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The Great Barrier Reef is home to six of the world’s seven turtle species; the Loggerhead, Green, Hawksbill, Olive Ridley, Leatherback and the endemic Flatback, which is only found in waters off Northern Australia.
Turtles have been in our ocean since the time of the dinosaurs, over 150 million years ago. They can live up to 100 years old and are air-breathing reptiles, meaning they must come to the surface every few hours to breathe. They also come ashore to lay eggs.
You can tell the sex of a Green Sea Turtle once they reach sexual maturity by looking at the tail of the turtle. Males have substantially longer tails housing the reproductive organs, sometimes extending past the hind flippers. Female tails usually don’t extend more than 10cm. If you get close enough to the underside of a turtle you can also make out their ‘Plastron Scutes’ (the segments on their under-belly). Males tend to have more concave Plastrons to make it easier to lie on the female shell during reproduction.
When female turtles are ready to lay nests, they return to the same beach from which they were born, up to 30 years earlier. They crawl ashore to dig an egg chamber 30-60 centimetres vertically deep into the sand to lay the clutch of eggs, which can comprise up to 120 eggs. As this is the only maternal care she provides her offspring, she spends a lot of time digging the perfect nest and burying the eggs, sometimes taking all night. The exhausted female then returns to the ocean.
During incubation, lasting up to 12 weeks, the temperature of the sand determines the sex of the hatchlings. A nest of 29.5 degrees will produce an even split of males and females, slightly cooler temperatures produces males, and nests hotter than 30 degrees will produce all females. On a full moon the hatchlings begin to emerge. Currently 99% of all Green Turtles born on the Northern Great Barrier Reef are female.
Using the light of the moon upon the ocean’s horizon, the little hatchlings begin their dash for the sea. During this short run, the hatchlings are besieged by birds, dogs, crabs and other predators out for an easy meal. Upon reaching the sea, the hatchlings embark on a swimming frenzy for the outer sea where they hide under seaweed and other flotsam, drifting on currents for the next 10 years or more. Once their shell is 20-40 centimetres in length, they return to inshore reefs to feed. They remain in these areas until they reach sexual maturity and the cycle begins again.
All turtle species within Australia are classified as endangered or vulnerable by the Australian Environment Protection and Biodiversity Conservation Act 1999. Scientists estimate that only 1 in 1,000 hatchlings will survive to reach sexual maturity, due to natural and human threats. Human influenced threats include climate change, marine debris, poaching of nests and boat strikes.
There are two breeding stocks of Green Turtles within the Great Barrier Reef, a northern stock and a southern stock. The northern subpopulation breed on Raine Island and Moulter Cay, while the southern subpopulation nest in the Capricorn/Bunker group of islands. Loggerhead turtles also nest in the Southern Great Barrier Reef Marine Park near the Capricorn/Bunker group of islands and off Bundaberg.
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