Ontogenic habitat shifts of the Iceland scallop, Chlamys islandica (Muller, 1776), in the northern Gulf of St Lawrence

The spatial distribution of the Iceland scallop, Chlamys islandica (Muller, 1776), was studied using scuba in an unharvested population in the Mingan Islands, northern Gulf of St. Lawrence. Size partitioning related to depth was evident, there being a change from predominantly small scallops (<30 mm in shell height) at 15 m to mainly large scallops (>60 mm) at 30 m in depth. Most small scallops were found in crevices under bivalve shells or rocks. Size partitioning did not appear to result from differential mortality among depths. Tethering trials suggested that size-specific predation pressure was similar between experimental depths (15 and 25 m). Vulnerability to predators and the frequency of spatial refuge use both decreased markedly with increasing scallop size and this suggested an ontogenetic microhabitat shift related to predation risk. The survival of small scallops in shallow waters (15 m) may be enhanced because of the abundance of bivalve shells that could provide refuges from predators. The depth distribution of rhodophytes, a settlement substratum, largely accounted for the distribution of recruits. We propose that size partitioning in this population results from higher settlement and survival in shallow water, followed by a gradual downslope movement with increasing scallop size.