization Assessment

The eastern oyster’s life history characteristics of high fecundity, multiple spawns per season, 

very early age at maturity, fast early growth, wide salinity and temperature tolerance, and ability 

to colonize a wide variety of habitats make the species inherently resilient to natural and maninduced mortality (Marsh 2004). These survival advantages are most prevalent along the South 

Atlantic coast and Gulf of Mexico where oysters grow to harvestable size in one or two years 

and have a six to eight month spawning season. Fast growth quickly replaces lost habitat and 

allows oysters to reach harvestable size before disease effects occur. Protracted spawning 

seasons increase the likelihood of recruitment success by providing more opportunities for larvae 

to encounter suitable environmental setting conditions by avoiding freshets and high turbidity 

caused by storms. The advantages are diminished at the northern end of the range where eastern 

oysters can take four to seven years to reach market size and the spawning season decreases to 

four to six weeks or less in New England and the southwestern Gulf of St. Lawrence (Powell et 

al. 1994; DFO 2005a). Nonetheless, oyster populations in northern latitudes still exhibit 

remarkable resilience (Jenkins et al. 1997). 

The Atlantic coast south of Cape Lookout and through the Gulf of Mexico appear to have 

avoided some of the extremely heavy utilization experienced by the area from Pamlico Sound to 

Long Island Sound. They were the last regions to reach historical peak oyster landings, and the 

intertidal oysters in this region were considered an inferior product and not harvested extensively 

at least until the early 1900s when canneries opened in the area. Harvest parameters in the Gulf 

of Mexico are currently less restrictive than those in the mid Atlantic area, but oyster populations 

there appear to be effectively managed and monitored so that harvest impacts are not substantial 

(Marsh 2004). 

Eastern oyster resources from Pamlico Sound to Long Island Sound appear to have suffered from 

long-term overutilization. Negative impacts from disease, pollution and non-fishing habitat 

losses are also major contributors to current low population levels. State managers in this region 

have attempted to protect public oyster stocks by conducting stock assessments, setting 

conservative harvest quotas, lowering daily catch limits, limiting harmful gear use and reducing 

harvest seasons. Attempts to restore oyster populations and rebuild the resource through general 

cultch planting, reef rebuilding and oyster sanctuaries/reserves are also becoming common 

management tools in this region. Oyster managers struggle with balancing the needs of the 

resource and the desire to preserve the industry. 

Oyster resources north of Long Island Sound are relatively small in comparison to other regions 

and much more susceptible to harvest pressure and environmental disturbances. Both of these 
46

causes had significant effects on oyster populations in this region. The Canadian Maritimes area 

maintains a small but persistent oyster fishery based on wild stocks and utilizes conservation 

measures similar to the mid Atlantic region. Wild harvest fisheries in the states north of Cape 

Cod declined very early and have been very low since the mid 1900s. Imported seed oysters 

were used in attempts to revive oyster production in this area but they failed and the oyster 

industry is now based largely on use of hatchery-reared stocks. 

Even though oyster harvests are at or near record low levels along the majority of the U.S. 

Atlantic coast, resource managers and independent experts surveyed by the BRT indicate that 

overutilization (overharvesting) is currently a minor threat to extant oyster populations, occurring 

only seven times in 286 threats listed for the 71 estuaries assessed by respondents. It is 

important to note that survey responses identified current threats to oyster populations and that 

historical decline may have been caused by practices that are not current threats. 

4.3 Predation and Disease

Predation 

The planktonic life stages of the oyster (free-swimming trochphore/ veliger) are subjected to 

predation from a myriad of filter feeding organisms from rotifers to ctenophores (comb jellies 

and sea walnuts). Soon after the spat cements itself on or near the bottom, it becomes part of the 

benthic food chain. A variety of crab species are known to forage on oyster tissues. As oysters 

grow and their shells thicken, they become better able to withstand predation. However, some 

species such as carnivorous gastropods (e.g., drills, conchs and whelks) are able to prey upon 

oyster populations on both the Atlantic and Gulf coasts, and star fish are a common predator in 

coastal locations. Populations in the upper regions of estuaries are protected from many of these 

higher salinity predators. Flatworm turbellarians of the genus Stylochus have inflicted 

significant damage to estuarine oyster populations on both the Atlantic and Gulf coasts as well. 

A number of fish species such as black drum and cownose rays occasionally cause extensive 

damage to oyster beds, and diving ducks have also been documented as consumers of oyster 

tissue (Galtsoff 1964). Black drum have been documented to heavily impact seeded oyster reefs 

in Louisiana in both spring and early fall (Brown et al 2003). Early post-settlement mortality up 

to 99% occurs for many reasons. Competition for space with other oysters will also reduce the 

total oyster population. As stated previously, the environmental plasticity of the oyster enables it 

to grow and thrive under conditions that might be considered extreme to other species. Oysters 

exist from deep, high salinity waters to intertidal flats where they are exposed to additional 

predators during low tide. Predation varies with surface roughness (i.e., interstitial space) of 

habitat, with predation rates being higher in areas with low relief (Luckenbach et al. 1999). 

There are many commensal organisms that make up a healthy oyster reef community. While 

many of these species reside on the outer surfaces of the oyster’s shell, some species such as 

boring sponges and clams and mud worms, perforate the inner shell surface causing the oyster to 

expend extra energy maintaining the integrity of the shell cavity. The boring sponge, Clionoa 

celata, occurs in the Atlantic, while boring clams affect oysters in both the southern Atlantic and 

in the Gulf of Mexico. A number of mud worm species in the genus Polydora infest the shells of 
47

oyster populations on both Atlantic and Gulf coasts (Galtsoff 1964). Many of these infestations 

are natural associations and in general, most oysters survive. Thus, these associations do not 

seem to be having an effect at the population level. 

Infectious Oyster Disease Agents 

The following diseases are recognized by the World Organization for Animal Health (OIE). OIE 

members have strict regulations on export of diseased oysters or product in order to reduce the 

spread to regions not presently infected by these organisms. Locally, the threat posed by disease 

is reduced by not moving infected seed between states. However, intrastate movement still 

occurs. Preventing accidental introductions and controlling vectors of introduction (e.g., ballast 

water) are of critical importance to reducing the spread of disease organisms. 

DERMO is a parasitic disease caused by Perkinsus marinus, which is a protozoan. The first 

major oyster pathogen to be identified was P. marinus (Levine 1978 = 

Dermocystidium marinum; Mackin et al. 1950 = Layirinthomyxa marina; Quick 

and Mackin 1971). Dr. John Mackin applied the first modern histological methods to 

 describe a parasite (still referred to as DERMO disease) associated with summer oyster 

mortalities in Louisiana and Virginia that had occurred in the late 1940’s. Later he showed, with 

histological sections of oyster tissues, the direct relationship of pathogen intensity and the death 

of the oysters (Mackin 1951). At the same time Drs. Willis Hewatt and Jay Andrews initiated 

their studies of oyster summer mortalities in Virginia and found wide spread oyster mortalities 

associated with this new pathogen (Hewatt and Andrews 1954; Andrews 1955). For over 50 

years, studies have continued to document the impact of this disease organism. The parasite 

infects oyster in the first year of life and continues to proliferate causing up to 50% mortalities in 

oysters carrying the infection into their second summer season and 80-90% mortalities by the 

third year. Very few oysters that are infected with this disease organism survive a forth season. 

This parasite inhabits the immune cells of the oyster and perhaps suppresses the immuno 

response. It reduces the effectiveness of the oyster phagocytes; thereby, overwhelming the 

oyster’s system and rendering it unable to fight off other opportunistic organisms.

Perkinsus marinus continues to cause significant mortalities along the Gulf and Atlantic coasts. 

Quick (1977) reported the range of P. marinus to be Massachusetts south to the Gulf of Mexico. 

Kern et al. (1973) reported the presence of P. marinus in eastern oysters introduced into the 

waters of Pearl Harbor, Hawaii. More recent reports extend its range from Maine (Burreson and 

Ragone-Calvo 1996; Ford 1996) to the Yucatan Peninsula (Burreson et al. 1994). Movement of 

infected seed oysters has resulted in the expansion of its range in the Chesapeake Bay. Sporadic 

reports of higher than normal mortalities occur from year-to-year within this reported range 

(Brousseau et al. 1998; Lewis et al. 1992). Reports of P. marinus in New England and Maine are 

probably the results of the movement of infected seed oysters. It has been generally accepted 

that cold conditions reduce the impact of, and unless there is extensive global climate warming, 

these northern extensions are not likely to remain. Conversely, should warming conditions 

persist, than one would expect to see even greater impacts on oyster populations throughout its 

range. High salinity and high temperatures elevate disease levels particularly during times of 

drought (Burreson and Andrews 1988).In addition to information presented to the BRT on the 

genetic variation reported between Atlantic and Gulf Coast oysters, there are reports of variation 
48

in pathogenesis and environmental tolerance in localized populations of P. marinus (Bushek and 

Allen 1996) and population variations in oyster disease resistance to DERMO (Gaffney and 

Bushek 1996) (See Section 7.1). 

Decades of studying P. marinus have provided significant information. Early partial culture 

provided easy diagnostic capabilities (Ray 1966). A number of techniques have been developed 

to propagate the histozoic stages of this parasite; thus, allowing in depth studies of its 

physiological needs, life cycle stages, other growth characteristics. P. marinus can withstand a 

wide range of temperatures and salinities. The parasite has a direct life cycle with infective 

stages passing from one oyster to the next via the water column. The most likely method of 

disease transfer is via the movement of infected oysters. Once a population is infected it will 

spread from oyster to oyster. Peak mortalities occur during the summer period when higher 

temperatures and salinities provide the optimal conditions for parasite growth. Efforts to deal 

with DERMO have been focused on managing for the harvest of the oyster rather than managing 

the oyster’s response. Because of the chronic nature of this parasitic disease, populations of 

oysters have the opportunity to spawn the first summer and others may be able to spawn a 

second or third time before succumbing to an infection. 

MSX is a protozoan disease caused by Haplosporidium nelsoni. This second lethal disease was 

first reported in oysters from Delaware Bay. Extensive mortalities were observed in the fall of 

1957 with massive mortalities occurring in the spring of 1958 and an unidentified organism first 

dubbed “MSX” for Multinucleate Sphere X (unknown), was associated with the dying oysters 

(Haskin et al. 1966). They reported oyster losses as high as 80% in some areas, killing both 

adult and juvenile oysters. Sindermann and Rosenfield (1967) reported oyster production in 

Delaware Bay fell from about 7.5 million pounds of shucked oyster meats prior to 1957 to less 

than 100,000 pounds by 1960. The parasite did not remain restricted to Delaware Bay; by 1959 

the impact of the disease was reported in the Virginia and Maryland waters of the Chesapeake 

bay (Andrews 1966). A concerted effort by state, university and Federal agencies lead to the 

identification of spore stages (Couch et al. 1966), and MSX was eventually identified and named 

Minchinia nelsoni (Haskin, Stauber and Mackin 1966) and later reclassified as Haplosporidium 

nelsoni (Perkins 1990). The mid-Atlantic region still suffers from the heaviest impact from 

MSX along the Atlantic coast, even though it has been reported from Maine to Florida. Range

expansion is probably due to the movement of infected seed and/or the natural movement or 

transplantation of a yet unidentified alternate host. Since first being reported, MSX has 

continued to be present in Delaware and Chesapeake Bay. The distribution and intensities of 

mortalities fluctuate from year to year mostly related to local salinity increases, brought on by 

drought conditions (Barber et al. 1997). MSX has recently reported to have invaded Maritime 

Canada (Stephenson et al. 2003) and has been documented in oysters from the Gulf of Mexico 

(Ulrich pers. comm. 2006). In a recent study, researchers were able to isolate and sequence H. 

nelsoni DNA from oysters that were collected from the Gulf of Mexico over the last 10 years. 

While the parasite has been in the Gulf for at least five to 10 years, it has not triggered an 

epizootic in the region which leads to questions regarding its intensity and the host-parasite 

relationship in subtropical latitudes (Ulrich pers. comm. 2006). 

Unlike Perkinsus sp, Haplosporidium nelsoni has never been cultured. It is not transmitted 

directly for one infected oyster to another. By studying populations of infected oysters, it is now 
49

known that to sustain infections in an oyster population, the salinity must consistently be above 

15 parts per thousand (ppt). This parasite infects, proliferates and kills the most oysters in areas 

where the salinity is generally above 18 -20 ppt. Proliferating parasites overwhelm the tissues of 

the oyster, resulting in a watery, emaciated individual that normally does not survive the 

infection. With heavy infections occurring in late summer, death is rapid. The time of infection 

to mortality is about six weeks. At sublethal levels of infection, the parasite interferes with the 

oyster’s metabolism, its natural defense mechanisms, and reduces fecundity (Barber et al. 1988a, 

1988b). Some oysters might not get a chance to spawn because MSX can kill zero year class 

juveniles before they have a chance to spawn. Initial infections occur in mid to late summer, 

with mortalities continuing until cooler waters slow metabolism of both the parasite and oyster. 

Heavily infected oysters may continue to die slowly over the winter period, and as water 

temperature rise in the spring, another period of mortality may occur. 

During drought years the impact of MSX is even greater, exposing oysters that normally live in 

lower salinity conditions to salinities that support infections by MSX. Infected oysters 

transplanted to new locations can initiate infections that are sustained in those populations. 

Shellfish managers have restricted the movement of oysters from areas of high infection to areas 

not known to be infected. Many believe that because it cannot be directly transmitted that an 

alternate or intermediate host is needed to maintain infections and spread the disease within a 

given oyster population. As drought conditions wane, survivors and their progeny may 

reproduce to re-establish oyster populations. During the wetter years that occurred during the 

1970’s, there was significant recovery of oyster populations that had been devastated during the 

1950-1960 MSX epizootic in both Delaware and Chesapeake Bays. Oyster recovery 

management programs have concentrated on moderate to lower salinity areas that are less likely 

to support the development of oyster diseases. 

There is growing evidence that this parasite is an invasive species introduced to the East Coast of 

the United States in one of the many tests to introduce the Pacific oyster (Crassostrea gigas) for 

aquaculture development. Recent molecular techniques (Burreson 1997) have shown that 

Haplosporidium parasites in Pacific oysters from Korea (Kern 1976) and Japan (Friedman 1996) 

were genetically identical to Haplosporidium nelsoni from Virginia. 

Malpeque Disease of Oysters is restricted to Atlantic Canada. It is limited to high salinity areas 

in southwest Nova Scotia and has not been reported in Bras d’Or Lakes, Cape Breton. The cause 

of the disease is unknown, but naïve oysters unexposed to the disease demonstrate up to 99% 

cumulative mortalities. Mortalities were first reported by Needler and Logie (1947) and later 

confirmed by Drinnan and Medcof (1961). Populations of oysters surviving the initial disease 

exposure are resistant to the disease and so are their progeny. Repeated testing shows continued 

presence of the disease and its ability to kill susceptible oysters, with no apparent disease in the 

existing oyster populations. Canada has restricted movement of oysters from these areas to 

prevent further spread to uninfected oyster populations. 

4.4 Regulatory Mechanisms 

Regulatory mechanisms for eastern oyster are most logically defined as habitat resource 

protection (preventative measures), fishery-specific, and conservation/replenishment based. 
50

Habitat measures are those defined at the federal, state or local level designed to protect aquatic 

resources (including benthic reef habitat and water quality) from various direct or indirect 

development impacts (e.g., impacts of channel dredging, onshore development, point-source 

runoff, etc.). Harvest measures are those intended to control or regulate the commercial or 

recreational catch of the species, and may or may not be resource conservation based. 

Conservation/replenishment measures are those intended to ensure the continuance of the fishery 

or habitat resource through various measures including setting aside no-harvest areas, requiring 

culling of shell during harvest, setting up programs to return shells from harvested product back 

to reef areas, or natural seed movement programs intended to support either habitat or fishery 

restoration. Additionally, the state shellfish control agencies are responsible for managing 

shellfish harvesting areas for public health protection, which may result in permanent or 

temporary closures due to the presence of toxic algal blooms, elevated fecal coliforms and/or 

Vibrio spp., or chemical contamination. These restrictions may have the ancillary benefit of 

protecting some populations in chronically contaminated areas from harvest. 

Federal

Throughout its range, the eastern oyster is not a federally managed fishery. Therefore, few 

federal regulations specifically address regulatory or management issues dealing directly with 

the eastern oyster. Oysters are primarily found in relatively shallow inshore locations. Thus, 

permit-requiring activities requiring consultation under Section 404 of the Clean Water Act or 

Section 10 of the Rivers and Harbors Act often involve the U.S. Army Corps of Engineers as the 

lead review agency, with various other federal agencies (typically NMFS and the U.S. Fish and 

Wildlife Service) providing resource protection recommendations. The Essential Fish Habitat 

consultation requirements of the Magnuson-Stevens Act and the Fish and Wildlife Coordination 

Act are intended to give each of the above listed federal resource agencies a mechanism to 

conserve and protect oyster reef habitat. 

State/Local

An exhaustive compendium of all state and local management tools is beyond the scope of this 

report. However, a brief overview of state and local regulations will be covered in the following 

sections. 

Resource Protection 

Similar to the regulatory process identified at the federal level, most states have a parallel 

environmental review and permitting process intended to protect nearshore aquatic habitat 

(including oyster reefs) from a variety of potential impacts. Individual states are delegated the 

authority and responsibility to enforce the provisions of the Clean Water Act pursuant to Section 

401 by regulating discharges into the nearshore environment. 

Harvest/Fishery Management 

Oyster harvest management is not typified by any consistent strategy or rule throughout the 

range. Overarching harvest regulations, specifically the opening or closure of some sites or the 

entire fishery, are based upon maintaining some degree of sustainability. More often, however, 

harvest regulations are centered more around controlling the daily harvest totals, effort, or 
51

efficiency (via gear restrictions). Few if any areas have harvest quotas that are defined based on 

any population census (fishery independent) data. Seasonal restrictions on harvest enacted in 

most states are not so much intended to promote or protect stocks of oysters during spawning, 

but rather have more to do with the marketability of the oyster when energy is diverted to 

gonadal tissue, reducing meat quality. 

Throughout the range, oyster habitat and oyster growing areas are variously classified as public 

grounds and private (or lease) grounds. Public grounds can be closed, open for recreational 

and/or commercial harvest, allow specific types of gear, have various minimal (and occasionally 

maximum) cull sizes, or other site-specific measures. In most areas the goal is to promote the 

sustainability of the commercial fishery, but resource conservation benefits are realized 

secondarily. Limiting gear type to less efficient gears (e.g., hand tonging) tends to reduce the 

economic viability of harvesting all available product, which helps to promote the maintenance 

of some standing stock on public grounds. Additionally, limiting the use of very efficient gear 

(such as large, powered dredges) that can significantly impact oyster reef habitat, limits the longterm impacts of harvest and helps to promote sustainability of the habitat. 

Privately held submerged lands and sovereignty submerged lands (leased grounds) typically have 

far fewer constraints as to regulations or restrictions on site-specific production, harvest, 

seasonality, or conservation/replenishment requirements. Most states, which lease submerged 

lands, apply specific terms and conditions to the use of those areas. 

Conservation/Replenishment 

Specific conservation and replenishment measures (as opposed to indirect results of fishery 

management measures) vary among states, and not all states have measures in place. Some 

states or localities have harvest surcharges that fund replenishment of historic bottom via 

cleaning of bars, relocation of buried shell, or the purchase and re-planting of shell initially 

removed as part of the fishery harvest. Virtually all states require all barren shell brought up as 

by-catch to the harvest be replaced back on the bottom. Other states have voluntary or mandated 

buy-back programs whereby processors are compensated for making harvested shell-by-product 

available to return back to productive bottom areas. 

Beyond the indirect result that fishery closures (for whatever regulatory reason) have in terms of 

creating non-harvest or sanctuary areas, some states have enacted programs or regulatory 

processes whereby small, isolated or large contiguous areas are set aside as off-limits to harvest 

(NCDENR 2001). This strategy is typically done to maintain a broodstock reserve area, whereby 

larger, more fecund oysters are intended to be retained to allow natural re-seeding of adjacent 

areas. Less frequently, historic oyster reef areas have been set-aside principally as habitat 

sanctuary areas, where the primary focus of the site is for the oyster reef to serve as benthic 

community habitat. In either case, both oyster-specific and ecosystem-specific benefits may be 

realized. 

There is conflicting information as to whether intentional bottom-disturbing activities (barcleaning, bagless dredging, suction dredging of sediments off of lightly buried shell, or the very 

act of harvesting and culling oysters via various gear types) benefits or impacts natural oyster 

reef habitat. Some advocates contend that this method cleans shell surfaces to allow a new 
52

settlement of oyster larvae, while others contend that these measures only result in additional 

destruction of three-dimensional habitat (Maryland DNR 2002). 

 International

Oyster resources in Mexico are managed by the Secretaria de Medio Ambiente Recursos 

Naturales y Pesca (SEMARNAP). In some areas of Mexico, the management of oyster beds is 

under the control of fishermen’s cooperatives. The Canadian oyster resources are managed by 

Canada DFO primarily through seasonal and gear restrictions. 

4.5 Other Natural and Manmade Impacts 

Introductions (C. ariakensis)

The state of Maryland and the Commonwealth of Virginia have proposed to intentionally 

introduce a non-native oyster species, Crassostrea ariakensis, into the Chesapeake Bay and other 

state tidal waters. This species, which is native to Asia, appears to have greater resistance to the 

pathogens responsible for MSX and Dermo. However, little is known about the life history and 

ecology of C. ariakensis in its native range, and current information is insufficient to predict the 

impacts an introduction of C. ariakensis might have on the native C. virginica oyster. 

In 2003 the U.S. Congress authorized the Army Corps of Engineers to prepare an Environmental 

Impact Statement (EIS) to examine both the risks and benefits of introducing this species to the 

Chesapeake Bay. The EIS is being conducted by the Corps as the lead federal agency, with the 

states of Maryland and Virginia serving as lead state agencies. The U.S. Environmental 

Protection Agency (EPA), National Oceanic and Atmospheric Administration (NOAA), and Fish 

& Wildlife Service (FWS) are cooperating agencies on the EIS. 

In 2004 the NOAA Chesapeake Bay Office (NCBO) initiated a Non-native Oyster Research 

Program funded at $2M annually to obtain the scientific information needed to prepare a 

thorough EIS. The program is aimed at research priorities recently identified by the National 

Research Council (National Research Council 2004) and the Scientific and Technical Advisory 

Committee of the Chesapeake Bay Program (STAC 2004), as well as guidance from the 

International Code of Practice on the Introductions and Transfers of Marine Organisms (ICES 

Code of Practice). This 3-year, $6M research program is scheduled to complete all the highest 

priority research needs by the end of 2007. 

Major research topics under investigation include: 

1. Understanding C. ariakensis within its native geographic range 

a. Taxonomy, population genetics 

b. Pathogens 

c. Ecology, reef building, phenotypic variation 

2. Potential for population growth and sustainability of C. ariakensis in Chesapeake Bay 

a. Demographic model 
53

b. Larval transport model 

c. Gametogenesis, fecundity, spawn cues, sex ratio 

d. Fertilization efficiency coefficient 

e. Larval temperature & salinity tolerances 

f. Larval mortality 

g. Larval physiology, behavior, metamorphosis 

h. Settlement cues, substrate preferences 

i. Juvenile mortality - mesohaline predation 

j. Juvenile mortality - polyhaline predation 

k. Juvenile/adult mortality - low DO, sediment 

l. Growth rate 

m. Triploids as surrogates for diploids 

3. Susceptibility of C. ariakensis to known disease-causing parasites and pathogens 

a. Bonamia spp. 

b. Herpes virus and vertical transfer 

c. Other viral pathogens 

4. Interactions between C. ariakensis and native oyster species 

a. Hybridization, gamete competition 

b. Competition (food, space, etc.) 

c. Spawning cues 

5. Human consumption risk 

a. Fecal coliform uptake, clearance rates 

b. Viral and protozoan human pathogens 

c. Post-harvest pathogens levels 

6. Potential for C. ariakensis to become a fouling nuisance 

a. Larval substrate preferences 

b. Fouling potential 

7. Ecosystem services and functions 

a. Reef building 

b. Filtration and water quality 

c. Food web dynamics 

8. Economic and cultural impacts 

Possible risks associated with an introduction of C. ariakensis have been described by the 

National Research Council (2004). Risk factors that could affect C. virginica include: 

Disease 

• A new disease-causing organism might be introduced that would negatively impact C. 

virginica. 
54

• C. ariakensis might serve as a reservoir for pathogens, thus increasing levels of infection for 

C. virginica. 

Direct Interspecific Interactions 

• C. ariakensis might out-compete C. virginica for limited hard substrates. 

• Cross-fertilization could result in reduced reproductive success for C. virginica (i.e., gamete 

sink) or hybridization. 

As the federal research program and EIS work continue, these factors will be evaluated to form a 

clearer picture of how C. ariakensis might impact native C. virginica populations in the 

Chesapeake Bay. 

 

 

“Harmful” algal blooms (HAB)

Oysters, and other bivalve mollusks, were historically-characterized as “symptomless” 

concentrators of toxins produced by “red-tide” dinoflagellates. Sophisticated surveillance and 

management programs have been instituted worldwide to prohibit shellfish harvest when toxic 

algae have the potential to contaminate shellfish tissues (Shumway 1996). More recently, 

detrimental effects of some microalgal taxa, including but not limited to those that produce 

toxins that threaten human consumers of shellfish harvested during red-tide blooms, have 

become recognized (Shumway 1996; Landsberg 2002). 

Over their extensive range, eastern oysters are exposed to several species of toxic or harmful 

algae, which tend to be more constrained geographically. In a north-to-south listing, the 

following HAB-oyster interactions occur; observed effects upon oysters are summarized: 

Alexandrium fundyense, A. tamarense: This is the well-known, “New England red tide” alga; 

two species have been implicated, but these may be conspecific. This dinoflagellate produces a 

suite of neurotoxins (termed ‘saxitoxins’) that interfere with sodium-channel transmission of 

nerve impulses in exposed vertebrates (Kao, 1993). Clinical effects include loss of motor control 

and respiratory failure in extreme cases (Backer et al., 2003). Eastern oysters retain toxins 

produced by these microalgae while feeding and are able to tolerate relatively-high tissue 

concentrations without suffering direct mortality. A recent study demonstrated adductor-muscle 

paralysis in oysters exposed to cultured A. fundyense (Hégaret et al., submitted), but respiratory, 

feeding, and hemocyte functions remained unaffected. Thus, A. fundyense does not appear to be 

acutely-toxic to oysters, but adductor-muscle paralysis may leave oysters more susceptible to 

predation, as they are unable to close the shell. A recent report indicated, however, that at least 

some vertebrate predators (in addition to humans) avoid eating saxitoxin-contaminated oysters 

(Kvitek and Bretz, 2004). 

Prorocentrum minimum: This dinoflagellate is a common bloom-forming species in coastal bays 

from New England through the Mid-Atlantic region. Production of mammalian toxins by this 

taxon is unclear, but no cases of human poisoning directly attributable to P. minimumcontaminated shellfish have been confirmed (Landsberg, 2002). P. minimum blooms – called 

“mahogany tides” in some places – have been associated with mass mortalities of oysters and 
55

other marine life (Heil et al., 2005). In some cases, environmental effects can be attributed to 

low dissolved oxygen following respiration of high-biomass blooms, but in other cases, direct 

toxicity was suggested (Grzebyk et al., 1997). Controlled laboratory studies have demonstrated 

acute and sub-lethal effects of P. minimum upon oysters and other mollusks (Wikfors, 2005); 

therefore, it appears likely that natural blooms impact survival, or at least health, of oyster 

populations. P. minimum blooms tend to be highly-localized and short in duration, and 

biological effects are dose and time-dependent. Thus, severe impacts are expected to be local 

and episodic. 

Heterosigma akashiwo: This raphidophycean flagellate has been recognized as a killer of finfish 

for many years (Honjo, 1994), but effects upon oysters and other mollusks have been studied 

only recently. The taxon affects fish chiefly by causing gill-tissue damage when reactive-oxygen 

radicals on the cell surface contact gill epithelial cells as fish respire. As is the case with finfish, 

mollusks respond to H. akashiwo by avoiding contact or mucous production at the gills when 

respiration becomes necessary (Wikfors, G.H., unpubl. obs. 2004). The taxon also has been 

shown to produce toxins (Khan et al., 1997) generally associated with some dinoflagellate taxa 

(gonyautoxins – see below), but biological effects of these toxins in H. akashiwo have not been 

demonstrated. H. akashiwo blooms tend to be even more localized and episodic than P. 

minimum, thus, they present a minor threat to oysters capable of minimizing contact through 

shell closure. 

Karlodinium veneficum: This dinoflagellate species (until recently, referred to as K. micrum) 

currently is emerging as an ichthyotoxic HAB in mid-Atlantic coastal bays and ponds (Deeds et 

al. 2002). Research in progress is showing biological effects of K. veneficum upon eastern 

oysters (Place, A., pers. comm. 2005), but the importance of this HAB in oyster habitats is not 

currently known. 

Aureococcus anophagefferens: Distribution of Aureococcus anophagefferens, originally 

recognized only on the northeast coast of the US (Anderson et al. 1993), now is recognized along 

the entire Atlantic coast of the US from Maine to Florida (Gobler et al. 2005). The Atlantic 

“brown tide” alga, a member of the Class Pelagophyceae, has been shown to interfere with 

feeding in mussels, scallops, and eastern oysters, sometimes causing mortalities (Gobler et al., 

2005). The hypothesized mechanism for effects in shellfish seems to involve an “anaesthetic” 

effect of A. anophagefferens cells on the ciliary motion by which bivalves move water through 

the gills for respiration and feeding (Gainey and Shumway, 1991). Mortalities appear to be a 

consequence of hypoxia in the palial cavity as water is not renewed (Bricelj et al., 2001). 

Mortalities of oysters associated with brown tide blooms have not been reported, but the mostdense blooms have occurred in areas where bay scallops and Northern quahogs are more 

abundant; these bivalves have been shown to suffer mortalities (Gobler et al., 2005). 

Pfeisteria piscicida and P. shumwaei: While not photosynthetic, these heterotrophic 

dinoflagellates generally are included in discussions of HABs. These species are thought to exist 

most of the time as predators on microalgae, and expression of toxicity and/or predation on fish 

and shellfish according to environmental cues not completely understood. Recently, predation of 

Pfeisteria cultures upon molluscan larvae has been demonstrated in the laboratory, and Pfeisteria

cultures have been shown to prey on post-set shellfish (Springer et al. 2002). Blooms of 
56

predatory Pfeisteria populations are relatively rare, considering that the species is present over 

much of the eastern oyster’s range. It appears that a narrow set of environmental cues is 

necessary for these species to express behaviors that make them a threat to bivalve mollusks 

(Burkholder and Glasgow, 2002). 

Alexandrium monilatum: Alexandrium monilatum is widely distributed in tropical and subtropical waters of the Gulf of Mexico and the southern Atlantic coast of North America, with 

reports of occurrence as far north as the Chesapeake Bay (Balech 1995). This dinoflagellate has 

very recently been recognized as ichthyotoxic, causing mortality in oysters and other mollusks 

(Pate, S. pers. comm. 2005); therefore, the environmental importance of oyster trophic 

interactions with this HAB are not fully understood. 

Karenia brevis: Contamination of oyster tissues with brevetoxins (previously referred to as 

‘gonyotoxins’) from the “Florida red-tide” dinoflagellate make oysters unfit for human 

consumption, but symptoms of stress or dysfunction in oysters exposed to this species have not 

been described (Landsberg 2002). 

Hurricanes 

Coastal estuaries in the Gulf of Mexico and South Atlantic, and in the mid Atlantic Bight to a 

lesser extent, are subject to hurricanes and the extreme climatic, hydrological, and environmental 

conditions associated with the passage of these weather phenomena. Hurricanes, tropical storms 

and associated flood events are essential elements in estuarine ecology. Oysters are inhabitants 

of coastal estuaries, and their broad environmental tolerances and prolific reproductive 

capabilities make them well suited to endure these short-term natural phenomena. 

Hurricanes have had devastating impacts on oyster production and its dependent economy (Engle 

1948; Ford 1970; Berrigan 1988; MacKenzie 1997c; Perret et al. 1999). Turbulent hydrologic 

conditions associated with hurricanes may result in habitat loss and damage to oyster reefs 

through various mechanisms; including destruction of reef integrity, removal of live oysters, 

burial, scouring, abrasion, and freshets (Berrigan 1988; Dugas et al. 1997; Perret et al. 1999). 

The severity of damage is often exacerbated or mitigated by local conditions; including tides, 

storm surge and rainfall. 

Extreme environmental, meteorological, and hydrological conditions associated with hurricanes 

are known to result in severe devastation to oyster reefs. Devastation includes losses or 

debilitation of oyster resources that may have short-term or long-term effects. Short-term effects 

may include loss of fishable stocks, disruption of recruitment, and loss of fishery revenues. 

Long-term effects may include the loss of all standing stocks, loss of recruitment, the cessation 

of fishing (harvesting), loss of reef integrity, and loss of reef ecology. There may be serious 

economic consequences for the dependent fishing community associated with both short-term 

and long-term impacts. 

Extreme conditions can result in physical damage to reefs and oyster populations. Severe 

hydrological conditions such as storm surge and wave action can scour oyster reefs, kill oyster 

spat and juvenile oysters, physically remove oysters from a reef, as well as destroy the structural 
57

integrity of a reef. Oysters may be washed from the reefs onto water bottoms that cannot support 

oysters where they become buried in soft mud and sediment. Under the most extreme conditions 

an entire reef can be destroyed by scouring, erosion and subsidence. In these cases, generations 

of reef development may be lost. Intertidal reefs and shallow subtidal reefs are frequently more 

susceptible to severe hydrological conditions. 

Excessive sedimentation over oyster reefs is also commonly associated with the severe 

hydrological conditions. The displacement of sediments and debris over oyster reefs results in 

burial and extensive mortality within oyster populations. Sedimentation results in long-term 

damage to reef integrity and functionality. It may take an extended period for sediments to erode 

from reefs, during which time oyster reproduction and growth are minimal. 

Oyster mortalities may continue after the direct effects of hurricanes, as unfavorable 

environmental conditions may persist long after the storm has passed. A combination of 

stressors, including rapidly changing salinity levels and water temperatures, decreased dissolved 

oxygen concentration levels, and high concentrations of potential contaminants may contribute to 

oyster mortalities as long as the conditions persist. Additionally, increasing amounts of 

freshwater will drain into the estuaries as flood waters recede in the drainage basins throughout 

the region, and prolonged periods of reduced salinity levels may also contribute to continuous 

mortalities. When adverse conditions are prolonged, there is an increased likelihood that natural 

reproductive cycles, larval development and spat setting will be affected. Critical spawning and 

setting peaks may be interrupted, and disruption of these cycles may have impacts on oyster 

populations that continue for several years. 

Oyster resources in the Gulf of Mexico have been directly impacted by a series of hurricanes in 

2004 and 2005, including Hurricanes Charley, Ivan, Dennis, Katrina and Rita. These hurricanes 

adversely affected most of the productive shellfish growing areas from Florida Bay to Galveston 

Bay. All of the Gulf States are engaged in assessing oyster resource losses and economic 

impacts to the oyster fishery resulting from these hurricanes. Preliminary information suggests 

that oyster resource losses range from extensive ecological damage to short-term disruption in 

local fisheries. In almost every case, the levels of damage are commensurate with the severity of 

storm conditions encountered and the proximity of the affected estuary to the storms’ path. 

Following hurricanes, oysters will repopulate estuaries when environmental conditions stabilize 

and suitable substrate is available. Under favorable conditions, oyster production may even be 

enhanced as a result of nutrient inputs and the depression of marine predators which may be 

associated with flood events. Also, available substrate on existing oyster reefs may be enhanced 

by wave action and scour which acts to expose clean substrate when conditions are not too 

severe. Numerous reports describe the re-establishment of productive oyster reefs after 

populations were decimated by hydrologic events associated with storms and floods (Hofstetter 

1981 1988; Berrigan 1988 1990; Perret et al. 1999). Full recovery of buried reefs is usually 

accelerated by restoration activities to enhance reef substrate. 
58

5. Aquaculture 

5.1 Cultivation and Aquaculture of Eastern Oysters 

Oysters have been subjected to some level of “domestication” since the time of the Roman 

Empire, when oysters were held in coastal ponds for convenient harvest as needed (Clark 1964). 

In the Western Hemisphere, Native Americans harvested intertidal eastern oyster populations 

prior to European colonization, and access to sub-tidal populations in the northern portion of the 

species’ range was limited by relatively-primitive harvesting tools – wooden rakes and tongs. 

Unsustainable harvest of natural oyster populations, enabled by importation of the dredge from 

Great Britain, led to declines in commercial production as early as the mid-eighteenth century in 

some coastal waters (Kochiss 1974). These declines led to directed efforts to increase the 

quantity and reliability of commercial oyster production (Belding 1912; Kochiss 1974). Two 

problems needed to be solved for effective management of oyster production: one was social – 

establishment of ownership of shellfish stocks in publicly-owned bottomlands, and the other was 

technical – improving recruitment of seed oysters. 

As most waters from which eastern oysters were traditionally harvested are under state or 

municipal jurisdiction, these governmental entities were responsible for establishing programs by 

which individuals could have exclusive rights to shellfish populations. In states that established 

programs for leasing bottomlands (e.g., Connecticut, Massachusetts), shellfishers had motivation 

to cultivate beds (alleviate siltation and eliminate predators) and harvest sustainably. 

Nevertheless, most commercial production on leased bottomlands has relied upon natural 

settlement of spat. 

Cultivation of oysters for human consumption is accomplished with a wide range of 

technological sophistication. The simplest form of oyster cultivation involves simply moving 

wild oysters from one place to another. Oysters can be collected in areas of high spat settlement 

or in waters closed to harvest because of sewage contamination and moved to bottomlands where 

sufficient growth and/or depuration occurs prior to harvest. When oysters are moved for 

purposes of depuration, this activity is generally referred to as “relay;” whereas, moving oysters 

for growth is referred to as “aquaculture.” Relocation of young oysters to growing beds also 

generally involves adjusting the population density, or proximity of oysters to each other. The 

oyster farmer attempts to distribute oysters such that competition for phytoplankton food does 

not limit growth, but otherwise to optimize production within the confines of his lease. Planting 

of oysters directly on the bottom at relatively high density may attract predators, such as crabs 

and starfish, leading to high predation loss to the oyster farmer. Thus, the next level of 

technology applied to oyster cultivation is protection of the beds from predators. Although 

various techniques have been considered, practically speaking, “mopping” for starfish (dragging 

the bottom with cloth string arrays to which the spiny skin of starfish adhere and killing the 

starfish on-board the boat with hot water) and “potting” (trapping) of crabs are most-commonly 

employed. 

In this context, production on leased beds of wild-caught oysters may not be considered 

“aquaculture,” in the strictest sense, but rather resource enhancement through habitat 

modification. 
59

One definition of domestication is: 

“that process by which a population of animals becomes adapted to humans and to the 

captive environment by some combination of genetic changes occurring over generations 

and environmentally-induced, developmental events re-occurring during each 

generation.” (Price 1984)

Thus, as developmental events are not induced during each generation, cultivation and protection 

of naturally-spawned, unselected oyster populations can be considered population enhancement, 

rather than aquaculture. Some alteration in the genetic structure of oyster populations 

“amplified” by mitigation of “natural” loss terms in population dynamics (burial, predation) can 

occur if oysters that would have been thusly removed before reproducing are permitted to spawn. 

Nevertheless, ultimate harvest of these oysters immediately upon achievement of commercial 

size would minimize the effects of this type of “aquaculture” upon un-cultivated oyster 

populations, while increasing commercial production. If anything, this “aquaculture” activity 

reduces harvest pressure on unmanaged, natural populations. This type of cultivation of wild 

oyster populations accounts for most current, commercial aquaculture production in the Gulf and 

northeast Atlantic coast states. 

Aquaculture production based upon “domesticated” breeds of oysters, selected and spawned in 

captivity on a continuing basis, is relatively new and small in scale (Committee on Nonnative 

Oysters in the Chesapeake Bay 2004), but provides a premium product, in terms of monetary 

value. The long-term investment in genetically-selected “breeds” of oysters and expense 

associated with captive breeding and rearing of seed justifies a greater investment in protection 

of livestock oysters being grown in captivity. Accordingly, on-bottom culture is being replaced 

by cage-culture technologies whereby predator exclusion is improved and cultivation (cleaning, 

moving) is facilitated. In regions where oysters achieve market size after first spawning, there is 

the potential for selected oysters to contribute to the gene pool of co-occurring, wild oyster 

populations; however, evidence from genetic analyses of populations on the Atlantic coast 

suggest that this has not occurred widely (Pers. Comm. Gaffney 2005). Further, reproductive 

isolation by culture of mainly sterile, triploid oysters – commercially motivated by faster growth 

rates – is increasingly being applied in oyster aquaculture (Baker 1996). Triploid oysters, 

whether produced by chemical induction or breeding of tetraploid males with diploid females, 

are extremely unlikely to contribute to the gene pool of local, wild oyster populations because 

gametogenesis and spawning of triploids is vanishingly rare. 

In summary, oyster aquaculture is practiced in two forms: one based upon collection of wild 

spat that amplifies natural production for commercial harvest with minimal effect on the local 

gene pool, and another that represents true “domestication” and is essentially reproductively 

isolated from wild populations. The main effect of aquaculture activities on oyster population 

biology is to provide a reliable, sustainable commercial harvest unaffected by fluctuations 

inherent in reproduction and recruitment of wild oyster stocks. 
60

6 Status of Population

In order to present information to assess the current status of eastern oyster populations 

throughout their range, quantitative and preliminary assessments for eastern oysters were 

reviewed and available information is incorporated below. However, the BRT determined that 

this information was insufficient to fully evaluate the status of the species. Thus, a survey was 

developed as an additional tool. 

6.1 Quantitative Stock Assessments

Quantitative population models are typically used to determine the ability of fisheries resources 

to sustain themselves considering all the factors affecting their survival. This type of analysis 

has not been applied to oyster populations until recently most likely due to lack of in-depth 

studies on important life history parameters (Rothschild et al. 1994) and failure of state agencies 

to gather necessary data. Chesapeake and Delaware bays and the public oyster grounds in 

Louisiana were the only areas found to have recent quantitative assessments of oyster population 

status. 

Rothschild et al. (1994) may have produced the first successful, modern attempt at applying 

well-established stock assessment techniques to analyze oyster population dynamics. They 

found substantial overfishing was occurring in Chesapeake Bay early in the 1900s and in 1990. 

Their efforts were complicated by a lack of in–depth studies on oyster size, growth, mortality, 

and reproduction (Rothschild et al. 1994). Jordan et al. (2002) analyzed Maryland oyster 

population data and found significant differences in age-length determinations that probably 

caused elevated mortality estimates by Rothschild et al. (1994). The Chesapeake Bay 2002 

Comprehensive Oyster Management Plan contains recommendations to improve biomass-based, 

quantitative population models (COMP 2002). The Chesapeake Bay oyster fishery is classified 

as overexploited with a low/depressed relative abundance (Marsh 2004). 

In one directed effort to conduct ongoing, quantitative stock assessment, the Haskin Shellfish 

Research Laboratory has been conducting oyster stock assessment workshops for the New Jersey 

Delaware Bay oyster beds since 1999 (HSRL 2005). The 2005 Stock Assessment Workshop 

report indicates that the market-size component of the oyster population is in a period of negative 

surplus production as a consequence of five years of low recruitment. This means that the 

market-size population of oysters is expected to contract in 2005 even in the absence of fishing. 

The harvest in 2004 was one of the lowest on record since 1953 (HSRL 2005). The report does 

not assign a stock status to the New Jersey oyster population; however, the management 

recommendations include establishment of specific management parameters conducive to stock 

rebuilding in times of surplus production including harvest limits for 2005 (HSRL 2005). 
61

6.2 Preliminary Stock Assessments

States and regions involved in fishery management planning processes may be required to make 

determinations of stock status without the data necessary to conduct a complete population 

assessment. In this case the available fishery and biological data, or proxies for that data, are 

used in a preliminary population assessment. Delaware, North Carolina, Texas and the Gulf 

States Marine Fisheries Commission produced oyster fishery management plans with this type of 

assessment of oyster population status. 

The Delaware Division of Fish and Wildlife uses a proxy based on the percentage of market 

oysters in the standing stock to ensure the sustainability of the State’s oyster population. A 

projected harvest quota is established by analyzing the percentage of direct market oysters 

greater than 2.75 inches on five primary Delaware Bay oyster beds. The lower 95% confidence 

limit, average, and upper 95% confidence limit on the annual survey index of market oysters per 

bushel serve as harvest control thresholds. The use of these thresholds allows for protection of 

oyster populations even though age structure, recruitment patterns, or disease dynamics may be 

changing. Delaware Division of Fish and Wildlife is currently recommending use of the more 

conservative upper confidence limit due to several years of low recruitment and an anticipated 

oyster population decline in their portion of the Delaware Bay (DFW 2004). 

North Carolina developed a fishery management plan for its oyster resources in 2001 and 

assigned a stock status of “concern” for the oyster population. Stocks designated as “concern” 

are those stocks for which an assessment is incomplete but that show from available data that 

overfishing is a threat. For the subtidal oyster populations in the Pamlico Sound region, the 

stock status factors causing concern were high P. marinus mortality, reduced spatfall, low 

CPUE, and suspected limited spawning stock. The intertidal populations in the southern part of 

the state showed evidence of high harvest pressure and significant habitat disturbance. Annual 

CPUE data are monitored to establish limits on the fishery. The oyster season is typically closed 

early in many areas due to depletion of harvestable oysters (NCDENR 2001). 

The Texas Oyster Fishery Management Plan was completed in 1988. Stock status could not be 

determined but fishing effort was thought to be occurring at a level that exceeded optimum yield 

(Quast et al.1988). A further statement on the condition of the resource concluded that, at the 

time, oyster abundance levels could not support increasing fishing effort. Factors involved in 

this determination included: overall depressed oyster abundance on harvestable reefs, reduced 

spring spat settlement peaks, and poor survival of spat and small oysters due to fishing pressure 

(Quast et al. 1988). Galveston Bay was closed five times between 1979 and 1988 because oyster 

reefs in the area were determined to be overworked and damaged (Quast et al. 1988). There has 

not been an update of the Texas Oyster Fishery Management Plan since 1988 (Marsh 2004) and 

Texas Parks and Wildlife Department’s participation in Gulf States Marine Fisheries 

Commission planning process provides more recent evaluation of the Texas oyster fishery. In 

order to help stabilize fishing effort, the 79th Texas Legislature (2005) established a moratorium 

on licenses in the Texas commercial oyster fishery; whereby, only those individuals holding a 

license on August 31, 2005 were eligible to purchase a license in subsequent years, and a license 

must be renewed each year to maintain eligibility. In conjunction with the license moratorium, 

the Texas Parks and Wildlife Commission reduced the daily limit of oysters from 150 sacks to 90 
62

sacks per day (110 pounds per sack) in an attempt to lengthen the productive part of the season 

and provide for a more stable price structure for oysters taken throughout the duration of the 

open season. 

 

“The Oyster Fishery of the Gulf of Mexico, United States: A Regional Management Plan” makes 

the finding that oyster abundance and range in the Gulf appear to be more limited by 

salinity/temperature regimes and available substrate for setting than fishing pressure (Berrigan et 

al. 1991). The plan goes on to state that overharvest may occur but that it is harvest above 

optimum yield only (Berrigan et al. 1991). Optimum yield is defined as: 

All the adult oysters of practical value and use that can be harvested from a given reef 

area provided: 1. The shell (or an equal or greater amount of other cultch material proven 

to be as effective as the whole oyster shells in catching and retaining spat) are returned to 

the reef in the same areas that harvest occurred; and 2. Freshwater from natural stream 

sources and runoff to the reef are maintained or restored in a manner that, a) eliminates 

contamination from harmful substances to the oyster or man (as a result of consumption) 

and b) optimizes salinity, temperature, water flow and nutrient conditions for oyster 

setting, growth and survival. (Berrigan et al. 1991) 

Powell et al. (1994) compared resistance to mortality between oyster populations in Chesapeake 

Bay and Galveston Bay using a time dependent energy flow model and found the Chesapeake 

Bay population required more conservative management measures possibly providing one 

explanation for the difference in the status of oyster populations in the two areas. 

6.3 Survey of Eastern Oyster Status 

This survey (see Appendix II) was developed as a telephone questionnaire for resource managers 

and independent experts and was designed to elicit responses from these experts as they pertain 

to individual estuaries, or shellfish growing areas (SGA), throughout the species range. 

Respondents were typically identified by members of the BRT due to familiarity with managers 

and experts in each state. Twenty resource managers and independent experts responded to the 

survey with information that covered 72 estuaries throughout the range of the eastern oyster 

(Appendix III). Given the numbers of estuaries in some states, there were many cases in which a 

member of the biological review team discussed the survey questions with the resource manager 

or independent expert and then allowed for the survey to be filled out at the respondents’ 

convenience. If necessary, the respondent was contacted to clarify any responses. The survey 

was meant to reflect the opinions of the respondent and may or may not reflect the official views 

of the agency or institution through which the respondent is employed. 

Survey responses were sought from both a resource manager and an independent expert from 

each state so as to avoid any perception of bias in regard to survey results. For instance, a 

resource manager may have the perceived bias of showing only the benefits of successful oyster 

resource management, while an independent expert (often an academic) may have the perceived 

bias of discussing only negative aspects of the oyster status in order to justify additional funding 

for research. Opposing responses between the resource manager and independent expert would 

have highlighted any potential bias; however, responses elicited by the survey showed no trend 
63

of consistent bias between respondents for individual states or estuaries. There were a few 

questions which often resulted in different responses; however, these differences are easily 

understandable in light of the lack of information available on oysters. The following are some 

examples of- and likely explanations for- such differences: 

• There were often differences in responses to the historic and current acreage present in 

each estuary but this often relies on best estimates and is highly dependent on the 

definition used to quantify such acreage. In some cases, acreage may be quantified as the 

total area available that could support oysters even if the presence/absence of live oysters 

is unknown. 

• There were often differing responses to the question “What, if any, do you perceive as the 

primary threats to the oyster population in this estuary/SGA?” In this case, differing 

responses may not necessarily indicate a disagreement between the resource manager and 

the independent expert. For instance, if each respondent had been privy to their 

counterpart’s responses, they may have agreed; however, given the wide range of threats 

that could impact oyster populations, it would be unreasonable to assume that 

independent responses would be exactly the same. 

• There were also differences in responses that seemed to be a result of a disparity in the 

information that each respondent had available to them. For instance, there were often 

differences in responses regarding the questions, “Has oyster restoration or enhancement 

work been done in this estuary/SGA?” and “If yes, is it conservation based or fisheries 

based?” One respondent might indicate that restoration was only fisheries based while 

the other respondent indicated that both conservation based and fisheries based 

restoration had occurred. In this case, it seemed most reasonable to conclude that 

differing responses were an indication that the first respondent was simply not familiar 

with the conservation based efforts. 

 

Shortcomings of the survey include a lack of responses from some regions (such as the 

Northeast) and a lack of independent expert responses from some states. However, it provided 

the most comprehensive review that could be accomplished within a short time period of oyster 

resource status throughout the species range in the United States. The BRT believes this 

represents the most current information available to assist in evaluating the status of the species. 

The following is the analysis of the survey results. 
64

Oysters were reported as “present” in nearly all estuaries. 

Figure 7. Response to the question: Are oysters present in the estuary? 

Sixty-seven percent of estuaries support harvest, while 23% do not, and 10% have seasonal 

harvest. 

Figure 8. Response to the question: Does the estuary support harvest? 

Based on fisheries-dependent data, respondents indicated that oyster populations are stable in 

53% and unstable in 47% of the estuaries where they occur. 
65

Figure 9. Response to the question: Based on fisheries dependent analysis, is the 

population stable? 

Based upon fisheries-independent data, oyster populations were judged to be stable in 60% and 

unstable in 40% of the estuaries in which they occur. 

Figure 10. Response to the question: Based on fisheries independent analysis, is the 

population stable? 

For the purposes of the survey restoration and enhancement were defined as follows: 

Enhancement: the manipulation of the physical, chemical, or biological characteristics of 

a site to heighten, intensify or improve specific function(s). Enhancement results in a 

change in function, but not a change in acreage. 

Restoration is an umbrella term that includes enhancement, creation and reestablishment. Creation is the manipulation of the physical, chemical or biological 

characteristics to develop oyster habitat where it did not previously exist. Reestablishment rebuilds oyster habitat where it once historically or formerly existed. 
66

Over 80% of estuaries were thought to have sustainable populations in the absence of restoration 

and enhancement activities. 

Figure 11. Response to the question: Is the population stable without restoration and/or 

enhancement? 

Similarly, the proportion of estuaries thought to have sufficient recruitment is 81% and 

insufficient recruitment is 19%. 

Figure 12. Response to the question: Is there sufficient recruitment? 

Respondents reported that restoration and enhancement activities are occurring in 65% of the 

estuaries where oysters occur. Restoration and enhancement of oyster populations is associated 

with two main management goals, fisheries and conservation – often for both in the same 

estuary. There were no restoration or enhancement activities identified in approximately 35% of 

estuaries. 
67

Figure 13. Response to the question: Is restoration or enhancement work conservation 

based or fisheries based? 

The importance of diseases or parasites impacting oyster harvest was highlighted in roughly half 

of the estuaries. 

Figure 14. Response to the question: Is oyster harvest impacted by disease/parasites? 

Possibly as a response to the threat of disease and parasite transfer, interstate transfer of oyster 

seed is widely regulated. 
68

Figure 15. Response to the question: Is interstate transfer of oyster seed regulated? 

Regulation within state boundaries of oyster seed movement was less-often subject to regulation. 

Figure 16. Response to the question: Is transfer of oyster seed within state boundaries 

regulated? 

Harvest is regulated by both size and bag limit, with no limit in only 24% of estuaries reported. 
69

Figure 17. Response to the question: Is harvest regulation by size? 

Figure 18. Response to the question: Is harvest regulation by bag limit? 

Overall, shellfish managers felt that oyster populations are being successfully regulated in their 

estuaries. 
70

Figure 19. Response to the question: Are regulations sufficient? 

 Recruitment 

Sufficient 

Pop. Stable 

(FisheryIndendent) 

Pop. Stable 

(FisheryDependent) 

Restoration Sustainable 

without 

Restoration 

Population 

stable (FisheryIndependent) 

0.44 (0.016) 

Population 

stable (FisheryDependent) 

0.31 (0.094) 0.30 (0.113) 

Restoration ? -0.35 (0.054) -0.47 (0.009) -0.45 (0.012) 

Sustainable 

without 

Restoration ? 

0.50 (0.005) 0.47 (0.009) 0.31 (0.096) -0.43 

(0.017) 

Regulation 

Sufficient ? 

0.56 (0.001) 0.48 (0.007) 0.25 (0.178) -0.35 

(0.058) 

0.73 (0.000) 

Table 3. Matrix of correlations between responses to Survey questions relevant to 

population stability and sustainability (correlation coefficients and probability values in 

parentheses). 

Significant correlations (p<0.05) reveal consistency in respondents’ views of oyster populations 

in the estuaries for which they were reporting. In estuaries characterized as having sufficient 

recruitment, oyster populations were judged to be stable, by fisheries-independent criteria and 

sustainable without restoration, and regulations were judged to be sufficient to maintain oyster 

populations. In estuaries where oyster populations were thought to be stable, by fisheriesindependent criteria, production was thought to be sustainable without restoration efforts, and 

restoration activities were less likely to be occurring than in estuaries with less-stable oyster 

populations. Similarly, in estuaries where oyster populations were judged to be sustainable by 
71

fisheries-dependent criteria, restoration activities were less likely than in estuaries with 

unsustainable fisheries. The strongest correlation obtained made the emphatic point that 

estuaries wherein regulation was sufficient were seen as sustainable without restoration. 

In summary: 

• Oysters are widely distributed and harvested throughout their range; they were reported 

to be currently present in all but one of the estuaries (Upper Laguna Madre); 

• The surveys provided few data regarding historic and current oyster reef acreage 

estimates; 

• Available fisheries dependent and independent data (e.g., long term quantitative stock 

assessments) are insufficient to assess stability of populations; 

• Surveys strongly suggest that recruitment is sufficient to maintain the viability of the 

populations throughout its range except in a portion of the mid-Atlantic (e.g., Long Island 

Sound, Peconic Bay, Hudson-Raritan Estuary); 

• Restoration and enhancement efforts for fisheries and conservation are occurring 

throughout the range, but are more common in the north and mid-Atlantic; 

• In estuaries where restoration and enhancement efforts are occurring, they are considered 

necessary to sustain populations in roughly half of the estuaries in the mid and south 

Atlantic regions (presumably, to support commercially viable populations); 

• In the North Atlantic (specifically, Connecticut and Rhode Island) and the Gulf of 

Mexico, restoration and enhancement efforts are not necessary to sustain biologically 

viable populations but are considered important to maintaining a fishery and conserving 

ecosystem services. 

6.4 Extinction Risk Considerations

1) The species displays a wide range of survival strategies (i.e., it is both a colonizer and an 

ecosystem engineer, and has a high reproductive potential). The eastern oyster’s ability 

to adapt to a wide range of environmental conditions (e.g. tolerance for low dissolved 

oxygen and wide ranges in salinity and temperature) makes the species resilient. 

2) The species inhabits a naturally-variable environment; evidence is that past local 

extirpations and colonizations have been common over geologic time. 

3) Threats are many, but none are overwhelmingly dominant or advancing at a rate that 

would threaten the viability of the species throughout its full range; however, there may 

be some threats that are significant at a regional or local level. 

4) Fishery harvest declines, often cited as cause for alarm, are widely-recognized as 

unreliable indicators of population trends. Landings data are more a metric of fishery 

success rather than species abundance. 

5) Restoration efforts for oysters are motivated by interest in reclaiming ecosystem services 

and/or sustaining fisheries, not by a perceived need to protect the species itself. 
72

6) Domestication and farming of reproductively-isolated breeds of eastern oysters is 

expanding to satisfy market demand, with the ancillary benefit of moderating harvest 

pressure on natural populations. 

7 Conservation actions

7.1 Past, Current, Anticipated Actions

Oyster reef restoration has been carried out since the late 1800’s for the purposes of maintaining 

harvestable stocks of oysters. This work has historically been accomplished by oystermen and 

state resource managers. However in the last decade, restoration of oyster reefs has become the 

focus of a wide range of restoration practitioners interested in restoring oyster reefs for the 

purposes of conservation and provision of ecosystem services. While the goals of maintaining 

harvestable stocks of oysters and provision of ecosystem services are not necessarily mutually 

exclusive, the scale, techniques and funding associated with the two types of restoration are very 

different. 

The history of oyster restoration through the late 1990’s has been summarized by Luckenbach et 

al. 1999. Case studies included in this compilation summarize the main goals of oyster reef 

restoration. Besides the small scale efforts by oystermen in the late 1800’s and early 1900’s to 

replace oyster shell harvested from reefs, most oyster restoration has historically been managed 

by the states. The focus of these restoration efforts has been to maintain or increase oyster 

habitat through placement of cultch in areas where commercial harvest has been successful. The 

loss of cultch typically resulted from harvest or natural disaster. 

Many cultch materials and placement techniques have been used with varying degrees of 

success. Cultch materials have included harvested oyster shell, Rangia clam shell (often mined), 

crushed limestone, crushed concrete, gypsum fly ash, calico scallop shell, and fossilized (mined) 

oyster shell. Most state managed efforts have focused solely on the placement of cultch 

materials; however, a few small-scale efforts have also included dispersal of oyster seed (small 

juvenile oysters). Dispersal of cultch material was historically contracted to oystermen using 

their own boats but much of the larger scale work is now done by blowing cultch off barges with 

high pressure water cannons. 

Funding has been provided for oyster reef restoration focused on maintenance of commercially 

harvested reefs through a number of sources, including taxes placed on harvested oysters. 

Federal relief following natural disasters such as hurricanes has also provided a large source of 

funds for areas in the northern Gulf of Mexico. For instance, following Hurricane Ivan in 2004, 

Federal relief funding in the amount of $9 million dollars was divided between the states of FL, 

AL, MS and LA for the purposes of oyster reef restoration. Most states also provide funding to 

support continuous and on-going restoration efforts and some states have also enacted laws to 

ensure that harvested oyster shell is returned to the state for restoration projects. 

It is difficult to estimate the number of acres that have been restored for the purposes of 

commercial harvest. Each state manages very unique programs with funding levels varying 

greatly between years. However, it can be easily concluded that state resource managers 
73

typically support restoration projects of a much larger scale than projects done for the purposes 

of conservation and ecosystem services. For instance, the State of Alabama has proposed to 

rehabilitate approximately 1,800 acres of “prime oyster bottom” through placement of cultch 

over a three year period beginning in 2005; whereas, oyster reef restoration for the purposes of 

conservation in Alabama have totaled less than 5 acres over the last 4 years. Louisiana also 

regularly plants cultch material on public oyster bottoms, with the most recent being the planting 

of nearly 250 acres in the spring of 2003. Since 1919, Louisiana has planted over one million 

cubic yards of cultch material on public water bottoms. 

 

As stated previously, in the last decade many restoration practitioners have come to support 

oyster restoration efforts for the purposes of conservation and provision of ecosystem services. 

Specifically, construction of oyster reefs is supported as an avenue for protecting biodiversity, 

regulating nutrients in estuaries through water filtration, protecting shorelines from erosion, and 

providing habitat for many estuarine species. Restoration projects accomplished with these goals 

in mind typically do not promote oyster harvesting on the restored reefs. 

A number of groups are now involved in oyster restoration with these goals in mind including 

state resource managers, non-profit “environmental” groups, colleges and National Estuary 

Programs. Projects of this type typically involve the local community through volunteer 

activities, education and/or outreach and scientific monitoring. In fact, many of these projects 

are scientifically monitored with the goal of gaining a better understanding of the numerous 

ecosystem services provided by oyster reefs. The hope is that more completely understood and 

quantified ecosystem services will result in a stronger argument for increased funding and 

support for these types of oyster restoration efforts. 

Cultch materials used for these types of restoration projects are similar to those used for 

restoration of commercial reefs, including fossilized oyster shell, harvested oyster shell, marl 

rock and concrete rubble. However, a variety of techniques are used for deployment based on 

the size/scale of the project. For instance, the South Carolina Oyster Restoration and 

Enhancement (S.C.O.R.E.) program typically creates reefs that are less than 0.1 acre in size by 

utilizing volunteers to collect discarded oyster shell from local seafood restaurants. Once the 

shell has dried in open-air bins, volunteers then place the “recycled” shell into mesh bags and 

deploy them along tidal creek shorelines to provide settlement area for oyster larvae. On the 

other hand, restoration in the Chesapeake could be categorized as both small scale (typically one 

acre or less) and large scale (typically greater than 5 acres). Small-scale projects have included 

planting shell in various thicknesses and in three-dimensional piles, experimenting with various 

alternative substrates (stone, marl, slag, concrete rubble, concrete forms, coal fly-ash, porcelain, 

surf clam shell, and ocean clam shell), various planting densities, and various oyster “strains” for 

disease tolerance performance. Large-scale projects typically include some combination of the 

following: a) "bar-cleaning" of remnant sites to remove old, potentially diseased oysters and 

sometimes to remove silt; b) planting of additional shell substrate (where needed); and c) replanting with hatchery-produced spat-on-shell (in areas where natural spatset is not likely or 

occurs infrequently). 

Grant funding is a common mechanism used to accomplish oyster restoration for the purposes of 

conservation and ecosystem services. Federal monies appropriated through NOAA are likely the 
74

largest source of such funds available on a nationwide basis. For instance, in 2005 the NOAA 

Restoration Center awarded $8.9 million through the Community-based Restoration Program 

(CRP) to support marine and anadromous fisheries habitat restoration projects. However, given 

the competitive nature of the funding assistance program, only a portion of the available funds 

will be used to support oyster restoration projects in any given year. Of the more than 900 

projects NOAA has supported since 1996, approximately 10% have benefited the eastern oyster. 

In the Chesapeake Bay area, the U.S. Army Corp of Engineers is also a significant source of 

funding for oyster restoration efforts. Since 2002, USACE funding has been on the order of $3 

million dollars per year, for restoration activities in Maryland and Virginia waters. The 

Maryland portion of these funds has targeted reconditioning reefs via placement of additional 

shell layers on historic oyster bottom. These sites are then typically seeded with hatchery 

produced disease-free seed oysters, produced at the University of Maryland hatchery facility 

with funds provided by the state of Maryland and NOAA (implemented through the Oyster 

Recovery Partnership). In Virginia, efforts have focused on developing tributary-intensive 

restoration plans, whereby large portions of the historic oyster bottom are reconditioned with 

shell, and smaller areas are densely seeded with specific genetic strains of oysters in the hope of 

developing a localized disease-resistant population. 

Estimates of the acreage of oyster reef restored for conservation have been calculated based on 

the assumption that the majority is supported through the NOAA and USACE programs 

described above. NOAA Restoration Center administers a number of programs and funding 

mechanisms, including the CRP (mentioned above), Directed Appropriations, and the Damage 

Assessment and Restoration Program. The CRP granting program has supported the restoration 

of more than 74 acres of oyster reef from Maine to Texas, the vast majority of which does not 

experience harvesting pressure. Through Directed Appropriations, funds appropriated through 

Congress for specific local programs and projects, the Restoration Center has supported more 

than 1,034 acres of oyster reef restoration. An additional 16.5 acres of oyster reef restoration has 

been accomplished through the Damage Assessment and Restoration Program, whereby NOAA 

acts on behalf of the public to restore natural resources injured by hazardous spills or groundings. 

Since 1992, it is estimated that the USACE has supported the restoration of approximately 80 

acres of oyster reef each year. While the acreage associated with these projects appears 

substantial, it should be clarified that the vast majority of the NOAA effort has been focused on 

the Chesapeake Bay including 40 of the 74 acres of oyster reef restored by the CRP and more 

than 680 of the 1,034 acres restored through Directed Appropriations. Also, given the fact that 

NOAA and the USACE often support different aspects of the same oyster restoration project, it 

is not possible to simply add the NOAA acres with the USACE acres to determine an overall 

acreage of oyster reef restored in the Chesapeake. Adding the two acreage estimates together 

would greatly overestimate the numbers of acres restored. 

Another difficulty in determining the effects of oyster restoration on the status of the species 

results from the fact that individual projects have vastly different success criteria; therefore, an 

acre of “restored” oyster reef does not always equate to an acre of oyster reef restored to full 

function. Even if full function had been achieved on all 1,124+ acres of restored oyster reef, it 

would be difficult to argue that these actions have had a measurable impact on the status of the 

eastern oyster species throughout its range. 
75

There are several major weaknesses associated with oyster reef restoration done for the purposes 

of conservation and ecosystem services. The first two of these are closely associated with the 

grant funding process. First, is a lack of long term scientific monitoring to evaluate project 

success. While most projects are subject to some degree of monitoring during the early months 

following restoration, funds are typically not available to monitor the restored reefs through time. 

This artifact of the grant mechanisms, which typically awards funds for a 1-2 year period, results 

in a lack of long-term knowledge about the effectiveness and long-term sustainability of such 

efforts. Second, is a lack of large-scale restoration projects to truly test the effectiveness of 

oyster restoration for the purposes of ecosystem services. It is simply not possible to quantify 

the water quality improvement in the Chesapeake Bay resulting from a 0.5 acre restoration 

project. Therefore, arguments to increase funding and public interest in oyster restoration 

focused on conservation are more difficult to fully support. Small scale efforts are an artifact of 

the grant process due to range of awards provided. For instance, the Restoration Center 

encourages projects to apply for funding in the range of $35,000 to $250,000. Only a few 

projects can be awarded funding in the higher range due to the limited amount of funding 

available for all project types so large scale projects are unachievable on the small budgets 

available. The third weakness identified is a lack of wide-ranging support from state resource 

managers and state regulations. Most state resource managers must follow statutes that only 

considered restoration for the purposes of commercial harvest. They are often limited in their 

ability to pursue and encourage restoration in areas that are not open for harvest. 

In 2003, the NOAA National, Maryland and Virginia Sea Grant Programs brought together 

scientists, resource managers, and industry representatives from across the nation to prioritize 

key research areas for restoration of oyster resources. Developing populations of oysters better 

able to survive MSX and Dermo was identified as a high priority (UM-SG-TS-2004-02, VSG04-01). Several disease resistant strains are under development and two, the Haskin CROSBreed 

and Andrews DEBY lines, are available for commercial production (Allen et al. 2003). These 

selected lines are widely recognized as good candidates for aquaculture operations due to their 

initial disease resistance and fast growth but they may not be sufficient for restoration of wild 

populations because they may not survive long enough to produce disease resistant progeny 

(UM-SG-TS-2004-02, VSG-04-01). It is also currently unknown if these disease resistant traits 

can be transmitted to the natural population (Allen et al. 2003, Angione 2005). Hatchery 

production might be a genetic bottleneck if supportive breeding is not properly carried out (Allen 

and Hillibish 2000). Despite concerns about decreasing genetic variability through hatchery 

propagation, regional scientific groups initially advocated large scale seeding of Chesapeake Bay 

reefs with these disease resistant stocks (Allen et al. 2003). More recently, however scientific 

guidance has recommended against this due to evidence that beneficial traits may not pass to 

intermixed (native and specific-strain) offspring. Disease resistant stocks have been used in small 

scale public oyster restoration programs sponsored by Maryland’s Oyster Recovery Partnership, 

the Chesapeake Bay Foundation, and the US Army Corps of Engineers in Chesapeake Bay 

(Allen et al. 2003). Research is underway to establish the molecular genetic profile for these 

disease resistant oysters. This profile can be identified in subsequent generations, and will 

indicate the degree to which the selectively bred broodstock contribute progeny to the next 

generation of oysters (Allen et al. 2004). 
76

8 Research Needs 

8.1 Gaps in Knowledge/Future Needs

1) Fishery independent surveys 

2) Further genetic analysis of population structure with a focus on local or regional 

adaptations 

3) Research on proximity-recruitment relationship 

4) Research on effects of combined and chronic stresses including changes due to future 

climate change 

5) Continued research on disease susceptibility and development of selectively bred 

disease tolerant strains 

6) Emerging role of endocrine disruptor pollutants upon population biology 

7) Delineation of oyster habitat 

8) Compatibility of information 

9) Continued ecological risk associated with other oyster or other alien species 

introductions 

10) Control and abatement of threats from all sources 

11) Develop a standardized monitoring protocol at a local or regional level 

12) Research on the effects of changes in coastal development and demographics. 

9 Conclusion

Based on the available information, the BRT concludes that the long term persistence of eastern 

oysters throughout their range is not at risk now or in the foreseeable future. This completes the 

BRT’s evaluation of the status of eastern oysters throughout their range. This comprehensive 

status review, as compiled and deliberated by the BRT, incorporates and summarizes the best 

available scientific and commercial data to date.
77

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Appendix I. Glossary of Terms

Allele- An alternative form of a given gene that differs from other alleles in DNA sequence or in 

effect on phenotype. 

Allele frequency- The percentage of all alleles at a particular locus represented by a particular 

allele in the gene pool of a given population. 

Allozyme- Allelic form of an enzyme encoded at a given locus. Allozymes usually are 

distinguished by electrophoresis and histochemical staining and are observed as a difference in 

electrophoretic mobility due to differences in net electric charge or molecular weight. 

Balancing selection- A type of natural selection in which a diversity of alleles is maintained by 

the changing action of selection in a dynamic environment. 

Cline- A gradual spatial variation in allele frequencies exhibited by a species along a line of 

environmental or geographic transition. 

Directional selection- A mode of selection favoring phenotypes at one end of the population’s 

phenotypic distribution. 

Denaturing Gradient Gel Electrophoresis (DGGE)- A molecular technique that works by 

applying DNA (or RNA) to an electrophoresis gel that contains a denaturing agent. Certain 

denaturing gels are capable of inducing DNA to melt at various stages. Sequence differences in 

otherwise identical fragments often cause them to partially melt at different positions in the 

gradient and therefore stop at different positions in the gel. By comparing the melting behavior 

of the polymorphic DNA fragments side-by side on denaturing gradient gels, it is possible to 

detect fragments that have mutations in the first melting domain. Placing two samples side-byside on the gel and allowing them to denature together, researchers can easily see even the 

smallest differences in two samples or fragments of DNA. Similar to SSCP. 

Ecosystem services- Protection of biodiversity; regulation of nutrients in estuaries through water 

filtration; protection from shorelines erosion; provision of habitat for many estuarine species. 

Effective population size (Ne)- The number of reproducing individuals in an ideal population 

that would lose genetic variation due to genetic drift or inbreeding at the same rate as the number 

of reproducing adults in the real population under consideration. Typically, Ne is less than either 

a population’s total number of sexually mature adults present or the total number of adults that 

reproduced. Effective population number can be defined either in terms of the amount of 

increase in homozygosity (inbreeding effective number) or the amount of allele frequency drift 

(variance effective number). 

Electrophoresis- A procedure for separation of charged molecules in an electric field, for 

example, for screening of allozyme variation. Refers to the electromotive force (EMF) that is 

used to push or pull molecules through the gel matrix; by placing the molecules in wells in the 

gel and applying an electric current, the molecules will move through the matrix at different 

rates, towards the anode if negatively charged or towards the cathode if positively charged (note 

that gel electrophoresis operates as an electrolytic cell; the anode is positive and the cathode is 

negative). 

Enhancement- the manipulation of the physical, chemical, or biological characteristics of a site 

to heighten, intensify or improve specific function(s). Enhancement results in a change in 

function, but not a change in acreage. 
101

Fixation Index (Fst)- The proportion of the variation at a locus attributable to divergence among 

populations. Measures the reduction in total expected heterozygosity of the entire system of 

populations due to random drift and other differentiating processes among isolated populations. 

Gel- The matrix used to separate the molecules. When separating proteins or small nucleic acids 

(DNA, RNA, or oligonucleotides) the gel is made with different concentrations of acrylamide 

and a cross-linker, producing different sized mesh networks of polyacrylamide. When separating 

larger nucleic acids (greater than a few hundred bases), the preferred matrix is purified agarose 

(which is a seaweed extract). In both cases, the gel forms a solid but porous matrix that looks and 

feels like clear jello. 

Gel electrophoresis- A group of techniques used by scientists to separate molecules based on 

physical characteristics such as size, shape, or isoelectric point. Gel electrophoresis is usually 

performed for analytical purposes, but may be used as a preparative technique to partially purify 

molecules prior to use of other methods such as mass spectrometry, PCR, cloning, DNA 

sequencing, or immuno-blotting for further characterization.

Genetic distance- An estimation of the number of allelic substitutions per locus that have 

occurred since separation of a population pair. 

Genetic drift- Random changes in allelic frequencies that occur in each generation due to 

natural sampling errors. The rate of genetic drift increases as effective population size decreases. 

Gene flow- Genetically effective migration; the movement of genes among populations of a 

species. 

Genetic marker – A genetic factor (e.g., a gene or other identifiable portion of DNA) or gene 

product that results from random mutations in the DNA sequence which act as genetic 

milestones. An observable characteristic useful for genetic analysis. 

Haplotypes- A specific mitochondrial DNA pattern or a collection of co-inherited nuclear DNA 

alleles or markers. 

Hardy-Weinberg Equilibrium- A model that predicts that for genetic characters the frequency 

of alleles remains constant from one generation to the next so long as the organisms are diploid, 

reproduce sexually, mate randomly, and the population is not subject to mutation, migration, or 

selection. 

Locus-The site that a gene or molecular sequence of interest occupies on a chromosome (plural 

is loci-multiple genes of interest). 

Mitochondrial DNA (mtDNA)- A small proportion (<1%) of the DNA of eukaryotic cells that 

is nonnuclear, located within organelles in the cytoplasm called mitochondria, and appears to 

have been endosymbiotic in early eukaryotic cells, as its genetic code differs from the 

“universal” genetic code. Animal mtDNA is a double-stranded, circular molecule usually 

ranging in size from 14,000 to 26,000 bp. 

Panmictic- Well mixed, referring to the set of genotypes in a population.

Polymorphism- Multiple alleles of a gene within a population, usually expressing different 

phenotypes. 

Polymerase Chain Reaction (PCR)- A molecular biological technique for amplifying (creating 

multiple copies of) DNA without using a living organism, such as E. coli or yeast. The technique 

allows a small sample of DNA to be copied multiple times so it can be used for analysis. 

Restoration- an umbrella term that includes enhancement, creation and re-establishment. 

Creation is the manipulation of the physical, chemical or biological characteristics to develop 

oyster habitat where it did not previously exist. Re-establishment rebuilds oyster habitat where it 

once historically or formerly existed.
102

Restriction enzymes- An enzyme that cleaves DNA molecules at specific recognition 

sequences; for example, the enzyme Alu I cuts DNA at the sequences AG↓CT. In the producing 

organism, restriction enzymes are a defense against foreign DNA. In molecular genetics, they 

are utilized in cloning and population genetic analysis. 

Restriction Fragment Length Polymorphism (RFLP)- Variation in the length of DNA 

fragments generated within a species when treating longer DNA segments with a restriction 

enzyme. The variants may be due to differences in DNA sequence at the recognition site where 

cleavage occurs (i.e., the enzyme does or does not cut) or to variations in length of the cleaved 

segment (often due to presence of varying members of tandem repeat motifs). 

Secondary structure- The helical, or twisting, appearance of the DNA (or protein) molecule. 

Single Nucleotide Polymorphism (SNP)- DNA sequence variation, occurring when a single 

nucleotide: adenine (A), thymine (T), cytosine (C) or guanine (G) - in the genome is altered. For 

example, a SNP might change the nucleotide sequence AAGCCTA to AAGCTTA. A variation 

must occur in at least 1% of the population to be considered a SNP. 

Single-Strand Conformational Polymorphism (SSCP)- A molecular technique based on the 

concept that single stranded DNA form unique secondary structures and that the secondary 

structures are dependent on the sequence. A single base substitution may create different 

secondary structures. PCR-amplified DNA is denatured and run on a gel, and single stranded 

DNA with different secondary structures will have different mobility on the gel. Similar to 

DGGE. 

 
103

Appendix II. Survey Form

Resource Managers Eastern Oyster Survey 

One questionnaire will be completed for each estuarine complex or Shellfish Growing Area 

(SGA) in the state. (Only one section on regulations will be completed unless they vary between 

SGAs.) 

State______ 

Based on the 1995 National Shellfish Register of Classified Shellfish Growing Waters, how 

many SGAs are in this state?____ 

Is there significant oyster acreage in your state unaccounted for in the Shellfish Register? 

Yes___ Estimate # acres_____ No____ 

Notes_________________________________________________________________________

________________________________________________________________________

Name of estuary/ SGA _______________________ 1. Are oysters present in this estuary/SGA? Yes_____ No _____ Unknown _____ 

2. Does oyster harvest take place in this estuary/ SGA? Yes _____ No _____ 

3. What percentage of reefs are in harvestable areas? _____% unknown _____ 

4. Is harvest allowed? Seasonal ___Year round _____ Not applicable____ Usual 

Season _________________________________________________________ 

5. Is your population estimate within the estuary/SGA stable based on fisheries dependent data? Yes 

_____ No _____ Unknown _____ 

Stable based on fisheries independent data? Yes____ No____ Unknonwn____ 

Notes__________________________________________________________________________________

_________________________________________________________________________________ 

6. Does sufficient recruitment take place within the estuary/SGA to sustain the viability of the 

population? Yes _____ No _____ Unknown _____ 

Notes__________________________________________________________________________________

_________________________________________________________________________________ 

7. Has oyster restoration or enhancement work been done in this estuary/SGA? Yes _____ No _____ 

Unknown _____ 

If yes, is it conservation based or fisheries based? _______________ 

If yes, would the population sustain itself without restoration efforts? Yes____ No___ 

Unknown____ 

Notes__________________________________________________________________________________

_________________________________________________________________________________ 

8. Do disease or parasites put the sustainability of harvested oysters at risk? Yes _____ No _____ 

9. Do disease or parasites put the sustainability of non-harvested oysters at risk? Yes __No _____ 

10. Do you have an estimate of “historic” total oyster acreage for this estuary/SGA or for the entire 

state? Yes _____ #acres _____ No _____ 

Notes__________________________________________________________________________________

_________________________________________________________________________________ 

11. Do you have an estimate of current total oyster acreage for this estuary/SGA or for the entire 

state? Yes _____ #acres _____ No _____ 

Notes__________________________________________________________________________________

_________________________________________________________________________________ 
104

Threats to oyster populations 

1. What, if any, do you perceive as the primary threats to the oyster population in this estuary/SGA?

(i.e. Habitat Threats; Overutilization threats; Predation & Disease threats; Regulatory threats; Other natural 

or manmade threats) Explain. 

Notes__________________________________________________________________________________

_______________________________________________________________________________________

______________________________________________________________________________

Regulations (*Please request an electronic copy or website for state regulations)

1. Does your state have harvest gear size and type restrictions? Explain 

________________________________________________________________________

____________________________________________________________ 

2. Are private leases allowed in your state? Yes _____ No _____ 

3. What is your states total lease acreage? _____# of acres or _____% of total. 

4. What is your states total harvestable public acreage? _____# of acres or ______% of 

total. 

5. Do you allow seeding of private leases from public reefs? Yes _____ No _____ 

6. Is there a minimum oyster size limit? Yes _____ No _____ What is it? _____ 

7. Are there daily sack limits on public reefs? Yes _____ No _____ 

Notes___________________________________________________________________ 

8. Do you feel that current regulations are sufficient to sustain oyster populations? Yes 

_____ No _____ 

Notes__________________________________________________________________________________

___________________________________________________________________________ 

9. Does your state regulate the amount of shell or cultch material that must be returned to the 

estuary after harvest? Yes _____ No _____ 

Notes__________________________________________________________________________________

___________________________________________________________________________ 

10. Does your state have regulations for inter-state oyster or seed transport/transplant? Yes____ 

No_____ If so, are they adequately enforced? Yes____ No____ 

Does your state have regulations for intra-state oyster or seed transport/transplant? Yes____ 

No____ If so, are they adequately enforced? Yes____ No____ 

Notes_____________________________________________________________________________________

____________________________________________________________________________________ 

Survey completed by: ________________ ___________________

Agency/Organization_____________Title: ___________________ 

Phone # ____________________E-mail:____________________ 

Biological Review Team Member: ___________ _____________ 

Date: ______________ 

Survey # _____ of _____ for State _____ 
105

Appendix III. 

This table shows each state and the number of estuaries/ SGAs for which responses were sought 

on the survey. It also shows if responses were successfully obtained from both a resource 

manager and an independent expert. 

State # estuaries/ SGA Resource Manager 

(Y/N) 

Independent 

Expert (Y/N) 

Alabama 2 Y Y 

Connecticut 1 Y N 

Florida 24 Y (for 24) Y (for 6) 

Georgia 8 Y Y 

Louisiana 8 Y Y 

Maine N N 

Massachusetts N N 

Mississippi 1 Y N 

Maryland 1 Y N 

New Hampshire N N 

North Carolina 2 Y Y 

New Jersey 2 Y (for 2) Y (for 1) 

New York 4 Y Y 

Rhode Island 3 Y (for 3) Y (for 1) 

South Carolina 7 Y N 

Texas 8 Y N 

Virginia 1 Y N