DNA metabarcoding reveals surprisingly diverse diet in under-ice polar cod during autumn

Abstract Polar cod ( Boreogadus saida ) functions as a key species in the Arctic food web and is one of the most abundant Arctic fishes. Despite its ecological importance, the feeding ecology of juveniles sampled under the sea ice is unknown. In November 2019, at the onset of sea ice formation, sea-ice associated polar cod and zooplankton samples were collected in the Beaufort and Chukchi seas. Stomach contents were highly digested and largely unidentifiable by morphological analysis. DNA metabarcoding, however, facilitated identification of most prey items to the species-level. Juvenile polar cod had a broad dietary spectrum and consumed at least 45 taxa, with calanoid copepods and ostracods being the most abundant prey taxa. The most frequent prey species were the pelagic ostracod Boroecia maxima and the calanoid copepods Calanus hyperboreus and Metridia longa . Contrary to expectations, the upper water column in the Alaskan Arctic below consolidated ice contained moderate abundances of energy-rich epipelagic zooplankton and under-ice fauna, particularly calanoid copepods and ice amphipods, in late fall. This zooplankton may provide a critical resource for late hatchers to survive in low-temperature habitats with low energy expenditure.