Variation in winter diet of southern Beaufort Sea polar bears inferred from stable isotope analysis

Ringed seals (Phoca hispida Schreber, 1775 = Pusa hispida (Schreber, 1775)) and bearded seals (Erignathus barbatus (Erxleben, 1777)) represent the majority of the polar bear (Ursus maritimus Phipps, 1774) annual diet. However, remains of lower trophic level bowhead whales (Balaena mysticetus L., 1758) are available in the southern Beaufort Sea and their dietary contribution to polar bears has been unknown. We used stable isotope (C-13/C-12, delta C-13, N-15/N-14, and delta N-15) analysis to determine the diet composition of polar bears sampled along Alaska's Beaufort Sea coast in March and April 2003 and 2004. The mean delta N-15 values of polar bear blood cells were 19.5 parts per thousand (SD = 0.7 parts per thousand) in 2003 and 19.9 parts per thousand (SD = 0.7 parts per thousand) in 2004. Mixing models indicated bowhead whales composed 11%-26% (95% CI) of the diets of sampled polar bears in 2003, and 0%-14% (95% CI) in 2004. This suggests significant variability in the proportion of lower trophic level prey in polar bear diets among individuals and between years. Polar bears depend on sea ice for hunting seals, and the temporal and spatial availabilities of sea ice are projected to decline. Consumption of low trophic level foods documented here suggests bears may increasingly scavenge such foods in the future.