The influence of oyster toadfish ( Opsanus tau) and crabs ( Callinectes sapidus and Xanthidae) on survival of oyster ( Crassostrea virginica) spat in Chesapeake Bay: does spat protection always work?

Abstract

Field experiments were conducted in central Chesapeake Bay to determine whether oyster toadfish Opsanus tau( Linnaeus ), mud crabs ( mostly Panopeus herbstii Milne-Edwards), and blue crabs ( Callinectes sapidus Rathbun) influence the survival of juvenile oysters ( Crassostrea virginica (Gmelin ) ), and whether interactions among these species modify their individual effects. Mesh-lined steel cages were used to manipulate toadfish and blue crab access to oyster spat and mud crabs. Oyster survival in the presence of toadfish was no different than in caged or uncaged controls. Spat set on cultch tended to have slightly lower survival in cages with only blue crabs than in other treatments. Survival of both cultchless spat and spat set on shells was negatively correlated with numbers of mud crabs in some, but not all, sets of replicates. Mud crabs were more abundant in August than in September each year and were also more abundant in 1988 than in 1987. Both temporal and habitat factors were probably responsible for the patterns of mud crab abundances. Results of 2 years of experimental studies suggest that the oyster toadfish is not an effective means of oyster predator control in the subtidal area of the Chesapeake Bay studied. These results contrast with those of studies conducted on oyster spat elsewhere and with tests of the use of toadfish to protect juvenile hard clams.