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-depen dent mortality of juvenile blue crabs utilizing nearshore shallow wate r as a refuge from cannibalism by large blue crabs. In a large laborat ory 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 cra b or in the presence of another small crab. Survival of juvenile crabs preyed upon by large crabs was significantly higher in laboratory tan ks with depth gradients providing a shallow refuge than in lab tanks w ithout depth gradients. In the laboratory, proportional mortality of j uvenile crabs was inversely density-dependent, indicating a type II fu nctional response of large crabs irrespective of the presence or absen ce of a shallow water refuge. In a non-vegetated subestuary of Chesape ake Bay, survival of tethered juveniles was significantly higher in sh allow (30 cm) than deep (70 cm) water of a nearshore zone, where canni balistic large blue crabs were their major source of mortality. Propor tional mortality in the field was also inversely density-dependent in both shallow and deep water. The persistence of a type II functional r esponse of predators in our laboratory and field experiments indicates that shallow water provides a significant but partially effective ref uge to juvenile blue crabs, although such inversely density-dependent mortality patterns indicate a potentially unstable predator-prey inter action. Under intense predation pressure from cannibalistic large crab s, this partially effective refuge in the nearshore shallows potential ly grants juvenile crabs a crucial reduction in mortality during their first year of life until they grow to a size large enough to obtain a n absolute refuge from predation.