DIVING BEHAVIOUR OF MIGRATING SEA TURTLES
Diving behaviour during oceanic migrations of green turtles
The diving behaviour of green turtles nesting on Ascension Island was monitored during the inter-nesting period and post-nesting migration to Brazil by means of satellite-relayed data on mean dive duration and number of dives in a given time interval. Further, time/depth recorders (TDRs) were also used on turtles returning to Ascension after experimental displacement.
During both internesting and migration turtles were stayed submerged for the vast majority (>95%) of the time, suggesting that they neither basked at the surface nor drifted passively during migration to any great extent. During postnesting migration submergences were shorter in duration than during inter-nesting (ca. 5 vs. 20 min), although TDRs also recorded longer (>20 min) submergences, especially made at night. TDR further revealed that travelling turtles mostly dive shallow (> 2m), reaching depths around 10-20 m only during more prolonged dives.
Diving behaviour during long-distance movements of leatherback sea turtles
The diving behaviour of leatherback turtles is particularly interesting, as these turtles dive continuously in search for prey and are thought to reach depths up to 1000 m. Four leatherback turtles moving in the Southern Indian Ocean were monitored in their diving behaviour. Satellite-relayed dive data on mean dive duration, number of dives and depth utilization were obtained for three turtles for up to 8 months, while an additional turtle was equipped with special satellite transmitters (SRDLs, manufactured by the Sea Mammal Research Unit of the University of St. Andrews) which also provided detailed information on the depths and dive profiles of the turtles.
Turtles mainly dove to depths <200 m, with maximum dive duration generally under 40 min, but cases of very deep (>500 m) and prolonged (>70min) dives were also recorded. Turtles exhibited large variations in their diving activity, as they displayed a diel rhythm (dives being longer and deeper at night) and changed their diving behaviour in different periods and in different parts of the route. Most dives were V-shaped, and several dives below 700 m were recorded.