Diving behaviour in two distinct populations of gravid Flatback turtles Natator depressus

This paper presents the first data on the diving behaviour of Flatback turtles, Natator depressus, between nesting events. Dive profiles were recorded in turtles from breeding populations at Curtis Island in Queensland and at Bare Sand Island in the Northern Territory, using Time-Depth Recorders (TDRs). Both populations displayed dive types typical of those described in other internesting sea turtles. Dives spent on the seabed were most prevalent, accounting for 57% of the time at depth. While on the sea bed, the turtles apparently remained inactive, cyclical changes in depth reflecting the tidal cycle. These inactive dives were long compared to those typical of other large sea turtles such as C. mydas and C. caretta, up to 98 min (mean 80±12 min), and with a mean and median of 50 and 52 min, respectively. In both populations, these dives occurred more commonly by day, by an average of 14%, and were most prevalent in the middle third of the internesting interval in most turtles, while the eggs were maturing. The turtles spent only 10% of their time at or near the surface and surface intervals rarely exceeded a few minutes, insufficient to be attributed to a need for either rest or thermoregulation. The Bare Sand Island population showed longer dive durations and proportionally more dives with a gradual ascent phase, a phase presumed to be a method of compensating for buoyancy loss and a way for turtles to conserve energy in mid-water while still moving around. Maximum dive depths of 29 m and 44 m were recorded at Curtis Island and Bare Sand Island respectively, probably reflecting differences in bathymetry.