Spatial variation in directional swimming enables juvenile sea turtles to reach and remain in productive waters

Abstract

Ocean currents play an important role in the movement and distribution of organisms and for small animals it is often assumed that their movements in the ocean are determined by passive drift. Here we challenge this assumption by conducting an experiment at the scale of an entire ocean basin to test whether small (35 cm) juvenile loggerhead sea turtles Caretta caretta move independently of ocean currents. By comparing the trajectories of 46 satellite tracked turtles (11 502 positions, 12 850 tracking days) with Lagrangian drifters (3 716 303 positions, 927 529 tracking days) and virtual particles tracked within the Hybrid Coordinate Ocean Model (HYCOM), we found that in certain areas turtles moved in a similar manner to ocean currents, but in other areas turtle movement was markedly different from ocean currents, with turtles moving to areas thousands of kilometres from where they would have drifted passively. We further found that turtles were distributed in more-productive areas than would be expected if their movement depended on passive transport only. These findings demonstrate that regional variation in directional swimming contributes to young sea turtles reaching more favourable developmental habitats and supports laboratory work suggesting that young turtles have a map sense to determine their location in a seemingly featureless ocean.