Commercial harvest and population structure of a northern quahog (Mercenaria mercenaria Linnaeus 1758) population in St. Mary's Bay, Nova Scotia, Canada

Innovative Fishery Products Inc. (IFP) has managed a 1682-ha northern quahog (Mercenaria mercenaria) lease in St. Mary's Bay, Nova Scotia, Canada, since 1997. This study describes the commercial harvest and age structure of the northern quahog population from St. Mary's Bay and provides estimates of total mortality and exploitation rates for the lease area. Overall, mean densities ranged from 48.3-88.4 individuals/m(2) for surveys conducted in June 2001 and 2002, and in May 2003. Catch and effort in the fishery increased from 1999 to 2001. The mean age to market was 7 y. Recruitment of spat (SL < 5 mm) was variable and age frequencies suggested immigration of juvenile quahogs (3-6 y old) onto the intertidal portion of the lease area. The abundance of large quahogs (SL > 60 min or > 8 y old) was low. Catch curve analyses resulted in a high estimate of total mortality (Z) for quahogs of ages 7-10. Commercial exploitation only represented 5% to 10% of the estimated standing stock of commercial size quahogs, which suggests that natural mortality may be high. However, field experiments conducted during 2003 suggested that summer survival (May to October) was high ranging from 93.8 +/- 1.54 to 97.6 +/- 2.14%. Causes of apparent high total mortality of adult quahogs are unclear, but winter-kill due to ice abrasion or scouring, predation, and the movement of quahogs from the lease area may be responsible.