High dive efficiency in shallow water

Dive studies across mammals, birds, reptiles, and fish often focus on deep dives, and shallow-water diving has tended to be overlooked. For air-breathers, foraging in shallow water poses challenges since the lungs generate buoyancy, and shallow divers must trade off the extent of inhalation against the negative buoyancy needed to avoid floating to the surface. Using high-resolution depth loggers, we addressed this knowledge gap around the ecology of shallow water diving at a foraging site for hawksbill turtles (Eretmochelys imbricata) where depth was typically <3m. Contrary to predictions, dive durations were long, particularly at night (mean dive duration per turtle: 17-61 min, n=12 turtles, n=2576 nocturnal dives), despite warm water temperatures (24-37oC). Dive efficiency (% time submerged) for hawksbills was 98%, the highest recorded for any air-breathing marine vertebrate including penguins (60-78%), seals (51-91%), cetaceans (68-87%), and other sea turtle species (68-95%). Hawk...