Thursday, April 16, 2015

Baby sea turtle mystery partly solved: they're independent swimmers not drifters

A great mystery of the marine world—what green sea turtles do as keiki—has partly been solved.

Researchers caught yearling turtles and attached tags that would slough off after a couple of months. 

They found the young turtles aren’t just drifting around—they’re actively swimming. Which is interesting, because nobody's been able to prove this before, and it contradicts conventional wisdom.

(Image: When buoys and tagged turtles were placed in the water together, they went different ways. Here, two blue lines track the buoys, and the green is the track of one of the turtles followed in this study. Credit: NOAA Fisheries.)

Nathan Putman and Katherine Mansfield published their results in the journal Current Biology. Their tagging included the green sea turtle, Chelonia mydas, which is the dominant nearshore turtle found in the Hawaiian Islands, as well as Kemp's ridley turtles.

The first couple of years of most turtle species' lives are sometimes called the “lost years.” The keiki hatch on beaches, scramble into the sea, and then disappear until they come back to they show up again as much larger animals.

“It has been widely assumed that turtles simply drift with ocean current,” says a NOAA press release on the study.

Putman, a sea turtle biologist with NOAA's Southeast Fisheries Science Center, and Mansfield, director of the University of Central Florida's Marine Turtle Research Group, set out to find out for sure.

Mansfield put solar-powered satellite tags on 44 wild-caught young turtles, and at the same time deployed drifting satellite buoys that would follow the currents. If the turtles and the buoys went the same places, then it would suggest the turtles simply followed the currents.

But they didn’t.

Within the first few days, some of the turtles were already 125 miles from the buoys. The young turtles were actively swimming and navigating independent of current flow.

Anyone who has seen freshly hatched green sea turtles flap and clamber up out of the sand and then down the beach can appreciate how animated they are. Their single-mindedness and independence seems to continue in the ocean.

“The results of our study have huge implications for better understanding early sea turtle survival and behavior, which may ultimately lead to new and innovative ways to further protect these imperiled animals," Mansfield said.

In the past, young turtles have sometimes been found downstream from nesting sites, suggesting they might move passively with currents. They’ve also been found collected in association with drifting organisms, like Sargassum seaweed, also suggesting they’re mainly drifters.

“Our data show that one hypothesis doesn't, and shouldn't, fit all, and that even a small degree of swimming or active orientation can make a huge difference in the dispersal of these young animals,” Mansfield said.

“We conclusively demonstrate that these turtles do not behave as passive drifters. In nearly all cases, drifter trajectories were uncharacteristic of turtle trajectories,” the paper said.

© Jan TenBruggencate 2015

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