Breakthrough as 12,000-mile loggerhead odyssey is mapped

Marine biologists believe they have solved a long-standing puzzle: what do endangered loggerhead turtles get up to in their early years, after hatching on an Australian beach?

The answer is, they hitch a lift on ocean currents that transport them all the way from Queensland across the Pacific to South America then back to Australia – a round trip of more than 12,000 miles.

The migratory behaviour of the juvenile turtles was confirmed by marine biologists at Queensland's James Cook University, who did genetic testing on loggerheads found stranded on the Australian coast and captured by long-line fishing vessels off Peru. Turtles at the two locations on opposite sides of the Pacific shared the same genetic background.

While loggerheads can be found around the world, in tropical and sub- tropical waters, scientists are particularly anxious to find out more about the Queensland nesting population, which has declined by up to 80 per cent over the past decade. The latest study has helped fill in the "lost years" of the juveniles, whose early movements were a mystery until recently.

The study, published in the scientific journal Proceedings of the Royal Society B, tracks the turtles from the time they emerge from their nests on Mon Repos beach near Bundaberg, in southern Queensland, during the southern hemisphere summer.

First, baby turtles head for the water and swim out to sea, according to the lead researcher, Michelle Boyle. Then, like the turtles in the animated film Finding Nemo, they hitch a ride south on the East Australian Current, taking them down the east coast towards Sydney.

From there, the loggerheads travel east, passing Lord Howe Island, north-east of Sydney, then the northern tip of New Zealand. They then pick up the Humboldt Current, traversing the South Pacific all the way to the coast of Peru and Chile. Later, they return to Queensland using another ocean system, the Southern Equatorial Current, researchers hypothesise.

In her paper, Dr Boyle, of the university's School of Marine and Tropical Ecology, says many questions remain to be answered. It is unclear how long the odyssey takes, for instance, and whether the loggerheads spend most of their time swimming or moving passively with the currents.

Scientists have no reliable means of calculating the age of turtles. But juveniles are just 2in long when they leave Australia and nearly 28in on their return. They remain in coastal waters off Queensland until they are ready to breed at the age of about 30.

Dr Boyle and her colleagues took tissue samples from the young loggerheads found off Australia and Peru and analysed their genetic make-up. She says the findings reinforce a need for international collaboration on conservation efforts.

The study had filled in some of the gaps about the turtles' early movements, she said. "No one knew where they went, or how long they went for."

While the population in eastern Australia has declined dramatically, its prospects look slightly better than a few years ago thanks to improved protection and the compulsory use of trawl nets in Queensland waters, designed to allow captured turtles to escape.

Australian turtles are not the only long-distance travellers; Californian loggerheads have been found to have a genetic affinity with those in Japan.

With their wide-ranging migrations, the species is vulnerable to accidental capture in nets and long-lines. But "turtle excluder devices" used in Queensland are still not mandatory everywhere. More than 100 species, including barnacles, crabs and algae, have been found living on turtles.