Feasibility of Using Small UAVs to Derive Morphometric Measurements of Australian Snubfin (Orcaella heinsohni) and Humpback (Sousa sahulensis) Dolphins

Analysis of animal morphometrics can provide vital information regarding population dynamics, structure, and body condition of cetaceans. Unmanned aerial vehicles (UAVs) have become the primary tool to collect morphometric measurements on whales, whereas on free ranging small dolphins, have not yet been applied. This study assesses the feasibility of obtaining reliable body morphometrics from Australian snubfin (Orcaella heinsohni) and humpback dolphins (Sousa sahulensis) using images collected from UAVs. Specifically, using a dolphin replica of known size, we tested the effect of the altitude of the UAV and the position of the animal within the image frame on the accuracy of length estimates. Using linear mixed models, we further assessed the precision of the total length estimates of humpback and snubfin dolphins. The precision of length estimates on the replica increased by ~2% when images were sampled at 45-60 m compared with 15-30 m. However, the precision of total length estimates on dolphins was significantly influenced only by the degree of arch and edge certainty. Overall, we obtained total length estimates with a precision of ~3% and consistent with published data. This study demonstrates the reliability of using UAV based images to obtain morphometrics of small dolphin species, such as snubfin and humpback dolphins.