The First Estimates of Body Lengths in Free-Ranging Narrow-Ridged Finless Porpoises Using Unmanned Aerial Vehicle Footage

This study presents the first estimate of body length in free-ranging narrow-ridged finless porpoises (Neophocaena asiaeorientalis), based on footage obtained using an unmanned aerial vehicle (UAV). UAV-captured videos were used to identify porpoise surfacing events, with the moment of breathing selected as optimal for morphometric analysis. Still images were extracted from these events, and body length (BL) was measured. To improve measurement accuracy, a linear regression model was developed to convert pixel lengths into real-world dimensions as a function of flight altitude, calibrated using reference objects placed both on land and at sea. Across 67 measured events, the estimation errors ranged from 1.6 to 3.0%. Repeated measurements from the same individuals yielded a mean standard deviation of 0.9 cm, and a coefficient of variation (CV) of 0.7%, indicating high precision. Estimated body lengths ranged from 93.8 to 168.0 cm. To the best of our knowledge, this study presents the first length distribution data for free-ranging finless porpoises. Furthermore, this study proposes a standardized and replicable approach for selecting frames in UAV-based morphometric analysis in small cetaceans. These results offer a valuable baseline for future assessments of health and reproductive status and may contribute to conservation strategies for this endangered species.