Comparison of beaver density estimates from aerial surveys of waterways versus transects

Historic beaver-sign (Castor canadensis) survey flights were often conducted over waterways to maximize beaver detections. However, densities determined from strip transect surveys are more useful to compare across and within study areas than waterway indices based on observations per distance flown because transects are more representative of the wider landscape. Yet, it is unknown if, and to what extent, aerial waterway surveys are reflective of transect densities. I conducted aerial surveys for active beaver sign each fall during 2015–2018 over two waterway routes and two corresponding strip-transect routes. The simple linear regression of transect densities on waterway densities (n=8) yielded a reasonable (R2=0.79) preliminary equation for converting historic waterway data to transect densities. Additionally, visual inspection indicated that converted waterway densities reasonably reflected the trend in transect densities in an area where the wider habitat was similar in terms of beaver harvest, land use, and proportion of water features. Although trend was well-reflected, individual waterway densities in this area were only 57-75% of transect densities. In other areas, where water features were limited, visual inspection suggested the trend of waterway densities was less reflective of transect density trend and individual waterway densities overestimated transect densities (up to 309%). Nevertheless, while transect densities are better for comparisons within and across study areas, waterway surveys are still important for timely and specific within-study area insights. This research provides useful benchmark examples of reliability regarding waterway observation indices converted to densities for conservation, research, and management of beaver and their ecosystems. Because these conclusions are based on a small sample, additional research is recommended to better define this relationship especially in areas with differing habitat, beaver harvest, land use patterns, etc.