Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


R. N. Lipcius


Various investigations of the population dynamics of blue crabs in Chesapeake Bay indicate that predator-induced mortality in the juvenile phase may determine year-class strength. In a tethering study, which spanned three seasons in shallow-water habitats of the lower York River, daily mortality rates of juvenile crabs were measured across three variables: crab size (30-70 mm carapace width), habitat type (seagrass, mud, and sand), and month (May-November). Vulnerability to predation was consistently lower for larger crabs, approaching a size refuge from predation at approximately 90 mm carapace width. Predation was most intense in unvegetated sand habitats, and significantly lower in seagrass and unvegetated mud. The data also reflected a strong seasonal pattern in predation potential which was correlated with water temperature. Predators contributing to this pattern were identified and assessed using an underwater video-recording system to monitor tethered crabs. While a variety of potential predators appeared and attacked crabs frequently, only two species had a measurable impact on crab survival, adult conspecifics and northern puffers, Sphoeroides maculatus. These results were confirmed in large laboratory tanks using untethered crabs as prey. These studies indicate that the mechanisms behind seasonal variation in predation pressure may include both physiologically-linked activity rhythms and seasonal migration of predators. The potential impact of seasonally-varying predation pressure on the life history of blue crabs was explored in a modeling exercise, using concepts of dynamic optimization. The results suggest that the blue crab may exhibit behavioral adaptations which are reflected in optimal biological timing of recruitment and growth, thereby enhancing survival through the juvenile phase.



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