Fishers (Pekania pennanti) are medium sized mustelids endemic to North America. Two fisher populations persist in Oregon: an indigenous population in southwestern Oregon, and a reintroduced population in the southern Cascade Mountains. Despite candidacy for listing under the Endangered Species Act, current information on fisher populations in Oregon is scarce. We conducted surveys using motion-activated cameras and scent detecting dog teams to assess the distribution of fishers. We quantified the potential for both populations to expand through time using a spatially explicit reaction-diffusion equation under varied initial conditions and growth scenarios. We deployed 1,855 camera survey stations equating to 591 sample units collecting 4,779,178 photographs. Detection dog teams surveyed 196 sample units. We detected fishers at 96 unique sample units and at 148 individual camera stations and 39 detection dog units, confirming the presence of the indigenous and reintroduced populations. The southern Cascade Mountains reintroduced population appears to have shifted or possibly occupy 67% less area than previously believed and the population extent was less than expected, except under our lowest growth model. We confirmed a larger indigenous population, but with a spatial extent 26% less than previously believed potentially as a result of recent wildfires or time-limited expansion. The indigenous population extent matched expectations under several growth scenarios suggesting multiple pathways to reach the contemporary distribution. There was no evidence of either population expanding into historically occupied forests. Understanding which factors limit fisher population growth could provide guidance for additional population supplementation efforts, categorize suitable habitat, and identify significant species interactions.