Climate change on sea currents is not expected to alter contemporary migration routes of loggerhead sea turtles

Abstract

For marine species, traveling with the current potentially reduces energetic costs. Still, the extent to which organisms adjust routes to follow current flow remains an open question. Moreover, the extent to which climate change is altering sea currents, and in turn species migration routes, remains unknown, representing a major challenge to spatial ecology and conservation efforts.We developed an approach to assess the extent to which projected optimal paths and corridors overlap with the observed migration routes of loggerhead sea turtles (Caretta caretta), minimizing exposure to opposing sea currents. To illustrate this approach, we used migratory tracks of the species traveling between breeding and foraging areas in the Mediterranean Sea. We calculated the energetic costs to sea turtles based on actual tracks and corresponding optimal paths. We also explored whether projected changes in ocean currents, driven by climate change, would alter the spatial patterns of optimal routes.The energetic cost of observed tracks was, on average, 1.25 times higher than that of corresponding optimal paths. While optimal corridors differed spatially to observed corridors used by loggerheads, some positive correlations still existed for three cases (rs= 0.43, 0.42, 0.30). Climate change projections showed no significant change to the migratory movement of sea turtles, as corridors for different climatic conditions overlapped by at least 70%.Our results show that loggerheads do not explicitly take advantage of ocean currents to facilitate long distance migrations and reduce energetic demands. The contemporary and future migration routes are characterized by similar energetic demands and together with their strong spatial overlap suggest that climate change is expected to minimally alter the species migration routes in the future. The approach presented here could be applied to different spatial scales and marine taxa, allowing possible mechanisms between sea currents (or other environmental characteristics) and species movements to be elucidated.