High dive efficiency in shallow water

Abstract

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 < 3 m. 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 (24–37 °C). 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%). Hawksbills usually dive for much longer (42–286% increase) than green and loggerhead turtles when depth and temperature are accounted for. Hawksbill turtles likely forage in very shallow water to reduce predation risk from sharks: of 423 hawksbills captured by hand, none had any evidence of shark attack, although large sharks were present in nearby deeper water. Our results challenge the prediction that shallow water dives by air-breathers will usually be short and open the way for comparative studies of the ecology of shallow water diving in a range of other taxa. Our work emphasises the likely importance of predation risk in shaping patterns of habitat utilisation.