Great Barrier Reef: Rising Temperatures Turning Green Sea Turtles Female

The Guardian

‘Complete feminisation’ of northern population is possible in near future, researchers find
Females in a Great Barrier Reef population of green sea turtles were found to make up 99.1% of juveniles, 99.8% of subadults and 86.8% of adults. Photograph: Christine Hof / WWF-Aus
Rising temperatures are turning almost all green sea turtles in a Great Barrier Reef population female, new research has found.
The scientific paper warned the skewed ratio could threaten the population’s future.
Sea turtles are among species with temperature dependent sex-determination and the proportion of female hatchlings increases when nests are in warmer sands.
Tuesday’s paper, from the National Oceanic and Atmospheric Administration, California State University and Worldwide Fund for Nature Australia, is published in Current Biology. It examined two genetically distinct populations of turtles on the reef, finding the northern group of about 200,000 animals was overwhelmingly female.
While the southern population was 65%-69% female, females in the northern group accounted for 99.1% of juveniles, 99.8% of subadults and 86.8% of adults.
“Combining our results with temperature data show that the northern GBR green turtle rookeries have been producing primarily females for more than two decades and that the complete feminisation of this population is possible in the near future,” the paper said.
The temperature at which the turtles will produce male or female hatchlings is heritable, the paper said, but tipped to produce 100% male or 100% female hatchlings within a range of just a few degrees.
“Furthermore, extreme incubation temperatures not only produce female-only hatchlings but also cause high mortality of developing clutches,” it said. “With warming global temperatures and most sea turtle populations naturally producing offspring above the pivotal temperature, it is clear that climate change poses a serious threat to the persistence of these populations.”
The lead author, Dr Michael Jensen from the National Oceanic and Atmospheric Administration, said the research provided a new understanding of what the turtle populations were dealing with.
Green sea turtles are among species with temperature dependent sex-determination. Photograph: Alamy
He said the findings were surprising and “a bit alarming”, with significant conservation implications.
“While we can hope there might be some cooler years to produce a few more males, overall we can expect the temperatures to increase,” he said.
Jensen said the researchers worked around “ethical implications” of past studies that required sacrificing some hatchlings to accurately determine sex ratios and pivotal temperature ranges.
This team instead studied more than 400 turtles at foraging grounds, gathering information on the sex of turtles from multiple generations.
“Knowing what the sex ratios in the adult breeding population are today, and what they might look like five, 10 and 20 years from now when these young turtles grow up and become adults, is going to be incredibly valuable,” Jensen said.
The research was facilitated through the Great Barrier Reef Rivers to Reef to Turtles project by the World Wildlife Fund Australia.
The chief executive of WWF Australia, Dermot O’Gorman, said it was yet another sign of the impact of climate change, following recent research that coral bleaching events were occurring far more frequently.
“We’ve had two years where we’ve had mass bleaching events on the Great Barrier Reef,” he told Guardian Australia. “That’s a very visible sign of the impact of climate change. But this is an invisible change. We can’t see the impact it’s having on a turtle population until a study like this shows some long-term trends.”
O’Gorman said more urgent action on climate change was clearly needed but conservationists were taking some practical measures, including trialling the use of shadecloth on known nesting beaches to lower the sand temperature, and reducing bycatch in the fishing industry.
“[Shadecloths] can be done in certain places but there’s a limit to the scale you can do that,” he said.
The green turtle is one of the most populous species of turtle in the world but the Great Barrier Reef settlement was significant and turtles were under enormous pressure outside Australian waters, O’Gorman said.
“An additional threat to them really does sound alarm bells,” he said. “Now every large reproductive male is going to be even more important.”


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