Western Mediterranean immature loggerhead turtles:: habitat use in spring and summer assessed through satellite tracking and aerial surveys

An aerial survey was conducted in early spring 2002 over the continental shelf of the Balearic Archipelago to study the distribution of neritic loggerhead turtles. Furthermore, five juvenile loggerhead turtles [straight carapace length (SCL) range 37.1-48.7 cm], were instrumented with transmitters and monitored during 2003 by satellite tracking to study habitat use over a broader geographical range. The distribution of the turtles over the continental shelf matched habitat availability, as defined by depth. However, those tracked by satellite spent most of the time in the oceanic waters of the Algerian basin and generally avoided continental shelf areas. In these turtles, average speed of travel and mean cosine of turning angle did not significantly differ between habitats, indicating that avoidance of shelf areas is not due to active habitat selection. On average, tracked turtles spent 35.1 +/- 19.7% of the time at the surface, although surface time was much greater in the turtle with the shortest carapace length, suggesting that this individual had limited swimming capacity. We conclude that the transition between passive drifting and active habitat selection occurs at an SCL of about 40 cm. The turtles followed tracks that matched prevailing currents, but on some occasions they also swam upstream. Hence, the distribution of late juvenile loggerhead turtles in the southern and central western Mediterranean may reflect a combination of passive drifting and active habitat selection.