During their oceanic movements, turtles cross widely distributed areas and certainly experience different environmental conditions. A number of physical and biological factors in the areas visited are likely to affect at-sea turtle behaviour, and may even determine the pattern of movement recorded. In particular, sea current circulation is likely to greatly affect the turtle spatial behaviour in the ocean, especially by influencing their actual movements and by determining the local availability of the patchily-distributed planktonic preys. In recent years, the development of remote sensing techniques through satellites allowed us to record several environmental parameters of the ocean, which, in turn, can provide reliable information about the main oceanographic features of a given area in a given time. For this, remotely-sensed data on sea surface temperature, sea height anomalies and clorophyll-a concentration, are particularly valuable. Trajectories of oceanographic surface drifters are also good indicators of sea current flow.
Matching satellite tracking data on turtle movements with remotely-sensed oceanographic and environmental data, could therefore be most useful to shed light on the factors influencing sea turtle behaviour in the ocean.
Oceanographic features and leatherback turtle routes
Leatherback turtles (Dermochelys coriacea) are particularly suited for this kind of analysis, as they are used to wander over immense oceanic areas while feeding on pelagic invertebrates. We have analysed the effects of oceanographic factors on the postnesting oceanic movements of three leatherback turtles satellite-tracked as they moved in the southwest Indian Ocean, an area dominated by the Agulhas Current, one of the most persistent and intense currents of the world.
By superimposing the turtle tracks on contemporaneous images of sea surface temperatures and sea height anomalies, we found that current-related features largely determined the shape of the reconstructed routes, with turtles moving along straight routes when in the core of the current (see figure left), and executing loops when within eddies (see figure right).
Large parts of the routes were strikingly similar to those of surface drifters tracked in the same region (see figure below).
These findings document that long-lasting oceanic movements of leatherback turtles are heavily influenced by the oceanic features of the areas crossed.