Too many to count: Using orthophotography to census an unharvested beaver (Castor canadensis) population in Ontario

Various field methods exist to monitor wildlife populations and estimate trends in their distribution and abundance. For American beaver (Castor canadensis), aerial surveys provide a means to obtain abundance data over large areas and typically involve observers searching watercourses and shorelines for active beaver presence. Here, we describe a systematic aerial photo census we designed and executed in autumn to quantify annual beaver abundance on the 184 km2 Michipicoten Island, Ontario from 2015–2019. Aerial photos were stitched together into orthophoto mosaics after each census and visually searched for beaver food caches, with each food cache representing an independent beaver colony. Our methods revealed that beaver colony abundance declined substantially from a peak of 6.1 colonies/km2 in 2015, the highest reported across North America, to 0.4 colonies/km2 in 2018. Beaver abundance remained low through 2019. Though photographing the entire isolated study area required relatively...