InXuence of oceanic factors on long-distance movements of loggerhead sea turtles displaced in the southwest Indian Ocean

The routes of Wve satellite-tracked loggerhead turtles (Caretta caretta), subjected to an experimental translocation away from their usual migratory routes, have been analysed in relation to the concurrent oceanographic conditions. Remote sensing data on sea surface temperature and height anomalies, as well as trajectories of surface drifters were used, to get simultaneous information on the currents encountered by the turtles during their long-range oceanic movements. Turtles mostly turned out to move in the same direction as the main currents, and their routes were often inXuenced by circulation features they encountered. A comparison between turtle ground speeds with that of drifters shows that in several instances, the turtles did not drift passively with the currents but contributed actively to the overall movement. Two turtles embarked on an oceanic crossing, probably induced by seasonal changes in surface temperatures, a crossing that was largely determined by the main currents existing in the area. Introduction Over the last two decades, the study of marine vertebrate movements has greatly beneWted from the remarkable development of several tracking techniques allowing reconstruction of animal routes even in remote regions otherwise not readily accessible to humans (e.g. Andrews et al. 2008; Bestley et al. 2008; Shillinger et al. 2008). For air-breathing animals such as sea turtles, satellite-based radio telemetry has become the state-of-the-art technique, with a wealth of data now available on the routes followed by the various species in various geographical areas, in diVerent stages of their life cycle and under both natural and experimental conditions (Godley et al. 2008; Luschi et al. 2003a). Since movements in the ocean never occur in a stationary medium, the reconstructed, ground-related trajectories of turtles derive from two diVerent components: that produced by the turtles’ active swimming and that induced by the water movement itself, i.e. the current drift (for review see Luschi et al. 2003a). The relative importance of the two components varies depending on a number of factors such as the turtle feeding activity and motivations, the strength of oceanic currents or even the ontogenetic stage of the turtles, with the youngest stages being more strongly inXuenced by currents (e.g. Hays and Marsh 1997). For ocean-dwelling species such as the leatherback turtle (Dermochelys coriacea), a fundamental role for marine currents in aVecting their movements has recently been documented, thanks to the fruitful integration of the satellite tracking of animal movements and the remote sensing of ocean current features. This powerful combination has allowed researchers to document that leatherback movements in the Indian and Atlantic Oceans are actually strongly dependent on the prevailing marine currents, up to the point that turtles often appear to be passively Communicated by R. Lewison. Electronic supplementary material The online version of this article (doi:10.1007/s00227-009-1321-z) contains supplementary material, which is available to authorized users. R. Mencacci (&) · E. De Bernardi · P. Luschi Department of Biology, University of Pisa, Via A. Volta 6, 56126 Pisa, Italy e-mail: rmencacci@biologia.unipi. it A. Sale Institute of Neuroscience, National Research Council, Via Moruzzi 1, 56100 Pisa, Italy J. R. E. Lutjeharms Department of Oceanography, University of Cape Town, Rondebosch 7700, South Africa