Stable Isotopes Reveal Variation in Consumption of Pacific Salmon by Brown Bears, Despite Ready Access in Small Streams

Abstract

Brown bears Ursus arctos consume a wide range of organisms, including ungulates and plants, but Pacific salmon Oncorhynchus spp. are especially important to their diet where their ranges overlap. Although some bears minimize antagonistic encounters with other bears or infanticide by avoiding streams where salmon spawn, studies generally assume that bears with ready access to salmon feed heavily on them. To test this assumption, and the hypothesis that male bears would feed more heavily on salmon than females (owing to their sexual size dimorphism), we collected hair samples from brown bears using barbed wire on six small tributaries of Lake Aleknagik, Alaska where adult sockeye salmon O. nerka are readily accessible and frequently consumed by bears. Analysis of DNA distinguished among the different bears leaving the hair samples, some of which were sampled multiple times within and among years. We assessed the contribution of salmon to the diet of individual bears using carbon and nitrogen stable isotope signatures. The 77 samples analyzed, from 31 different bears over four years, showed isotopic ratios consistent with reliance on salmon, but the wide range of isotopic signatures included values suggesting variable, and in one case considerable, use of terrestrial resources. Stable isotope signatures did not differ between male and female bears, nor did they differ between two sides of the lake, despite marked differences in sockeye salmon density. The hair samples were collected when salmon were present, so there was some uncertainty regarding whether they reflected feeding during the current or previous season. Notwithstanding this caveat, the results are consistent with the hypothesis that salmon were sufficiently available to provide food for the bears, and that the considerable isotopic variation among bears with access to salmon reflected their age, status, and behavior.