Rare in the West, the Pacific fisher is a subspecies of fisher found in other U.S. regions, including the Northeast. Its decline, and now perilous status, has led it to be a candidate species under the federal Endangered Species Act. Yosemite represents the northern limit of distribution of the small and isolated Southern Sierra Nevada population, which is estimated to number between 125 to 250 adults.
In spring 2011, Yosemite National Park, in conjunction with the UC-Berkeley Sierra Nevada Adaptive Management Program (SNAMP), has successfully identified the park's first Pacific fisher den. Scientists used radio-telemetry to track the movements of a female Pacific fisher from the Sierra National Forest northward into the southern portion of Yosemite. Remote motion-triggered cameras confirmed that this female fisher is denning in the park.
To help save a key forest predator, Yosemite National Park's wildlife biologists have deployed motion-activated cameras in targeted locations of the park with the hope of detecting the elusive Pacific fisher (Martes pennanti pacifica) and identifying necessary habitat requirements to give the park's small fisher population its best chance at survival.
This two-year camera study was initiated to determine the distribution, abundance, and northern range limit of the Pacific fisher in Yosemite National Park. In collaboration with the California Department of Fish and Game, UC-Berkeley, the Yosemite Conservancy, and the Central Sierra Environmental Resource Center, biologists have combined research efforts to achieve the collective goal of determining the region-wide status of the southern population of Pacific fisher. Highly advanced remote motion-sensing camera stations were strategically placed in suitable habitat throughout the park with the objective of determining fisher presence. The first year of this camera survey study (winter 2009-2010) focused efforts in the southern portion of the park and documented fisher presence at 20 camera stations, with fisher activity concentrated in and around the Mariposa Grove of Giant Sequoias and along Highway 41 south of Chinquapin. Evaluation of images obtained from the camera stations suggest that five to eight individual animals have been detected. This first survey year collected more fisher detections in Yosemite National Park than any previous study.
For the duration of the second survey year (winter 2010-2011) in this region-wide project, efforts have been focused north of the Merced River, in the western portion of the park, as well as along the Glacier Point Road. These surveys have contributed critical information toward our understanding of habitat suitability for fisher in Yosemite, particularly in less accessible areas in the western part of the park north of the Merced River where limited data exist. "If we understand fisher habitat needs," said Steve Thompson, chief of Wildlife Management for Yosemite's Resources Management and Science division, "we can manage the forest ecosystem in ways that could restore forest attributes to help the fisher in the re-colonization of their former range."
The sharing of fisher information is highly valuable to a wide range of agencies as options for statewide management status of the fisher are evaluated and planned in the coming years. Yosemite's second year of surveys also brings unprecedented findings: for the first time in the history of Yosemite National Park, a female Pacific fisher with kits (young) has been located in a den tree in the park. In collaboration with UC-Berkeley, this fisher's movements are being tracked very carefully so that the park can do everything it can to protect the fisher family from any disturbance.
The Pacific fisher once ranged from British Columbia, south through Washington, Oregon and northern California, reaching its most southern extent in the Sierra Nevada. This carnivore has declined to roughly 50% of its historical range in California, and only two native populations remain today—one around the Western California/Oregon border, and one in the Southern Sierra Nevada. As scientists study the state's two separate Pacific fisher populations, wildlife biologists believe the Sierra Nevada fisher population has been physically—and genetically—separated from the fisher on the Northwest Coast for more than 5,000 years.
A medium-sized member of the weasel family, the fisher inhabits mid-elevation forests (from 4,000 to 7,000 feet) associated with large mature trees, and eats everything from birds to small mammals to fruit and fungi. It is known as one animal clever enough to prey regularly on porcupine. The loss of the Pacific fisher would affect the natural balance between predators and prey. "Anytime you lose any species from the ecosystem, it is a tragedy," Thompson added. Fishers are threatened by low reproductive capacity, reduced genetic diversity, predation, disease, and habitat loss. In Yosemite, fishers are frequently killed by cars, which, with such a small population size, could be a critical cause of mortality that limits population size and distribution.
Female Pacific fishers can breed as early as 1 year old, but most do not successfully reproduce until age 3. Female fisher, however, do not produce young each year. In fact, reproductive rates fluctuate widely; anywhere from 15% to 75% of mature females will give birth in any given year, with typical litter sizes varying from one to four kits. Following a gestation period of about 40 days, females give birth and raise their kits in dens, usually tree cavities. Some females have one maternal den while others have multiple dens among which they will move their kits. (Dens in the Sierra Nevada have been located in tree cavities in both live and dead trees-in species including black oaks, white fir, incense cedar, ponderosa pine and sugar pine.) Kits are born blind and helpless-completely dependent on their mother's milk for the first eight to 10 weeks. Kits begin to crawl after three weeks, can open their eyes after seven weeks, and can climb after eight weeks. After five to seven months, kits leave the maternal den and their mother to venture beyond the territory of their birth and upbringing.
Other Sierra Nevada-related studies are taking place in an attempt to save this important mammal.
In the West, Pacific fishers continue to gain scientific attention to facilitate their recovery. Historic trapping prior to 1946 contributed to population declines. Other Western parks study the Pacific fisher, including Olympic National Park where a 2008 reintroduction of the species occurred as part of a State of Washington effort. In the East, fishers have survived better, perhaps, due to their slightly larger size and due to reintroductions, such as a 1994 Pennsylvania project.
Endangered Status: The Pacific fisher is a federal candidate species under the Endangered Species Act, and a California species of concern. View Yosemite's special-status species.