Pacific cod or tikhookeanskaya treska (Gadus macrocephalus) in the Chukchi Sea during recent warm years: Distribution by life stage and age-0 diet and condition

Many fish species have moved poleward with ocean warming, and species distribution shifts can occur because of adult fish movement, or juveniles can recruit to new areas. In the Bering Sea, recent studies document a dramatic northward shift in the distribution of Gadus macrocephalus (Pacific cod in English and tikhookeanskaya treska in Russian) during a period of ocean warming, but it is unknown whether the current northward distribution shift continues into the Chukchi Sea. Here, we use catch data from multiple gear types to present larval, age-0, and older Pacific cod distributions from before (2010 and 2012) and during (2017, 2018, and 2019) recent Chukchi Sea warming events. We also report on the habitat, diet, and condition of age-0 Pacific cod, which were present in the eastern Chukchi Sea in recent warm years (2017 and 2019), but were absent in a cold year (2012). We hypothesize that age-0 recruitment to the eastern Chukchi Sea is associated with recent warm temperatures and increased northward transport through the Bering Strait in the spring. Age-0 fish were present in both benthic and pelagic habitats and diets reflected prey resources at these capture locations. Age-1 Pacific cod were observed in the western Chukchi Sea in 2018 and 2019, indicating possible overwinter survival of age-0 fish, although there was little evidence that they survive and/or remain in the Chukchi Sea to age-2. Observed low lipid accumulation in age-0 Pacific cod from the Chukchi Sea suggests juvenile overwinter mortality may be relatively high compared to more boreal regions (e.g. Gulf of Alaska). Adult Pacific cod were also observed in the Chukchi Sea during 2018 and 2019. Although densities in the western Chukchi Sea were very low compared to the Bering Sea, the adults are the first known (to us) records from the Chukchi Sea. The increased presence of multiple age-classes of Pacific cod in the Chukchi Sea suggests poleward shifts in both nursery areas and adult summer habitat beyond the Bering Sea, but the quantity and quality (e.g. summer productivity and over -wintering potential) of these habitats will require continued surveys.