DYNAMICS OF CRUSTACEAN PREDATION ON THE NORTHERN BAY SCALLOP, ARGOPECTEN IRRADIANS IRRADIANS (CRABS, PELECYPODS, POPULATION, CONNECTICUT)

The present study examined the seasonal effects of a predatory crab assemblage on a population of northern bay scallops, Argopecten irradians irradians, in the Poquonock River, Connecticut. Factors contributing to seasonal changes in predation intensity were evaluated through scallop and crab population surveys, mark-recapture studies, laboratory predation experiments, and a newly-developed Index of Potential Predator Importance.^ Following settlement of larval scallops in mid-July, juvenile growth was (TURN)10 mm/month until November. Growth ceased in December, then resumed in May at a rate of 4 mm/month. Scallop densities decreased sharply during the month after juvenile recruitment was observed, then declined more gradually.^ Seasonal trends in crab abundance closely followed changes in water temperature. Peaks in species abundance were staggered. Diel abundance of Pagurus longicarpus, Carcinus maenas and Libinia spp. varied significantly in different months. Green crabs were significantly larger in night collections.^ Laboratory experiments demonstrated that rates of predation declined as scallop size increased or crab size decreased. Predation rates were consistently low on scallops larger than 40 mm, but scallops achieved only a partial size refuge from crab predation because even the largest individuals ((TURN)60 mm) were sometimes attacked successfully. Predatory methods employed to subdue scallops of progressively larger shell size (crushing, partial crushing, chipping, prying) were highly consistent among different crab species.^ Monthly mark-recapture studies revealed predation was most intense on small juvenile scallops during late summer/early fall, then declined steadily as scallops grew to 40 mm by December. Predation intensity on larger individuals was consistently low throughout the year. The primary factor affecting seasonal predation patterns was scallop size.^ Linear regressions comparing composite monthly values of the Index of Potential Predator Importance with observed levels of predation intensity were highly significant. Carcinus maenas was potentially the most important bay scallop predator during virtually every month of the study, but Callinectes sapidus, Cancer irroratus, and Libinia emarginata were potentially important during certain months.^ Predation was the major cause of juvenile and adult bay scallop mortalities in the Poquonock River. Stranding, burial, extreme temperatures, and low salinities were of relatively minor importance.