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Abstracts
Session: Regional Updates
Chesapeake Bay and the Mid-Atlantic - Oyster Population Estimation in the Chesapeake Bay and Strategies for Restoration
Presented By:
Roger Mann, Virginia Institute of Marine Sciences, rmann@vims.edu
Steve Jordan, MD Department of Natural Resources, US-Environmental Protection Agency
Gary Smith, MD Department of Natural Resources
Kennedy Paynter, University of Maryland
James Wesson, Virginia Marine Resources Commission
Mary Christman, University of Maryland
Jessica Vanisko, MD Department of Natural Resources
Juliana Harding, Virginia Institute of Marine Sciences
Kelly Greenhawk, MD Department of Natural Resources
and Melissa Southworth, Virginia Institute of Marine Sciences
Steve Jordan, MD Department of Natural Resources, US-Environmental Protection Agency
Gary Smith, MD Department of Natural Resources
Kennedy Paynter, University of Maryland
James Wesson, Virginia Marine Resources Commission
Mary Christman, University of Maryland
Jessica Vanisko, MD Department of Natural Resources
Juliana Harding, Virginia Institute of Marine Sciences
Kelly Greenhawk, MD Department of Natural Resources
and Melissa Southworth, Virginia Institute of Marine Sciences
One of the principal goals of the Chesapeake Bay 2000 Agreement was a 10-fold increase in the biomass of the Bay oyster population by 2010. A collaborative between researchers in Maryland and Virginia was initiated to estimate baseline oyster population, and establish the monitoring, data management and data analysis frameworks for measuring progress toward the oyster restoration goal. Two estimators of the size of the oyster population in the Bay were developed as goals, these being the absolute number of oysters, and their biomass.
Both Maryland and Virginia support oyster fisheries. The Maryland fishery is almost exclusively a public access fishery. Virginia has an historical mix of a direct public access fishery and a private fishery based on leases of bay bottom. The leased private grounds can be planted with spat and harvested throughout the year once they reach the legal marketable size. Landings in Virginia illustrate a long-term decline and are at a level <1% of the historical high of the 1880's. Landings in Maryland have also declined rapidly from historic levels, then stabilized in the early 1990's, but are again in a state of decline. In both states the cumulative impacts of the diseases, MSX (Haplosporidium nelsoni) and Dermo (Perkinsus marinus) on harvest have been and continue to be substantial. Fishery dependent data contributes to population estimation in Maryland; however, the collapse of the fishery limits the use of such data in Virginia.
In Maryland, the Department of Natural Resources conducts an annual fall dredge survey at 43 sites that have been used to track changes in the oyster population as well as disease intensity and prevalence from 1990 to present. These sites are representative of the diverse types of oyster bars and have been supplemented with four additional sites that are closed to the fishery. They collectively comprise Maryland's sentinel sites and will be the focus of long term monitoring as part of the 2010 goal evaluation. Continuing field studies are focused on developing estimates of oyster dredge efficiencies that can be used to quantify the oyster population. In Virginia a combination of long term dredge data covering extensive areas has been combined with more focused patent tong surveys using stratified random sampling designs. The patent tong surveys provide a continuous data series for the James River for the period 1994- present. Long term data from the James using both dredge and tong data has been examined for application in developing a conversion function to estimate absolute abundance from dredge data. This conversion is the focus of continuing work. Virginia has identified 32 sentinel sites for long term monitoring of progress towards the 2010 goal, these being a combination of both historical sites and new sites in recent restoration areas. Identification and quantification of currently oyster habitat in both states remains an important aspect of this project in that these are estimated to be only modest fractions of historical habitat. The continued incorporation of refined habitat data through acoustic surveys will serve to improve population estimates.
Oyster density from fishery independent MD dredge surveys is estimated to range from 1.35 to 2.54 oysters per m2, as estimated using the total population size extrapolated over an estimated area of high and low quality habitat available in the bay. In terms of biomass, it is reasonable to assume that the dry tissue weight of an average oyster is 1 g, so the total biomass would be between 5 x 106 and 6 x 108 kg. These estimates include all small (<76 mm) and market (>76 mm) oysters, but do not include spat. Estimates of the standing stock of market oysters in 2000 were 702,000 bushels for Maryland. Using fishery independent and dependent surveys, the 1994 baseline population estimates range from 0.29 to 1.00 x 109 oysters, with a mean of 0.64 x 109 oysters in Maryland. In Virginia, intensive patent tong surveys in the James River between 1993 and 2000 indicate a fairly stable population of 4.41 and 6.30 x 108 oysters with a current standing stock of approximately 365,000 bushels. Depending on location market oysters vary from <1% to 42% of the standing stock by number. Extrapolation of dredge data for larger spatial areas in other rivers and regions in the Virginia portion of the Bay contribute to grand total estimates for the Virginia productive bottoms that vary from 5.31 x 109 to 6.00 x 1011 oysters. Present productive oyster ground in Virginia is well below the total surveyed by Baylor. The current calculations, which are arguably very optimistic, use only the modest values for high and moderate potential bottom area, as described by Haven et al. (1981); together they constitute only 24.85% of the total Baylor area within the Virginia portion of the Bay. Sub-market (<76mm) size oysters constitute a mean of 79% of the total (spat not included) for the period 1993-2001 in Virginia populations, a much higher percentage than in Maryland. Consequently biomass estimates for oysters in Virginia, which vary from 2.16 x 106 and 2.45 x 108 kg, are comparable to those for Maryland even though Virginia has an estimated higher population.
Young of the year recruitment, commonly termed spatfall, is assessed in both states by fall dredge surveys. In addition, Virginia employs shell string surveys. The Maryland Spat Index follows recruitment at the 43 sentinel stations for a period of over 60 years. The majority of years since 1986 show values in excess of the long-term median, indeed six years are above the 75% quartile; however, the past three years are generally between the 25% quartile and median. The comparable Virginia Spat index calculated for the past 16 years for 5 stations in the James River shows slightly higher median and quartile values than the Maryland long term record. VA indices within the past decade have generally remained between the 25% and 75% quartile. Both Maryland and Virginia enjoyed high spat indices in 1991, but the 1997 high value in Maryland was not reflected in Virginia where the lowest index in the 16-year record was recorded. The diseases, MSX (Haplospridium nelsoni) and Dermo (Perkinsus marinus) remain as threats to oyster populations in both states, and a substantial portion of natural mortality is directly attributable to one or both diseases. VIMS has maintained a disease monitoring program for MSX since 1960. A general trend of increasing maximum prevalence is observed over the 40-year period with intervals of decreased activity in the early 1970s, 1980s, and 1990s. Recent years have been characterized by very high maximal values. Maryland, generally observing lower salinity than Virginia across its 43 sentinel sites, has recorded considerably lower MSX prevalence since 1991, although this has notably increased above the average in the 2000-2002 period. The cumulative impacts of disease on long-term population trends are evident in recent analysis of stock assessment in the Maryland region of the bay. In the low salinity (<12 ppt) zone populations are moderate and stable in the face of limited disease pressure (MSX is rare and Perkinsus only in low intensity). In the mid salinity zone (12-14 ppt) a declining population from 1985 through 1994 was followed by a modest recovery in 1995-1999. By contrast no recovery has been observed in the high salinity zone (>14 ppt) where MSX in enzootic and Perkinsus causes consistently high mortality. The Virginia oyster resource is all predominantly in the > 14ppt region. Over the 1991-2001 period Maryland surveys have recorded sustained prevalence of a marked increase in mean intensity in 2000-2002, and mortality in excess of 30%. Comparable data for Virginia have been collected along a salinity gradient in the James River from Deep Water Shoal to Wreck Shoal in a downstream direction.
Both Maryland and Virginia record their survey data as oyster per unit area, and size frequency as 5mm size classes. Size frequency data is being used in a length-based analysis of individual oyster growth. Oyster growth provides an essential link between spat settlement and the fishable stock. In order to evaluate potential oyster management options, it is essential to understand population dynamics and quantify parameter such as growth, natural mortality (M), fishing mortality (F), and recruitment (R). Additionally, size frequency data will be used to estimate M in Virginia populations, where F is negligible and in Maryland within fishery closures. These data can then be compared to box (articulated oyster shells) counts that comprise present annual mortality estimates in Maryland, but are believed to be biased high due to dead oyster shells that fail to disarticulate within a year. Refined oyster population parameters can be used within a framework to examine the efficacy of various restoration and management options. In stable populations knowledge of M can allow prediction of year strength recruitment in older year classes with the option to effect area management wherein submarket size class strength, after debiting for M, can facilitate prediction of harvestable stock (F, fishery mortality) for the subsequent year. The only stable population in Virginia, that is a population with minimal disease mortality, is in the low salinity sanctuary of the upper James River. Estimates for mean annual M values for 1, 2, 3 and 4 year olds are 0.16, 0.19, 0.55 and 0.14 respectively based on transformation of size demography to age demography using unpublished growth data and annual survey data. However, extensive analysis of historical survey data for sub market and market oysters in concert with fishery landings suggest that submarket abundance is not a statistically robust predictor of fishery landings in subsequent years. Why is this? It is because consistent heavy disease pressure has effectively reduced F to near zero values, while M is both spatially and temporally variable depending on salinity and temperature as promoters of disease pressure - although disease is not the only contributor to high values of M. A collective failure of surveys, population dynamics and disease dynamics studies to date has been our inability to formulate estimates of M (by size and age class), F and R in time and location specific manners in order to develop and implement rebuilding plans in the manner employed by the regional fishery management councils. Development of robust estimates of M and strategies to lower M and thus facilitate rebuilding through increasing R MUST BE THE COLLECTIVE HIGHEST PRIORITY OF BOTH POPULATION DYNAMICS AND DISEASE RESEARCH. Only through this strategy will there be any progress towards the 2010 goal. Further, we must all be aware that this highly visible barometer of restoration is also a commentary on our ability as a research community to respond to a defined biological problem.
Future management would benefit from definition of biological reference points, including over-fishing. A broader goal might also include developing a Total Allowable Catch (TAC) for Maryland alone, Maryland and Virginia together as part of a bi-state fishery, Virginia alone, and defined management zones based on salinity as a proxy for disease pressure. Once the above have been completed, evaluation of alternative reference points might provide new options for fishery management in concert with rebuilding to the 2010 goal. Finally, recognizing the unique value of shell as a habitat limiting resource for oysters, it is critical to improving R, a cost - benefit analysis of various methods of use of the shell resource is recommended. The suggested analysis will provide a framework for evaluation of introduction of oysters with lowered disease susceptibility on the genetic composition of the population in the Bay, with the long-term impact on estimation of M in the presence of continued disease challenge. Long-term data provide the basis to develop population models, examine possible stock recruit relationships, and examine the sensitivity of management options to variation in R, M and F.
ACKNOWLEDGEMENTS:
The authors wish to acknowledge funding sources for theses studies including National Oceanic and Atmospheric Administration (NOAA) Oyster Disease Program Grant number NA26FL0385-01, the EPA Chesapeake Bay Program, the NOAA Chesapeake Bay Stock Assessment Program grant number (NOAA AWARD no. NA07FU0539), the State of Maryland through the Maryland Department of Natural Resources, and the Commonwealth of Virginia. through both the Virginia Marine Resources Commission and the Virginia Institute of Marine Sciences.
LITERATURE CITED:
Haven, D. S., J. P. Whitcomb and P. Kendall. 1981. The present and potential productivity of the Baylor Grounds in Virginia. Va. Inst. Mar. Sci., Spec. Rep. Appl. Mar. Sci. Ocean. Eng. No. 243: 1-154.
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