DOI: 10.5061/dryad.c2fqz61c1
title: Data from: High dive efficiency in shallow water

description: 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
 &lt;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.
description: High-resolution time-depth recorders (TDRs) were deployed on
 hawksbill turtles foraging in Turtle Cove at the southern tip of the
 horse-shoe-shaped lagoon of Diego Garcia atoll in the Chagos Archipelago,
 Indian Ocean (7.42oS, 72.46oE), during October 2012 (n=5) and February to
 March 2021 (n=7). Turtles were captured at low tide in shallow water
 (&lt;1 m) by approaching slowly and carefully from behind before
 seizing the carapace, whereupon turtles were carried ashore. The curved
 carapace length (CCL) and curved carapace width (CCW) were measured using
 a flexible tape, and mass recorded with a suspended scale (Pesola Macro
 line 50 kg scale, Pesola AG, Switzerland). Maximum precision TDRs (G5 data
 loggers, 10 bar sensors with 0.03 m resolution, Cefas Technology Limited,
 Lowestoft, UK) were attached proximally to the trailing edge of flippers,
 using wire ties to fix them onto flipper tags placed at the thin junction
 between adjacent thick flipper scales (Supplementary Information Fig. S1).
 The TDR data sampling rate was every 1-5 seconds for pressure/depth and
 every 30-60 seconds for temperature. TDRs were recovered after various
 intervals when the turtles were next encountered in the cove (SI Table
 S1). The first four hours of each deployment were excluded in case of
 altered behaviour following capture and attachment (Thomson et al. 2012).
 Overall data analysis and visualisation were performed in R (R Core Team
 2021). Zero-offset correction was scripted directly in base R. Individual
 dive characteristics and post-dive surface intervals (PDSI) were measured
 using MultiTrace Dive (Jensen Software Systems, Hamburg, Germany) with a
 minimum duration of 5 min and a threshold depth of 0.2 m, as has been
 described for shallow divers by Hays et al. (2007).
description: Any software that can open .txt files, for example Excel, R,
 etc.