Seasonal changes in diets of coastal and riverine mink: The role of spawning Pacific salmon

Feeding niches of riverine and coastal mink (Mustela vison) in southeast Alaska differ in prey composition and abundance and diving medium during spring and summer. In autumn, however, the upstream migration of spawning Pacific salmon (Oncorhynchus sp.) creates a pulse of food for mink. We hypothesized that diets of coastal and riverine mink, and therefore their stable isotope ratios (delta(13)C, delta(15)N), would differ significantly during periods when salmon were absent, but that salmon carcasses would constitute a large portion of the diet of both groups during the salmon spawning season. Stable isotope analyses of clotted blood cells from 24 live-captured mink and muscle tissue from 25 mink carcasses were used to indicate the composition of diets of individual mink in 1992 and 1993. These isotope values were then compared with stable isotope ratios of prey, using a multiple-source mixing model. Our results indicate that riverine mink depended on salmon (carcasses and fry), with little seasonal or individual variation, whereas coastal mink relied on intertidal organisms in spring and summer, with measurable (<25%) consumption of salmon carcasses when they became available in autumn. Coastal and riverine mink in southeast Alaska differ strongly in their diets in spring and summer, with both groups relying on the abundant salmon carcasses during autumn.