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Abstract
In past years, satellite telemetry had a key role in the study of marine vertebrates and it is particularly suitable for sea turtles, who emerge from the water to breathe. The combination of satellite telemetry and oceanographic data enables us to improve our knowledge about sea turtles and to quantify the influence of oceanic currents on sea turtles tracks in order to understand if they adapt their movements with respect to currents.
This technique, previously performed on other sea turtles species, is particularly interesting if applied to leatherback sea turtles (Dermochelys coriacea) who have strictly pelagic habits and use to spend most of their life in the open ocean, travelling for hundreds of kilometers looking for their pelagic preys.
In the present work we analyze the routes of two groups of leatherback turtles.
To the first group belong 4 individuals of post-nesting turtles that occupy the south- western part of Indian ocean; to the second one belong 9 turtles, including 8 adult females and a male, that inhabit the Northern Atlantic.
The reconstructed turtles’ routes were analyzed in relation to oceanographic data which allowed us to estimate the speed and the direction of the current.
Since the trajectory of an animal, travelling into a moving medium, can be affected by the movement of the medium itself, the routes are
4 the result of two components: the first one is turtle’s active swimming and the second one is related to currents drift.
In order to discriminate the influence of oceanic currents from the movement component derived by turtle active movement a vectorial analysis was applied to the routes filtered locations. The current speed was detected through oceanographic data and the track speed was estimated by computing the distance between two consecutive locations and then dividing these distances by the time elapsed. Finally turtle active movement was calculated as the vectorial difference between the track speed and the current speed. Furthermore, the routes were divided into various legs, according to oceanographic data, in order to investigate leatherback turtles behavior depending on different situation.
We divided the routes into linear legs where we expected a travelling pattern in search of zooplankton, and convoluted legs, where we supposed to find an intensive foraging activity. In conclusion, the present analysis has shown that leatherback turtles move actively during their long movements which are substantially influenced by surface currents. These results showed that leatherback turtles are able to perform movements in accordance with current flow either when turtles were subjected to current which changes in intensity and direction.
