Using stable isotopes to understand the feeding ecology of the Hokkaido brown bear (Ursus arctos) in Japan

Abstract

Interactions between brown bears (Ursus arctos) and anadromous salmon (Oncorhynchus spp.) constitute a unique energy pathway that facilitates nutrient cycling between marine and terrestrial ecosystems. Previous studies have documented variation in salmon consumption by brown bears; however, few have addressed potential anthropogenic factors influencing consumption. We assessed diet of brown bears on Hokkaido Island, Japan, using carbon and nitrogen stable isotopes to determine the effect of demographic (age and sex) and environmental (developed and undeveloped area) factors on salmon consumption. We collected thigh bones from 190 harvested bears from 1996 to 2011 and samples of their major dietary foods from 2009 to 2011, and we then estimated the potential contributions of these foods to the diets of brown bears using a Bayesian mixing model. Brown bears consumed more herbs, fruits, and corn than terrestrial animals or salmon at the population level. However, the dietary contribution of salmon varied widely among bears; in some cases, it comprised >30% of the total diet. Salmon consumption also varied by bear age class, sex, and location. Low salmon consumption by adult females with cubs suggested avoidance of salmon-spawning areas to minimize risk to their cubs. Bears inhabiting undeveloped areas were more likely to consume salmon than those inhabiting developed areas, suggesting that human activities restrict brown bears' salmon consumption. The lower salmon intake of Hokkaido brown bears compared with Alaskan brown bears may be attributed in part to extensive human development on Hokkaido Island, including in-stream structures that preclude salmon migrations and agricultural crops that provide an alternative food subsidy.