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Martes pennanti - Fisher

Skull Pictures:     
Dorsal      Posterior mandible
Lateral      Ventral

Physical Description:

The fisher (Martes pennanti), is the largest member of its genus.  The physical characteristics that are common to most fishers are dark brown to almost black in hue with lighter shades of brown and tan colors on the breast (Herdmann 2000).  The fisher also has silver and gold colored hairs on the front of the face along the side of the head to the top of the neck and to the tail.  These hairs are called guard hairs and have no mentioned purpose other then camouflage.  The ventral abdomen of the fisher is white and has other light colored markings (DNR 1993).  The thickness of the coat changes with the seasons.  The tail can go from bushy in the winter to thin in the months of September and October. The fisher has short legs and a long body like most mustelids.  Fishers have five digits on each foot, which they use to help them walk in a plantigrade motion, and their feet are large for their body size.  Males are larger than females in body mass as well as in body length, and in most cases males are almost twice the size of females in body weight.  Males weigh between 3.5 and 5.5 kilograms whereas females weigh 2.0-2.5 kilograms.  Male fishers are on average 90-120 cm long and females range from 75-95 cm.  They have good night vision with the green eye shine (Powell 1993).  Fishers have large canines for biting a holding onto prey and carnassials for sheering flesh (Powell 1993). They have 38 teeth with a dental formula of I 3/3 C 1/1 P 4/4 M 1/2 with the 4th upper pre-molar and the 1st lower molar being the carnassial teeth.  Also, the last upper molar is constricted, a trait common to all mustelids.


The fisher was historically common to the wooded areas of Wisconsin.  Before the arrival of Europeans the fisher ranged all throughout Wisconsin.  By the early 1900’s most of the population across the state was killed off by a combination of logging, forest fires, and trapping for their fur.  The last wild Wisconsin fisher was seen in 1932 until reintroduction started in the late 1950’s and 60’s when the Department of Natural Resources and the U.S. Forest Service reintroduced the animal to Wisconsin.  Kohn (1989) states that sixty animals from New York and Minnesota were released into the Nicolet National Forest, and sixty from Minnesota were released into the Chequamegon National Forest.  Along with this the U.S. Forest Service established “Fisher Management Areas” to reduce accidental deaths due to trapping in the areas around the fisher release sites.  The fisher is now a common site in the area of Northern Wisconsin.

Today the fisher has a range as far south as Shawano County and spanning from Marinette to Burnett counties.  The estimated population of fishers in Wisconsin is 6000 animals.  There is a 12,000 square mile area where fishers are common.  This area is in the northern third of the state that extends as far south as Langlade County.  A 7000 square mile area where they are fewer in numbers runs from as far north as Bayfield County to as far south as Menominee County (Kohn 1989).  That population fluctuates according to Powell (1993) just like all other species of martens. Estimates were based on track counts and trapping reports from the DNR between 1976 and 1991.

The trapping of these animals is allowed in four regions throughout Wisconsin.  The regions are in the northern part of Wisconsin near Florence, Vilas, Dodge, and Price counties.

Ontogeny and Reproduction:

Breeding starts when the female fisher goes into estrus.  This is visible by the enlarging of the vulva and lasts about one week (Powell 1993).  A study has also stated that a second estrus happens ten days after the original mating season.  In the wild and in captivity, an increase in activity occurs in early March marked by the males using urine, feces, and musk glands to mark territories as well as enlargement of the testes and production of sperm (Powell 1993).  This breeding season lasts through the end of April.

Female reproduce as early as one year of age.  The blastocyst remains inactive for ten to eleven months and then attaches and develops (Linzey 2002).  After parturition in the early spring the female copulates again ten to twelve days later.  The average litter size is between 2.7 and 3.9 kits per litter.  They are born with their eyes and ear closed, and little patches of gray hair on their bodies.  By two weeks their hair is silver and gray and a week later the color becomes darker.  Fishers are weaned by 80 days when they leave the den (Powell 1993).

The life expectancy for fishers in the wild is about ten years (Powell 1993).

Ecology and Behavior:

Martes pennanti is active during both day and night.  According to Powell (1993), the mammal has spurts of activity at three or four times a day that last between 2 and 5 hours.  The times of most activity are around sunrise and sunset. Activity patterns may change seasonally with more daylight activity during the summer than the winter (Powell 1993).  Most of the activity is spent looking for food.  The fisher uses fur and retractable claws on its paws to keep their footing on slippery terrain as well as in the forest on logs, rocks, and sand.  They also seem to bounce when they run, although their gait is not a bounding one.  Fishers run with their two front feet together slightly offline and the same with the two back feet (Powell 1993).   Males move more then female according to Kelly (1977).  Fishers forage for food by constantly zigzagging in different directions to catch prey.  They search for prey in blown down trees, brush, and other dense cover areas to get at prey that is not available to other carnivores (Coulter 1966).  The fisher preys on smaller herbivores such as snowshoe hares, mice, porcupines, and squirrels  (Powell 1993).  Birds, such as ruffed grouse, are taken too. Fishers have also been known to eat other carnivores and berries as well  (Powell 1993).  Fishers kill with a bite to the back of the neck to animals like hares and shake or swallow mice and other small mammals.

Fisher rest in logs, branches, nest of bird and mammals, and holes in the ground.  Most of these places are temporary, but some are used numerous times (Powell 1993). They can climb to get food in trees.  Despite the name, fishers are not known for swimming.  They have been reported as swimming only if the river is easily crossed quickly by Kelly (1977).

The average size of home ranges for male fishers is 38 square kilometers, and the female average is 15 (Powell 1993).  To mark these home ranges fishers use their strong smelling anal scent gland to produce a territory marking scent.  Powell (1993) suggests that this is because food is a limiting resource and food is “patchy” in these areas.  Usually same sex territories do not overlap but territories of opposite sexed fishers overlap many times.  The fisher has no natural predators.  Human trapping is responsible for most of their deaths in the wild.         

Literature Cited:

Coulter, M. W.  1966.  Ecology and management of fishers in Maine.  Ph.D. thesis, St. Univ. Coll. Forest., Syracuse University, Syracuse, NY.

Herdman, E.  2000.  Fisher: 1-5.  < fisher/fisher.htm>.   

Kohn, B.E.  1989.  History and Status of Wisconsin’s Fisher Population.  Wisconsin Department of Natural Resources.  Number 23.

_____.  1993.  The Fisher In Wisconsin.  Wisconsin Department of Natural Resources.  Madison, WI.

Kelly, G. M.  1977.  Fisher (Martes pennanti) biology in the White Mountain National Forest and adjacent areas.  Ph. D. thesis, University of Massachusetts, Amherst.

Linzey, D.  2002.  Martes pennanti:1-5.  <>.

Powell, R.A.  1993.  The Fisher Life History, Ecology, and Behavior.  University of Minnesota Press.  Minneapolis, MN.  

Reference written by Marcus Wahleithne,  Biology 378 student.  Edited by Christopher Yahnke.
Page last updated 4-28-04.

©1993- University of Wisconsin-Stevens Point