Engineering a better future for Raine Island Green Turtles

During my childhood, a favourite bedtime story was The Story of Sarli, the Barrier Reef Turtle, a captivatingly illustrated book romanticising a turtle’s journey just off our shores. Fast forward a halfcentury and Sarli’s life story rewritten today would be more fraught, never more so than at the headquarters for Green Turtles on the northern Great Barrier Reef at Raine Island.

Turtle hotspot

Raine Island is a vegetated coral cay sitting on the outer edge of the Great Barrier Reef, some 600km NNW of Cairns. In addition to being a significant seabird rookery, it is the world’s largest remaining nesting population of the endangered Green Turtle. Around 40,000 females lay eggs on Raine Island every year, with a record 20,000 mother turtles competing for a nesting space on a single night.

In the 1990s, lower nesting success and higher mortality of adults were observed on Raine Island. These worrying trends spawned an ambitious conservation project to give the turtles a helping hand. The Raine Island Recovery Project (RIRP) began in 2015 as a partnership between governments, scientists, industry and indigenous owners. Its mission was to protect and restore the island’s critical habitat to ensure the future of Green Turtles as well as seabirds.

Nesting nightmare

Researchers discovered that the nesting conditions for Green Turtles on Raine Island had become unfavourable. A change in the beach profile, as well as a reduction in suitable habitat available for nesting led to mother turtles struggling to find a spot to nest and disturbing previously laid clutches of eggs. Further, inundation of nesting areas during high tides made the sand unsuitable for incubating the eggs.

“Operation Sand Dune” was initiated to replenish sand to the nesting areas on the upper level of the beach. Subsequent reprofiling of the beach increased the sand depth and raised nesting areas above tidal influence. This engineering work required heavy machinery and the operations were closely monitored given Raine Island’s status as a scientific national park. Ultimately, about 40,000m3 of sand was relocated resulting in a doubling of suitable habitat for turtle nesting. The reprofiled areas have remained stable for five years, improving the chances for more and more offspring to be born on the beach in the future.

Fateful falls

In seeking a free spot to nest, turtles were venturing into the centre of the island to lay. On their return to the sea in the dark early morning, the exhausted mothers were becoming entrapped by, or falling down the metre-high phosphate rock ‘cliff’ that separates the beach from the island’s central platform. Ending belly up, the weary turtles were unable to right their 150kg torsos and inevitably perished. Each nesting season, this death trap claimed up to 2000 turtles.

The RIRP tackled the problem of beach mortality in two practical ways. Firstly, as a reactive approach, stranded nesting turtles are assisted to reach the water. However, Raine Island is uninhabited, and rangers or scientists are only infrequently present. A more proactive approach was needed, consisting of building a low fence along the cavernous clifftop to prevent falls. On their return to the sea after laying, tired mother turtles are prevented from falling by the safety barrier. They navigate along the fence line to exit from where they arrived the previous night, by sandy corridors where the treacherous rock walls are absent.

To date, 1.7km of turtle fencing has been installed and 700 stranded turtles rescued. Regular monitoring has established that these two interventions are reducing the number of females lost to beach accidents. Given that each mother lays a clutch of about 100 eggs up to six times in a single breeding season, minimising beach deaths will contribute in the longer term to the sustainability of the turtle rookery.

“It takes a planet to raise a green turtle”

Now in its ninth year, the success of the RIRP would give a modern-day Sarli a better chance of laying a healthy clutch of eggs and returning safely to the ocean. Indeed, it is estimated that 640,000 additional hatchlings have emerged from nests on Raine Island, with an extra 4.6 million baby turtles expected over the next 10 years.

Alas, for the tiny hatchlings, the lottery of life has only just begun. Clambering out of the nest, they must run the gauntlet of the waiting predators on the beach and the shallows of the reef flat. In fact, only about seven per cent survive to swim safely offshore.

While conservation projects such as the RIRP are making a difference to successful nesting locally, the future of Sarli’s descendants still looks bleak. Female Green Turtles reach sexual maturity at 25-35 years old when they migrate back to their natal beach for nesting. In those intervening decades, they can spend significant time in other territorial waters and open seas where turtle protection safeguards can be quite limited. Actually, only one in a thousand hatchlings that reach the sea survives to mate.

Furthermore, the sex of a turtle hatchling is determined by nest temperature with warmer sand temperatures resulting in more females being born. Last year, it was estimated that 99 per cent of the hatchlings on Raine Island were born female. Such skewing of the gender ratio is an invisible impact of environmental change and feminisation of turtle populations threatens the longevity of the species.

In addition to the existential threats in the sea where turtles spend most of their lives, their short but crucial nesting phase on land at their natal beach makes them highly vulnerable to changing environments. Ground-breaking ventures like the RIRP are playing their part to secure the future of Green Turtles.