Pacific Fisher Description and Biology

A fisher's head close up
 

Pacific Fishers

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Biology and Description
Fishers are tree-dwelling mammals that have long, slender bodies, long bushy tails, and short muscular legs with large feet and retractable claws, making them climbing specialists. Fishers are very agile and have specialized hind legs with feet that turn nearly backwards, giving them the ability to climb down trees head first.

Fishers are highly associated with older, mid-elevation (4000 to 7000 feet) forests and prefer areas with large conifer and hardwood trees, interspersed with plenty of younger trees and downed logs. This complex forest structure is important, especially for female fishers who have their babies (called kits) in cavities of older trees and use the dense understory to forage for food and hide from predators. Fishers use multiple den trees when raising their kits and will move the kits to different dens to keep them clean, healthy, and safe from predators. Fishers in the southern Sierra Nevada most often use cavities and hollowed out areas of trees such as black oaks, incense cedar, white fir, ponderosa pine, and sugar pine to make their dens.
 
Fisher at night with food in its mouth
Hunters by nature, fishers feed on a wide variety of rodents, rabbits, and birds but sometimes they supplement their diet with fruits and fungi. Fishers are even wily enough to eat a porcupine if one crosses its path. Similar in size to a domestic cat, fishers weigh between 2 and 5.5 kilograms (4 to 12 pounds) and are members of the mustelid family along with otter, mink, marten, wolverine, and badger. The fur of a fisher is very thick, soft, and shiny, has a distinct musky smell, and ranges in color from light blond to a rich, dark brown.

Fisher reproduction is a unique process and is sometimes considered a limiting factor in their population viability. Fishers exhibit a polygnous mating system, mating with multiple individuals in one breeding cycle/season. Further, female fishers have a reproductive strategy called delayed implantation in which the eggs are fertilized during mating season between March and April, but the embryo does not begin to develop until approximately 10 months later. Although delayed implantation allows the female to arrest pregnancy during years of high stress, it also contributes to the lengthy reproduction time of the species. After implantation, the gestation period is approximately 40 days, so kits are born between February and April (one year after mating and just before the new mating season begins). The average litter size for fishers in the southern Sierra Nevada are two kits that are born blind and completely helpless. Their eyes open between 6 to 8 weeks and weaning occurs at 8 to 10 weeks. Kits remain with the mothers until the fall. Female fishers can mate within their first year, but most do not successfully give birth until the second or third year.

Fisher reproduction strategies contribute to home range size differences between male and female fishers. Average male home ranges (2998 hectares) are more than twice the size of average female home ranges (527.5 hectares) in the Sierra Nevada, since males must maintain access to multiple females during the breeding season to ensure successful mating.

Last updated: August 7, 2017

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