Annual and spatial variation in the condition and lipid storage of juvenile Chukchi Sea gadids during a recent period of environmental warming (2012 to 2019).

Abstract

The Arctic is undergoing dramatic environmental change with decreasing sea-ice extent and increasing summer temperatures. The late summers of 2017 and 2019 on the eastern Chukchi Sea were anomalously warm, nearly 4 °C warmer than the previous 30-year average. Increased ocean temperatures can affect the energetics of North Pacific fish by increasing their metabolic demands and via shifting fish prey assemblages. Here we describe the total lipids as well as fatty acid (FA) trophic markers in juveniles of two Arctic gadids (polar cod, Boreogadus saida and saffron cod, Eleginus gracilis) as well as two sub-Arctic gadids (walleye pollock, Gadus chalcogrammus and Pacific cod, Gadus macrocephalus) collected on recent ecosystem surveys spanning the north Bering and Chukchi seas. Fifty percent of the variance in the lipid composition of gadids was accounted for by species-specific differences, while ecosystem measurements such as bottom temperature, large > C3 stage Calanus abundance, and surface temperature were found to independently account for 25%, 12% and 10%, respectively. Allometric relationships in lipid storage revealed that polar cod have a different lipid profile than other gadids, suggesting a species-specific life-history strategy for high lipid storage that is an adaptation to Arctic environments. Both polar cod and saffron cod had reduced lipid storage in 2017 compared to fish collected in earlier years. Polar cod in 2017 were significantly lower in total lipid, triacylglycerols (TAG), diatom- (16:1n-7/16:0) and Calanus-sourced (∑C20+ C22) FAs over the Chukchi Shelf. Juvenile gadids showed interspecific differences in the spatial distribution of high lipid individuals, with polar cod having the highest lipids in the northern ice-associated regions of the Chukchi Sea and walleye pollock in the southern Chukchi Sea. In 2019, polar cod's distribution had shifted north such that they were only abundant in the northern Chukchi Sea, where they maintained higher region-specific lipid storage than in 2017. It is concerning that reduced lipid content in polar cod was associated with elevated water temperatures, given predicted continued warming in the Chukchi Sea. Energetic changes in juvenile gadids may be associated with future increased natural mortality rates for regional populations (e.g. overwintering) and unstable foraging value for birds and mammals in the Arctic.