Population structure and recruitment of the bivalve Arctica islandica (Linnaeus, 1767) on Georges Bank from 1980-1999

摘要

Arctica islandica is a commercially important bivalve from the North Atlantic Ocean. Due to its slow growth rate and longevity, long-term studies are needed to understand its population dynamics, particularly recruitment. This study describes the spatial distribution and population structure of A. islandica on Georges Bank, USA from 1980-1999. Results are based on samples collected with a hydraulic clam dredge during National Marine Fisheries Service ocean shellfish surveys. In all surveys from 1980-1999, individuals were abundant on the south flank of Georges Bank between 60 and 75 m depth. In the 1980s, size distributions in tows were typically unimodal, and the population was dominated by large individuals, 75-90 mm in length. In contrast, bimodal size distributions were observed at many stations in the 1990s, and small individuals (<70 mm in length) often represented 20-50 percent of the catch (by number). These small individuals were the result of spawning in the 1980s. Ages of clams were estimated from shell banding at one station in 1994. Most individuals had less than 40 bands, and individuals between 21 and 28 were rare. Among young individuals, modes were centered on 7 and 12 bands; assuming band formation is approximately annual, thissuggests recruitment by cohorts spawned during 1986 or 1987 and 1981 or 1982. These results imply that annual recruitment of small A. islandica on Georges Bank has been highly variable during the last 40 years and suggest an increase in recruitment to the fishery after 1990.