Though fishers Pekania pennanti were extirpated from most of their historical range in the United States, they have successfully repatriated many areas, and are now legally harvested in the Great Lakes region. Recent harvests and winter track surveys, however, suggest fisher populations may again be decreasing in northern Wisconsin. We investigated potential factors contributing to this purported decrease by quantifying fisher diet in northern Wisconsin via stable isotope analysis (δ¹³C and 6¹⁵N) and examining body condition of individual fishers using measurements of fat depots. Diet reconstruction revealed small mammals and porcupines constitute the largest portions of fishers' diets in Wisconsin. Body condition analyses showed that contemporary fishers carry significantly less body fat compared to Wisconsin fishers two decades ago and to two other regional populations. Our findings contrast with past regional studies that report snowshoe hares and white-tailed deer carrion as the largest components of fisher diet. We suggest the diet and body condition of fishers in Wisconsin could be due to low availability of preferred prey other than porcupines on the landscape, which may be contributing to the recent putative population decrease.