SURVIVAL ANALYSIS OF TETHERED JUVENILE SEA SCALLOPS PLACOPECTEN-MAGELLANICUS IN-FIELD EXPERIMENTS - EFFECTS OF PREDATORS, SCALLOP SIZE AND DENSITY, SITE AND SEASON

The impact of predation on survival of juvenile sea scallops Placopect en magellanicus has important implications for the population dynamics and aquaculture of this species. In field experiments using tethered juvenile scallops, we examined the effect of biological and physical f actors, such as prey size, predator and prey density, water temperatur e (season) and site, on mortality rates of scallops in Lunenburg Bay, Nova Scotia, Canada, The major predators of juvenile scallops were cra bs Cancer irroratus and sea stars Asterias spp. In an experiment manip ulating scallop density, crab predation rate increased significantly w ith scallop density, but crab density did not, suggesting a functional response. Sea star predation rate on scallops was not affected by sca llop density. In a series of experiments with different size classes o f scallops tethered at different sites and seasons, and in different d ensities of surrounding scallops and predators, the effects of covaria tes on scallop survival times were assessed using the statistical proc edure of survival analysis. Since the tethering technique affected cra b and sea star predation differently, the competing causes of scallop mortality were analysed separately. Water temperature, site, and scall op density were the important variables affecting crab predation. Ther e was a significant interaction between temperature and site, such tha t crab predation on scallops increased with temperature at one site an d was independent of temperature at the other site. Predation by crabs also increased with scallop density and, to a lesser extent, with cra b density. Water temperature and scallop size were the primary variabl es affecting sea star predation. Sea star predation on scallops increa sed with temperature and decreased with scallop size. The results of t his study suggest options for minimizing predation of scallops by crab s and sea stars in bottom culture operations.