Seasonal diets of mink and martens: Effects of spatial and temporal changes in resource abundance

Abstract

Seasonal changes in food availability and feeding habits of mink (Mustela vison) and martens (Martes americana) on Chichagof Island, Southeast Alaska, were studied through the analysis of natural abundance of stable isotopes. Dependence of the two species on marine-derived nutrients, carried to the terrestrial system via the upstream migration of spawning Pacific salmon (Onchorhynchus sp.), was investigated. Twenty-four mink and 75 martens were live-trapped repeatedly in early summer (prior to salmon runs), early autumn (post salmon runs), late winter, and in spring (during the mating season). A blood sample was obtained from each individual. In addition, 25 mink and 165 marten carcasses were obtained from trappers during late autumn 1991-1994. Concurrently, prey availability was monitored, and tissues from prey were collected. The abundance of stable isotopes in prey tissues and blood samples were compared, indicating 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, but fed on salmon carcasses when they became available in autumn. In addition, analysis of blood progesterone revealed that timing of reproduction in female mink appear to be shifted, so that lactation coincided with the availability of salmon carcasses. In contrast, martens showed individual variation in their diets, with some individuals feeding exclusively on terrestrial organisms, while the diets of others include salmon carcasses. Incorporation of salmon in the diet depended largely on availability of small rodents and location of the martens home range on the landscape. Although salmon carcasses are not a preferred food item for martens, they act as a suitable alternative to maintain body condition and allow successful reproduction even in years when preferred food is not readily available.