Migrating Turtles

How migrating animals like sea turtles navigate hundreds to thousands of kilometres across the open ocean has intrigued biologists since Charles Darwin. But some sea turtles might not really know where they’re going, new research suggests.

Analysis by an international team of scientists has mapped the movements of hawksbill turtles as they swam from their nesting grounds in the Chagos Archipelago to foraging sites also in the Indian Ocean.

It found the turtles often travelled in circuitous routes when migrating short distances, suggesting the animals’ navigational sense is relatively crude while in the open ocean.

The turtles typically travelled twice the required distance to their target locations. One individual swam 1,306km to reach an island that was a mere 176km away – travelling more than seven times the beeline distance.

The team tagged and tracked via satellite 22 hawksbill turtles after they had finished nesting.

Generally, sea turtles do not forage and nest in the same geographical area. These animals would have already undergone a migration from their foraging grounds, a mating season, and have laid multiple clutches of eggs before preparing for the return trip.

Chair in marine science at Deakin University and the study’s first author, Prof Graeme Hays, said if the turtles were perfect navigators, they would probably travel in direct paths from their nesting sites to foraging areas in search for food. “Those turtles that we’re tracking – they most probably hadn’t eaten for four or five months,” he said.

Previous research has suggested that turtles likely imprint on the magnetic field of their birth area – where they later return to lay eggs – and detect changes in the Earth’s magnetic field as a means of navigating through the ocean.

Hays said the new study suggested the turtles “almost certainly are using a geomagnetic map, but it’s a fairly coarse resolution”.

“So it doesn’t allow pinpoint straight-line migration, but it does tell them when they’re getting a long way off route,” he said.

Hawksbill turtles typically migrate distances of about 150km, a modest distance compared with the migration of green turtles, Hays said.

“For green turtles that nest in the Chagos Archipelago … we’ve tracked them going almost 5,000km to their foraging grounds,” he said. “They’ll swim all the way across the Indian Ocean to the mainland African coast.

“Although it’s a long journey, in a sense it’s actually quite an easy navigational task because all the turtle has to do is swim vaguely westwards and it’ll eventually hit Africa.”

Though hawksbill turtles were making far shorter migrations in comparison, they had the tricky navigational task of locating small, specific places like remote isolated islands or submerged banks.

The new research suggests the turtles’ geomagnetic map sense is not fine-grained enough to locate specific targets.

When closer to their intended locations, the animals likely use other navigational cues including sense of smell and visual landmarks, Hays said. “In the final stages, they can smell an island that they’re headed to.”

“As they get some sort of visual landmark, for example, the water starts to get a bit shallower and they can see the seabed, then they probably got some sort of cognitive map of that area. They could probably just recognise the seafloor, just like you would recognise visual landmarks in the area where you live.”

The research, published in the Journal of the Royal Society Interface.