Stable isotope differences of polar bears in the Southern Beaufort Sea and Chukchi Sea

The life-history, genetic, and habitat use differences between the 2 polar bear (Ursus maritimus) subpopulations in Alaska, USA, have been used to determine the geographic border separating them, but it has sparked a debate of the correct placement of the border for several years. Recently, the Southern Beaufort Sea (SBS) polar bear subpopulation has declined because of sea ice loss, while the Chukchi Sea (CS) subpopulation appears stable. To provide additional information about potential differences between the SBS and CS subpopulations, such as differences in prey sources, we used stable isotope analysis of carbon and nitrogen from bone collagen of polar bears in these 2 neighboring subpopulations. We analyzed polar bear bones from 112 individuals collected from 1954-2019. Our purpose was to determine if the SBS and CS subpopulations could be distinguished based on the stable isotope signatures of bone collagen. A difference >1 parts per thousand in stable carbon isotope (delta C-13) values suggests a change in carbon sources, such as nearshore to offshore, while a 3 parts per thousand change in stable nitrogen isotope (delta N-15) values equates to a change of about 1 trophic level. Our study indicated a difference in delta C-13 values (P <= 0.001) but not delta N-15 values (P = 0.654) between the CS (-13.0 +/- 0.3 parts per thousand and 22.0 +/- 0.9 parts per thousand, respectively) and SBS bears (-14.7 +/- 1.3 parts per thousand and 22.2 +/- 1.0 parts per thousand, respectively). Our findings indicate that the 2 subpopulations are consuming similar high trophic level prey, while feeding in ecosystems with different delta C-13 baselines. We performed a logistic regression analysis using delta C-13 and delta N-15 values of the polar bears to predict their placement into these 2 subpopulations. Using Icy Cape, Alaska as the geographical boundary, the analysis correctly placed polar bears in their respective subpopulations 82% of the time. Overall accuracy of placement changed to 84% when using the current geographical boundary at Utqiavik, Alaska. We predicted samples collected from the Wainwright, Alaska region as 58% CS and 42% SBS polar bears. This suggests that the area between Wainwright and Icy Cape is a polar bear mixing zone that includes bears from both subpopulations. Bone collagen has a long-term, potentially life-long, stable isotope turnover rate, and our results could be used to determine the association of harvested polar bears to Alaska subpopulations, thus aiding in transboundary harvest quota management.