Rising temperatures in the Great Barrier Reef aren’t just leading to coral bleaching – the sea turtle population is experiencing a significant impact as well.

The warmer climate has turned 99 percent of sea turtle hatchlings female, according to a study published in Current Biology.

Sea turtles have temperature-dependent sex determination (TSD), meaning the sex of an individual turtle is determined by the incubation temperature during embryonic development. In short, warmer temperatures produce females, while more moderate temperatures result in males.

In Australia, two genetically distinct breeding populations of green sea turtles are found at opposite ends of the Great Barrier Reef. Raine Island, located in the northern portion of the Great Barrier Reef, is home to one of the largest green turtle populations in the world, with an estimated female population size in excess of 200,000 nesting females, according to the study.

Raine Island has also been affected by rising ocean and shore temperatures moreso than the group of turtle nesting islands further south, which have remained cooler in temperature.

The researchers found that the northern Great Barrier Reef nesting beaches were extremely female-biased, with 99.1 percent of juvenile sea turtles being female. Additionally, 99.8 percent of “young adult” and 86.8 percent of adult-sized turtles that hatched from the same beaches were female.

The findings show that the northern population of turtles have been producing primarily females for more than 20 years.

But at the cooler nesting beaches in the southern Great Barrier Reef, the effects weren’t as dramatic. About 65 to 69 percent of the turtles originating from the cooler beaches were female.

“Combining our results with temperature data show that the northern GBR green turtle rookeries have been producing primarily females for more than two decades and that the complete feminization of this population is possible in the near future,” wrote the researchers

To conduct the study, the researchers had to solve a major challenge – determining the sex of hundreds of sea turtles in an easy, non-invasive way.

Identifying a sea turtle’s sex via physical observation can only be done after it has reached maturity. Therefore, the team took blood samples and analyzed hormone levels to determine the sex of the turtles. They also used genetic markers and mixed-stock analysis (MSA) to link turtles foraging in the Great Barrier Reef to the nesting beach that they hatched from. This is the first time this type of analysis has been done with Great Barrier Reef turtles, according to the researchers.

Female sea turtles can lay about 100 eggs at a time, and can repeat the process several times over the course of one nesting season. The eggs incubate for about 55 days. Once hatched, the babies enter the sea to forage and mature, but then return to the nesting beach to produce a new class of offspring.

Raine Island is nearly 750 miles north of the southern group of nesting islands, which are located near Brisbane. If temperatures continue to rise in upcoming years, the risk of female-only offspring production in the Great Barrier Reef will also elevate.

“Although increased breeding frequency, as well as polygynous behavior of male turtles, may help mitigate skewing offspring sex ratio, it is unknown how many (or what minimum proportion of) males is sufficient to sustain sea turtle populations,” writes the team.

The findings also highlight concerns about sea turtle populations in other parts of the world that don’t exhibit such a “strength in numbers” as the Great Barrier Reef turtle communities.

“Our study highlights the need for immediate management strategies aimed at lowering incubation temperatures at key rookeries to boost the ability of local turtle populations to adapt to the changing environment and avoid a population collapse—or even extinction.”