Satellite tracking devices have shed light on an astonishing trans-Atlantic odyssey undertaken by the leatherback turtle, one of the world's most ancient species, as it goes on a feeding foray before breeding, scientists said on Wednesday.

Biologists at the University of Exeter in southwestern England attached the tiny trackers to 25 female turtles at their breeding grounds in Gabon, central-west Africa, and then monitored their movements over the next five years.

Three migratory routes emerged as the turtles headed for food-rich waters in the Atlantic, building reserves over the next two to five years before returning to Gabon to reproduce, they reported.

One route led to a circular zone in the mid-Atlantic between central Africa and Brazil, and another route went far south, beyond the Cape of Good Hope.

A third went straight as an arrow across the Atlantic to the coast of South America, an ocean-spanning 7,563 kilometres (4,699 miles).

"Despite extensive research carried out on leatherbacks, no-one has really been sure about the journeys they take in the South Atlantic until now," said Matthew Witt of the university's Centre for Ecology and Conservation.

"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."

The findings, published by Britain's Royal Society in its journal Proceedings B, also showed that the turtles also crisscross routes used by longline trawlers.

These are ships that trail baited hooks for fish but often end up snaring hapless turtles, as well as albatrosses.

"All of the routes we've identified take the leatherbacks through areas of high risk from fisheries," said Witt's colleague, Brendan Godley.

"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."

The largest, farthest-ranging and deepest-diving turtles on Earth, leatherbacks can reach up to two metres (6.5 feet) in length and exceed 900 kilos (2,000 pounds) in weight.

Their numbers have remained relatively stable in the Atlantic, but have declined alarmingly in the Pacific. Their decline has been blamed on accidental catch by trawlers and loss of breeding grounds to shorefront development.