Alarming green turtle baby boom an all-female affair

By Tracey Ferrier, AAP 16 November 2021
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The world’s largest green turtle rookery, near the tip of Australia, is only churning out girls but scientists are working on a plan to bring the boys back.

On a small island near the tip of Australia, a warped kind of baby boom is about to begin.

Virtually every green turtle hatchling that will emerge this season from thousands of nests on remote Raine Island will be female.

The alarming gender imbalance affecting the world’s largest green turtle rookery was first reported to the scientific community in 2018.

But it’s clear the phenomenon began decades earlier because green turtles from the northern Great Barrier Reef, from juveniles to adults, are now predominantly female.

Researchers have linked the imbalance to global warming. Nests on Raine Island simply get too hot at the critical point of incubation, when sex is determined, to produce males.

It’s a dire problem but scientists attached to WWF-Australia’s Turtle Cooling Project believe they know how to resolve it.

Earlier this year, the team shared the results of a small, controlled study involving genetically distinct but similar green turtles from the southern part of the Great Barrier Reef.

On Heron Island, researchers were able to show that sea water could be used to cool nests without significantly harming embryos by drying out eggs too much.

The results could be a game changer because turtles like to nest in remote locations and many, like Raine Island, do not have trees to provide shade cover or fresh water for nest-cooling purposes.

The next step in the complicated mission to get northern green turtles back into gender balance will be a larger seawater irrigation trial on Raine Island.

WWF scientist Caitlin Smith leads the turtle cooling project. She says it won’t happen in time to influence the gender mix of this year’s hatchlings, which will dig their way out between early December and April.

But she’s hopeful it will be place for the 2022/23 hatching season and by then scientists should have another piece of the complex puzzle – how many males does a green turtle population actually need?

Too few is a problem, but so is too many. That’s where University of Queensland PhD candidate Melissa Staines comes in.

She spent a good chunk of this year flying drones over mating green turtles in the southern part of the Great Barrier Reef, where the population is increasing and there are no concerns about gender distortion.

Heron Island was chosen for the drone project because its sand temperatures are relatively cool, and it’s a popular courtship area for amorous turtles.

“By analysing drone videos, we will be able to determine the ratio of breeding males to females in a healthy and growing population,” Ms Staines says.

The work will help inform the seawater irrigation to be done on Raine Island, ensuring nests are managed to produce the right number of males and females.

The results will also provide a useful benchmark for rookery managers elsewhere around the world, including in the Asia Pacific region where gender imbalances have been observed.

“Rookery managers will be able to establish if a breeding population is feminising, prompting action such as sea water irrigation and shaded hatcheries to increase male hatchling production,” Ms Staines says.

The Turtle Cooling Project is a partnership involving WWF-Australia, the University of Queensland and the Conflict Islands Conservation Initiative, with funding support from furniture company Koala.