Leatherback turtles and their jellyfish prey (Dr Matt Witt)
31st March 2012
Of the 7 species of marine turtles, 3 are seen regularly in UK waters: the Leatherback, Loggerhead and Kemp’s Ridley turtles. However, Britain also has interests in overseas waters, such as the Caribbean, and so the work of the Marine Turtle Research Group of Exeter University, based at Tremough, encompasses the world’s oceans. For our speaker, Dr Matthew Witt, the principal study area has been the beaches of Gabon, West Africa. Secluded and little frequented, (although with oilfields offshore), these are the nesting grounds for the world’s largest population of Leatherback turtles. Hitherto this community has been one of the least studied, but by attaching transmitters to the turtles’ backs it has been possible to track their movements. The devices, which have a battery life of 180 days, transmit automatically to a satellite each time the animal surfaces. Combining the results for a 5-year period it is clear that the turtles from Gabon seek the food-rich areas along the west coast of Africa from Cape Town to the equator, then west along the Southern Equatorial Current, while some cross the relatively barren southern ocean to forage off south Brazil.
Turtles evolved from land reptiles which returned to the sea and had become fully adapted to the marine environment by the Cretaceous period (145 – 65 Ma), such that they are now cosmopolitan and occupy all marine habitats except polar. Belonging to the same taxonomic class as marine iguanas and sea snakes, they play key ecological roles, for example by having specialised diets or by grazing on sea grass. Hawksbill turtles eat mainly sponges, Leatherbacks eat jellyfish, while Loggerheads eat anything, such as fish discards, molluscs and crabs. Factors which cause their lives to be at risk include being caught in fishing nets and lines, degradation of breeding grounds and the harvesting of individuals and eggs. Turtles can live for 60 to 80 years. They are slow to mature, taking 20 to 25 years to reach adulthood, at which stage they migrate to their natal areas to breed. The male then returns to a foraging area while the female goes to the beach where it was born, scrapes a deep hole in the sand above high water mark, lays its eggs and then fills in the hole with sand. During adult life they breed every 1 to 4 years and in each nesting year multiple egg clutches are laid, perhaps 60 – 100 eggs every 2 weeks. After incubating for 50 to 70 days all the hatchlings from one clutch emerge together, at night, and rush for the sea, hoping to avoid gathered predators such as dogs, snakes, cats, lizards and birds. Their sex has been determined by the heat of the sand: temperatures above 260C favour the development of females, below that, males. Once in the sea, the hatchlings are distributed by coastal and ocean currents and begin their long period of maturing, although, because of natural predation, perhaps only one in a thousand will survive the 20 years to adulthood. By far the worst predator, however, is man, who kills turtles at their most vulnerable and most critical stage, when the females return to lay; and digs up their eggs.
Four species of marine turtle are rarely or never seen in UK waters. The Green turtle, which can be 4 ft (1.2 m) long and weigh 300-400 lb, is found around the globe within the tropics. It feeds on sea grass and has numerous nesting sites, such as Ascension Island, the Galapagos and NE Australia, but is in serious decline because of man’s exploitation for its flesh and eggs. Another tropical species is the Hawksbill, smaller than the Green turtle with a shell length of about 2 ft (60 cm). It frequents coral reefs where it feeds on sponges, crabs, molluscs and jellyfish. Its flesh is said to be unpalatable and may be poisonous, but the animal’s carapace is in demand as the source of “tortoiseshell”. The Olive Ridley turtle appears to be confined to the S. Atlantic. The Flatback is a rarity, found only in N. Australian waters.
The earliest record of a turtle in UK waters was in 1756, off Lands End. Since then, studies have shown that the principal distributor for Loggerhead and Kemp’s Ridley turtles is the North Atlantic Gyre, an ocean current which issues from the Gulf of Mexico and in a 7-year journey sweeps north past the Florida and Carolinas coasts (which contain the largest population of Loggerheads), then east towards Spain and back to the Gulf. A branch on the north side, the Gulf Stream, carries warm waters to Europe. These currents transport juvenile and immature turtles in their pelagic stages. Over the past 100 years there have been a few hundred UK sightings and strandings of Loggerheads, mostly 15-20 cm long but a few measuring more than 60 cm and perhaps representing a double circuit of the gyre. Full size animals can be 40 inches (1 m) long and weigh 300 lb. Sightings occurred throughout the year, but there were more in the winter months, which also saw a higher percentage of deaths, since smaller animals are more vulnerable to falling temperatures. The records for each decade since 1900 indicate very low numbers in the 1960s to 1980s, because of losses to fishing in the USA. Subsequently there has been a dramatic increase in UK Loggerheads, all recorded on the west and south coasts of Britain and Ireland.
The Kemp’s Ridley is a small turtle about 1 ft (30 cm) long which ranges from the eastern seaboard of the USA to W. Africa and W. Europe. Formerly numbering hundreds of thousands from its breeding ground off Rancho Nuevo in Mexico, by the late 1980s it was down to a few hundreds and heading for extinction. Its eggs had been taken for food and its skins for leather goods. Conservation and re-stocking have restored numbers to a few thousand. There have been several tens of sightings and strandings in UK waters over the last century, just a few each decade with none in the 1950s and 1980s. They occurred in winter months and most of the animals were 20-25 cm long, with a high proportion dead.
Leatherbacks are the largest of the sea turtles. They can grow to a length of 9 ft (2.7 m) with a flipper span of 9 ft and can weigh up to 1800 lb. Their carapace is made of hundreds of bony plates covered with leathery skin. All turtles are ectothermic, their body temperature controlled by the environment, but the huge body size of the Leatherback, combined with metabolic adaptations to conserve heat, gives it a wider global range than others into waters as cool as 100C. In the Atlantic it spreads from the tip of S. Africa to north of Scandinavia. Leatherbacks feed on jellyfish, salps and similar soft-bodied zooplankton and, being strong swimmers, they have no difficulty in following the areas of high food productivity as these move with the seasons. Satellites have tracked them from Venezuela to the Rockall Bank and from Trinidad and Tobago to the Flemish Cap region east of Newfoundland, thence to the Azores and to waters off Mauretania and Iberia. Unlike hard-shell turtles, which arrive in UK waters incidentally throughout the year in the Gulf Stream, Leatherbacks come with intent, seeking rich food areas during the warm summer months of June to October. More than 800 sightings and strandings have been recorded over the last century with a steeply increasing trend over the last 30 years. Predominantly they are on the west coasts of Britain and Ireland but some occur on the east coast.
Matthew made several points in summary. Increased sightings of the three species over the last few decades might be due to (a) better conservation, (b) climate change, (c) greater awareness. Climate change affects the distribution of jellyfish and hence Leatherbacks. Equally, over-fishing upsets the food chain allowing jellyfish numbers to increase. Hard-shell turtles are carried to the UK throughout the year by the Gulf Stream and suffer most mortality during winter months. Leatherbacks are seasonal, coming in the summer to eat. Sightings and strandings are concentrated on the west coasts of Britain and Ireland, but some Leatherbacks venture to the east coast. Most sightings in Cornwall are on the north coast and embayments appear to be important.
The HMCG is extremely grateful to Matthew for his informative and thoroughly fascinating talk and for amending the above report.