The regularity of dive performance in sea turtles: a new perspective from precise activity data

Air-breathing divers are assumed to dive and surface in a way that maximizes their realized benefit. Dive duration in diving animals seems to depend on the purpose of the dive. Although the purpose of dives in most marine mammals and birds is undoubtedly foraging, this is not always true for so-called ‘surfacers’ that spend most of their time underwater for various purposes (e.g. foraging, resting, mating and migration). In this study, we investigated the relationships among dive duration, the amount of activity per unit time, the air volume in the lungs and postdive surface duration of four hawksbill turtles, Eretmochelys imbricata, which are surfacers, using multisensor data loggers. Our results demonstrated that hawksbills generally perform dives with consistent relationships between dive duration, the amount of activity per unit time and the air volume in the lungs, and that dives are followed by short periods at the surface, indicating that hawksbills rarely perform anaerobic dives. This longer dive duration compared to the length of surface resting indicates that the diving strategy of hawksbill turtles maximizes their time spent submerged. This strategy seems unique to sea turtles and surfacers in general.