Similar numbers of female and male green and hawksbill turtles are hatching in the Coral Sea’s Conflict Islands, new research suggests, despite global heating increasingly leading to “extreme feminisation” of sea turtles.
Sea turtles are particularly susceptible to the effects of global heating because their sex determination is temperature dependent, with the proportion of female hatchlings increasing when nests are warmer.
Analysing two islands within Papua New Guinea’s Conflict Group, researchers estimated that between 1960 and 2019, an average of 46.2% of turtle hatchlings have been female – a finding scientists welcomed but described in the study as “likely rare in the global context”. Over the same period, sand temperatures have increased by about 0.6C.
In stark contrast, 2018 research found that at a similar latitude, on Raine Island in the northern Great Barrier Reef – the largest green turtle rookery in the world – warm nest temperatures had resulted in more than 99% of hatchlings being female.
“It’s likely that a male-producing site like the Conflict Islands is going to be critical to survival of the future nesting populations within the Coral Sea,” said Melissa Staines, the study’s first author and a PhD student at the University of Queensland.
Staines said that the proportion of female hatchlings had increased in recent years but it was likely that sites with balanced sex ratios were rare.
The researchers believe the discrepancy in turtle sex ratios between the Conflict Islands and Raine Island – which are about 850km apart in the Coral Sea – is likely because the island group has high rainfall and abundant shade, whereas Raine Island has no trees.
“All of the islands within the [Conflict Islands] atoll are heavily shaded by tropical rainforests, pandanus palms and coconut palm trees,” Staines said. “Providing full shade throughout the day across most of the islands can really significantly keep that temperature nice and cool.”
The study, published in the journal Global Change Biology, also suggests the islands “will also be moderately safe guarded against extreme feminisation in future decades”, but projects that over the next 80 years 76% to 87% of future hatchlings may be female on average.
“Female dominance in the next 80 years may still be quite adequate for that population,” said Christine Madden Hof, the global marine turtle conservation lead at the World Wide Fund for Nature (WWF) and a co-author of the paper.
“Climate change isn’t impacting this population at a threatening stage right now. We need to keep a mindful and a watchful eye on it, and it may need intervention in the future.”
The Conflict Islands study is part of a wider Asia Pacific-wide project focusing on how temperatures impact turtle populations. The team also has temperature gauges deployed in Malaysia, the Philippines, Vanuatu, Tonga and New Caledonia.
“It is really important to be able to identify where those male-producing populations are, because these are the ones that might really sustain some of our more regionally based populations,” Madden Hof said.
WWF-led research on a turtle nest cooling project has found that one-off applications of seawater can temporarily cool nests by 2C to yield more male hatchlings.
In research published last month in the journal Marine Ecology Progress Series, scientists found that applying different volumes of seawater in tests on Heron Island increased the proportions of male hatchlings between two- and fivefold.
Some seawater applications resulted in a small decrease in hatchling success, but at rates that “didn’t raise significant concern for us as scientists”, Madden Hof said.
Previous research has found that warming seas will probably threaten sea turtle survival even if the reptiles lay their eggs at cooler times of year to adapt to global heating.
Of the seven living marine turtle species, two are listed on the International Union for Conservation of Nature red list as critically endangered, one is endangered, and two are considered vulnerable.