We always hear people exclaiming about their wonderful encounters with a dolphins, and the feeling that they seem to sense you, watch you and enjoy you as much as you enjoy them. Well the same can be said about the turtle. Their gracefulness, sometimes playfulness and curiosity make them a much sought after sight on the Great Barrier Reef.
Turtles have a large shell called a carapace, four strong, paddle-like flippers and like all reptiles, lungs for breathing air. The characteristic beak-like mouth is used to shear or crush food. Young marine turtles drift and feed in the open ocean. When they are about dinner plate size, turtles settle near inshore feeding grounds. Marine turtles grow slowly and take between 30 and 50 years to reach sexual maturity. They live for years in the one place before they are ready to make the long breeding migration of up to 3000 kilometres from the feeding grounds to nesting beaches.
Of the seven species of marine turtles in the world, six occur in Australian waters, these include:
- Loggerhead turtle,
- Green turtle
- Hawksbill turtle
- Leatherback turtle
- Olive Ridley turtle
- Flatback turtle
Marine turtles have lived in the oceans for over 100 million years and are an integral part of the traditional culture of many coastal indigenous peoples throughout the world.
All marine turtle species are experiencing serious threats to their survival. The main threats are pollution and changes to important turtle habitats, especially coral reefs, seagrass beds, mangrove forests and nesting beaches. Other threats include accidental drowning in fishing gear, over-harvesting of turtles and eggs, and predation of eggs and hatchlings by foxes, feral pigs, dogs and goannas.
There are only a few large nesting populations of the green, hawksbill and loggerhead turtles left in the world. Australia has some of the largest marine turtle nesting areas in the Indo-Pacific region and has the only nesting populations of the Flatback turtle.
In Australia, all species of marine turtles are protected under various State and Territory legislation and the Commonwealth's National Parks and Wildlife Conservation Act 1975. Due to increasing threats to marine turtles, five of the six species which occur in Australian waters are listed under the Commonwealth's Endangered Species Protection Act 1992 (ESP Act).
The sea turtle has nested on parts of Queensland's coast for thousands of years. To watch a sea turtle nest is to gain an insight into one of nature's rituals. After reaching sexual maturity, marine turtles breed for several decades, although there may be intervals between breeding of two to seven years. Courtship and mating take place in shallow waters near the nesting beach. Females often mate with more than one male. After mating, the males return to the feeding grounds.
Between nesting efforts, female turtles gather adjacent to the nesting beaches. Each breeding season, nesting females return to the same area, thought to be in the region of their birth. A female green turtle usually lays six clutches of eggs at two weekly intervals. When ready to lay eggs, the female turtle crawls out of the sea to above the high water mark, usually about one hour before to about two hours after the night high tide. In preparation for nesting, the female turtle scrapes away loose sand with all four flippers to form a body pit. She then excavates a vertical pear-shaped egg chamber with the hind flippers.
Often, the sand is unsuitable for nesting, especially if it is too dry, and the turtle moves on to another site. For most turtle species, digging the nest takes about 45 minutes. It then takes another 10 to 20 minutes to lay the clutch of leathery shelled eggs. Each clutch contains about 100 white, spherical, "ping-pong" ball sized eggs. After laying, the turtle fills the egg chamber with sand using the hind flippers, and then fills the body pit using all four flippers. The turtle finally crawls back to sea, entering the surf about one to two hours after emerging. Green turtles may take longer to nest.
Incubation time and sex of the hatchlings depend on the temperature of the sand. Warm, dark sand produces mostly females and the eggs hatch in seven to eight weeks. Eggs laid in cool, white sand mostly result in males and the eggs take longer to hatch.
The hatchlings then take a few days to dig their way through the sand to the surface. When leaving the nest, usually at night, hatchlings head for the low elevation horizon of the ocean. Hatchlings can be easily disoriented and attracted to bright lights such as street and house lights. This contributes to many hatchling deaths as does crabs and sea bird attacks. However most hatchlings at least reach the sea and then during their first few hours in the water, they are also faced with heavy predation by sharks and other fishes. All in all the hatchlings have a low chance of survival, with perhaps only one in 1000 possibly reaching maturity.
Marine Turtles and Indigenous Culture
Marine turtles have important cultural and social values for Aborigines and Torres Strait Islanders living in coastal areas of northern Australia. They are an essential food item for some of the island communities in the Torres Strait where there are few other sources of fresh red meat. Eggs of marine turtles are also an important source of protein. Torres Strait Islanders have one of the highest seafood consumption rates in the world.
Green turtles are hunted more regularly than the other species. Hawksbill turtles are rarely hunted because they can be poisonous or unpleasant to eat. Most turtles are taken in the later part of the year, during the breeding season. In a few communities, marine turtles are taken in large numbers for traditional feasting. Harvested turtles and eggs are shared equally among relatives and friends of the hunters. Turtle oil is used as a medicine or tonic.