Shallow water as a refuge habitat for fish and crustaceans in non-vegetated estuaries: an example from Chesapeake Bay

Abstract

Abundances and size-frequency distributions of common epibenth~c flsh 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 absence of submerged aquatic vegetation (SAV), inter-and intraspccific size segregation occurred by depth from May to October, 1989-1992. Small species (Palaemonetes pugjo, Crangon septernspjnosa, Fundulus heteroclitus, F majaljs, Rhithropanope~ls harrisii, Apeltes quadracus, Gobiosorna boscj) were most abundant at water depths <70 cm. Furthermore, the proportion of small individuals decreased significantly with depth for 7 of 8 species, with C. septemsp~nosa being the exception, exhibiting no size change with increasing depth. These distributional patterns were related to depth-dependent predalion risk. Large species (Callinectes sap~dus, Leiostomus xanthurus, and Micropogonias undulatus), known predators of some of the small species, were often most abundant in deep water (>70 cm). In field experiments, mortality of tethered P pugio (30 to 35 mm), small E heteroclitus (40 to 50 mm), and small C. sapjdus (30 to 70 mm) increased significantly with depth. Wc hypothesize that predation risk was size-dependent, creating the observed intra-and interspecific size differences among depth zones. For C. sep.temspinosa, burial may modify this size-dc>pendency and create the unusual absence of intraspecific size increase with depth. Historically, f ? pugio and Fundulus spp. (and other small species) were not restricted to shallow (<70 cm) waters and were abundant in deeper SAV beds, which provided a structural refuge from predators. Since the recent demise of SAV in Chesapeake Bay, our results indicate many small species have shifted their distributions and now utilize primarily shallow water as an alternate refuge habitat.