Mesoscale eddies, surface circulation and the scale of habitat selection by immature loggerhead sea turtles

Here we tested whether the dispersal of loggerhead sea turtles in the western Mediterranean was affected by the pattern of surface currents and whether temporary mesoscale eddies were favoured habitats by comparing the distribution of ten satellite-tracked turtles (average straight carapace length = 46.3 cm; range: 37.1–61.1 cm) with satellite images indicative of the position of mesoscale eddies and the Algerian current. The tracking period averaged 120.9 ± 67.0 SD days. The comparison of turtle tracks with satellite images revealed that only 9.5% of the fixes were associated with mesoscale eddies or the Algerian current. However, 28.3% (range: 0–60.0%) of the maximum longitudinal displacement of each turtle occurred while swimming in the Algerian current. Furthermore, accelerated eastward dispersal with the Algerian current was the most likely reason why turtles did not penetrate deep into the Alboran Sea. Conversely, mesoscale eddies were not involved in any net latitudinal or longitudinal displacement of the turtles. Track analysis revealed that only a few turtles were involved in fine habitat search, as the speed of travel and the cosine of the turning angle seldom correlated once the drift associated with the Algerian current and with mesoscale eddies had been removed from the analysis; only the turtles approaching the limits of the Algerian basin modified their swimming behaviour, thus indicating habitat selection. These observations and the results of the simulation experiments revealed that turtle dispersal within the western Mediterranean was a combination of: (1) passive dispersal within a region limited by physical barriers (the Balearic Archipelago, Sardinia, Corsica, and Sicily) and influenced by a strong current (the Algerian current) and (2) fidelity to regions filled with recent Atlantic water. However, turtles fail to recognise and remain in small, discrete mesoscale eddies, thus suggesting that habitat selection in these turtles appears to occur at the scale of hundreds rather than tens of kilometres.