Diving efficiently contributes to the overall fitness of marine animals. Consequently, marine mammals maximise foraging time to increase the energetic gain from each dive. The distribution and density of food are key drivers of depth utilisation. For dugongs (Dugong dugon), depth utilisation during foraging is largely determined by the depth of seagrass communities, their main food source. Dugongs must therefore balance the costs of reaching a feeding patch with the energetic gains obtained, by increasing their diving efficiency.
We compared dive efficiency per dive function and investigated how efficiency changes with depth to understand factors that may influence dugong foraging time. We assessed dive efficiency using a time-based efficiency: Dive efficiency= (bottom phase duration/dive duration + post-dive duration). Mean efficiency during foraging dives (73.1% ± 13.8% (S.D.)) was higher than during resting (59.1% ± 21.3% (S.D.)) and swimming dives (53.3% ± 19.5% (S.D.)). The efficiency of shallow foraging dives (<6 m) decreased with increasing maximum dive depth: at the 0.95 quantile, diving efficiency decreased by 1.1% with every 1 m increase in depth. These results suggest that declining shallow seagrass beds are likely to reduce diving efficiency, highlighting that dive efficiency is an important factor influencing dugong depth utilisation.