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New research shows seawater irrigation could help reverse male sea turtle drought

Co-Authored by Hayley Versace

Climate change is causing the “feminisation” of green turtle populations in far north Queensland, but a new study shows seawater irrigation could potentially reverse the male drought.

The research, published in Marine Ecology Progress Series, reveals that a single application of seawater can theoretically cool a sea turtle’s nest long enough to create male hatchlings.

In 2018, scientists revealed that more than 99% of green turtles being born in the northern Great Barrier Reef are female.

The incubation temperature of eggs determines the sex of sea turtles with a warmer nest resulting in more females.

Scientists blame rising temperatures, caused by climate change, for the lack of male hatchling production prompting fears for the future of one of the largest green turtle populations in the world.

That led to the Turtle Cooling Project, a partnership between the World Wide Fund for Nature-Australia, The University of Queensland, and the Conflict Islands Conservation Initiative with funding support from furniture company Koala.

Erecting shade structures, and freshwater or seawater irrigation, are possible ways to cool turtle nests to generate some males.

But on remote islands closer to the equator – and Raine Island where most northern Great Barrier Reef green turtles nest – building shade structures is not always practical, and freshwater is in short supply. That leaves seawater irrigation, however, too much salt can fatally dehydrate eggs.

Previous laboratory studies found the period when hatchlings turn either male or female is when the embryo is between 33% and 66% developed and it was assumed that the temperature had to remain low throughout this time to produce male hatchlings.

However, recent Turtle Cooling Project work led by University of Queensland’s Ellen Porter, discovered that some male hatchlings can be produced if the temperature drops for as little as 3 days when embryos are between 40% and 50% developed.

This suggests the window of time needed to change sex is much shorter than previously thought. It indicates males can be created by cooling a nest for days instead of weeks, which makes such an intervention easier to carry out.

WWF-Australia Marine Species Conservation Project Officer Caitlin Smith, lead author of the paper published today, conducted research on Heron Island in the 2019-20 nesting season to determine if seawater could cool nests without harming embryos.

Nests were given a one-off application of either 50 litres of seawater or 50 litres of freshwater, equal to about 100-120mm of rain. Control nests received no irrigation.

Both the seawater and freshwater-treated nests saw an immediate 1.3°C drop in temperature which lasted four days, while control nest temperatures remained constant.

In freshwater-treated nests, 83.8% of eggs successfully hatched, 71.2% in seawater-treated nests, and 63.5% in control nests.

Seawater irrigation was also trialled in the Conflict Islands, Papua New Guinea. Sand temperatures in this region cooled by about 2°C when seawater irrigation was applied.

“Our results are really promising. A one-off seawater application cooled nests, for potentially long enough to create males, and did not significantly impact embryonic survival. Despite our concern about dehydration, the seawater-treated nests were actually more successful than the control nests,” said Ms Smith.

“Seawater now needs to be trialled on a larger scale at other rookeries, like Raine Island, that are in critical need of male production.

“Because of human-caused climate change, northern Great Barrier Reef green turtles need help but there’s still time to save them,” she said.

Co-author Hayley Versace, operations manager for the Conflict Island Conservation Initiative, said: "Using seawater irrigation in remote regions, such as the Conflict Islands, is a simple yet proactive hands-on approach that can help people like us in remote locations secure the future of the nesting turtle populations where fresh water and other resources are limited".

Co-author Dr David Booth, School of Biological Sciences, University of Queensland, said there are concerns that climate change is happening too rapidly for turtles to adapt by nesting further south or at a different time of year.

“If we reach a situation where no males are being produced then the northern Great Barrier Reef population of green turtles will go into a rapid decline.

“Seawater irrigation is worth further investigation as a way to ensure at least some males are being produced and buy turtles some time.

“The most important action to increase male sea turtle production is to stop rises in global temperature,” Dr Booth said.

Mitchell Taylor, the founder and CEO of Koala, said the company had been working with WWF-Australia since 2017 and wanted to support innovative research to help endangered green sea turtles.

“We’re excited to be a supporter of this project and to raise awareness about feminisation. As a business with purpose it’s important for us to support groundbreaking projects like this to ensure that these majestic creatures have a strong future,” Mr Taylor said.

“Our mission is to help people and our planet thrive. When the opportunity came up to assist another threatened species - in addition to the iconic koala - we were moved to do what we could to help.

“I’m thankful that we have been able to donate $1.5 million to date to WWF-Australia and that these funds have been put to such great use,” he said.

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