Effects of shallow water refuge on behavior and density-dependent mortality of juvenile blue crabs in Chesapeake Bay

This study experimentally tested aspects of behavior and density-dependent mortality of juvenile blue crabs utilizing nearshore shallow water as a refuge from cannibalism by large blue crabs. In a large laboratory tank with a depth gradient, individual juvenile blue crabs shifted their depth utilization from deep and medium depths to shallow water in the presence of a large crab, but not in the absence of another crab or in the presence of another small crab. Survival of juvenile crabs preyed upon by large crabs was significantly higher in laboratory tanks with depth gradients providing a shallow refuge than in lab tanks without depth gradients. In the laboratory, proportional mortality of juvenile crabs was inversely density-dependent, indicating a type II functional response of large crabs irrespective of the presence or absence of a shallow water refuge. In a non-vegetated subestuary of Chesapeake Bay, survival of tethered juveniles was significantly higher in shallow (30 cm) than deep (70 cm) water of a nearshore zone, where cannibalistic large blue crabs were their major source of mortality. Proportional mortality in the field was also inversely density-dependent in both shallow and deep water. The persistence of a type II functional response of predators in our laboratory and field experiments indicates that shallow water provides a significant but partially effective refuge to juvenile blue crabs, although such inversely density-dependent mortality patterns indicate a potentially unstable predator-prey interaction. Under intense predation pressure from cannibalistic large crabs, this partially effective refuge in the nearshore shallows potentially grants juvenile crabs a crucial reduction in mortality during their first year of life until they grow to a size large enough to obtain an absolute refuge from predation.