Effects of climate variations on pelagic ocean habitats and their role in structuring forage fish distributions in the Bering Sea

Abstract

This paper examines how climate variations influence the boundaries of suitable ocean habitat, and how these changes affect the spatial distribution and interactions between forage fishes in the southeastern Bering Sea shelf. The study focuses on the summer distributions of forage fish age-0 and age-1 walleye pollock, Theragra chalcogramma, and capelin, Mallotus villosus, observed during National Marine Fisheries Service summer acoustic trawl, surface trawl and bottom trawl surveys conducted in the Bering Sea between 2004 and 2009. We compare the responses of these forage fish to climate-induced shifts in ocean habitats. Habitat boundaries were defined using key explanatory variables including depth, bottom temperature and surface temperature, using general additive models. Bathymetry, bottom temperature and frontal zones formed boundaries between different groups of forage fishes. Age-0 pollock were dispersed throughout the middle domain (50–100m depth) in well-stratified regions. In cold years the highest densities of age-0s were found in the southern regions of the middle domain waters in waters warmer than approximately 1°C. In contrast, age-1 pollock were observed on the sea floor over the middle domain and in midwater in the northern outer domain in cold years and more broadly dispersed across the middle and outer domain in warm years. The demersal concentrations of age-1 pollock in the middle domain shows age-1 pollock tolerate a wide range of bottom temperatures. Midwater and demersal distributions of age-1 pollock exhibited a patchier distribution than age-0 pollock. Midwater concentrations of age-1 pollock tended to be associated with the outer domain and regions where higher levels of lower trophic level production are expected. Capelin were concentrated in the inner domain, a well-mixed region. The overlap of age-1 pollock and capelin was higher in cold years than in warm years.