The use of orthoimagery and stereoscopic aerial imagery to identify muskrat (Ondatra zibethicus) houses

The muskrat (Ondatra zibethicus) is considered a ubiquitous inhabitant of wetlands across Canada and the United States, but recent studies indicate that muskrat populations in many parts of North America have experienced substantial declines over the last 40–60 years. Monitoring of muskrat abundance is therefore an important task for wildlife managers, but traditional methods such as house counts conducted during ground-based surveys can be labor-intensive and time-consuming. Poor conditions or a lack of access may limit how much of a wetland can be surveyed. Aerial imagery has previously been used to census a diverse array of wildlife populations but is not yet a common tool for muskrat surveys. To investigate the accuracy of this alternative survey method, we collected aerial imagery from coastal wetlands along the north shore of Lake Ontario during the winter of 2014 for examination in both 2D orthorectified and 3D stereoscopic formats. We compared muskrat house counts obtained from imagery to counts recorded by ground survey crews in the same wetlands during the same winter. We found no significant difference between mean muskrat house counts obtained by ground survey crews and orthoimagery observers. In contrast, stereoscopic imagery observers overestimated mean house counts compared to ground survey crews, which we interpret was due to an increase in false positives. Our results indicate that orthoimagery is a promising tool for assessing muskrat occupancy, provides comparable broad-scale results to traditional ground survey methods, and may be preferable to wildlife managers for a variety of reasons.