Mortality risks and limits to population growth of fishers

Abstract

Fishers (Pekania pennanti) in the west coast states of Washington, Oregon, and California, USA have not recovered from population declines and the United States Fish and Wildlife Service has proposed options for listing them as threatened. Our objectives were to evaluate differences in survival and mortality risk from natural (e.g., predation, disease, injuries, starvation) and human‐linked causes (e.g., rodenticide exposure [toxicants], vehicle strikes). We monitored survival of 232 radio‐collared fishers at a northern and southern study site in the Sierra National Forest, California. We retrieved fisher carcasses, and used necropsy examinations to determine causes of death. We estimated survival and mortality rates using the Kaplan–Meier estimator and nonparametric cumulative incidence functions, respectively, and integrated risk‐specific survival rates into a Leslie matrix to evaluate how population growth (λ) might improve if management can reduce mortality from any of the risks. We determined cause of death for 93 of 121 fishers, and annual survival was 0.69 for all fishers, and 0.72 for female fishers. Mortality rates were 19.5% for predation; 2.5% for disease, injury, and starvation; and 1.9% for toxicant exposure and vehicle strikes. Predation rates were similar between sexes, and relative risk was 51% lower in fall and winter compared to spring and summer. Combined mortality from disease, injuries, starvation, vehicle strikes, and toxicants was 4.4%, and 11 times higher for males than females. We estimated a base λ at the northern site of 0.96, which had the potential to increase to 1.03 or 1.11 if predation were reduced by 25% or 50%, respectively, whereas λ could increase to 0.97 in the absence of deaths from all other risks except predation. Predation was the dominant limiting factor in our study population, and was also the most common mortality risk for fishers in the 3 West Coast states (67%), followed by disease, injury, and starvation (12%) and vehicle strikes (8%). Direct mortality from toxicants appeared limited to fishers in California. Our results identified the need for continued and potential expanded restrictions on habitat disturbance in fisher denning habitats. Research is needed on contact rates between larger predators and fishers, including whether fishers are more vulnerable to attack in open forests or along roads used by coursing predators. © 2015 The Wildlife Society.

A tale of two populations: vital rates of fishers in British Columbia, Canada

“…This is at odds with findings from other fisher populations (Krohn et al 1994) and other populations of mustelids exposed to fur harvest (Krebs et al 2004) where males, particularly subadults, are generally more at risk to trapping mortalities (Koen et al 2007). By comparison, in populations that are not exposed to commercial fur harvest, predation is the primary source of mortality (e.g., California; Sweitzer et al 2016 a ), which was the second‐most common mortality source in the populations that we studied.…”

Establishing Bayesian priors for natural mortality rate in carnivore populations

“…Maximum age in this population was 9 years for both sexes ( n = 18 males, n = 32 females; M. Gabriel, Integral Ecology Research Center, personal communication; Gabriel et al ). The population was unexploited, with some anthropogenic mortality noted from road traffic collisions and toxicant poisoning (Sweitzer et al ).…”

“…Mean annual adult survival rate in the fisher population was estimated across sexes as 0.69, or for females as 0.72 (Sweitzer et al ). Sweitzer et al () estimated survival rate using the Kaplan–Meier estimator modified by Pollock et al (). The female‐only survival rate estimate enabled us to compare model predictions for a sexually dimorphic species using female‐specific mass and maximum age data.…”

Data from camera surveys identifying co-occurrence and occupancy linkages between fishers (Pekania pennanti), rodent prey, mesocarnivores, and larger predators in mixed-conifer forests

“…The study area was centered in the California Wildlife Habitat Relations (CWHR) Sierran mixed-conifer forest habitat type ( http://www.dfg.ca.gov/biogeodata/cwhr/wildlife_habitats.asp ). Additional details on the diversity of trees and shrubs, and historic and current land use within the study area were provided elsewhere [9] , [10] , [11] .…”