Comparison of Three Methods for Aging the Iceland Scallop Chlamys islandica

Abstract

The Gilbert Bay (GB) Marine Protected Area (MPA) on the southeastern coast of Labrador was established in 2005 to protect a resident population of Atlantic cod (Gadus morhua). Commercial dredging for Iceland scallops (Chlamys islandica) in the bay was permitted to continue under MPA regulations, but was excluded from areas of cod spawning. The average size of Iceland scallops harvested in GB is smaller than in the Strait of Belle Isle approximately 150 km to the south. Local fishermen have suggested this is due to slow growth, and this prompted the present study to test the hypothesis that Iceland scallops grow more slowly in GB than in the Strait of Belle Isle. Scallop growth was parameterized as change in shell height with age. Three methods of aging individual scallops were used: interpreting external growth rings on the shell, counting growth increments on the calcareous portion of the shell hinge ligament, and counting internal shell layer growth zones. Counting ligament growth increments and internal shell layer growth zones gave similar estimates of age. In comparison, interpreting external growth rings on the shell underestimated the age of individual scallops. If the ligament method and the internal shell layer method of aging are accurate (scallops of known-age were not examined), scallops in GB require eleven years of growth to recruit to the fishery (attain a shell height of 60 mm). This is substantially longer than the 6-y period estimated using the external growth rings method of aging. Growth rate parameters of Iceland scallops in GB, the Strait of Belle, and in the Nuuk area of West Greenland were found to be similar. The relatively small shell size of scallops presently harvested in GB is not due to anomalous growth, but more likely attributable to repeated dredging of scallop beds in the past and removal of the larger individuals.