Conservationists say they are now convinced that loggerhead turtles routinely migrate across the Pacific from Japan to Mexico, and then swim back.

The distance involved - 12,000km each way - makes this one of the longest migrations recorded.

It is appreciably longer than the 10,000km each way that gray whales cover on their annual trip between Mexico and the Arctic.

More proof needed

In 1996-97 Dr Wallace Nichols, of the University of Arizona, tagged and tracked an adult female loggerhead from Baja California in Mexico to the species' nesting grounds in Japan.

Because mature turtles usually return to the beach where they hatched to lay their own eggs, Dr Nichols concluded that juvenile loggerheads must also be swimming from Japan to Mexico. But he was unable to prove it.

Earthwatch researchers with a turtle in Mexico (Photo Earthwatch)

Then his luck changed. He was recently given a tag by a Mexican fisherman, who had found it on a loggerhead. It bore a return address in Japan.

The fisherman had kept the tag for five years before reporting it, as he feared the penalty for taking an endangered species.

Dr Nichols says the tag was from a turtle hatched on Yakushima island, where a third of Japan's loggerheads nest.

The turtle spent a year at the Okinawa aquarium before being released in 1988. It was captured in Mexico six years later.