Status of Fishers in Glacier National Park, Montana

Abstract

Fishers (Pekania pennanti) are a mid-sized forest-dwelling carnivore in the family Mustelidae, once widely distributed across the northern US and Canada. Between 1800 and 1940, they were extirpated across much of their former range through commercial fur harvesting and habitat modification. Glacier National Park (GNP), which maintains a wildlife sighting database containing public and agency sighting and track records of Fishers from 1947 through the present, presumed Fishers extant in the park. Over 2007–2012, GNP made opportunistic efforts to collect Fisher DNA in hair traps but were unsuccessful. In 2013 and 2015, GNP conducted a more intensive effort to document the occurrence of Fishers by systematically placing baited hair traps within putative Fisher habitat across the park. These efforts were also unsuccessful and I show through application of Bayes' Theorem that a resident population of Fishers in GNP is highly improbable. Past credible sightings of Fishers in GNP were probably animals dispersing from occupied habitats to the west of GNP, and I suggest that regular occurrence of deep, unconsolidated winter snow may have prevented Fishers from occupying GNP in the past.