The importance of for the feeding success of young polar cod: a circumpolar synthesis.

Understanding the feeding ecology of polar cod () during its first year of life is crucial to forecasting its response to the ongoing borealization of Arctic seas. We investigated the relationships between diet composition and feeding success in 1797 polar cod larvae and juveniles 4.5-55.6 mm standard length (SL) collected in five Arctic seas from 1993 to 2009. Prey were identified to species and developmental stages when possible, measured, and their carbon content was estimated using taxon-specific allometric equations. Feeding success was defined as the ratio of ingested carbon to fish weight. Carbon uptake in polar cod larvae < 15 mm was sourced primarily from calanoid copepods eggs and nauplii which were positively selected from the plankton. With increasing length, carbon sources shifted from eggs and nauplii to the copepodites of , and spp. copepodites were the main carbon source in polar cod > 25 mm and the only copepodite positively selected for. spp. copepodites became important replacement prey when left the epipelagic layer at the end of summer. was the preferred prey of polar cod, contributing from 23 to 84% of carbon uptake at any stage in the early development. Feeding success was determined by the number of prey captured in larvae < 15 mm and by the size of prey in juveniles > 30 mm. As Arctic seas warm, the progressive displacement of by the smaller could accelerate the replacement of polar cod, the dominant Arctic forage fish, by boreal species.