Community structure of eastern Bering Sea epibenthic invertebrates from summer bottom-trawl surveys 1982 to 2002

Abstract

Catches of invertebrates from the annual summer bottom-trawl surveys in the eastern Bering Sea between 1982 and 2002 were analyzed to describe the composition and spatial distri- bution of epibenthic invertebrate communities. A persistent characteristic is distinct inshore and offshore community types separated by the dynamic oceanographic inner front that generally co- incides with the 50 m isobath. This typical spatial distribution of the 2 communities corresponds closely with surficial sediment type and previously reported patterns for groundfishes and infaunal invertebrates. The biomass of the inshore assemblage is overwhelmingly dominated by the sea star Asterias amurensis, whereas Gastropoda, Paguridae and the snow crab Chionoecetes opilio domi- nate the offshore assemblage. Variations in the typical inshore- offshore pattern occurred in 1982- 84, 1987- 88, 1998- 99 and 2001- 02, when there were substantial reductions in the spatial extent of the inshore community, especially in the Bristol Bay area. During these reductions, epiben- thos in Bristol Bay shifted from the inshore type to either offshore or an undefined community type. In general, reductions in the inshore domain were correlated with a mean bottom temperature in the survey area that was higher than normal in the preceding summer. Extreme El Nino events coincided with sizable contractions of the inshore community in 1982- 84 and 1998- 99. Spatial variability in the epibenthic communities may thus reflect the influence of environmental changes on interannual and decadal scales. Evidence suggests that mobile taxa, especially crabs, may be migrating offshore toward cooler waters, thereby rearranging the epibenthic communities.