NATURAL AND HUMAN-LINKED MORTALITY RISKS AND LIMITS TO POPULATION GROWTH OF FISHERS IN CALIFORNIA

Rick A. Sweitzer; The Great Basin Institute; sweitzerrick@gmail.com; Viorel D. Popescu, Craig M. Thompson, Kathryn L. Purcell, Greta M. Wengert, Mourad W. Gabriel, Leslie W. Woods, Reginald H. Barrett

Fishers in Washington, Oregon, and California have not recovered from population declines and the U. S. Fish and Wildlife Service has proposed to list them. We evaluated survival and mortality risks among 232 radiocollared fishers in the Sierra National Forest. We estimated mortality rates using non-parametric incidence functions, and integrated risk-specific survival rates into a Leslie matrix to evaluate how population growth (λ) might improve if management can reduce mortality. Cause of death was determined for 93 of 121 fishers, and annual survival was 0.72 for female fishers. Mortality rates were 19.5% for predation, 2.5% for disease+injury+starvation, and 1.9% for toxicant exposure+vehicle strikes. Predation rates were similar between sexes, but predation risk was 51% lower in fall and winter compared to spring and summer. The mortality rate from disease+injuries+starvation+vehicle strikes+toxicants was 4.4%, and 11 times higher for males than females. We estimated a base ÃŽÂ» of 0.96, which had the potential to increase to 1.03 or 1.11 if predation were reduced by 25% or 50%. Lambda could increase to 0.97 in the absence of all other risks besides predation. Predation was the dominant limiting factor, and research is needed on habitat conditions that facilitate contact between fishers and larger predators

Ecology and Conservation of Mesocarnivores