Trophic basis of dominant amphipods in the gray whale feeding grounds near northeastern Sakhalin Island (the Sea of Okhotsk) inferred from fatty acid and stable isotope analyses

The shelf waters off northeastern Sakhalin Island are the main feeding ground for the endangered western subpopulation of the gray whale Eschrichtius robustus. Amphipods, which dominate the benthic communities, are key dietary sources for the gray whales foraging in this area. To elucidate the trophic base supporting the large local amphipod biomass, fatty acid (FA) compositions and the stable isotope ratios δ13C and δ15N of the five most abundant amphipod species in the area were studied. The FA compositions were unusually similar (85% similarity) across the amphipod species although these species were represented by suspension-feeders, deposit-feeders and carnivores. The FAs of diatom origin predominated in the FA profiles of all amphipods and decreased in accordance with the increase in trophic position as determined by δ15N values and the FA trophic markers of carnivorous feeding. These results suggested that diatoms are the single, main trophic basis that underpins the food web of the feeding grounds on the northeastern Sakhalin shelf. Further studies on factors influencing the local production of large diatoms on the Sakhalin shelf can be important for the prognosis of future changes of foraging resources of the gray whales. •In the present study, five amphipod species were well separated into three groups by their FA composition and δ15N values.•These groups correspond to different trophic guilds of Amphipoda: suspension feeders (group 1, Ampelisca eschrichtii); deposit feeders (group 2, Monoporeia affinis and Eogammarus schmidti); and scavenger/predators (group 3, Anonyx nugax and Onisimus derjugini.•The similar FA and carbon isotopic compositions for the northeastern Sakhalin shelf feeding ground amphipods reflected homogeneity of end-members (primary production sources) with the food web dominated by one endmember.