The development of early diving behavior by juvenile flatback sea turtles (Natator depressus)

Abstract

The flatback turtle is the only species of marine turtle that lacks an oceanic phase of development in its early life history. Instead, the turtles grow to maturity in shallow turbid shelf waters of tropical to subtropical Australia. We studied the development of diving behavior in neonate flatbacks to determine whether diving under those ecological conditions resulted in differences from leatherbacks (Dermochelys coriacea) and green turtles (Chelonia mydas) at the same age when diving in clear, deep oceanic waters. Data were obtained from flatbacks that varied in both age (1–7 weeks) and mass (38–100 g). Each turtle towed a miniature time–depth tag during a single 30-minute trial in shallow (≤12 m) turbid shelf waters near Townsville, Queensland, Australia. In total, 192 dives were recorded from 22 turtles from 4 nests. Most dives were short (<100 seconds) and shallow (<4 m), but even young turtles could dive to the bottom. The most common flatback dives had V- or W-profiles, whereas, in leatherbacks, most dives were V-profiles, and, in green turtles, the dives were either V- or U-profiles. Routine flatback dives were accomplished by swimming slowly (like leatherbacks), but, when sufficiently motivated, flatbacks could swim faster (>1 m/s) than green turtles. They could also make repeated deep dives after surfacing only briefly to replenish their oxygen supply. Changes in performance (longer, shallower dives) were correlated with increases in mass but not age. We hypothesize that, as neonates, flatback dives enable the turtles to 1) search efficiently for prey throughout the water column under conditions of limited visibility, 2) minimize surface time so that even in murky water the turtles can return to previously attractive locations, and 3) swim rapidly to evade their predators.