Ontogenetic patterns in lipid and fatty acid biomarkers of juvenile polar cod (Boreogadus saida) and saffron cod (Eleginus gracilis) from across the Alaska Arctic

Characterizing the condition of fish in dynamic seasonal environments requires an understanding of their energy allocation strategies. Both polar cod ( Boreogadus saida ) and saffron cod ( Eleginus gracilis ) are important mid-trophic fish in Alaska Arctic waters, and changes in their lipid allocation could have important implications for their overwintering survival as well as their energetic value for predators. We used a combination of laboratory and field approaches to describe allometric relationships in lipid storage of polar cod and we then explored spatial patterns in field-caught juvenile gadid condition during 2012 and 2013. Lipid density in wild juvenile Arctic gadids increased with size leading into the first overwintering period, but age-1 + fish showed a reduction in lipid density with size prior to the 2nd overwintering period. Using the residuals from the underlying allometry of total lipid and fatty acid density in each species, we were able to develop a condition metric which was then explored in relation to spatial patterns in large Calanus glacialis copepodite (stages C3 and older) abundance and thermal conditions measured in the field. Fatty acid biomarkers from the total lipid pool indicated that polar cod have a higher reliance on calanoid copepods than saffron cod. Collectively, these data suggest polar cod and saffron cod will likely respond differently to regional warming depending upon the shift in the zooplankton communities, such that the energetic contribution of these fish to higher trophic levels could be transformed with future ocean warming.