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 densitydependent, 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. Refuges provide prey means of escape from predation and thus are an important stabilizing factor in predator-prey interactions (Murdoch and Oaten, 1975). The stabilizing role of refuges from predation has received considerable theoretical and empirical attention (Cern and Fraser, 1983; Werner et aI., 1983; Schmitt and Holbrook, 1985; Abrams, 1987; Gilliam and Fraser, 1987; Murdoch and Bence, 1987). Most theoretical considerations have focused on "absolute refuges," where prey are completely invulnerable to predation (Murdoch and Oaten, 1975). Yet many field studies indicate that refuges provide "partial" escape from predation along a gradient of reduced predator access or efficiency (Werner et al., 1983; Mittelbach 1986; Dill, 1987; Dill and Lima, 1990). The refuge function of a habitat often changes during ontogeny of prey and is a crucial component of nursery habitats that may limit juvenile survival (Werner and Gilliam, 1984). Estuaries serve as major nurseries for juveniles of many species of coastal fish and decapod crustaceans (McHugh, 1976; Heck and Thoman, 1984). In estuarine ecosystems, submerged aquatic vegetation (SAV) is a major habitat with structural complexity that provides critical refuge to early life stages and has profound effects on prey survival from mobile predators (Heck and Thoman, 1981; Penry, 1982; Wilson et al., 1987). However, little is known about the refuge function of estuarine habitats other than SAV beds. Coarse woody debris also can provide a significant source of habitat complexity and refuge for prey in estuarine ecosystems, but this structure is highly patchy in its distribution and has been reduced significantly in recent decades (Everett and Ruiz, 1993).