Third rare La Niña event in a row could lead to more coral bleaching along WA's Ningaloo Reef, scientists say

Marine scientists are worried confirmation of another La Niña weather event this year will lead to more coral bleaching along the north-west coast of Western Australia over summer.

While there are some signs of recovery, researchers have confirmed significant bleaching events along the World Heritage-listed Ningaloo Reef — including Turquoise Bay and Coral Bay.

Research scientists James Gilmour and Luke Thomas from the Australian Institute of Marine Science are monitoring sections of the Marine Park for coral bleaching.

"The common thread is climate change. Oceans are warming, and no reef is really immune," Dr Thomas said.

Further south, scientists have detected between 20–30 per cent coral bleaching in Turquoise Bay.

But there are signs of recovery in the wake of last summer's marine heat wave.

Marine Park coordinator Sallyann Gudge from the Department of Biodiversity Conservation and Attractions (DBCA) has been regularly monitoring the area.

"There's a couple of corals that didn't recover to the full extent, but there's definitely growth there, and we are seeing some baby corals and recruits, so that's a really great sign as well," she said.

A double whammy for the reef

Warm ocean temperatures and light winds created a double impact on WA's Ningaloo Coast earlier this year.

A marine heatwave caused bleaching at Bundegi and Turquoise Bay, while an unrelated coral spawning event at the same time led to a mass fish kill at Coral Bay in April.

Ms Gudge said the mass fish kill occurred due to oxygen depletion in shallow waters.

"The corals' eggs and reproductive cells come to the surface and cause a slick, and usually that just gets dispersed in the ocean," she said.

"But this year, the way the weather was, it actually just pulled it into Bill's Bay, and it stayed there over the course of a week.

"And those conditions caused the oxygen to be depleted out of the water.

"Unfortunately, the corals there got stressed and bleached also."

Not the first time

The DBCA say since they began monitoring the area, Bill's Bay has experienced fish kill and coral bleaching due to anoxic events (absence of oxygen) in 1978, 1989, 2002, 2005, 2008, 2019 and 2022.

And they say marine heat wave events have caused coral bleaching at the Ningaloo previously, particularly in 2011 and 2013.

While there are signs of recovery, scientists are concerned this third La Niña weather event in a row, confirmed by the Bureau of Meteorology, will again lead to higher ocean temperatures and light winds along the north-west coast this summer.

Dr Gilmour said mass bleaching occurred once the temperature hit two degrees above normal for several weeks.

However, he said, a bleached coral wasn't necessarily a dead coral.

"It can be any kind of stress, but it's usually warm water where corals expel or spit out the tiny algal cells that live inside their tissue. And the algal cells really provide a food source to the corals," he said.

"It's called bleaching because you can see the white skeleton through the translucent tissue.

"But if severe heat persists for many weeks, then the coral will most certainly die."

Take care of our reef

As ocean temperatures continue to rise, Dr Thomas says they are using DNA technology to help find heat-resilient coral at the Ningaloo Reef and learn from it.

"[We are] trying to focus on how temperature tolerance varies across the reef," Dr Thomas said.

"What are the mechanisms driving this variation? Can we harness this natural variation?

"Can we find pockets of heat-tolerant corals?"

Coral is already renowned for being fragile, but with sections of the reef in recovery mode, the DBCA is urging people to take care while visiting the reef.

"Please don't stand on the coral. Don't touch the coral. And look and enjoy, but don't take," Ms Gudge said.

For now, the DBCA and research scientists say they will continue to monitor the Ningaloo Reef this summer.