How do young sea turtles behave during their early days in the open sea? This question has been difficult to answer until now, as there were no suitable methods for attaching trackers to small, rapidly growing marine animals for long periods of time. Now researchers have developed such a technique and used it to observe 21 young green sea turtles by satellite for almost half a year. The movement patterns of the animals are therefore more complex than previously assumed. Instead of letting themselves drift passively, they swim actively in preferred areas. The Sargasso Sea in the North Atlantic is therefore a particularly important habitat for the endangered species.
As “green turtles” they were hunted until they were almost extinct. Today they are threatened by climate change and the destruction of their habitat: Green sea turtles (Chelonia mydas) are endangered and are under international protection. Information about how the turtles live, migrate, and reproduce is important in any effort to conserve the species. But this data is sketchy. In particular, little is known about the early life phase of marine animals. One reason for this: Since the young turtles grow quickly, it is difficult to attach satellite trackers to their shell that will stick to it for a long time, but not affect the animal’s growth or other well-being. Previous findings are therefore mainly based on short-term observations and sightings.
New tracking method
A team led by Katherine Mansfield from the University of Central Florida in Orlando has now developed a tracking method specifically for young green sea turtles and used it to track 21 individuals for up to 152 days. All turtles were collected on a Florida beach immediately after hatching and reared in large tanks at the university’s marine laboratory for three to nine months until they weighed at least 300 grams. Then the researchers equipped them with solar-powered satellite trackers and released them near their birth beach in the Gulf Stream.
Mansfield and colleagues already had experience with tracking sea turtles through previous studies with loggerhead turtles (Caretta caretta), which they also observed via satellite as young animals. “The shell of the green sea turtle has a different structure and a waxy surface, so we had to develop a new method for attaching the tracker,” the researchers report. After several unsuccessful attempts, in which the trackers fell off again after a few days, the researchers found a solution: Before they applied the special adhesive, they lightly sanded the waxy layer on the tank. Only then did they attach the satellite tracker. According to the researchers, this method can also help in the future to observe green sea turtles for several months.
Lake Sargasso as a rearing habitat
“Our data show that the behavior of green sea turtles is more complex than previously thought,” report Mansfield and colleagues. While it was previously assumed that the animals were mainly driven passively, the new data clearly show that the animals are actively oriented and swim out of large currents such as the Gulf Stream. A particularly popular destination is apparently the Sargasso Sea, a marine area in the Atlantic Ocean east of Florida, which takes its name from the abundant brown algae of the Sargassum genus. “Two thirds of our tracked green sea turtles were in the waters of the Sargasso Sea when their tags stopped broadcasting,” the researchers report. “This indicates that many Atlantic green sea turtles use the Sargasso Sea as a breeding habitat.”
The Sargasso Sea offers the young turtles plenty of food, a certain protection from predators and a good temperature to grow. “The turtles in this study stayed in the ocean’s surface layer and likely received thermal benefits from exposure to direct sunlight on the ocean surface,” the researchers said. How long the turtles stay in the Sargasso Sea and why many but not all individuals have swum there is still unclear. It would be conceivable, for example, that smaller, weaker swimmers initially stay in the larger currents. The researchers want to clarify this in future studies with more green sea turtles of different ages.
Findings for the protection of species
The researchers also compared their results with previous findings on loggerhead sea turtles from the same region. Accordingly, the routes and movement patterns of the two species differ, but for both the Sargasso Sea is an important stop in the early life course. “Now that more and more data are available on the first years of sea turtles in the North Atlantic, it is clear that the Sargasso Sea is an important development and rearing habitat for sea turtles,” the authors write. That is why it is particularly important to preserve this habitat. “Understanding early behavior and identifying early development habitats are critical to the successful management and conservation of this endangered species,” said Mansfield and colleagues.
Source: Katherine Mansfield (University of Central Florida, Orlando) et al., Proceedings of the Royal Society B: Biological Sciences, doi: 10.1098 / rspb.2021.0057