Development and Validation of Girth‐, Volume‐, and Mass‐Based Body Condition Indices for Free‐Swimming Dugongs, via Drone Photogrammetry

Drone photogrammetry provides a minimally invasive morphometric tool to assess growth and energy reserve in marine mammals. Converting drone‐derived measurements (e.g., body width) into body condition indices (BCIs, residuals from expected body girth, volume, or mass) requires prior knowledge of normative body shape. We developed and validated drone‐based BCIs for dugongs in Moreton Bay, Australia. Measurement errors in body girths and body mass were quantified using 25 dugongs with direct measurements. BCIs were determined for 513 dugongs using drone‐derived measurements and tested for their ability to detect a known temporal trend (decline in mature male BCI during spring). Maximum girth was estimated with a mean absolute error of 8 ± 3 cm (1.26% ± 3.64%) and body mass with 17.3 ± 11.9 kg (4.69% ± 3.0%). Maximum girth at length increased relatively rapidly until 1.75 m of body length and then more gradually, indicating that dugongs became more rotund early in life and more streamlined thereafter. Adult BCIs were significantly lower in late spring than in late summer and late autumn. Drone‐based photogrammetry accurately estimated girth and body mass and detected temporal variation in BCIs. When using a girth‐based BCI for dugongs, separate analyses for early growth stage are recommended.