Data from: 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%). 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 emphasizes the likely importance of predation risk in
shaping patterns of habitat utilisation.