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.

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.

Researchers were able to follow the journey leatherback turtles make from Africa to the South American coast using transmitters. Chlorophyll A is used by scientists as a proxy for the amount of nutrition in an area

Matthew Witt, a researcher at the Centre 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."

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.

The data showed that Regab took 150 days to swim 4215 miles, arriving in the waters off Brazil. The deepest dive was 1,080 metres, 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 kilometres 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.

The data from the research project will be used to try to prevent the potential decline of leatherback turtles. "If you look at the Pacific Ocean, the population there has undergone a huge decline – greater than 98% [have gone] in 30 to 40 years," said Witt. "The population is bordering on extinction; there are only hundreds of females left, rather than many tens of thousands."

The exact cause of the dramatic fall in Pacific numbers is not clear, but turtles can get caught on the hooks used to catch tuna, or under large gillnets. In both cases, the turtles are held under the water and drown. "It's all accidental bycatch, but it still has really significant impact on the population," said Witt. "The one risk that doesn't seem to exist in central Africa is egg-harvesting. This is a good thing because you can just keep refreshing the populations with the females and the new eggs they lay."

Banning fishing in the areas where turtles live is not always necessary, said Witt. "There are a whole range of strategies people have been developing over the last five to 10 years, such as changing the shape of hooks, changing the colour of the glowsticks and changing the bait types. It still maintains fisheries' catch rates, but it reduces sea turtle bycatch."

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 programme, 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 fertilised.

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 generalise, 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 journeysas juveniles. Witt's research has created a live map of creatures around the Atlantic, updated at www.seaturtle.org.