(Image credit: Gustavo Stahelin, UCF MTRG; Permit number NMFS-19508)
Green sea turtles venture into the open ocean immediately after hatching on the Florida coast, and then seem to vanish for a spell — now, new tracking data shows that, after surfing the Gulf Stream northward, many turtles drop out of the current to enter the Sargasso Sea, an oasis of cozy seaweed and plentiful food.
In the new study, published Tuesday (May 4) in the journal Proceedings of the Royal Society B, scientists attached solar-powered satellite tags to 21 green sea turtles (Chelonia mydas) of “toddler” age, meaning about 3 to 9 months old. The young turtles weighed just over 10.5 ounces (300 grams) and their shells measured about 5 to 7 inches (12 to 18.6 centimeters) long; tagging such small creatures presented a huge challenge, both due to their initial size and the fact that they grow and change shape fairly rapidly, compared with mature animals.
Despite the hurdles, to better protect sea turtle populations, “we really need to get tags on some of these little guys,” said first author Kate Mansfield, director of the Marine Turtle Research Group and an associate professor in the biology department at the University of Central Florida. Young turtles’ migration into the open ocean is often referred to as “the lost years,” since scientists know so little about what the animals get up to before they return to the coast as “teenagers.”
“Sea turtles, in general, don’t reach maturity for at least a couple decades. And those years leading up to when they become adults, we don’t know much about them,” Mansfield said. Now, thanks to the tracking data, “the Sargasso Sea is emerging as an important habitat for sea turtles in their early life stages,” marking the region as critical for conserving the species, she said.
The adventures of turtle toddlers
The Sargasso Sea, named for a free-floating genus of brown algae called Sargassum, is the only sea whose edge is defined by ocean currents, rather than land boundaries, according to the National Oceanic and Atmospheric Administration (NOAA). The sea sits within the so-called Northern Atlantic Subtropical Gyre, a big circle made up of four major ocean currents: the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east and the North Atlantic Equatorial Current to the south.
In past studies, researchers caught sight of young green sea turtles, as well as loggerheads (Caretta caretta), passively drifting in these currents, often on a mat of Sargassum, Mansfield said. These observations, as well as sightings of the Florida-borne turtles in the east Atlantic, hinted that turtles may just make a big loop around the gyre before heading back to the U.S. as juveniles.
But Mansfield and her colleagues wanted to get hard evidence of this migration, while also pinning down how long the turtles typically stay offshore. Are they away for one year, “or are they out for a decade? These are pretty fundamental questions,” said study author Jeanette Wyneken, a professor of biological sciences at Florida Atlantic University.
Satellite tags designed for adult turtles “looked like a brick” about the size of a cell phone, Mansfield said; but with the advent of small tags, about the length of a finger segment, the team could start tracking young turtles, she said. They began with loggerheads and tagged 17 turtles in a 2014 study; they found that, rather than passively drifting along, the loggerheads frequently exited the currents to swim toward warm, nutrient-dense waters, including those in the Sargasso Sea, they reported in the journal Proceedings of the Royal Society B.
Green sea turtles may be similarly active swimmers once they reach the open sea, the team thought, so they set out to repeat the study with C. mydas hatchlings.
The team collected the hatchlings from Boca Raton, located on the southeast coast of Florida, and then brought them back to the lab to rear for several months. Though the teams’ satellite tags are small, they’re still too large for freshly hatched turtles, Mansfield noted. The team initially attempted to attach the tags using the same adhesive they had used for the loggerheads, a kind of manicure acrylic that eventually peeled off as the animals grew.
“But that didn’t work with the green turtles,” Mansfield said. The texture of a young green turtle’s shell feels “waxy,” somewhat like a human fingernail doused in cuticle oil, whereas loggerhead shells aren’t so slick, Wyneken said. The team tried many adhesives — those used to cement fillings in teeth, those used to attach theatrical prosthetics to skin, you name it — before finding one that would stick to the slippery turtles.
That ended up being a marine urethane adhesives, normally used to seal boats; the glue is flexible enough to stretch as the turtles grow, but once the turtles reach a certain size, it pops right off.
After ensuring the adhesive was safe and sticky enough, the team released their turtles into the western Atlantic and tracked them for an average of 66 days; they were able to follow several turtles for more than 100 days, and one for 152 days. They found that their turtles mostly swam near the surface of the ocean, similar to the loggerheads, and also coasted along the Gulf Stream. However, in general, the turtles dropped out of the Gulf Stream and adjacent North Atlantic Current sooner than the loggerheads did.
About two-thirds of these green sea turtles then high-tailed it toward the Sargasso Sea, where they stayed until their tags ceased to transmit; this hinted that the Sargasso serves as an appealing nursery for the turtles.
“That habitat, it makes sense,” Mansfield said. The seaweed provides camouflage for the teeny turtles, while also slowing the flow of water and thus allowing the sun to heat its surface. Being cold-blooded, sea turtles require warm water to survive and their growth slows significantly when they get too chilly. Other juvenile marine animals, such as shrimp, crab and fish, also grow up in the Sargassum and provide food for the growing turtles.
But while a proportion of young sea turtles flock to the Sargasso, there’s still the question of why some swim toward the sea while others remain in the current, Mansfield said. It may be linked to the fact that the quantity of Sargassum and its distribution along the eastern U.S. varies by season, according to a 2011 study in the International Journal of Remote Sensing.
So while some turtles encounter the Sargassum in the Gulf Stream and follow it into the Sargasso Sea, “there may be some that miss that Sargassum boat and hang out in the currents,” Mansfield said.
And when the turtles do reach the Sargasso Sea, another question emerges: How long do they stay?
“I would guess those animals would be out there for maybe two to three years,” Wyneken said, but that’s a “pure guess,” she added. “If you’re in a safe place, why leave it?” Mansfield said, echoing Wyneken’s sentiment. Green sea turtles typically return to Florida as juveniles and remain in the coastal habitat until adulthood, feasting on algae, seagrasses and jellyfish. Until they reach that critical transition point, the Sargasso Sea likely fills many of the turtle toddlers’ needs.
But to know exactly how long they remain in the seaweed-laden habitat, scientists will need more tracking data, Mansfield said. Now that they’ve tracked turtles on their way to the Sargasso, the team could potentially search for turtles already living there and attach tags to them on-site, she said. Regardless of how long the turtles stay, the Sargasso appears to be an important habitat for the young animals. To ensure the turtles grow up to lay their own eggs in the future, the sea must be conserved, she said.
“The Sargasso Sea can’t become another garbage dump. It has to be recognized as a habitat that’s important to imperiled species,” Wyneken said. “It’s probably not just a green turtle story,” given that other juvenile marine animals also grow up among the seaweed, she added.
Originally published on Live Science.