Tiny sea turtles are swimmers, not drifters

It has long been thought that after sea turtles hatch and make their run for the sea, the tiny reptiles passively drift through the oceans, their destination driven by the currents, not the turtles themselves. No one really knew where the turtles went, and their early lives were nicknamed the “lost years” in recognition of human ignorance.

Then researchers attached itty-bitty satellite trackers to young loggerhead sea turtles, and last year those scientists reported that the animals sometimes swim. Loggerheads aren’t the only species doing that; young green and Kemp’s ridley sea turtles also swim, Nathan Putman and Katherine Mansfield of the National Marine Fisheries Service in Miami report in the May 4 Current Biology.

Putman and Mansfield collected 24 green and 20 Kemp’s ridley turtles that were only a year or two old from the Gulf of Mexico. The animals were still small, with carapaces measuring only 14 to 30 centimeters. (The carapace of a green sea turtle can reach 1.5 meters when full grown, and a Kemp’s ridley turtle can grow to more than a meter.) The researchers attached satellite trackers to the turtles and released groups of them back into the gulf. With each release, the scientists also set loose a pair of drifters — objects such as five-gallon buckets adorned with satellite trackers — so they could compare the turtles’ movements with something that can’t swim.

“If the movement of turtles were primarily the result of ocean circulation processes, we would expect swimming speeds to be similar for drifters and turtles,” the researchers note. But that’s not what they saw. When they plotted out the paths of the turtles and the drifters, there were distinct differences — the turtles moved faster than the drifters. They also moved in slightly different directions, with the greens going eastward and the Kemp’s ridleys heading north.

The green sea turtles were faster than the Kemp’s ridleys, the satellite tracking revealed. That speed may help the green sea turtles to disperse farther, the scientists say. This species — the largest of all the sea turtles — migrates long distances. Kemp’s ridley sea turtles, in contrast, mostly stick to the Gulf of Mexico. They can mosey along a bit more slowly.

The turtle speeds were slow (green sea turtles moved at a median velocity of 0.168 meters per second, and Kemp’s ridleys at 0.155 meters per second), and they were at the lower range reported for similar size loggerhead turtles. With such similarities between the three species, other young sea turtles of other species may also be able to control their movements through the oceans. And perhaps none of them are the passive drifters they were so long assumed to be.