Spatio-temporal variations of sea star Asterias spp. distributions between sea scallop Placopecten magellanicus beds on Georges Bank

摘要

Presently 80 % of the biomass of sea scallop Placopecten magellanicus on Georges Bank is located within 3 large areas closed to fisheries. Sea stars Asterias spp., primary predators of scallops, are also aggregated within these closed areas. As prey becomes depleted within one scallop bed, sea stars may move to another food source, possibly to another scallop bed. We tested the hypothesis that sea star aggregations moved from one scallop bed to another within the Nantucket Lightship Closed Area (NLCA) on Georges Bank. We video surveyed 204 stations in the NLCA from 1999 to 2006 using a 1.57 km grid-centric systematic sampling design. The center of sea star abundance was calculated by averaging the sea star freguency-weighted latitude and longitude for all stations. Using multivariate analysis of variance and all-pairs comparisons (Hotelling's T~2), shifts in the center of sea star abundance were determined by assessing if the locations of the 2 aggregations were different. The sea star center of abundance, standard ellipse and 95% confidence ellipse were superimposed on the scallop density distribution maps to determine the spatial overlap. The distributions of sea star aggregations in the NLCA significantly shifted between consecutive years from 1999 to 2006 and overlapped with areas of high densities of scallops. Shifts in the center of abundance reflect changes in distribution possibly resulting from movement, recruitment and mortality. As sea stars aggregate in these areas presumably due to high abundances of scallops, sea star movement between the scallop beds may increase natural mortality rates of the scallop population on Georges Bank.