Community-level response of fish larvae to environmental variability in the southeastern Bering Sea

Abstract

Oceanographic conditions in the southeastern Bering Sea are affected by large-scale climatic drivers (e.g. Pacific Decadal Oscillation, Aleutian Low Pressure System). Ecosystem changes in response to climate variability should be monitored, as the Bering Sea supports the largest com- mercial fishery in the USA (walleye pollock Theragra chalcogramma). This analysis examined shifts in larval fish community composition in the southeastern Bering Sea in response to environmental variability across both warm and cold periods. Larvae were sampled in spring (May) during 5 cruises between 2002 and 2008 using oblique 60 cm bongo tows. Non-metric multidimensional scaling (NMDS) was used to quantify variability and reduce multi-species abundance data to major modes of species composition. Generalized additive models (GAMs) characterized spatial and temporal differ- ences in assemblage structure as a function of environmental covariates. We identified a strong cross- shelf gradient delineating slope and shelf assemblages, an influence of water masses from the Gulf of Alaska on species composition, and the importance of nearshore areas for larval fish. Species assem- blages differed between warm and cold periods, and larval abundances were generally greater in warm years. High abundances of walleye pollock in warm years contributed most to differences in Unimak Pass, outer domain, and shelf areas (geographic areas in the study region defined based on bathymetry). Sebastes spp. contributed to differences over the slope with increased abundances in cold years. We propose that community-level patterns in larval fish composition may reflect species- specific responses to climate change and that early life stages may be primary indicators of environ- mental change.