Secret voyages of leatherback turtles revealed using transmitters

On 2 February 2009, at 4am, a turtle known as Tika set off from the coast of Gabon, west Africa. She spent almost six months swimming across the Atlantic, a 5,000-mile (8,000km) journey to the coast of South America. At the moment she is probably somewhere off Brazil, eating jellyfish and building herself up.

In about March next year, she'll begin her journey back to Africa, and, if all goes well, she'll then build a nest and lay her eggs in the sands of the Mayumba national park in Gabon. And this will be just one of many 10,000-mile round trips she makes in her 50-year life.

Scientists know all of this because, for the first time, they have tracked the journeys taken by leatherback turtles as they cross the Atlantic Ocean, with Tika travelling the furthest of the 25 females that were followed in a study lasting more than five years. She, along with another female called Regab, ended up in the waters off Brazil, Argentina and Uruguay. Others stayed closer to Africa, but still their journeys lasted for months and they swam thousands of miles. One, named Caroline by researchers, swam around the middle of the Atlantic for more than a year and a half, clocking up more than 7,000 miles, before returning to breed.

British scientists have discovered that the gigantic females can swim for thousands of miles in a perfectly straight line. Following a course that would be the envy of a state-of-the-art cruise liner, the turtles make the transatlantic journey from Central Africa to South America using the shortest possible route.

The maps of their journeys, published today in the Proceedings of the Royal Society B, will be an important means by which to document and conserve the rare creatures in the Atlantic Ocean, according to the scientists involved. In the Pacific, numbers of leatherback turtles have plummeted in the past few decades, as they are caught and drowned in fishing nets.

Matthew Witt, a researcher at the Center for Ecology and Conservation at the University of Exeter, led the project. "Despite extensive research carried out on leatherbacks, no one has really been sure about the journeys they take in the south Atlantic until now," he said.

"What we've shown is that there are three clear migration routes as they head back to feeding grounds after breeding in Gabon, although the numbers adopting each strategy varied each year. We don't know what influences that choice yet, but we do know these are truly remarkable journeys – with one female tracked for thousands of miles travelling in a straight line right across the Atlantic."

Of the three discovered migratory routes, one included a 4,699 mile journey straight across the Atlantic from Gabon to the coastal waters off southern Brazil and Uruguay that took 150 days.

Over five years, the researchers tagged 25 females with satellite tracking devices strapped or drilled to their shells as they left their hatching grounds in Africa to seek food across the Atlantic

Each turtle was fitted with a simple transmitter on her back, powered by four lithium camera batteries. This sent signals to a satellite receiver every time the creature came up for air on its travels across the open ocean. (Left: turtle hatchings head to ocean from nest)

The data showed that Regab took 150 days to swim 4215 miles, arriving in the waters off Brazil. The deepest dive was 1,080 meters, by a turtle called Darwinia, who was also headed to South America.

As well as South America, Witt's team identified two other migration routes. One saw turtles swim to the coast of South Africa, while the other led them around the middle of Atlantic Ocean. "Although sometimes they're in the middle of nowhere, hundreds of kilometers from any coastal features, they have plonked themselves in the middle of a food hotspot," said Witt.

In each case, the turtles swim thousands of miles to stay within food-rich areas of the oceans. Typically, a mature individual could stay swimming around the migration routes for up to five years, building up food reserves, before returning to their birthplace in Gabon to reproduce.

Brendan Godley, of the University of Exeter and a co-author of the work, said all of the routes identified by researchers take the leatherbacks through areas at high risk from fisheries, so there was a real danger to the Atlantic population.

"Knowing the routes has also helped us identify at least 11 nations who should be involved in conservation efforts, as well as those with long-distance fishing fleets. There's a concern that the turtles we tracked spent a long time on the high seas, where it's very difficult to implement and manage conservation efforts, but hopefully this research will help inform future efforts to safeguard these fantastic creatures."

Howard Rosenbaum, director of the Wildlife Conservation Society's Ocean Giants program, agreed. "This important work shows that protecting leatherback turtles – the ancient mariners of our oceans – requires research and conservation on important nesting beaches, foraging areas and important areas of the high seas.

"Armed with a better understanding of migration patterns and preferences for particular areas of the ocean, the conservation community can now work toward protecting leatherbacks at sea, which has been previously difficult."

All at sea

Leatherbacks are born on the beach and, in the case of those in the Atlantic, that means somewhere along the coast of southern Gabon in the Mayumba national park. Hatchlings run straight into the sea and spend their first few years being wafted around the dominant ocean currents, not yet strong enough to swim as they please.

"While you're a hatchling and growing over the first five to 10 years, you're just being swept along the south Atlantic," said Matthew Witt, of the University of Exeter. "Once you get to a stage where you can maintain your own position and be a bit more autonomous, that might become your foraging ground as an adult."

Leatherbacks prey on jellyfish and other soft-bodied sea creatures, and can live for 40 to 50 years. When turtles reach sexual maturity about 10 to 15 years after they have been hatched, they swim back to their birthplace to reproduce. Male turtles stay in the water all their lives, unlike females, which head towards land when they are fertilized.

Females can mate every year and, in each clutch, will lay about 100 eggs in the sand of a beach near their own birthplace. These will hatch within 70 days. "To generalize, all leatherbacks in the south Atlantic typically come from central and West African beaches," said Witt. "Most nesting of leatherbacks happens in Gabon on those central west African beaches."

Because they live predominantly in the open ocean, there is little opportunity to see the young individuals and plot out their journeys as juveniles. Witt's research has created a live map of creatures around the Atlantic, updated at www.seaturtle.org.

Source:
The Guardian,"Secret voyages of leatherback turtles revealed using transmitters", accessed January 6, 2011
The DailyMail, "How turtles can conquer Atlantic by swimming thousands of miles in a perfectly straight line", accessed January 6, 2011