For years, researchers in Australia have struggled to accurately count thousands of green sea turtles that come to Raine Island, the largest green turtle rookery in the world. Now, researchers have used drones and splotches of temporary white paint to get an accurate count of the endangered turtles and the results nearly double prior estimates, reports Amy Woodyatt for CNN. The drone footage used by the scientists provides stunning aerial views of an estimated 64,000 turtles circling the small cay waiting to lay their eggs.
The footage may show an astonishing congregation of sea turtles arriving by sea and burying their eggs in the sand, but not all is well at Raine Island. Despite the apparently tremendous numbers, the turtles’ rookery has not been producing that many hatchlings and many adult turtles have been dying on shore, according to the Raine Island Recovery Project.
A 2015 paper found that, between 2011 and 2015, the likelihood for a Raine Island egg successfully hatching was just 12-36 percent, compared to a typical success rate of more than 80 percent in the rest of the world. The research attributed much of the decline in hatchlings to rising sea levels, which now routinely flood the nesting beaches, potentially drowning the next generation of turtles. Climate change is predicted to continue increasing the global sea level, which is bad news for the turtles and the size of the island.
Raine Island is a remote coral cay situated at the northern edge of the Great Barrier Reef. The authors of the 2015 paper write that if greenhouse gas emissions continue to rise rapidly, that human-caused climate change could erase nearly 30 percent of Raine’s 79-acre area by 2100.
However, the authors of the 2015 paper note that saltwater inundation alone may not explain the massive decline in hatching success.
Up to 2,000 adult turtles also die on the island every year, according to the Recovery Project. Many of the nesting adults find themselves fatally flipped over after falling off the cay’s mini-cliffs or die of heat exhaustion after running into trouble on beach rocks.
The Recovery Project is attempting to remedy these issues by installing fencing to keep adult turtles from getting themselves into danger and by adding sand to the beaches to raise them high enough to avoid being inundated by seawater.
But to figure out if these measures are having a positive impact on the green sea turtle population, scientists need accurate population estimates.
Since 1984, estimates of the number of nesting turtles at Raine Island relied on human observers spotting turtles from boats. But researchers started to wonder if drones and underwater video might provide more accurate and cost effective estimates of the huge numbers of green turtles that nest at Raine Island each year.
To compare the methods, the team carried out counts using all three techniques, counting the turtles using drones, underwater video and observers on boats, according to the new paper which published this week in the journal PLOS ONE.
Counting via the old method involved marking the turtles’ shells with stripes of temporary, non-toxic white paint when they came ashore so the researchers could tell the ones who already nested apart from those which had yet to lay eggs. Researchers then counted the thousands of painted and unpainted turtles from boats to estimate their numbers. The drone and underwater video-based counting methods involved analyzing the footage frame by frame in the lab to count the turtles, according to a statement.
After comparing the three methods, researchers found the drone footage was the most efficient and effective counting method, according to the statement. The extraordinary footage, captured in December 2019, provided an estimate of up to 64,000 green turtles around the island, 1.73 times the figure produced by previous estimates, CNN reports.
The team attributed the underestimates of the old method to the fact that it’s easier for observers to spot the marked turtles than those without the white stripes, creating a bias in the count. Researchers say the results will be applied retroactively to adjust prior population estimates as well as to direct future conservation of the turtles.
“This research is of prime importance to the understanding and management of the vulnerable green turtle population,” says lead researcher Andrew Dunstan of the Queensland Department of Environment and Science in the statement. “In the future, we will be able to automate these counts from video footage using artificial intelligence so the computer does the counting for us.”