New study shows stable fisher population in the Southern Sierra Nevada

After experiencing years of population decline on the West Coast, a recent study examining fisher populations found that -- at least in the southern Sierra Nevada -- the animal's numbers appear to be stable. Scientists from the U.S. Forest Service's Pacific Southwest Research Station (PSW) and the Pacific Southwest Region collaborated to monitor the distribution of fishers across a 7,606-square-mile area in the southern Sierra Nevada. They used baited track-plate stations -- an enclosure where the fisher leaves a sooted track print as it walks through -- at 223 locations across three national forests. Over an eight-year period, from 2002-2009, they found that the fisher population in the southern Sierra Nevada neither increased nor decreased.

The findings are relatively good news for the cat-sized relative of the weasel family. The forest-dwelling fisher (Martes pennanti) once lived throughout most of the mountains in northern California and the Sierra Nevada, and in the Rocky Mountains, Cascades and Coast ranges. But many populations were eliminated or declined due to commercial trapping and clear-cut timber harvesting. Fishers have been reintroduced at a few locations in the western U.S., but only two native populations -- both centered in California -- remain. The small population of fishers in the southern Sierra has been estimated, by other methods, to be approximately 250 individuals.

"This study is encouraging in that it demonstrates that we can monitor a fisher population over a large area, with simple methods," said Bill Zielinski, a PSW research ecologist who coordinated the analysis of the research data. "It is also encouraging that we did not discover a decreasing trend in the population, but eight years is a relatively short period."

Zielinski noted that given the short time period of this study, the effects of Forest Service management actions to protect fishers and their habitat cannot yet be fully assessed. Further study, over a longer time period is necessary to fully understand the efficacy of these conservation measures. Other factors unrelated to habitat, including road kill and the illegal use of rodenticides which poison the fishers, must also be considered when evaluating the population.

"We hope that we can continue to monitor the fisher, so that we can witness an increase in the population as public and private groups collaborate to institute measures to protect them from various threats," he said.

PSW statistician James Baldwin, along with collaborators Richard Truex, Jody Tucker and Patricia Flebbe from the Pacific Southwest Region, contributed to the study.