May 18, 2011
Yosemite National Park is home to more than 400 species of animals, including a number of species that qualify for listing under the Endangered Species Act. The park provides habitat for a wide variety of wildlife due to its undisturbed ecosystems in the Central Sierra Nevada. Wildlife species in Yosemite range from aquatic invertebrates to large mammals.
One of the many mammals that roam Yosemite’s forests is the Pacific fisher (Martes pennanti). The fisher, a medium-sized member of the weasel family, is a candidate species for listing as threatened or endangered under the federal Endangered Species Act. Yosemite National Park, in conjunction with the U.C. Berkeley Sierra Nevada Adaptive Management Program (SNAMP) Fisher Project, has successfully identified the park’s first Pacific fisher den.
The female Pacific fisher using this den was originally captured as a juvenile in October of 2009 in Sierra National Forest, directly south of the park. SNAMP researchers have been able to continuously track this fisher using radio-telemetry since her initial capture. After remaining near her capture site for almost a full year, she moved northward toward Yosemite. This female fisher recently moved her kits (young) from their den in Sierra National Forest where they were born, to a den in the southern portion of Yosemite National Park. This is typical behavior for a female fisher with newborn kits. The female will likely move her kits one to two more times to different dens during the next month.
Female Pacific fishers can breed as early as one year of age, but most do not successfully reproduce until three years of age. The fisher discovered at the park successfully had kits at two years old. Following a gestation period of about 40 days, the female typically gives birth to one to three kits in a hollow cavity of a tree. Kits are born blind and helpless, and completely dependent on their mother’s milk for the first 8 to 10 weeks. Kits begin to crawl after about 3 weeks, can open their eyes after about 7 weeks, and can start to climb after 8 weeks. At about 4 months, kits are more mobile and are able to travel with their mother on hunting trips. Fisher eat small mammals such as mice and squirrels, and are even known to be one of the few animals to prey on porcupines. After approximately 5 to 7 months, the kits leave the maternal den and their mother and travel out of the territory of their birth and upbringing.
The Pacific fisher is a western subspecies that once ranged from British Colombia, south through Washington, Oregon, and northern California, and reached their southern extent in the Sierra Nevada. Pacific fisher have declined to only 50 percent of their historical range in California, with only two native populations separated by approximately 260 miles remaining today. One population is located around the western California-Oregon border, while the other is located in the southern Sierra Nevada in California. Yosemite represents the northern boundary of the small and isolated southern Sierra Nevada population, which is estimated to be between 125 to 250 adults.
The Pacific fisher is threatened by low reproductive rates, reduced genetic diversity, predators, disease, and habitat degradation. Trapping prior to 1946 also contributed to population decline. Recently, road-kill has become a concern as several Pacific fisher have been found dead along the roadways in the park over the past decade after being hit by cars.
In order to promote continued fisher recovery, partnerships have developed between the NPS, the Yosemite Conservancy, U.C. Berkeley, the U.S. Forest Service, the U.S. Fish and Wildlife Service, and the California Department of Fish and Game.
This important recovery work has been made possible with support from Yosemite Conservancy, the Wilderness Society, and the Aspenwood Foundation.
For more information on Pacific fishers, please visit https://www.nps.gov/yose/naturescience/fishers.htm