After a baby green turtle crawls out of its nest on a moonlit night and makes its way to the surf, it disappears at sea for years until returning to land. Until recently, biologists figured that the turtles drifted on the major currents of the open ocean. But new research, which used satellite tags to track young green turtles for the first time, suggests that the middle of those swirling currents acts as a previously unknown turtle nursery.
“That gyre acts as the walls of a playpen, and we really need to factor that into our management of the north Atlantic species,” says Katherine Mansfield a turtle biologist at the University of Central Florida, whose research was published today in Biological Sciences.
Like most other sea turtles, green turtles head out to the open ocean to grow, returning to coastal waters only years later as adults. Researchers call that time at sea the “lost years,” since we know very little about where the young turtles go, or even how long they’re away.
Most of what we did know is inferred from “opportunistic sightings of these little baby turtles in the open ocean,” says Mansfield. Baby green turtles have been spotted in the Gulf Stream, the fast-moving river of warm water that curls up the East Coast of the US, as well as in the currents on the eastern side of the Atlantic.
So the existing hypothesis was that the baby turtles mostly went with the flow, following big ocean currents as they circled the North Atlantic.
The big challenge is attaching solar-powered trackers to young turtles with six-inch-long shells. “At that age, the bony plates of their shell, similar to the bony plates of baby skulls, haven’t fully fused together,” Mansfield says. If the researchers attached the trackers with a tough epoxy, they would end up deforming the turtles as they grew.
They tried flexible glues instead, but that ended up flaking off the shells, which are made of keratin. That’s the same material that makes up human fingernails, which gave them another thought. “Jeanette [Wyneken, a co-author,] and I kind of looked at each other,” Mansfield says. “She called her manicurist and asked, ‘What would you do for peeling nails?’”
The manicurist recommended an acrylic base coat. That turns out to be perfect for loggerhead, Kemp’s Ridley, and hawksbill turtles. “But the green turtles were another issue,” Mansfield says. “We consulted with a dentist, and tried the same bonding adhesive as dentists used to use for crowns. Finally, we went to a local hardware store and grabbed a bunch of stuff that didn’t say ‘carcinogen.’”
A particular marine adhesive ended up doing the job, holding the tracker in place for three months before finally falling off as the turtle grew.
When the researchers were finally able to watch their cohort of 21 tagged turtles swim, they found that while many of them spent some time riding the Gulf Stream north, 14 of the 21 ended up swimming into the Sargasso Sea.
The Sargasso Sea, a roughly two million square mile area roughly surrounding Bermuda, doesn’t have land borders. Instead, it’s surrounded by the major north Atlantic currents. The sea inside is relatively calm, and huge mats of the brown sargassum seaweed build up on the surface, giving the region its name.
Biologists best guess is that there are fewer predators in the open ocean than in coastal waters, making it a more appealing refuge. And young sea turtles of all kinds tend to hang out around the sargassum mats, since they harbor all kinds of life in the otherwise empty open ocean.
“We call it the golden floating rainforest of the Atlantic,” says Tess Mackey, program manager for the Sargasso Sea Commission, a nonbinding conservation collaboration between 10 countries bordering the waterway. “If you look at a photo of a sargassum mat, it’s like a scavenger hunt, seeing the small crabs and shrimp and fish that are living in there.”
The seasonal movement of sargassum around the Atlantic currents even lines up with the yearly turtle hatch, although it’s not clear if the two are related.
“It makes sense that turtles would associate with sargassum, because it provides them with a little bit of protection,” Mansfield says. “If you have a shark or dolphinfish or mahi mahi, it’s going to be swimming under those mats. Lots of those turtles have camouflage, and they blend in nicely with the sargassum.”
Green turtles aren’t the only threatened species that use the Sargasso Sea as a nursery. American and critically endangered European eels both travel to the area to spawn, although the actual act has never been documented. And other fish, both economically important varieties and those unique to the Sargasso Sea, make their homes in the mats.
“My favorite fish, the sargassum frogfish, has modified fins to crawl around in the sargassum,” Mackey says.
But the sea also collects plastic from the surrounding currents, much like the Pacific Garbage Patch. “Marine life like turtles that are sheltering and feeding in the sargassum mats can get tangled in it, or even ingest it,” Mackey says.
The Commission is preparing to work on a large-scale assessment of the value of the Sargasso Sea ecosystem. “This research is really helpful for us to have new evidence about green turtles on why the Sargasso Sea is vitally important for marine biodiversity in general,” Mackey says.
So far, it appears that this behavior is unique to North American green turtles—their South American relatives don’t have a similar nursery. “Our sample sizes are small, it’s expensive getting tags on these little turtles,” Mansfield says. “Really what it’s telling us is we need to look at this more closely, and hopefully put more tags on turtles, to really get at these differences, and to see if there is a target destination for the animals.”