Turtles

Green turtle / Chelonia mydas/ Family - Cheloniidae

The highlight of a visit to the Great Barrier Reef may well be spotting a marine turtle. Of the seven species of marine turtles, six are found in Queensland's shallow coastal waters. Green, Hawksbill and Loggerhead turtles are the most frequently sighted but Flatback, Leatherback and Olive Ridley turtles are also occasionally seen.

All year round in the protected, shallow reef waters, divers and snorkellers can catch sight of juvenile and adult turtles swimming about or feeding on plants and small animals. Around October, some visitors might be lucky enough to see turtles mating in the calm, protected reef waters. The most common encounter however, is between November and February, when adult female turtles come out of the water to nest on land. Between mid-January and late April, turtle hatchlings might be seen emerging from their nests and rushing into the sea.

Where turtles breed

Turtles breed mostly in the northern and southern regions of the Great Barrier Reef. In the southern rookeries, marine turtles nest only in the summer months, but in the northern rookeries some nesting takes place all year round, with a summer peak.

Breeding behaviour

When adult male and female turtles come together to mate in the waters near nesting beaches, they do not feed at all. In fact, their feeding grounds might be as far away as Papua New Guinea, Solomon Islands, New Caledonia or Indonesia. Female turtles can mate with a number of different partners, storing the sperm for later use. After mating, male turtles return to their feeding grounds while the females stay in the area for nesting.

Several weeks after the first mating, female turtles go ashore to nest. They usually do this at night when the tide is high. They drag themselves high up the beach and dig a nest for their eggs. Loggerhead and green turtles lay about 100 round, white eggs about the size of ping-pong balls, and flatback turtles lay larger eggs in clutches of about fifty. No marine turtle species stays with or cares for its young.

Each female lays several clutches of eggs in the one season. This takes place about every two weeks during the breeding season and usually on the same beach. Two to eight years pass before a female turtle is ready to breed again. Each turtle returns to breed in the region where it was born.

When a turtle comes ashore to nest, the weight of her body and the action of her flippers and tail carve out tell-tale tracks in the sand. The tracks resemble one-metre wide tractor tracks. One-way tracks suggest that a turtle is in the process of egg-laying somewhere near the top of the beach.

Egg laying takes 1-3 hours from start to finish. First the turtle digs strongly with her front flippers, sending the loose sand flying everywhere. Then she positions her tail over the shallow nest and, very slowly and methodically, she uses her two hind flippers to carve out a vertical pear-shaped egg chamber. Shielding the hole with these flippers, she then lays her leathery-shelled eggs inside the warm, moist chamber. The hind flippers are used to cover the eggs with sand and flatten out the nest. When the work is all done the turtle returns to the sea.

The early years of life

The eggs incubate in the nest for 7-12 weeks. The temperature of the surrounding sand determines the sex of the offspring. Male hatchlings result from cool nests (25oC), while females are produced from warm nests (31oC). Both sexes may occur in nests at intermediate temperatures.

As a group, the hatchlings dig their way up through the 50 cm or more of sand to the surface. Then, generally in the early evening, the sand will erupt and the hatchlings will scurry down to the sea. Many hatchlings do not survive the early stages of life. Some are eaten on the beach by ghost crabs, dogs, feral pigs and foxes; others are taken in the coastal shallows by gulls, fish, sharks and crocodiles.

Feeding

Different species of turtles feed on different things. Green turtles eat algae and seagrass; loggerhead and olive ridley turtles eat shellfish and crustaceans; hawksbills eat algae and sponges; and flatbacks eat sea pens, sea cucumbers and soft corals. Leatherback turtles feed almost exclusively on jellyfish.

Turtle conservation

At all stages of its life, a marine turtle is vulnerable to predators. Immature and adult turtles can be attacked by large cod, groupers, sharks, crocodiles and killer whales. However, their most significant predators are humans. In many countries turtles are harvested for their meat, thick scales (tortoiseshell), leather and eggs. While they are protected in their breeding grounds of the Great Barrier Reef, they are regarded as a prize catch in neighbouring countries where they have their feeding grounds. Indigenous inhabitants of the Great Barrier Reef may be permitted to hunt turtles for food.

Globally, turtle populations are declining and will continue to do so unless there is international cooperation in their management. Here are some ways that you can help.

When travelling overseas, don't eat local dishes such as turtle soup and eggs. Reject turtle products such as tortoiseshell jewellery, turtle leather shoes and bags, cosmetics made from turtle oil, and turtle souvenirs.

When visiting the Great Barrier Reef, switch off unnecessary lights on beaches near turtle rookeries so that emerging hatchlings won't be confused.

Don't throw any rubbish into the sea. Turtles can become entangled in fishing gear and can easily mistake floating plastic bags for the jellyfish of their diet.

Discourage dogs, foxes and pigs from roaming free on mainland beaches. They destroy turtle nests and hatchlings.

Take care and reduce speed when boating in shallow waters where turtles may be feeding. Propeller damage to turtles is severe.

If you are trawling, fit a turtle excluder device, or TED, to your trawling gear. Turtles and other large animals that are accidentally trapped are quickly released from the net before they drown.

If you are an indigenous hunter, take turtles outside of the breeding season. The turtles should be less than 80 cm in shell length and try not to take adult females. Take only what you need.

Where and when to see turtles

The best places within the Great Barrier Reef area for visitors to observe turtles nesting are Mon Repos or Lady Elliott Island near Bundaberg, and Heron or Curtis Island near Gladstone. Rangers from the Queensland Parks and Wildlife Service (QPWS) and trained volunteers are often present to act as guides and to answer questions. QPWS also produces wildlife information sheets detailing other accessible breeding areas. Nesting turtles can usually be seen nightly from mid-November until late February. Seeing hatchlings emerge is most likely between mid-January to late March.

Turtle feeding can be viewed readily by scuba divers and snorkellers in the Capricorn-Bunker Group reefs. Turtles can also be viewed at Reef HQ, Townsville. Visitors to Brisbane can see the exciting, permanent display Living with our Marine Reptiles at the Queensland Museum. It realistically shows the life histories of these creatures and focuses on the processes that continue to threaten their population numbers.