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Thirteen loggerhead turtles Caretta caretta were released (10 from Naples, Italy, 2 from Monastir, Tunisia, 1 from Gallipoli, South Italy) with satellite relay data loggers (SRDL) to elucidate their overwintering behaviour. Nine turtles were successfully tracked throughout the winter, while 4 SRDLs failed to transmit after short deployment periods. Of these 9, 4 remained within 80 km of the release site, 3 travelled to a distant overwintering site, and 2 continued to move and did not remain within 80 km of a specific site. Apart from these differences, all turtles stayed near the coast and dedicated most of their time to dives lasting 3 h and longer. Maximum dive durations ranged from 270 to 480 min and were highly correlated with water temperatures, which fell below the supposed 15°C threshold for sea turtle hibernation in all overwintering sites. Median dive depths were between 4 and 24 m and were, thus, well within the mixed layer, as revealed by temperature profiles, which also were relayed by the SRDLs. No evidence was found that the turtles preferred warmer temperatures to overwinter in, because the range of temperature was very narrow on both the horizontal and the vertical scale of their movements. Despite the long resting phases and the low temperatures (minimum = 11.8°C) all turtles retained activity to some degree, at least to commute between the depth of resting and the surface to breathe. While the degree of winter dormancy is certainly affected by temperature, turtles were by no means obligatory hibernators, and their ability to move and even forage during the winter may be important for their growth and maturation rates, as well as their reproductive output.

The first published record, from the early 1970s, of hibernation in sea turtles is based on the reports of the indigenous Indians and fishermen from Mexico, who hunted dormant green turtles (Chelonia mydas) in the Gulf of California. However, there were no successful attempts to investigate the biology of this particular behaviour further. Hence, data such as the exact duration and energetic requirements of dormant winter submergences are lacking. We used new satellite relay data loggers to obtain the first records of up to 7 h long dives of a loggerhead turtle (Caretta caretta) overwintering in Greek waters. These represent the longest dives ever reported for a diving marine vertebrate. There is strong evidence that the dives were aerobic, because the turtle surfaced only for short intervals and before the calculated oxygen stores were depleted. This evidence suggests that the common belief that sea turtles hibernate underwater, as some freshwater turtles do, is incorrect.

No abstract

SUMMARYMarine turtles spend more than 90% of their life underwater and have been termed surfacers as opposed to divers. Nonetheless turtles have been reported occasionally to float motionless at the surface but the reasons for this behaviour are not clear. We investigated the location, timing and duration of extended surface times (ESTs) in 10 free-ranging loggerhead turtles (Caretta caretta) and the possible relationship to water temperature and diving activity recorded via satellite relay data loggers for 101-450 days. For one turtle that dived only in offshore areas, ESTs contributed 12% of the time whereas for the other turtles ESTs contributed 0.4-1.8% of the time. ESTs lasted on average 90 min but were mostly infrequent and irregular, excluding the involvement of a fundamental regulatory function. However, 82% of the ESTs occurred during daylight, mostly around noon, suggesting a dependence on solar radiation. For three turtles, there was an appreciable (7°C to 10.5°C) temperature decrease with depth for dives during periods when ESTs occurred frequently, suggesting a re-warming function of EST to compensate for decreased body temperatures, possibly to enhance digestive efficiency. A positive correlation between body mass and EST duration supported this explanation. By contrast, night-active turtles that exceeded their calculated aerobic dive limits in 7.6-16% of the dives engaged in nocturnal ESTs, probably for lactate clearance. This is the first evidence that loggerhead turtles may refrain from diving for at least two reasons, either to absorb solar radiation or to recover from anaerobic activity.