Sarah Bayer - Fish and Invertebrate Ecology
Texas A & M University, Corpus Christi
Abstract of Research
Large scale spatiotemporal variation in blue crab abundance within Chesapeake Bay
The blue crab population has severely declined in the past fifteen years. Adult female spawning stocks suffered greatly from 1990-1991 and since then have been steadily declining. The population is now recruitment limited and we must find what the crabs are utilizing in terms of food resources and habitat so we can help increase the blue crab stock. We expect to see higher crab densities near the mouth of the bay and lower densities in the northern bay. As crabs recruit they settle in the nearest suitable habitat, but if densities are too high they will continue to ride the currents and tides northward up the bay. The main objective of this project was to see where the crab abundances were by comparing spatial patterns throughout the Chesapeake Bay in 2006 and 2007.
Rivers throughout the bay were chosen and enhanceable units (EU) within each river were identified. Twelve sled tows each 100m and six seines covering 113m2 were used to collect blue crabs along with fish and clams. In 2006 there was a strong south to north trend with crab densities being higher in the southern most rivers and densities decreased in the northern rivers. There was a significant difference among rivers (ANOVA p < 0.05) with a nested EU. Ryan-Einot-Gabriel-Welsch multiple range post-hoc test identified the two southern most rivers (Patuxent River and Little Choptank River) to be different from each other and different from all other rivers in 2006. There was a significant difference among rivers (ANOVA p < 0.05) with a nested EU in 2007. The southern most rivers had higher crab densities than the northern rivers. Ryan-Einot-Gabriel-Welsch multiple range post-hoc test identified Tangier Sound to be different from all other rivers. Spatial variation with each river in 2006 showed crab densities to be higher near the mouth of the river and decrease up river. However, Patuxent River showed higher crab densities to be up river. This is possibly due to a slight salinity gradient because it is a large river whereas a smaller river would have a steep salinity gradient. There was no significance in crab density between 2006 and 2007, however there was a significance with the interaction of year and river with a nested EU. West River and Rhode River showed a significant increase in crab density from 2006 to 2007 (ANOVA p = 0.01) and Little Choptank River showed a significant decrease in crab density (ANOVA p = 0.002). Overall trends showed crab densities in the northern most rivers to increase indicating the movement of crabs with a salinity gradient. There was no significance between man made and natural habitat structures. Crab density did not vary with the slope of the bathymetry.
Overall crab density did not vary between years; however the densities did vary among rivers. The higher densities were near the lower bay and decreased northward. From 2006 to 2007 the crab densities increased in the northern most rivers possibly due to salinity gradients. This trend may be a year effect due to lack of rainfall in 2007 so there was not an inflow of fresh water. More data will be collected to better determine spatial analysis and what factors are affecting the blue crab abundance throughout the Chesapeake Bay.