Aerial photogrammetry techniques using drones to estimate morphometric measurements and body condition in South American small cetaceans

Aerial photogrammetry protocols using drones have proved efficient in monitoring free-living cetaceans. In this work an aerial photogrammetry protocol using a drone was applied to estimate morphometric measurements and body condition variation in two endangered cetacean species, franciscanas (Pontoporia blainvillei, n = 122) and Guiana dolphin (Sotalia guianensis, n = 41). Sampling occurred in winter and summer in Babitonga Bay, southern Brazil. A 115 cm length object was used to calibrate the scale of measurements at different altitudes. A scale calibration model based on eGSD regression by flight altitude was developed (RMSE = 0.039, r(2) = 0.9902, Delta AICc = 0.00) and used to convert measurements from pixels to centimeters. Five morphometric measurements (body length - BL, widths at anterior pectoral fin - AP, axilla - AX, anterior dorsal fin - AD, and posterior dorsal fin - PD) were taken from each animal to infer body condition, in which, the widths in the anterior portions of the body as % BL, can be an auxiliary tool in the evaluation of variation in the body condition of the small cetaceans studied. The aerial photogrammetry protocol allowed for estimating morphometric measurements and monitoring seasonal changes in the body condition of the small cetaceans.