Virginia Institute Marine Science
Population Assessment of Eastern Oysters (Crassostrea virginica) in the Seaside Coastal Bays
Project Description as Proposed:
Declines of oyster populations and commercial harvest from the seaside have followed similar patterns, though not as severe, as those in Chesapeake Bay. High prevalence of Dermo disease (Perkinsus marinus) and MSX disease (Haplosporidium nelsoni) coupled with over harvest and habitat destruction have dramatically reduced populations. Nevertheless, there are several promising signs that significant enhancement of the population could be achieved with well conceived restoration efforts. Recruitment rates remain high and rapid growth allows oysters to reach reproductive size prior to disease mortality.
To plan a more a comprehensive restoration effort we need an estimate not only of the current standing stock of oysters, but also of their spatial distribution in the coastal bays. This is easier said than done in the complex of habitats that make up the coastal bays. Oysters in the area are naturally found in several intertidal habitats - patch reefs, fringing reefs and isolated, small clumps in the marsh. In addition, private lease holders create a variety of habitats for planting and rearing oysters that include both subtidal and intertidal habitats. An increasing amount of man-made structures, such as rip-rap and bulkheads provide habitat for oysters. Traditional stock assessment methods have involved only determining the density of oysters on "public" oyster reefs and restoration sanctuary reefs. Arguably, the majority of oysters in the region are not counted by this method.
Obtaining reliable estimates of the distribution and abundance of oysters on the seaside are beyond the scope of VMRC's resources and until recently posed several technical challenges. Fortunately, the Virginia Institute of Marine Science (VIMS) now possesses the tools to develop reliable population and distribution estimates for oysters on the seaside. These tools include aerial observations, ground-level sub-meter accuracy Global Positioning Systems (GPS), and an ArcView-based Geographic Information System (GIS). Combined with well designed sub-sampling schemes, these tools will allow VIMS to develop distribution maps and obtain accurate estimates of basin-wide distribution and abundance of oysters throughout the entire seaside. This 2-year project will provide spatially-explicit estimates of oyster populations throughout the region that can be used to help guide management and restoration efforts.
Spatially explicit distribution of C. virginica will be determined in two phases. In the first year of this project, oyster habitats (shell reefs/beds, marsh, and manmade structures) will be mapped in progressively finer resolution utilizing: 1-meter geo-referenced aerial images in an Arcview-based GIS; aerial surveys; and field mapping/ground-truthing (by boat and on foot) using sub-meter surveying GPS technology (Trimble XRS/Ranger TsCe). This initial mapping phase, which will include preliminary data on oyster presence and density, will contribute to developing an appropriate and rigorous oyster sampling design during year 2 of the project. During the second year, quadrate and transect sampling will be conducted in a stratified random design to provide estimates quantitative the abundance, density and size distribution, of oyster populations.
Mark Luckenbach, (757) 787-5816; firstname.lastname@example.org
1/1/2007 - 12/31/2007; Project Completed
Final Product Received:
"Population Assessment of Eastern Oysters (Crassostrea virginica) in the Seaside Coastal Bays: Habitat Mapping" (pdf)
Project Summary Provided by Grantee:
Declines of oyster populations and commercial harvest from the seaside coastal bays of Virginia's Eastern Shore have followed similar patterns, though not as severe, as those in Chesapeake Bay.
Nevertheless, there are several promising signs that significant enhancement of the population could be achieved with well conceived restoration efforts. To plan a more comprehensive restoration effort, estimates not only of the current standing stock of oysters, but also of their spatial distribution in the coastal bays are needed.
The overall objective of this project is to develop a spatially explicit oyster stock assessment of this region in a Geographic Information System (GIS). It will be based on habitat-specific oyster densities and will be undertaken in two phases, each taking one year to complete.Â This data summary pertains to Phase I (habitat mapping) that was completed in 2007.
Virginia Institute of Marine Science (VIMS) staff used National Wetlands Inventory (NWI) habitat data as the base map for several important oyster habitats (marshes, intertidal flats and subtidal bottom). These GIS polygons were checked with 2002 1-m aerial imagery from the Virginia Base Mapping Project (VBMP). Additionally, oyster patch reefs were mapped utilizing VBMP images during 30 aerial surveys at an altitude of 100 m within 1.5 hrs of low tide. The reefs were subsequently manually digitized in GIS. Ground-truth measurements showed >90% accuracy for reef identification. Additionally, man-made shorelines (e.g. bulkheads) were manually digitized using VBMP images. Overall, 9,600 habitat polygons covering 92,846 hectares (928 km2 or 358 mi2) were delineated in this project along with 41.4 km of manmade shoreline features.
Each of the above habitat polygons were assigned to one of 10 oyster-relevant habitat categories based on habitat type and tidal inundation: High Marsh, Low Marsh, Flats-Marsh, High Tidal Flats, Low Tidal Flats, Subtidal Bottom, Patch Reef, Small Patch Reefs, Manmade or Non-Oyster. Low Tidal Flats dominated the study area (37.9%) while High Marsh (19.7%) and Subtidal Bottom (19.7%) were also important. Approximately 225 and 316 hectares (0.2% and 0.3% of the study area) of Patch Reefs and Small Patch Reefs, respectively, were mapped.
The prevalence and spatial patterns observed in this study show that the coastal bay system on the seaside of Virginia's Eastern Shore has a complex arrangement of potential oyster habitats. This work, together with Phase II of the project, will provide valuable information that will help oyster restoration managers make decisions on where to best allocate limited resources. They should also be helpful with designing future research projects, both in terms of base maps on which to build, and also by generating questions based on the observed spatial patterns of oyster populations within this complex landscape.
Results of this study have been disseminated as a written report and GIS shapefiles to the funding agency.
Disclaimer: This project summary provides the federal dollars initially awarded to the grantee. Due to underexpenditure or reprogramming of grant funds, this figure may change. For more information on the allocation of coastal grant funds, please contact Laura McKay, Virginia Coastal Program Manager, at 804.698.4323 or email: Laura.McKay@deq.virginia.gov
A more detailed Scope of Work for this project is available. Please direct your request for a copy to Virginia.Witmer@deq.virginia.gov