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 little
time and effort, even when beaver density was very high, post-census
processing work was time-consuming. Lessons learned will improve the
efficiencies of our future censuses and aid other researchers. Our method
is advantageous over traditional aerial wildlife surveys in that it
provides a digital and visual record that can be used for additional
analyses.