A Comparison Of Dredge And Patent Tongs For Estimation Of Oyster Populations

Exploited oyster stocks on public grounds in Virginia waters are subject to regular surveys effected using a traditional oyster dredge and, more recently, patent tongs. Dredges provide semiquantitative data, have been used with consistency over extended periods (decades), and provide data on population trends. Surveys with patent tongs provide absolute quantification (number of individuals per unit area) of oyster stocks but are more labor intensive. Absolute quantification of dredge data is difficult in that dredges accumulate organisms as they move over the bottom, may not sample with constancy throughout a single dredge haul, and may fill before completion of the haul thereby providing biased sampling. Selectivity of dredges versus patent tongs with respect to oyster demographics has not been rigorously examined. The objective of this study is to compare demographic oyster data collected at the same sites in the same years from both gear types. Data for the study were taken from 1993 to 2001 surveys conducted in the James River, Virginia, by the Virginia Institute of Marine Science and the Virginia Marine Resources Commission wherein the same stations were sampled by both techniques. Dredge surveys give data in oysters per bushel and assume no selective retention of live oysters with respect to shell substrate by the dredge. Patent tong surveys provide data as per tong estimates of oysters by size class and shell by volume. The hydraulically operated, 1-m square tong used in VMRC/VIMS surveys is designed to sample on and below the reef surface and include elements of buried shell that are probably not well sampled by a dredge, although the sampling ensures collection of all oysters within the tong mouth. Oysters collected by both gear types were classified as small (25-75 mm) or market (\u3e75 mm SL) for comparisons across methods. Shell volumes collected in patent tong surveys were standardized to bushel increments assuming 35.28 L of shell per bushel. The summary plots of mean values from 1993 to 2001 and 1998 to 2001 illustrate differences related to sampling gear. More shell per unit oyster (lower bushel counts) are observed in a patent tong sample. The appropriate model for attempting to fit a predictive line is open to debate, and will be influenced by patent tong penetration as determined by the degree of consolidation of the underlying substrate. The available data do not strongly support the ability to predict a relationship between dredge and patent tong population estimates at this time

Oyster Planting Protocols To Deter Losses To Cownose Ray Predation

The utility of shell overlays to oyster (Crassostrea virginica) plantings as a cownose ray (Rhinoptera bonasus) predator deterrence mechanism was examined. Typical industry practice of oyster seed planting was followed in an experimental design employing treatment areas of 0.5-1.0 acre (0.2-0.4 hectare). Areas were prepared in the Lower Machodoc Creek, Virginia, by the initial application of shell to insure a stable substrate under planted seed oysters. Seed oysters were planted using standard industry methods. Experimental areas were located, two upstream and two downstream, of a constriction in the Lower Machodoc that dictated differing physical environments in the respective locations with downstream locations being more exposed to northeast wind-driven stresses and, historically, a greater incidence of ray predation. Once oysters were planted, two of the areas, one upstream and one downstream of the aforementioned constriction, were additionally treated with a shell overlay as a predation deterrent. Oyster seed were planted in the experimental plots in February 2012. Market oysters were harvested from the experimental plots in December 2013 and January 2014. Final harvest data demonstrated that shell overlays do not offer additional protection to planted oyster seed with respect to possible cownose ray predation. Evidence of predation in the form of characteristically broken oyster valves were recorded in all treatment areas. Concurrent stomach content analysis of rays captured at the study location and observations of fouling community associated with the cultured oysters taken during the harvest operation indicate broad dietary preferences for rays when such a variety exists in the foraging region. For rays, oysters are not the singular preferred diet item, although localized and intensive feeding on oysters remains an option with a wide foraging range. Areas without overlay demonstrated higher production than those with shell overlay. Shell overlays are not recommended as predator deterrents for cownose rays in large deployments of unprotected oyster seed