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Great Basin Institute Research May Help Potentially Threatened West Coast Fishers

Great Basin Institute Research May Help Potentially Threatened West Coast Fishers

For the last seven years, GBI conservation biologist Rick A. Sweitzer, PhD, has been studying the population dynamics of a small, isolated population of fishers (Pekania pennanti) in the southern Sierra Nevada, California near Yosemite National Park.  Fishers in this region are struggling to exist in the face of threats from logging, forest fires, collisions with vehicles on highways, and poisons being used around illegal marijuana grow sites between Yosemite and Sequoia and Kings Canyon national parks.

As a member of the USDA Region 5 Forest Service Fisher Technical Team, Dr. Sweitzer has contributed analyses and results from his research and helped produce a Fisher Conservation Assessment that will guide fisher management in forest ecosystems across California.  The work of Dr. Sweitzer and his cooperators with the USDA Forest Service Pacific Southwest Research Station on the biology of fishers has been valuable and timely.   This because on October 7, 2014, the U.S. Fish and Wildlife Service (USFWS) proposed to list the west coast population of fisher as a threatened species under the Endangered Species Act (ESA).

Fishers are medium-size mammals native to North America. Members of the mustelid family (aka weasels), they are related to the smaller American Marten (Martes americana) and the larger wolverine (Gulo gulo). Fishers are forest-dwelling carnivores that prey on squirrels, rabbits and hares, and even the well-armored North American porcupine (Erethizon dorsatum). Like all predators, Pacific fishers provide a vital service in controlling prey populations, and their extinction could significantly disrupt trophic interactions in forest ecosystems (i.e., uncontrolled prey species can damage the health of the larger plant community, impacting all species in the forest).





A female fisher descending a den tree.
Credit: R. Sweitzer.






Pacific fishers once inhabited forested landscapes from northern British Columbia southward to California’s Sierra Nevada. However, their historic range has shrunk by nearly 50%, leaving only two native populations in California: one around the western California/Oregon border and the other in the southern Sierra.Although laws passed in the 1940s banned the trapping of Pacific fishers, the loss of forest habitat to development, logging, forest fires, along with fishers’ low reproductive rate, their diminished genetic diversity, exposure to disease, rat poisons, and collisions with vehicles on tourist-laden roadways in places such as Yosemite National Park, have put the fisher population in peril.

This map shows fisher habitat in the West.
Dark pink = historic range.
Blue = Reintroduced populations.
Green = Current distribution.






The ESA listing proposal has been a long time coming, because a petition to list the species in the region was first received in December 2000. The USFWS proposal identifies “the potential of direct and indirect exposures from the illicit use of anti-coagulant rodenticides on public and community forest lands within fisher habitat is a significant threat to the species … and some types of timber harvest and alteration of fisher habitat will continue to be a concern.”

Dr. Sweitzer has been studying fishers in the Sierra National Forest as part of the University of California Sierra Nevada Adaptive Management Fisher Project from September 2007 to June 2013.  In July 2013, the Reno-based Great Basin Institute entered into a cooperative agreement with the USDA Forest Service Pacific Southwest Research Station to support Dr. Sweitzer’s analyses assessing how current forest management intended to reduce risk of large wildfires and improve the health of forest ecosystems may impact the viability of fisher populations in the southern Sierra Nevada.  This work is being done as part of GBI’s Research Associate Program.

Learn more about fishers and the known threats to their survival, as well as a link to the USFWS listing proposal here:

You can also contact Dr. Sweitzer at 775-674-5478, or Rick would be happy to provide first-hand insight on the challenges to fishers in the West Coast Population region, including details on the science, conservation planning and management in California in particular.

And for a beautiful and interesting video that explains trophic interactions—albeit in Yellowstone National Park and not the Sierra Nevada—click here.

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