Anticoagulant rodenticides may affect fisher population trends in the northeastern United States

Regrowth of forests across the northeastern United States in recent decades has allowed for range expansion of many forest‐dependent mammals in the region. However, these contemporary forests have smaller patch sizes, putting humans in closer proximity to previously remote forested areas, and different species compositions and structure compared to historical forests. These conditions pose an increased risk of exposure to anthropogenic disturbances and contaminants for forest‐dwelling wildlife. We evaluated the relationship between contemporary forest conditions and anticoagulant rodenticides (ARs) on recent population trends for fishers (Pekania pennanti). We acquired a snapshot of recent population trends for fishers across New York, Vermont, and New Hampshire, USA, using standardized catch‐per‐unit‐effort (CPUE) data from the regulated fall harvest spanning 2016‐2020, when harvest regulations remained consistent across the states. We used regression models to relate the probability of CPUE increase to measures of forest habitat (age, structure, productivity), length of winter snow cover, and human disturbance (land use patterns and AR detection). The top model indicated that the probability of CPUE increase declined 43% for every 10% increase in the percent of the population exposed to at least 1 AR and by 24% for every 1‐day increase in the length of winter snow cover, while measures of forest conditions had no significant relationship. The relationship between AR detection and suppressed CPUE increase is compelling, as few mammalian studies effectively link the detection of AR residues in animal tissues to population‐level performance in the wild. Nevertheless, longer time frames are required to fully understand population trends and their drivers across heterogeneous landscapes. We recommend the fisher as a model species for longer‐term monitoring of AR detections and for comparing outcomes of measures potentially designed to reduce AR impacts on wildlife in this region.