Orientation of migrating leatherback turtles in relation to ocean currents

During their offshore movements, leatherback turtles, Dermochelys coriacea , associate frequently with ocean currents and mesoscale oceanographic features such as eddies, and their movements are often in accordance with the current flow. To investigate how individual turtles oriented their ground- and water-related movements in relation to the currents encountered on their journeys, we used oceanographic techniques to estimate the direction and intensity of ocean currents along the course of 15 leatherbacks tracked by satellite during their long-distance movements in the Indian and Atlantic Oceans. For all individuals a non-negligible component of active swimming was evident throughout the journeys, even when their routes closely followed the currents, but overall the turtle water-related orientation was random with respect to current directions. For turtles in the North Atlantic, the ground-related movements largely derived from the turtles' active swimming, while in the Indian Ocean currents contributed substantially to the observed movements. The same pattern was shown when distinct parts of the routes corresponding to foraging bouts and travelling segments were considered separately. These findings substantiate previous qualitative observations of leatherback movements, by revealing that turtles were not simply drifting passively but rather swam actively during most of their journeys, although with a random orientation with respect to currents. Our analysis did not provide any indication that leatherbacks were able to detect the current drift they were exposed to, further highlighting the navigational challenges they face in their oceanic wanderings.