The lost years of the loggerheads

Research published today in the Proceedings of the Royal Society B reveals where young loggerhead sea turtles disappear to during their ‘lost years’.

Adult and juvenile loggerheads are often sighted on the south-east coast of Florida where many nest on coastal beaches. However, after hatching and making their risky journey into the surf baby turtles are rarely spotted again until they have grown to around 40cm. The hatchlings swim out into the open ocean until they are big enough to deter some predators who might easily pick off smaller turtles. In the past the young turtles’ open-ocean habitats had made them logistically and technologically impossible to track. Harnessing new technologies this is the first study to follow neonate turtles and see where they disappear off to in their lost years.

The team behind the study, led by Katherine Mansfield of the University of Central Florida, tested hypotheses based on the little available data scientists have about loggerheads in this stage of their life. The rarity of hatchling sightings on the continental shelf led scientists to hypothesise that young loggerheads remain exclusively off shore and that they stick to circular North Atlantic currents as they migrate toward the Azores, Maderia and Cape Verde which are known oceanic developmental habitats for sea turtles. To find out where the turtles go, the team used remote satellite tracking. They attached solar-powered transmitters to 17 turtles collected from nests along the south-east coast of Florida. The team reared the turtles in the laboratory until they were 11-18cm long before releasing them in the Gulf Stream off the Floridian coast.

Monitoring the turtles for between 27 and 220 days the team tracked them as they travelled distances from 200 to more than 4300km. The scientists found that all the turtles travelled North and remained within or close to the Gulf Stream immediately after they were released and tended to travel in clockwise direction around the circular North Atlantic Subtropical Gyre currents which move clockwise out into the Atlantic and back towards the US’s south-eastern coast. Some turtles moved out of these Gyre currents into the centre; an area called the Sargasso sea. The team suggest that the turtles’ movements could be linked to the seasonal drift of Sargassum, a type of macro-algae that floats in large mats. The algae travels from the Gulf of Mexico along the eastern coast of the US before moving south into the Sargasso sea. The researchers think that the turtles that left the currents of the Gyre might have been following floating mats of Sargassum to take advantage of the habitat they offer, in particular the warmth the mats trap at the water surface close to them.

For young turtles, staying warm is of upmost importance. Warmer temperatures help their skeletons grow quicker, making them increasingly less vulnerable. The team suggest that the routes the turtles took could have been closely linked to where they could find warmer habitats to boost their growth. Testing Sargassum mats in the lab the team found that the algae helped to warm up the temperature of sea water by trapping the sun’s energy just under the water surface.

Tracking the neonate turtles the team say that, despite previous assumptions, not all loggerheads stick to the Gyre currents during their ‘lost years’. Some leave the currents and travel into the Sargasso sea. The team conclude that turtles choose sea surface habitats that provide a ‘thermal benefit or refuge to young sea turtles, supporting growth, foraging and survival’. These warmer habitats are vital for the turtles to thrive, aiding the growth of the neonate loggerheads so that once they are large enough they can return to the coast much less vulnerable than when they left as hatchlings.