The behavioural choices of green turtles nesting at two environmentally different islands in Taiwan

Endangered marine animals that are capable of changing behaviour in response to different environmental conditions and still reproduce successfully may have better survival chances in the long term. Although the inter-nesting behaviour of sea turtles has been studied extensively since the development of animal borne telemetry devices, there is still much to learn about their behavioural strategies across a range of different environments. Here we compared the diving behaviour of green turtles, Chelonia mydas, nesting at two geographically distinct islands of Taiwan (Wan-an and Lanyu) using time–depth recorders. Data records for 9 out of 10 turtles (4 from Wan-an and 5 from Lanyu) revealed conformity in their “rest to save energy” behaviour by spending 55% of their time on the sea floor although suitable places at Lanyu are limited. One turtle, however, left the island's nearshore waters and made deep (≥5m down to 64m) pelagic dives during the night and shallow (<5m) diurnal dives with gliding phases during the ascent. The dive profiles resembled those recorded from green turtles during oceanic migration, and may be attributable to multiple activities including travelling, mid-water resting and, possibly, foraging at night on macroplanktonic organisms. Therefore, a heterogeneous inter-nesting habitat may provide enough resources to suit at least two different behavioural strategies that should maximise reproductive output — either “conserve energy through resting” or “gain energy through foraging”.