Biological and physical variables associated with aggregations of the giant scallop Placopecten magellanicus

Abstract

To identify factors that may determine where aggregations (beds) of giant scallops (Placopecten magellanicus) develop, we compared physical and biological conditions in two scallop beds in Port Daniel Bay, Baie des Chaleurs, Gulf of St. Lawrence, with conditions in seven adjacent areas. Gravel substratum predominantly characterized scallop beds. Physical hydrographic conditions and the intensity of asteroid predation were similar within scallop beds and in surrounding areas with few scallops. High P. magellanicus spat settlement density was not consistently correlated with existing scallop aggregations, but filamentous organisms, on which scallops preferentially settle, were more abundant in scallop beds, possibly enhancing recruitment. Scallop growth rate varied among areas and was intermediate within scallop beds. Winter flounder (Pseudopleuronectes americanus) and Atlantic wolffish (Anarhichas lupus) preyed on P. magellanicus and may influence scallop distribution. Scallop tethering experiments indicated that risk of predation was low within scallop beds compared with adjacent areas. Scallop mortality was positively correlated with mean density of both rock crab (Cancer irroratus) and American lobster (Homarus americanus). We conclude that gravel substratum, low decapod predation, and presence of filamentous flora and fauna are critical factors determining scallop aggregation location.