Behavioural responses of predatory crabs and sea stars to varying density of juvenile sea scallops

The effect of density of juvenile sea scallops (Placopecten magellanicus; 0-45 scallops m^-2) on behavioural responses of predatory crabs (Cancer irroratus) and sea stars (Asterias vulgaris and A. forbesi) was examined in an experiment using tethered scallops in Lunenburg Bay, Nova Scotia, Canada. Within experimental plots (4 m²), mortality of tethered scallops, density of shell remains of unattached scallops, and density of predators were monitored to determine the functional and aggregative responses of crabs and sea stars. The cause of mortality (crab or sea star predation) could be identified from the scallop shell remains. Predation rate on scallops increased significantly with scallop density, but predator density did not. Thus, predators displayed a functional response, and not an aggregative response. Proportional mortality of scallops indicated that crab predation tended to be positively density-dependent, whereas sea star predation was density-independent. The results of this study are relevant to bottom culture operations and indicate that crabs would have a greater impact than sea stars on seeded populations of scallops.