Research has shown that even some of the most stress-resistant WA coral is susceptible to bleaching.

Researchers have examined the impact of a 2016 mass bleaching event on reefs in Western Australia.

They found significant bleaching occurred in the inshore Kimberley region, despite Kimberley corals being known as some of the hardiest. They also found mild bleaching at Rottnest Island, but that the Ningaloo Reef escaped bleaching.

The 2016 mass bleaching event was the most severe global bleaching event ever recorded, but with climate extremes becoming more frequent, this record may not stand for long.

Coral bleaching is triggered by a range of abnormal environmental conditions, including heightened sea temperatures and acidity levels.

When coral gets uncomfortable, it expels tiny photosynthetic algae called zooxanthella that normally exists in a symbiotic relationship with the coral animal.

These algae give coral their colour, so when they are missing the coral turns white, known as ‘bleaching’.

Bleached corals can recover if their environment improves and they allow zooxanthellae to recolonise.

The research team – featuring experts from the University of Western Australia, ARC Centre of Excellence for Coral Reef Studies, and Western Australian Marine Science Institution - was led by UWA’s Dr Verena Schoepf and masters student Morane Le Nohaïc.

They conducted surveys on the health of coral reefs along the Western Australian coastline from tropical to temperate locations.

“We found a concerning 57 to 80 per cent of corals on inshore Kimberley reefs were bleached in April 2016 – this included Montgomery Reef, Australia’s largest inshore reef,” Dr Schoepf said.

“Our research also found that there was mild bleaching at Rottnest Island – 29 per cent of corals were moderately bleached.

“Ningaloo Reef, a UNESCO World Heritage site, escaped bleaching, but had some temperature-unrelated coral mortality. Temperate corals at Bremer Bay (Southwest) experienced no bleaching.”

Dr Schoepf said bleaching patterns were consistent with patterns of heat stress across WA.

“This is the first documented regional-scale bleaching event in WA during an El Nino year and the first time we have been able to measure the percentage of impacted corals in 2016,” she said.

“Coral reefs in WA are now at risk of bleaching during both El Nino years, such as in 2016, and La Nina years, such as 2010/11. But the geographic footprint differs – the northwest is at risk during El Nino years, whereas Ningaloo Reef and reefs further south are at risk during the La Nina cycle.

“As bleaching events become more common in the future, it is critical to monitor how bleaching events impact coral reef resilience, and how long it takes reefs to recover from such catastrophic events.”

The study is accessible here.