When foraging, sea turtles presumably give little thought to the physics of ocean vortices. But scientists who study the reptiles want to know how mesoscale eddies, vortices 50–100 km in radius that transfer heat in the oceans, influence the distribution of turtle populations.
Now Peter Gaube at the University of Washington and colleagues have found that loggerhead sea turtles (Caretta caretta) modify their behavior to position themselves inside eddies. Colleagues from Uruguay and Brazil fixed satellite transmitters to juvenile turtles, including the one pictured above, that journeyed to the confluence zone of the Brazil and Malvinas currents in the southwestern Atlantic Ocean. For five years the team tracked the turtles’ positions and those of untethered buoys known as drifters that are administered by the National Oceanic and Atmospheric Administration. The researchers used satellite observations of sea-surface height to identify and track eddies.
The data showed that turtles avoided the peripheries of anticyclonic eddies. They were 83% more likely to be found in the interior of the eddies than in the outermost regions, whereas the buoys were scattered about all regions. Once the turtles found the sweet spot of an eddy, they seemed to try to stay there: Relative to the local current, they swam more slowly in the interior (~17 cm/s) than they did when caught in the periphery (~29 cm/s). Gaube proposes that the turtles were seeking the gelatinous gastropods that favor the anomalously low chlorophyll concentrations and warm water in the interior. That hypothesis is bolstered by the fact that the turtles did not target the colder waters at the center of cyclonic eddies.
The study establishes a methodology for investigating how aquatic organisms make use of ocean currents, for identifying critical habitat in the open ocean, and for protecting endangered species. Such research is becoming increasingly important as ocean conditions change due to natural and anthropogenic factors. (P. Gaube et al., PLOS ONE 12, e0172839, 2017.)
