The world's largest turtle is a frequent visitor to Ireland, and we could be seeing more of the species in the near future writes ANTHONY KING
THE GIANT LEATHERBACK marine turtle is a regular visitor to Irish waters and climate change seems likely to increase this animal's visits. Yet few people recognise the largest turtle in the world as local fauna to Ireland, according to Prof Graeme Hays.
Recently, he gave a lecture on the habits of the leatherback at the National Museum of Ireland in Dublin, organised by the Irish Naturalists' Journal. It provided a fascinating view of this remarkable mariner, which can cover 10,000km a year.
Hays is a leading expert on sea turtles. His group in Swansea University, Wales, has collaborated with scientists in University College Cork to gain insight into the behaviour of leatherbacks and their mysterious migrations.
Much was learned by satellite tagging and tracking two turtles caught off the Irish coast in 2005 and 2006.
The leatherback weighs up to 1,000kg and swims thousands of miles, from nesting beaches in South America and the Caribbean to the North Atlantic. Commuting over such distances is worth the effort, because there is so much jellyfish for them to eat at high latitudes, according to Hays.
"They come when the jellyfish are most abundant, which is the summer and autumn, and when the water is warmest," he says.
With global warming, the turtles have pushed further north in recent decades and Hays predicts that more of the population will visit Irish waters in future years. Leatherbacks tolerate colder water than any other marine turtles, but avoid water below 15 degrees.
The turtles tracked as part of the research collaboration swam up to 70km a day, but often remained in one area, presumably due to an abundance of their jellyfish prey.
The extent of their diving behaviour was unexpected, says Hays. "The fact that they repeatedly dived, typically to 50 and 200 metres, was a surprise."
One of the tagged animals performed the deepest dive recorded for a turtle - 1,280 metres off Cape Verde.
Leatherback turtles are almost unique in the world's oceans in predating jellyfish. "It's an unusual prey type because it's so low in energy," explains Hays. Predators get 100 times less energy from jellyfish per gram than from fish, he says, so a turtle must eat hundreds of kilos each day. "Leatherbacks also probably have a very low metabolic rate, so their fuel economy is very good," he adds.
Dr Tom Doyle of University College Cork mapped the distribution of jellyfish in Irish waters and recorded leatherback sightings as part of his PhD research. He discovered that barrel jellyfish, Rhizostoma octopus, grow prolifically in certain areas and turtles are drawn to these blooms. The barrel jellyfish can span a metre across and weigh 40kg.
Leatherbacks do not have the typical hard turtle shell; instead, their carapace has the consistency of a "scuba-diver wearing a neoprene wetsuit", says Hays. This may be a weight-saving adaptation, which is important, since they can swim 10,000km in a year.
The soft shell is also their Achilles heel: it can snag on bait hooks in long-line tuna and swordfish fisheries. They get hooked, dragged down and drown, explains Dr Doyle. In coastal waters, the turtles can become entangled in nets and various ropes in the water. Hays says it is a question of fishermen being aware of the problem, and frequently checking nets and ropes to release turtles. The species is critically endangered.
Leatherbacks are a potential draw for ecotourism. "A 500kg turtle is impressive, so, in the same way you have seal- and dolphin-watching trips, there's no reason you can't have turtle-watching trips," Hays says.
The mistaken view of leatherbacks as tropical arose because they come ashore to lay eggs in the tropics - their eggs require warm sand to develop. "When you buy a pocket guide to the fauna of Ireland, it should always include leatherback turtles," says Hays.
Last week's lecture noted a special publication on turtles in the Irish Naturalists' Journallater this year by Dr Simon Berrow of the Shannon Dolphin and Wildlife Foundation and naturalist Gabriel King, a pioneer of Irish turtle studies.
Anyone who finds a stranded turtle should contact a local aquarium or Tom Doyle at the Coastal and Marine Resources Centre in UCC.
For more information on Irish turtle research, or to report
sightings, visit
www.turtle.ie
.
Tom Doyle's research on turtles and jellyfish is funded by
the Irish Research Council for Science, Engineering and
Technology