Shifts in bowhead whale distribution, behavior, and condition following rapid sea ice change in the Bering sea

Abstract

Prior to 2017, the winter (January–March) distribution of the Bering-Chukchi-Beaufort (BCB) Sea population of bowhead whales (Balaena mysticetus) was centered in western Bering Sea shelfwaters characterized by high concentrations of sea ice. This area was largely devoid of sea ice during the winters of 2017–19 and satellite tagging studies show that the distribution of whales shifted northwards into the Chukchi Sea during 2018 and 2019 (no winter data were available for 2017). Here, we examine how bowhead whale movements and behavior changed in response to potential ecological drivers by linking satellite tag data, collected during 2009–2019, with contemporaneous sea ice concentration data and numerical simulations of regional oceanography. We used step selection and resource selection functions to examine how whales responded to changes in sea ice concentration, modeled ocean temperature, salinity, velocity, and the local gradients in these variables. Analyses indicated that whales selected areas of intermediate ice concentrations (∼65–85%) at distances ∼150–250 km from the southern ice edge. Dive data suggest that, prior to the ice decline, whales spent more time near the seafloor and less time near the surface. Interestingly, we found evidence that whales may track the diel vertical migration (DVM) of krill, but only after sea ice declined. We found no change in body condition of yearling or subadult whales harvested at Utqiaġvik following 2017; however, post-weaning whales had a statistically significant decline in body condition, which could be due to changes in sea ice or could reflect negative density dependence given increases in whale abundance and density. The BCB bowhead whale population may be approaching carrying capacity and abundance is thought to have reached or surpassed what existed prior to commercial whaling. At present, overwintering in the Chukchi Sea has few negative consequences for this population and we expect over wintering within the Chukchi Sea will become more common as winter sea ice declines in the Bering Sea.