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 (Mu ller, 1776), was studied using scuba in an unharvested population in t he Mingan Islands, northern Gulf of St. Lawrence. Size partitioning re lated to depth was evident, there being a change from predominantly sm all 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 re sult from differential mortality among depths. Tethering trials sugges ted that size-specific predation pressure was similar between experime ntal depths (15 and 25 m). Vulnerability to predators and the frequenc y of spatial refuge use both decreased markedly with increasing scallo p 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 cou ld provide refuges from predators. The depth distribution of rhodophyt es, a settlement substratum, largely accounted for the distribution of recruits. We propose that size partitioning in this population result s from higher settlement and survival in shallow water, followed by a gradual downslope movement with increasing scallop size.