SHALLOW-WATER AS A REFUGE HABITAT FOR FISH AND CRUSTACEANS IN NONVEGETATED ESTUARIES - AN EXAMPLE FROM CHESAPEAKE BAY

Abundances and size-frequency distributions of common epibenthic fish and crustaceans were compared among 3 depth zones (1-35, 35-70, 71-95 cm) of the Rhode River, a subestuary of Chesapeake Bay, USA. In the ab sence of submerged aquatic vegetation (SAV), inter- and intraspecific size segregation occurred by depth from May to October, 1989-1992. Sma ll species (Palaemonetes pugio, Crangon septemspinosa, Fundulus hetero clitus, F. majalis, Rhithropanopeus harrisii, Apeltes quadracus, Gobio soma bosci) were most abundant at water depths < 70 cm. Furthermore, t he proportion of small individuals decreased significantly with depth for 7 of 8 species, with C. septemspinosa being the exception, exhibit ing no size change with increasing depth. These distributional pattern s were related to depth-dependent predation risk. Large species (Calli nectes sapidus, Leiostomus xanthurus, and Micropogonias undulatus), kn own predators of some of the small species, were often most abundant i n deep water (> 70 cm). In field experiments, mortality of tethered P. pugio (30 to 35 mm), small F. heteroclitus (40 to 50 mm), and small C . sapidus (30 to 70 mm) increased significantly with depth. We hypothe size that predation risk was size-dependent, creating the observed int ra- and interspecific size differences among depth zones. For C. septe mspinosa, burial may modify this size-dependency and create the unusua l absence of intraspecific size increase with depth. Historically, P. pugio and Fundulus spp, (and other small species) were not restricted to shallow (< 70 cm) waters and were abundant in deeper SAV beds, whic h provided a structural refuge from predators. Since the recent demise of SAV in Chesapeake Bay, our results indicate many small species hav e shifted their distributions and now utilize primarily shallow water as an alternate refuge habitat.