Sea turtle boom could hide population collapse

Around the world, many conservationists are celebrating the increasing growth of the sea turtle population. Cape Verde, in West Africa, now has 100 times more loggerhead turtle nests each year than in 2008. But scientists warn that this apparent success could hide a looming population collapse.

Using drone surveys and 15 years of nesting data, scientists at Queen Mary University of London report that increasing numbers of nests could be misleading. As global warming causes temperatures to rise, more and more sea turtle eggs develop into females. Without enough males, even a seemingly prosperous population can collapse. Their findings, which still need to be peer-reviewed, were published Jan. 20 on bioRxiv.org.

“We think it’s a mirage,” says Christophe Eizaguirre, a conservation geneticist at Queen Mary University. Conservation efforts, such as marine protected areas, fishing regulations, and protection of egg clutches, help populations, but may not give the complete picture.

As with some other reptiles, such as crocodiles, the sex of a sea turtle is depending on the incubation temperature of the egg. Eggs that develop in warmer sand become females, while cooler conditions create males. “The way we describe it in my lab is ‘hot girls and cool guys,'” says Jeanette Wyneken, a biologist at Florida Atlantic University in Boca Raton, who was not involved in the study.

Using drones, scientists photographed the breeding population and found a female-to-male ratio of 9 to 1. “We can tell females from males by the tail,” says biologist Fitra Arya Dwi Nugraha, also from Queen Mary University. “Males have longer and thicker tails.”

A temperature of around 29 degrees Celsius is thought to produce a roughly equal distribution between male and female newborns. “We don’t know what a perfect population should look like,” Eizaguirre says.

The authors hypothesize that the extreme trend in favor of female loggerheads (Caretta caretta) who then lay eggs inflates the number of nests, making the population appear healthier than it is. Without enough males to support reproduction, population growth could quickly disappear, although it is difficult to predict when this critical point might arrive.

Wyneken is concerned about the methods used in the study, particularly regarding the determination of the ratio. Although adult males can be identified by drone, it is more difficult to tell with subadult individuals, which can be comparable in size to females. “It’s possible that they consider some immature males to be females,” she says. “The 9-1 may be more asymmetric than normal if it has this error.” It is more accurate, she says, to confirm the sex of newborns through laparoscopic surgery.

Turtles have evolved strategies to protect themselves against female bias: males may mate more frequently than females, females store sperm to maximize the number of clutches of eggs they can fertilize, and both sexes mate with multiple partners. Some conservation initiatives move eggs to hatcheries to give them the best chance of survival by protecting them from predators, poachers and environmental threats. If something goes wrong, “you put all your eggs in one basket,” Wyneken says.

Having too many women in global populations would be a concern, scientists agree. “You expect more women,” Wyneken says, but “seasons where we have 100 percent women, over and over again, or 98 percent women? That’s not sustainable.” These significantly skewed populations are already appearing in some warmer regions, such as the northern beaches of the Great Barrier Reef.

The authors hope their work will prevent people from ending conservation initiatives thinking they have worked. “That’s probably not the case,” Eizaguirre says. Ongoing, adaptive conservation efforts are essential to give turtle populations time to respond to changes in their environment. “What we really don’t want is for the effort to stop.”