Survival of reintroduced fishers among differing sympatric predator and prey assemblages

For many wildlife species, reintroduction is necessary to re-establish populations in areas of their historical range where they have been extirpated, but reintroduction efforts are often expensive, time-consuming, and unsuccessful. A more complete understanding of the factors affecting restoration success is important for responsible stewardship and optimizing outcomes. Fishers (Pekania pennanti) are a commonly reintroduced carnivore in North America, but differences in predator and prey assemblages among release sites may contribute to variation in the success rates of such efforts. We examined how predator and prey occurrence and relative abundance influenced survival rates in reintroduced fisher populations in the southern and northern Cascade Mountains, Washington, USA. We compared survival rates from telemetry data between the 2 areas and used independent detections of prey and predators at 190 remote camera stations to assess how sympatric species related to near-term (1–2 years post-release) fisher survival. We released 81 fishers, of mixed age and sex (majority ≤2 years old), into the South Cascades between December 2015 and January 2020 and released 89 fishers into the North Cascades between December 2018 and February 2020. Using radio-telemetry data, we estimated 365-day post-release survival as 0.65 (95% CI = 0.54–0.79) in the South Cascades and 0.31 (0.21–0.48) in the North Cascades. The relative abundance of important fisher prey species was significantly higher in the South than in the North; notably, snowshoe hares (Lepus americanus) were detected at a rate of 5.11 (±0.86 SE)/100 trap nights in the South versus 1.13 (±0.25)/100 trap nights in the North. Relative abundance of potential fisher predators did not differ significantly between study areas. Our findings are consistent with the survival of reintroduced fishers being affected by differences in prey assemblages across release sites, though other differences between the sites may also play a role in fisher survival. Future reintroduction efforts may benefit from preliminary assessment of prey abundance prior to release site selection.