Warming seas will likely threaten sea turtle survival even if the animals lay their eggs at cooler times of year to adapt to climate change, new research suggests.
Modelling by Australian scientists at Deakin University has found that marine turtles are unlikely to be able to change their nesting behaviour enough to mitigate the effects of higher sea surface temperatures.
Analysing 58 nesting sites globally that cover all seven marine turtle species, the researchers suggest that if the reptiles laid their eggs earlier to avoid hotter beaches, any shift would at best mitigate against 55% of a 1.5C increase in sea temperatures.
The sex ratio of turtle hatchlings is closely determined by the temperature of the nest in which the eggs are incubated, with warmer nests yielding more females. Higher temperatures have also been linked to lower hatchling success rates.
In a “doomsday scenario”, said study co-author Prof Graeme Hays, “there may be only females being produced from certain populations, which obviously would then lead to population extinction”.
Previous research has linked warmer seas to overwhelmingly female green sea turtle populations in parts of the Great Barrier Reef. A 2018 study found that females accounted for 99.1% of juvenile turtles originating from nesting beaches in the north of the reef.
Hays emphasised the new research wasn’t “all doom and gloom”, but highlighted the potential need for conservation strategies to cool the temperatures at nesting beaches.
“There’s lots of good news going on around the world with sea turtles,” he said. “In a number of sites around the world, populations are going up; conservation is working really successfully.
“With the climate warming feminisation [research], what we’re trying to do now is to get the information so that we can prevent problems down the line … [so] if turtles do need help down the track, we’re not caught off-guard.”
Sea turtles tend to nest in the area they hatched from, a behaviour known as natal philopatry. It means, the researchers suggest, that they “cannot readily change their range to accommodate warming temperatures”.
The study, published in the journal Royal Society Open Science, used a projected 1.5C increase in sea temperatures, “as this warming is very likely to happen before the end of the century”.
It was also based on existing research showing that warming temperatures can result in earlier nesting. A study on loggerhead turtles in the Mediterranean had previously found that in extreme cases turtles laid eggs about 18 days earlier when waters warmed by 1C.
Christine Madden Hof, the global marine turtle conservation lead at the World Wide Fund for Nature (WWF), said: “We need more data to understand how the different species of marine turtles in different localities around the world are going to fare in the face of climate change.
“Where we know those populations aren’t going to fare well, we should be really seriously considering scientifically based [interventions].”
WWF is a collaborator on trials that have successfully used seawater and freshwater to artificially cool turtle nests.
Increasing shade cover on beaches using marquees or by planting vegetation has also been suggested as an approach that could improve sex ratios. It would require care to ensure that nesting habitats were not disrupted, Hays said.
“You’re playing god when you put an intervention in place,” Hof said. “It might be that a really cool rainy year or number of years might produce enough males to sustain the population, so we don’t want to intervene when it’s not necessary.”
WWF is also currently investigating how many male turtles are needed at a minimum to sustain populations.
Of the seven living marine turtle species, six are included on the International Union for Conservation of Nature red list of threatened species.
