Diet of female polar bears in the southern Beaufort Sea of Alaska: evidence for an emerging alternative foraging strategy in response to environmental change

Polar bear (Ursus maritimus) diet may become more variable in some Arctic regions due to climate warming and altered sea ice habitat. We surveyed carbon and nitrogen stable isotope profiles of five polar bear tissues sampled from adult females in the Southern Beaufort Sea of Alaska in order to assess inter-tissue isotopic variability and to determine whether any dietary shifts are occurring in this population. We did not detect any significant shifts from historical means in population-level tissue stable isotope values. A number of sectioned hair samples, however, were significantly depleted in N-15 relative to the mean. We hypothesized that lower hair delta N-15 values were due to the consumption of bowhead whale (Balaena mysticetus) tissue. Telemetry data showed that polar bears with N-15-depleted hair sections were located on multiple dates near known subsistence-harvested bowhead whale bone piles and had spent 90 % of the prior year within 50 km of the shore. Bears with hair section delta N-15 values at or above the mean spent no time near bowhead whale bone piles and less than half of the year nearshore. An isotopic mixing model estimation of diet proportions determined that bowhead whale comprised approximately 50-70 % of fall diet for bears with lower hair delta N-15 values. We conclude that these results offer emergent evidence of an alternative foraging strategy within this population: 'coastal' bears, which remain near to shore for much of the year and use bowhead whale bone piles when they are present. In contrast, 'pelagic' bears follow a more typical strategy and forage widely on sea ice for seals.