Influences of temperature, predators, and competitors on polar cod (Boreogadus saida) at the southern margin of their distribution

Polar cod (Boreogadus saida) is the most abundant and ubiquitous fish species throughout the Arctic Ocean. As such, they serve an important ecosystem role linking upper and lower trophic levels and transferring energy between the benthic and pelagic realms. Our objective is to explore what limits the southern distribution of polar cod in Pacific and Atlantic sectors by examining time series of survey and oceanographic data. We quantify the variability in the southern extent of the polar cod distribution in the Bering and Labrador Seas, and determine mechanisms (bottom temperature and potential predators: Pacific cod Gadus macrocephalus, Atlantic cod Gadus morhua, and Greenland halibut Reinhardtius hippoglossoides and competitors: capelin Mallotus villosus and walleye pollock Gadus chalcogrammus) driving the variability. When temperatures were lower, polar cod occupied larger areas and had higher abundances in both regions, suggesting that as temperatures increase with climate warming the range of polar cod is likely to contract. Temperature had a much larger impact on polar cod abundance than competitor abundance and predator abundance, especially in the eastern Bering Sea. However, when we included data from northern and eastern Bering Sea in 2010 and 2017, polar cod were less likely to occur in warmer waters when either Pacific cod or walleye pollock were present. Northward range expansions of subarctic Pacific cod and walleye pollock may further restrict polar cod distributions.