Green Turtle

Green Turtle - ancient traveller of global seas

(photographs by Leen Charle Christiaens, Mnemba Island)

Marine turtles are related to tortoises (which are strictly terrestrial) and terrapins (which live in fresh water) with seven species occurring in warmer seas around the world. The Green Turtle is the largest of the hard-shelled turtles which are characterised by having their vertebrae and ribs fused to the underside of a bony-plated carapace. Adults usually attain a weight of 140 kilograms and measure about 1 metre in length, but some specimens assume greater proportions.

Green Turtles occur in tropical and subtropical waters around the globe, with nesting beaches in 80 different countries from Costa Rica, Senegal and Tanzania, to Malaysia, Australia and Peru. The species has suffered a major population crash in the past 100 years and is now regarded as 'endangered globally' according to the IUCN's Red Data List. Estimates indicate that the world population of Green Turtles has been reduced by about two thirds over the past three generations of these long-lived creatures.

Ocean Wanderers

Green Turtles are highly migratory and undertake complex movements and migrations over thousands of kilometres. Hatchlings leave the beach on which their mother nested and swim far offshore to drift in major current systems. Here, among beds of floating sargassum seaweed, they feed on marine invertebrates such as immature crustaceans, macro-plankton and algae. The developing turtles live in the deep ocean for at least twenty years until they reach sexual maturity. At this stage, their diet has changed into a strictly herbivorous one. Adults of both sexes congregate in shallow waters where their favoured seagrasses are abundant, to court and mate close to nesting beaches. Remarkably, Green Turtles return to the very beach on which they were born, having the extraordinary capacity to remember the precise locality of a birth-site they left as infants at least twenty years earlier.

From egg to infant

Adult females emerge from the surf at night to excavate a burrow, lay her clutch of eggs, bury them with sand, and drag her heavy-body back into the sea. Deep sand at the highest point of the beach where vegetation begins, is usually chosen. This is a laborious and tiring procedure for the turtle which uses its large paddle-shaped front flippers to great effect. Females often lay three clutches of between 100 and 115 eggs per season, but as many as nine clutches have been recorded. The turtles do not mate every year, however. An incubation period of approximately two months passes before the hatchlings emerge, unaided by their mother which plays no parental role. After digging their way through the sand, the little turtles scamper to the surf, sometimes running a gauntlet of crabs and other predators. In common with crocodiles and some other reptiles, the sex of the baby turtles is determined by the length of incubation and the nest temperature, with females arising from longer periods and warmer nest sites. The real challenge for the hatchling turtles comes when they must swim frenetically for 24 hours or more to get beyond the strong offshore currents. Large fish are among the abundant predators. For those little turtles which survive this ordeal, and make it out into the deep ocean, the cycle of life begins again.

Threats from Mankind

The greatest threat facing Green Turtles and the reason for their population collapse in all but a few parts of the world, is direct human predation. Eggs are harvested from nesting beaches, and the egg-laying females are killed when they are most vulnerable. Harvesting of eggs remains legal in many countries and in those where it is outlawed, it is difficult to police. Other factors which impact on marine turtles are habitat degradation at nesting beaches, and incidental by-catch by commercial fisheries.

Sanctuary at Mnemba

In Tanzania and other East African countries, Green Turtles have few safe nesting sites. At Mnemba Island, north-east of Zanzibar, CC Africa staff have been protecting and monitoring Green Turtle nest sites since 1996, and the surrounding reefs and shallow seas where the adults mate and feed have now been officially proclaimed as the Mnemba Island Marine Conservation Area. Guests at Mnemba Island Lodge have the chance to witness the ancient ritual of a female Green Turtle depositing the next generation of her kind in the pearly white beach.

- Duncan Butchart -

Posted: Other by CC Africa, Date: 21 November 2006