Northern elephant seal UAS mass estimates
Beltran, Roxanne (2020), Northern elephant seal UAS mass estimates, Dryad, Dataset, https://doi.org/10.7291/D15D56
Unmanned aerial system (UAS) photogrammetry offers a method that is safer for both animals and researchers and is logistically simpler than traditional weighing methods (Fiori et al. 2017). Additionally, UAS photogrammetry facilitates larger sample sizes because it allows measurement at larger spatial scales, thereby increasing statistical power (Sweeney et al. 2015). However, UAS photogrammetry requires calibration and validation prior to use in order to assess the error relative to known mass measurements. Species-specific calibration of appropriate metrics (e.g., footprint area (Christiansen et al. 2016)) is necessary to account for body shape differences (e.g., “peanut head” syndrome) (Miller et al. 2012, Joblon et al. 2014). Further, the accuracies of photographs taken in overhead and oblique positions are rarely compared due to the challenge of weighing and repeatedly photographing uniquely identified individuals (Fearnbach et al. 2018). Northern elephant seals (Mirounga angustirostris) offer a unique opportunity to calibrate UAS photogrammetry to measure body mass. Elephant seals undergo dramatic changes in mass throughout the year and consistently haul-out to breed and molt at Año Nuevo Natural Reserve, California, USA (37.11° N, 122.34° W) (Le Boeuf and Laws 1994) (Figure 1). Our objectives were: 1) to evaluate the accuracy of UAS photogrammetry for estimating mass in adult female northern elephant seals and 2) to examine the effect of body position on UAS mass estimates. Our error estimates were comparable to other ground-based studies that used multiple photogrammetric measurements to estimate mass (Table 1) and slightly higher than the only other overhead drone study (Krause et al. 2017), possibly due to lower camera resolution. This error is also slightly higher than previous mass estimates derived from morphometric measurements obtained during manual procedures (2.8% for Weddell seals Leptonychotes weddellii (Shero et al. 2014) and 4% for elephant seals (Crocker pers. comm)) but results in significantly less disturbance. Errors in estimated mass were marginally better when we used dorsal footprint area (R2= 0.895) than lateral footprint area (R2= 0.822), suggesting that our UAS mass estimates are robust to changes in body position. In summary, photogrammetric measurements from a single, vertical image obtained using UAS provide a promising approach for estimating the body mass of pinnipeds and similar approaches can be used to find species-specific calibration equations. Mass measurements can inform ecosystem-based resource management (Boyd et al. 2006) by providing information about the inter-annual productivity of the ocean environment and in turn individual, population, and ecosystem-level health in marine mammals (Krause et al. 2017).