Warmer beaches threaten WA’s flatback turtle

Beaches that are nesting grounds for Western Australia’s impressive flatback turtle are perilously close to temperatures which interfere with hatchling survival.

Dr Malindi Gammon, from the University of Western Australia’s School of Biological Sciences, says the flatbacks (Natator depressus) are very vulnerable to climate change because their eggs are laid in underground nests, and the temperature of the sand is critical.

“Previous studies have shown a decline in emergence success from nests with sand temperatures above 31.5°C,” Gammon says. “The sex of a hatchling is also determined by the nest temperature with mostly females produced from warmer nests.”

In the latest study of the Pilbara sites in WA published in Global Ecology and Conservation, most active nesting beaches had a low ‘thermal risk’ but a handful of beaches in the north-easterly region had very high exposure threat, including Cemetery Beach in Port Hedland, where average sand temperatures are already over 32°C.

“Under the recent climate, emergence [hatching] success averaged 76% but varied between sites,” Gammon tells Cosmos. “Modelled hatching success exceeded 80% at the coolest site, yet reached only 50% at the warmest.

“Sand temperature is the major driver of hatching success, and hatching success markedly reduces when incubation temperatures exceed about 33-35°C.

“Under a future climate scenario where average air temperatures are 2°C warmer than the present, 63 percent of eggs are expected to successfully hatch. With 4°C of warming, the number of eggs that are expected to successfully hatch falls to just 37 percent. Almost all hatchlings in these scenarios will be female.”

Flatbacks are listed as “Vulnerable” under the Australian Commonwealth’s Endangered Species Protection Act.

Unlike other sea turtle species that lay 100-200 eggs per nest, this species lays an average of 50 per nest. Their eggs and hatchlings however, are proportionally larger than other species, which may aid hatchlings in evading predators.

Gammon says systematic monitoring of flatback turtles has occurred for the last two decades in WA but it’s hard to know exactly how big the population is.

“Due to the remote location of the populations, their complex life history and the long-lived nature of the species, monitoring has not yet been conducted long enough to make accurate estimates of the total population in Western Australia.”

One estimate in 2020 suggested about 27,000 female flatbacks nest every year in WA and around 17,000 nest at North West Shelf nesting beaches.

Gammon points out that hatching success also depends on other factors including the location of a nest on the beach, and that nests below the high-tide can be periodically flooded, leading to clutch failure.

“The likelihood that a hatchling survives to adulthood is extremely low,” says Gammon.

“Hatchlings are very difficult to track through their lifecycle, so survival rates are hard to accurately quantify but estimates range from 1 in 1,000 to 1 in 10,000 hatchlings surviving to adulthood.

“If the sand is too dry the eggs can dry-out and do not hatch. Even if an egg hatches and a hatchling successfully leaves the nest, on the beach hatchlings are predated by sea gulls, crabs, and feral cats and foxes.

“Once in the water, they are predated by fish and seabirds.”

The research was partly funded by the WA Department of Biodiversity, Conservation and Attractions. It’s web site says flatback turtles are unique in that they only nest on mainland and island beaches in Australia, and mostly remain within the waters of the continental shelf.

There are seven distinct genetic stocks, including the Pilbara stock which is the focus of the present study.

Strategies for managing or mitigating the effects of climate change are difficult to implement.

Sea turtles have a behaviour called ‘natal homing’ where females return near to where they hatched, to lay their eggs. Gammon says this behaviour limits their ability to relocate nesting sites across large distances .

“While there is the potential for sea turtle populations to expand the boundaries of their nesting range, often towards cooler beaches, range expansion is a slow process, initiated by wandering females or those that nest opportunistically in other areas close to their foraging grounds,” she says.

“Further, the likelihood of range expansion can be greatly reduced due to other human impacts, such as coastal and industrial development, on otherwise suitable nesting beach locations”

“Colonisation of new habitats is what allowed sea turtles, with their 100 million years of evolutionary history, to survive previous climatic changes similar in magnitude to today.

“However current climate change is occurring at a much faster rate and sea turtles may not be able to adapt fast enough.”

“Climate change is one of several anthropogenic threats to sea turtles globally. Other significant threats include habitat loss from coastal and industrial development, ingestion and entanglement in plastics, and predation,” Gammon says.

She says local communities around turtle playgrounds have a role to play, including as individuals.

Protection could include strategies to minimise coastal and industrial development, such as legal protection at the State and National level for beaches deemed the least vulnerable to climate change; efforts to eradicate feral cats and foxes at rookeries; and the reduction of artificial lighting impacts along the coastline.

There might also be physical actions communities can take in the region to help the turtles.

“It may be necessary to mitigate the impacts of climate change at some of the warmest beaches. Some approaches which have been proven to reliably cool nest temperatures include seawater irrigation and shade,” says Gammon.

“Seawater irrigation requires applying water directly to the nest at a key period during embryonic development to cool the incubation temperature of embryos, increasing male production and hatching success.

“Nest shading through artificial structures also works to decrease incubation temperatures, balancing sex ratios and increasing hatching success.

“Crucially, cooler nesting sites which will continue to support male production under future warmer climates will become increasingly important and protecting these beaches against additional anthropogenic impacts (such as coastal development and artificial light pollution) should be a priority.

“These mitigation strategies are resource intensive and may be hard to implement on remote rookeries, including the many island beaches used for nesting by flatback turtles in the Pilbara.”