Vol.:(0123456789) 1 3 Polar Biology (2020) 43:1121–1140 https://doi.org/10.1007/s00300-020-02648-9 ORIGINAL PAPER Ontogenetic patterns in lipid and fatty acid biomarkers of juvenile polar cod (Boreogadus saida) and safron cod (Eleginus gracilis) from across the Alaska Arctic Louise Copeman 1,2,3  · Mara Spencer 3  · Ron Heintz 4,5  · Johanna Vollenweider 4  · Angie Sremba 2  · Thomas Helser 6  · Libby Logerwell 6  · Leandra Sousa 7  · Seth Danielson 8  · Alexei I. Pinchuk 8  · Benjamin Laurel 3 Received: 14 August 2019 / Revised: 17 December 2019 / Accepted: 11 March 2020 / Published online: 4 April 2020 © This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply 2020 Abstract Characterizing the condition of fsh in dynamic seasonal environments requires an understanding of their energy allocation strategies. Both polar cod (Boreogadus saida) and safron cod (Eleginus gracilis) are important mid-trophic fsh 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 feld approaches to describe allometric relationships in lipid storage of polar cod and we then explored spatial patterns in feld-caught juvenile gadid condition during 2012 and 2013. Lipid density in wild juvenile Arctic gadids increased with size leading into the frst overwintering period, but age-1 + fsh showed a reduction in lipid density with size prior to the 2nd overwintering period. Using the residu- als 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 feld. Fatty acid biomarkers from the total lipid pool indicated that polar cod have a higher reliance on calanoid copepods than safron cod. Collectively, these data suggest polar cod and safron cod will likely respond diferently to regional warming depending upon the shift in the zooplankton communities, such that the energetic contribution of these fsh to higher trophic levels could be transformed with future ocean warming. Keywords Gadid · Early life history · Lipids · Chukchi sea · Calanus · Trophic biomarker Introduction Juvenile fsh living in seasonal environments face trade-ofs in energy allocation between immediate growth and storage for future demands (e.g., overwintering). These conficting energy requirements have important consequences for fsh ftness and are likely variable depending on local physi- cal (i.e., temperature, currents) and biological (i.e., food quantity and quality) parameters. Juvenile fsh in temperate regions generally follow an energy allocation strategy that includes the following: (1) prioritizing growth during larval stages, (2) trade-ofs between growth and lipid storage in early juvenile stages depending on predation pressure and This article belongs to the special issue on the “Arctic Gadids in a Changing Climate”, coordinated by Franz Mueter, Haakon Hop, Benjamin Laurel, Caroline Bouchard, and Brenda Norcross. * Louise Copeman Louise.Copeman@noaa.gov 1 College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Newport, OR 97365, USA 2 Cooperative Institute for Marine Resources Studies, Oregon State University, Newport, OR 97365, USA 3 Present Address: Alaska Fisheries Science Center, NOAA National Marine Fisheries Service, Newport, OR 97365, USA 4 Alaska Fisheries Science Center, NOAA National Marine Fisheries Service, Juneau, AK 99801, USA 5 Sitka Sound Science Center, Sitka, Alaska 99835, USA 6 Alaska Fisheries Science Center, NOAA National Marine Fisheries Service, Seattle 98115, WA, USA 7 North Slope Borough, Department of Wildlife Management, Barrow, AK 99723, USA 8 College of Fisheries and Ocean Sciences, University of Alaska, Fairbanks, AK 99775, USA