Uneaten foods are usually cached for later use by fishers and the species will sometimes deposit their musk on the uneaten portions to discourage other animals. Fisher also eat carrion and substantial amounts of wild berries in season. It was once thought that fisher required mature forests, but the species can thrive in newer second growth forest, favoring large tracts of pine, spruce, aspen, birch and cedar swamps. Good populations are considered to be one fisher per 10 square miles. As the only consistent predator of porcupines, fisher provide a service by controlling this species, as well as the damage that the porcupines inflict upon forests.
Fisher contribute to the overall health of prey species when their densities are light or moderate.
Dense populations of fisher create problems for a variety of other species, including other furbearers. Fisher do have to abandon territories as food sources become depleted and less mobile furbearers can become malnourished when competing for the same prey species as fisher. Predation upon fisher by other predators is not thought to be serious. A lynx or bobcat will rarely kill a fisher and wolves have been known to kill fisher caught out on frozen lakes. Young fisher are killed at times by large owls, eagles and coyotes.