Leatherback turtles in Cornwall

Also known as leathery turtle and luth.

People in the UK imagine that turtles live elsewhere, but in fact leatherbacks are clearly a part of the fauna of the Celtic Sea. We may even have an important summer concentration just off the edge of our continental shelf.

Leatherback turtle
photograph by Jayne Herbert

The leatherback is the largest living turtle. Among the seven or eight species of sea turtle it is the most pelagic, swims fastest and dives deepest.

The weight normally ranges from 200 to 700kg. The largest leatherback ever recorded was a male of 914kg and 2.6m length that stranded on the west coast of Wales in 1988.

The distinctive carapace has a rubber-like texture, is about 4cm thick and is made primarily of tough, oil-saturated connective tissue. The front flippers are proportionally longer than in any other sea turtle, and may span 2.7m.

The core body temperature of adults in cold water can be up to 18 degrees C above the surrounding water. The turtles can rest submerged for two hours, and can dive to 4,000 feet.

Leatherback turtles feed on jellyfish, comb jellies and pelagic tunicates. They are immune to coelenterate toxins. Their success in feeding on food of such low energy density may be due in part to their ability to find deep-water jellyfish very efficiently from their bioluminescence.

All sea turtles lay their eggs in nests dug in open, sandy, tropical beaches. Leatherbacks differ from other sea turtles in their low beach fidelity, which hampers population estimation. Age at sexual maturity is unknown. Small turtles are rarely seen after they have left the beaches and their habitat is unknown.

The leatherback ranges as far as the Arctic Circle. They are regularly seen in the Celtic Sea, with occasional sightings from the coasts of Cornwall and Devon and a long history of strandings. These have been documented by Roger Penhallurick: 1988 was a peak year with 18 stranded, caught or seen; similar numbers were reported in 1990. The record suggests some correspondence between turtle and jellyfish numbers around Cornwall.

By-catches in the tuna drift-net fishery by Cornish boats were estimated several years ago at a rate of four per 1,000 km hours of net immersion. This was a remarkably high figure, being about one fifth of the capture rate of dolphins in the same nets, despite the fact that the slower movements of turtles might be expected to put them at less risk. It suggested a high density of turtles in the area of the fishery. This by-catch has been one factor in the decision of the EU to close this fishery in 2001.

During the summer, leatherback numbers are known to rise along the east coast of the US from Maine to Florida. A high-seas aggregation of leatherbacks is known to occur in the Pacific at 35°-45°N, 175°-180°W, but no definite Atlantic aggregations have been identified.

In 1982, Peter Pritchard estimated that 115,000 adult female leatherbacks existed worldwide and that roughly half of them were probably nesting on the Pacific coast of Mexico. In 1992 the estimate was of 20,000-30,000, but estimates are quite uncertain.

Nick Tregenza

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