New research helps piece together the ‘lost years’ of green sea turtles

May 4, 2021
Green sea turtles are less subject to ocean currents than previously thought. (Gustavo Stahelin)

Green sea turtles are less subject to ocean currents than previously thought. (Gustavo Stahelin)

Researchers have gained a rare glimpse of the early lives of green turtles that challenges previous hypotheses that the reptiles passively drift along ocean currents, information that could help conserve this endangered species.

Sea turtles spend the first few years of their lives in the open ocean, a period that has been shrouded in mystery because of challenges associated with tracking and observing the young reptiles. Using a new method for attaching satellite trackers, researchers reported Wednesday in Proceedings of the Royal Society B that baby green turtles swim to the Sargasso Sea, a calm expanse in the Atlantic Ocean, rather than drifting around it.

The green turtle, Chelonia mydas, is one of seven species of sea turtle, all of which are endangered, threatened or critically endangered.

"They're very long-lived animals — they live as long as humans, at least — and they don't reach maturity until age 25 to 30. We really don't know anything about the turtles' early years, and because of that, it's termed the sea turtle 'lost years,' because they're kind of lost to our understanding and knowledge," said Kate Mansfield, the study's lead author, who is an associate professor of biology and the director of the Marine Turtle Research Group at the University of Central Florida. "It's really important to understand those formative early juvenile years, because if we don't protect them and don't know how to protect them, then we're ultimately impacting the future of the populations."

In the North Atlantic, female green turtles lay their eggs on sandy beaches along the U.S. East Coast, from Florida to Virginia.

"They hatch out as these tiny little guys that are probably about half the size of a deck of cards," Mansfield told The Academic Times. "They emerge from their nests and crawl down the beach and enter the ocean. And then they swim like crazy for about 24 to 48 hours to get offshore and likely away from nearshore coastal predators, things that would eat them as a little turtle snack."

Until turtles show up again in coastal habitats as larger, salad plate–sized juveniles, little is known about their lives in the open ocean, Mansfield explained. Other researchers have hypothesized that green turtles and other sea turtle species spend this time passively drifting around the circular system of currents that form the North Atlantic Subtropical Gyre until the turtles are large enough to return to shallower waters. But according to Mansfield, there was little data to confirm this idea.  

In 2014, Mansfield and her colleagues published a study about the lost years of baby loggerhead sea turtles. About half of the tagged animals actively swam to the Sargasso Sea, which is found at the center of the circular gyre currents, disputing the idea that baby turtles are mere drifters. 

"It was unexpected and not something that anyone had really thought about before," Mansfield said. "It really opens up this perspective that the Sargasso Sea could be a very important sea turtle nursery habitat for many of the turtles in the western Atlantic that hatch off the U.S. coast."

To examine whether the lost years of green turtles looked similar to those of loggerheads, the researchers used solar-powered satellite tags to track 21 juveniles. After collecting turtle hatchlings from beaches in Florida and rearing them in the laboratory for several months, the team attached the tags to their subjects' shells.

"One of the biggest challenges that we had was figuring out a way that we could safely put a tag on these little turtles," Mansfield said. "Turtles at that size are growing so quickly that anything that we glued to the back of the turtles would just fall right off."

In their previous work on loggerhead turtles, Mansfield and her colleagues overcame a similar challenge with the help of a manicurist.

"Sea turtle shells are made of keratin, which is the same thing as our fingernails and toenails," Mansfield explained. "We were able to figure out a way to glue a tag onto the turtles by first sealing their shell with a little bit of manicure acrylic, which would prevent it from peeling."

But this creative solution didn't work on green turtles because of differences in shell texture between the two species.

"It still is keratin, but it almost feels waxy. It's hard to describe," Mansfield said of green turtle shells. "We tried a lot of different adhesives. We even enlisted the help of a dentist and tried the bonding material used for crowns and a couple of other options, but nothing worked."

Eventually, the team did find something that worked. With the use of a marine adhesive, they successfully affixed the tags for up to three months, without stunting the turtles' growth or limiting feeding or movement.

With tags firmly in place, the turtles were released offshore from beaches where they were born. Then, using satellite-relayed data, the team monitored the animals' movements remotely for up to 152 days.

The researchers found that about two-thirds of the tracked animals moved out of the ocean currents and entered the Sargasso Sea, suggesting that green turtles actively orient toward this region and, like loggerhead turtles, could have more complex behaviors than previously supposed.

Because sea turtles have complicated life cycles, requiring land, ocean and coastal waters, they face threats in each of these habitats, according to Mansfield. Pollution, plastics, fishing and climate change are a few of the things that harm these charismatic creatures. 

The study's findings suggest that the Sargasso Sea is a key nursery area not only for loggerhead turtles but for green turtles as well, Mansfield said.

"[The study] is really illuminating the Sargasso Sea as a very important area that we need to consider for protection or for more research about the importance of this area to our populations of sea turtles," she said. "I believe the Sargasso Sea Commission is working towards trying to establish the Sargasso Sea as a World Heritage site for the [International Union for Conservation of Nature]. So this will hopefully provide some information and data that will support the importance of this area as a resource for a number of species, particularly protected species."

The Sargasso Sea is named for the mats of Sargassum algae that float on the water's surface in this region. 

"Sargassum floats at the sea surface, and it entrains a lot of little fish larvae and young juvenile fish species and crabs, so it provides protection and food for the turtles," Mansfield said. "So it kind of makes sense that they would be hanging out there."

In future work, Mansfield and her team will look more closely at associations between green turtles and Sargassum mats. They also plan to examine finer-scale movements of turtles to understand what they are doing on a day-to-day basis.

The study, "First Atlantic satellite tracks of 'lost years' green turtles support the importance of the Sargasso Sea as a sea turtle nursery," published May 5 in Proceedings of the Royal Society B, was authored by Katherine L. Mansfield, University of Central Florida; Jeanette Wyneken, Florida Atlantic University; and Jiangang Luo, University of Miami.

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