Rapid condition monitoring of an endangered marine vertebrate using precise, non-invasive morphometrics

Understanding causes of population change is critical for conservation. Quantifying these causes can be difficult, especially for hard to sample animals like marine vertebrates (e.g. pinnipeds). One solution is to investigate spatiotemporal differences in a species' body condition by measuring body size and mass. Collecting traditional morphological measurements is risky and labour intensive, making less invasive and more efficient techniques desirable. Using Australian sea lions (Neophoca cinerea) of known size and mass as a case study, we tested the suitability of using drone-derived photogrammetry to estimate morphological measurements and assess body condition. Drone-derived measurements were precise and without bias. Animal mass was highly correlated with the 2-dimensional and 3-dimensional measurements of simplified area and volume, explaining > 77% and > 84% (all P < .01) of the variation in mass, depending on the age-sex class. The juvenile class exhibited the strongest associations (both 2D and 3D R-2 > 0.99). Using each measurement as a proxy for mass, we calculated body condition indices for each class by standardising the variables by animal length. Photogrammetric indices ranked individuals comparably to those generated from ground-collected data (r(s) = 0.77-1, depending on age-sex class). Our technique provides a workflow for the non-invasive collection of morphometric data to quantify animal condition, which is transferrable to other pinniped species with species-specific calibration. It will also facilitate the efficient collection of morphometric data of vertebrates from remotely sensed imagery.