Montana Field Guide

Fisher - Martes pennanti

The fisher is a medium-sized mammal with a long, low stocky body. The tail is relatively long and heavily furred. They have a pronounced muzzle and large rounded ears. In winter, fishers are dark brown to black with light colored hairs around the face and shoulders (Powell 1993). The undersides are uniformly brown, however individually unique patterns of white or cream can occur on the chest, underarms or genital region (Powell 1993). The summer pelage is more variable and lighter in color. Molt occurs once per year in late summer and early autumn (Powell 1993). The feet are large and have 5 retractable, but not sheathed, claws (Powell 1993). Fishers are highly sexually dimorphic with males averaging nearly twice the size of females. Male fishers generally weigh between 3.5 and 5.5 kilograms with females weighing between 2.0 and 2.5 kilograms (Powell 1993).

Fishers are not easily confused with other mustelids in Montana. They are much larger than the mink and larger and much darker than the slightly smaller marten. Fisher are smaller than wolverines and have a longer tail and a lower, longer appearance overall.

Montana Range

Fishers are non-migratory, but may make extensive movements up to a maximum of 40 kilometers in 3 days (Hash 1979).

Although they are primarily terrestrial, fishers are well adapted for climbing. When inactive, they occupy dens in tree hollows, under logs, or in ground or rocky crevices, or they rest in branches of conifers (in the warmer months). Fishers occur primarily in dense coniferous or mixed forests, including early successional forests with dense overhead cover (Thomas et al. 1993). They commonly use hardwood stands in summer but prefer coniferous or mixed forests in winter and avoid open areas. Optimal conditions for fishers are forest tracts of 245 acres or more, interconnected with other large areas of suitable habitat. A dense understory of young conifers, shrubs, and herbaceous cover is important in summer.

Forest structure, which affects prey abundance and vulnerability and provides denning and resting sites for fishers, is probably more important than tree species composition (Buskirk and Powell 1994). Forest structure can be characterized by a diversity of tree shapes and sizes, understory vegetation, snags and fallen limbs and trees, and tree limbs close to the ground (Buskirk and Powell 1994).

Young are born in a den in a tree hollow (usually), or under a log or in a rocky crevice. Large snags (greater than 20 inches diameter at breast height) are important as maternal den sites (Thomas et al. 1993).

Diet consists primarily of mammals (small rodents, shrews, squirrels, hares, muskrat, beaver, porcupine, raccoon, deer carrion); also birds and fruit. Snowshoe hares are an important dietary item for fishers in Montana, as is deer carrion (Foresman 2001). Fishers are also well known for their skill at killing porcupines and have been implicated as the only predator capable of regulating porcupine populations (Powell 1993).

Fishers are generally solitary except during the breeding season. Home ranges have been estimated at 10 to 800 square kilometers by snow tracking, and 7 to 78 square kilometers by telemetry using a minimum convex polygon model. Generally, the ranges of adults of the same sex do not overlap. In Maine, home ranges of females were stable between seasons and years, but males moved extensively in late winter and early spring and their ranges shifted between years. In New Hampshire, mean annual home range was about 15 to 25 square kilometers, with daily movements of 1.5 to 3.0 kilometers. In southern Quebec, mean home range size was 5.4 square kilometers for females and 9.2 square kilometers for males (Garant and Crete 1997). Fishers have been recorded moving 90 kilometers in 3 days (Nowak 1991).

Population density in favorable habitat has been estimated at up to about 1 per 3 to 11 square kilometers in summer, and 1 per 8 to 20 square kilometers in winter (Arthur et al. 1989). In southern Quebec, density was estimated at about 3 individuals per 10 square kilometers; the high density was attributed to the absence of trapping (Garant and Crete 1997).

No specific information on fisher reproductive biology is available for Montana. Information gathered in other portions of the range suggests that fishers breed in late February to April or March to May. Females probably mate within days of giving birth. The implantation of the embryo is delayed for 11 months after mating. Parturition occurs approximately one month after implantation and renewed development of the embryo (Foresman 2001). Litter sizes average about three throughout the range. Births occur primarily from March to mid-April (sometimes in February or May in some areas). Young are mobile by 8 weeks and are weaned in 2.5 to 4 months. Separation from the mother occurs in the fifth month, in late summer or early fall. In Maine, young are weaned from mid-May to early June and are independent probably in late August or early September (Arthur and Krohn 1991). Fishers are sexually mature in 1 to 2 years but not all adult females will breed in a given year.

Fishers were extinct in Montana by the 1930's (Foresman 2001). Reintroduction efforts in 1959 and 1960 in Lincoln, Granite and Missoula counties resulted in the establishment of populations in those counties. More recent reintroductions were made in the Cabinet Mountains between 1988 and 1991 (Foresman 2001). The species is currently managed as a furbearer with a limited harvest of 7 animals. Consult Montana Fish, Wildlife and Parks for current trapping regulations. Additional information concerning fisher management can be found in Ruggiero et al. (1994).