A Queen's researcher has used satellites to track the elusive leatherback turtle, finally explaining why these creatures dive to incredible depths that would crush most creatures
THOSE INVOLVED in extreme sports are left to marvel at the leatherback turtle, an extreme free-diver that can reach depths of up to 1,250m on a single breath of air. Now satellite technology has helped marine biologists explain why the remarkable leatherback risks life and limb to plumb these depths.
There have long been questions about this risky behaviour, explains Dr Jonathan Houghton, a lecturer in the school of biological sciences at Queen's University, Belfast. Why would the leatherback occasionally face such demanding dives when almost everything they need is located within just a few hundred metres of the surface?
That was not the question being pursued by Houghton when he and colleagues at Swansea University started attaching satellite "data loggers" to a group of 13 leatherbacks in their study published recently in the Journal of Experimental Biology. Rather, they wanted to find ways to protect this rare and endangered turtle from extinction.
"The original reason we put the transmitters on them was not because of deep diving but because the turtles were being badly hit by fishing," he explains. They didn't touch bait but were at risk of being "foul hooked" by long-line tackle set for mackerel. They decided to study migration patterns as the turtles left nesting sites on the Caribbean islands to travel to rich feeding grounds in the north Atlantic to see how to avoid these accidental killings.
Earlier work was based on using an early generation of data loggers that remained with the animals for about two weeks before breaking free relatively close to the shore for subsequent collection. These provided limited information however and gave only tantalising glimpses into their spectacular deep-diving behaviour, Houghton says.
"We knew about this deep-diving behaviour but only close to the beaches," he explains. "We have known that they do this for 20 years, but no one had been able to get enough data in to look at this properly."
The current generation of data loggers, however, transmit to satellite all of the time, providing information about location, temperature, dive depth and duration. "The transmissions occur in real time as they go along," he says.
The team catalogued data from 26,000 dives of all depths made by the turtles as they traversed the thousands of miles between the Caribbean and the north Atlantic, where the animals travel to feed on jellyfish. "It was only when we had all the data in that we were able to look at this."
Deep diving is very rare, with only 95 of the 26,000 dives catalogued reaching deeper than 300m, they found. It is also highly risky behaviour given the crushing pressures at 1,250m, which reaches 122 atmospheres or about 125 bar. "It is a formidable pressure down there. The only way they can get away with it is their flexible shell."
Their data helped to eliminate some of the theories linked to deep diving. One held that the turtles were escaping predators, but the data showed they weren't running, they spent time idling on the surface both before and after deep dives, preparing for, then resting after the dive.
Another suggested they were cooling off, but temperatures only marginally decrease below 350m, Houghton says. This left the team facing the third hypothesis, the turtles were after food. But why dive when there is such bounty close to the surface?
The answer came because Swansea had experience of jellyfish ecology through research collaborations with scientists at University College Cork. There are few shallow-water jellyfish available until the turtles reach the north, but all along the route from tropical to northern waters there is a collection of unusual deep-water jellyfish including salps, pyrosomes and siphonophores.
These form large colonies and live at great depths during the day but rise closer to the surface at night. "We suddenly realised their deep dives only occurred during the transit phase," Houghton says. "That was the real moment it dawned on us."
They now believe the turtles dive only to locate areas where the deep-water jellyfish are plentiful. The turtles then wait at the surface for the jellyfish to rise at nightfall. This matches the data captured by satellite which shows the turtles remaining in an area for days before travelling on to the next mid-Atlantic feeding station on their way north.
