Thursday, 1 March 2012

Sea Turtles Ride the Waves

Talk about getting your kicks. Newly hatched leatherback sea turtles born on beaches in Costa Rica ride the ocean's Route 66, zipping away from shore—and away from predators—on fast and seasonal currents, a new study suggests. Insights into where these endangered animals swim during their first months of life may help conservationists keep them safe during this dangerous time.

Youth isn't kind to leatherback turtles (Dermochelys coriacea), which are facing extinction in the eastern Pacific Ocean as beach resorts and hotels spring up. "The most vulnerable part of a sea turtle's life is the first few years," says Frank Paladino, a marine biologist at Indiana-Purdue University in Fort Wayne. First, the bite-sized reptiles face the daunting crawl from their hatching sites to the surf; many are picked off by crabs and great blue herons along the way. Once in the water, things don't get easier. Predators still abound, and fishing nets snare an unknown but likely large number of juvenile turtles. In all, only about 1 in every 1000 young leatherbacks will return to the beach on which it hatched nearly a decade later to breed, he says.

Researchers still know very little about this important period in a turtle's life, not even where the animals go once they leave the beach. One reason is that hatchling leatherbacks, weighing only about 40 grams, are too small to tote satellite tracking devices, Paladino says. So, in the new study, George Shillinger, a marine biologist at Stanford University in Pacific Grove, California, and colleagues tried a different tack. They followed ocean currents, not young turtles. Because these reptiles are small and relatively weak, their earliest migrations could be decided largely by ocean patterns.

And for turtles born on Playa Grande in Costa Rica, one of the last leatherback strongholds, currents may be taking them far, the team found. Each year, winter winds whip up large eddies that last up to 6 months. To test how much these looping ocean highways might be buoying young turtles away from their hatching sites, the researchers first designed a computer simulation of ocean conditions near the turtle beach between 2000 and 2008. Within the program, they dropped virtual floating markers-similar to food coloring—into the water to stand in for the swimming turtles. Then the team watched where the simulated currents took those markers.

Starting in January—about when leatherbacks first poke out of their eggs—the markers quickly drifted to the deep sea. Based on the simulations, by early June many would have traveled nearly 1500 kilometers from shore, the group reports online today in the Proceedings of the Royal Society B. Faux turtles that launched from beaches farther to the north or south, however, largely sputtered, hugging the land. It's probably no accident that leatherbacks favor Playa Grande over these other beaches, Shillinger adds. Ocean highways likely provide a quick escape route from herons and other predators, meaning that turtles from Playa Grande have better odds of surviving their dicey youth.

Knowing a bit more about where turtles swim before they hit adulthood may be critical in protecting the species, Shillinger adds. If scientists map leatherback migration routes, for instance, they can work to keep fishing boats out of those channels.

"This is certainly a good step in understanding the dynamics of hatchlings," says Paladino, who was not involved in the study.

Kenneth Lohmann, a marine biologist at the University of North Carolina, Chapel Hill, agrees. But, he says, currents are only part of the story. Leatherbacks, after all, aren't rubber duckies. Despite their miniature fins, young sea turtles paddle more than many people think they do, he says, meaning these animals could head in directions not decided solely by ocean conditions. "They don't just drift passively," he says. "They take an active role in guiding themselves through the ocean."

Correction: This article has been amended to reflect the following: George Shillinger is affiliated with Stanford University in Pacific Grove, not Palo Alto, California. Additionally, markers in his computer simulation began drifting out to sea in January, not April. In the third and fourth paragraphs, the word "waves" has been replaced with "ocean patterns" and then "currents," respectively. Lastly, the second paragraph from the top and third paragraph from the bottom have been changed to clarify that fishing presents a bigger threat to juvenile turtles than to hatchling turtles.

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