Turning down the heat

A study trial on the temperature of sea turtle nests has revealed that shade cloth can effectively turn down the heat. 

The findings, collated following a trial study on Pennefather Beach in western Cape York Peninsula last year, found that a shaded nest had a maximum temperature difference of  6.58 degrees when compared to a control site with no shade.

“This is an encouraging result,” Cape York NRM Western Cape Turtle Threat Abatement Alliance (WCTTAA)  Coordinator Dr Manuela Fischer said. 

“It means we can now conduct a more long-term and comprehensive study across several beaches to record the effects of using shading to protect these nests from rising sand temperatures and the impacts this has on hatchlings.”

The initial trial was triggered by research that found rising nest temperatures were skewing the sex ratio of baby turtles, resulting in fewer male hatchlings. 

Temperature-dependent sex determination is particularly important in the endangered olive ridley sea turtle (Lepidochelys olivacea) species. 

The incubation temperature at which a 1:1 male-to-female sex ratio is produced - the so-called pivotal temperature - for olive ridleys is known to be about 30°. Temperatures higher than the pivotal temperature produce females and lower temperatures produce males.

“A concern is that sex ratios will become increasingly female-biased,” Manuela said. “It is possible that low numbers of males in the turtle population will reduce the fertilisation capacity and the loss of genetic variation, which therefore may lower the capacity to adapt to environmental changes.”

Funding from the Australian  Government’s Marine Turtle Climate Change Resilience and Nest Protection grant supported the trial study of 2023 and will continue to fund the follow-up research until March 2025. 

“With this money, we have been able to buy the necessary equipment to shade the nests and record the temperatures,” Manuela said. 

“Ultimately, the research will determine what impact climate change has on sea turtles and predict hatchling sex ratio, so we can implement appropriate management actions, such as lowering the temperature in the nests.”

The “we” in the research are the members of the WCTTAA, which is a group of six Indigenous Land and Sea Rangers on the Western Cape Peninsula (from Apudthama Land Trust and the Northern Peninsula Area Regional Council [NPARC], Aak Puul Ngantam [APN], and the Aboriginal Shire Councils of Napranum, Mapoon, Pormpuraaw, Aurukun and Kowanyama. They work to protect endangered sea turtle nests from feral animals, the impacts of marine debris, and human beach traffic.

Over the past 11 years, it has reduced turtle predation from up to 100 % to under 30%, through the use of purpose-built aluminium cages secured over the top of the nests at targeted rookeries, education campaigns on rubbish and human impact, and intense “turtle camps” during which nests are monitored and results recorded in a database for national research. 

The 2023 trial involved deploying temperature loggers at five nests on Pennefather Beach near Napranum. One logger was placed above the nest chamber,20 cm below the sand surface. The nest was protected by a cage and covered with secured shade cloth. Another logger was placed next to the nest at the same depth but not covered with shade cloth to act as a control site.  The loggers collected temperature data points every 30 min, for two months.

“On average the temperature in the nest was 1.82 degrees cooler in the shaded area, and the maximum difference we found was 6.58 degrees,” Manuela said.

“This was great news, as the method of shading was simple and effective.”

This year’s research will use  42 data loggers across at least three different beaches, mainly targeting olive ridley nests. Loggers will be dropped into the nest chamber when the female turtle is laying the eggs to minimise disturbance of the nest later. 

The loggers can collect data every 30 minutes for approximately one year.

Rangers will be involved in building cages, equipping cages with shade cloth and deploying cages and temperature data loggers on-site.

“It is going to be a busy season this year and I can’t wait to see what we discover,” Manuela said. 

The Turtle Climate Change Resilience project is funded by the Australian  Government’s Marine Turtle Climate Change Resilience and Nest Protection Grant