Riding on the fast lane: how sea turtles behave in post-nesting migration

Abstract. Sea turtles are known as powerful swimmers. How do they behave when riding in strong currents during their migrations? In this study, three, satellite-tagged, post-nesting green turtles travelled from Lanyu Island, east of Taiwan, partly within the Kuroshio to their foraging sites approximately 1000 km away in the Ryukyu Archipelago. Their swimming behaviors were analyzed by comparing their migration velocities estimated from Argos tag data with ocean currents derived from a data simulation model and from AVISO advection estimates. Results suggest that the turtles take advantage of Kuroshio during the initial portion of their migration routes. They must then make a great effort to swim eastward, at speeds over 1 m s⁻¹, toward their foraging sites to avoid being carried off course by the strong current. The cues that might cause the change in swimming direction were evaluated with a Principle Component Analysis. The factors considered are ambient current velocity, wind, eddy activity (vorticity), magnetic field (latitude) and water temperature. The analysis shows that the ambient current and water temperature are negatively correlated with the eastward swimming velocity. This suggests that the changes in ocean current and a drop of water temperature, likely due to eddies impinging on the Kuroshio, may trigger the eastward swimming. Despite the differences among migratory routes of three Argos-tagged turtles after leaving the Kuroshio, they all reached foraging sites in the same general area. That suggests there may be more complex cues that guide the turtles to their foraging sites during their post-nesting migrations.