Facts of marine turtles:
Marine turtles have lived in the oceans for over 100 million years. They are an integral part of the traditional culture of many coastal indigenous peoples throughout the world.
Marine turtles migrate long distances between their feeding grounds and nesting sites. They 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.
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.
Of the seven species of marine turtles in the world, six occur in Australian waters:
- Flatback turtle (Natator depressus)
- Green turtle (Chelonia mydas)
- Hawksbill turtle (Eretmochelys imbricata)
- Leatherback turtle (Dermochelys coriacea)
- Loggerhead turtle (Caretta caretta)
- Olive Ridley turtle (Lepidochelys olivacea)
In Australia, all species of marine turtles are protected under various State and Territory legislation and the Australian Government's Environment Protection and Biodiversity Conservation Act 1999.
Due to increasing threats to marine turtles, all the six species which occur in Australian waters are listed under the Australian Government's EPBC Act. The loggerhead and olive ridley turtle are listed as endangered under this Act which means that the species may become extinct if the threats to its survival continue. The green, leatherback, hawksbill and flatback turtles are listed as vulnerable which means that they may become endangered if threats continue. The Act identifies the need to prepare a recovery plan and specifies the content of the plan.
Marine turtles are recognised internationally as species of conservation concern. Six of the species found in Australia are listed in the 2000 IUCN (World Conservation Union) Red List of Threatened Animals.
All marine turtle species occurring in Australian waters are listed under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). In addition, all marine turtles occurring in the Indo-Pacific region are a priority for conservation under the Convention on the Conservation of Migratory Species of Wild Animals (the Bonn Convention or CMS).
- Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES)
- Convention on the Conservation of Migratory Species of Wild Animals (the Bonn Convention or CMS
- Indian Ocean South-East Asian Marine Turtle (IOSEA) web site
Biology of marine turtles:
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.
After reaching sexual maturity, marine turtles breed for several decades, although there may be intervals between breeding of two to seven years.
When breeding, nesting females return to the same area, thought to be in the region of their birth. As hatchlings, they become imprinted to the earth's magnetic field and, possibly, the smell of the waters adjacent to the nesting beach which allow them to successfully complete their migration.
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. They return to the same beach to lay consecutive clutches. 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.
While on the beach, fluid hangs from the turtle's eyes. This is a concentrated salt solution which helps to remove excess salt ingested by the turtle from drinking sea water. This solution also washes the eyes free of sand.
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.
Most hatchlings reach the sea although crabs and sea birds attack them on the beach. During their first few hours in the water, these young turtles face heavy predation by sharks and other fishes.
Research:
Detailed research, population modelling, and turtle tagging and tracking activities are being carried out in Australia and the Indo-Pacific region to assist with turtle conservation programs.
During summer at selected turtle breeding areas, each nesting marine turtle is tagged with a non-corrosive metal tag carrying an identification number. Later recaptures allow scientists from the conservation agencies and universities to monitor marine turtle populations, migration patterns, breeding activity, growth and mortality factors.
Genetic studies help to identify separate breeding populations and which population a particular turtle comes from, especially those captured in distant feeding grounds. They also provide information on the genetic variability within each population.
