We knew Ningaloo’s coral bleaching was severe. But what we found 6 months later was still a shock

The heatwave in Western Australia last summer broke records. As marine scientists, we were deeply concerned about whether Ningaloo’s corals would survive. We were prepared for the worst, but what my colleague and I found when we returned to assess the damage months later was still a shock.

It was the longest-lasting, largest and most intense underwater heatwave ever recorded in WA. And it hammered the coral at Ningaloo Reef, part of the World Heritage-listed Nyinggulu or Ningaloo Coast, in the northwest of the state. (The word Ningaloo comes from the Aboriginal name Nyinggulu, which means a promontory or headland).

For coral, sustained heat stress is measured using degree heating weeks (DHW). At 4 DHW, coral bleaching is likely. At 8 DHW, many corals are at risk of bleaching and dying. At 20 DHW 80% mortality is predicted. At Ningaloo Reef last summer, more than 20 DHW were recorded.

In October, we returned and found that two in every three corals in the shallow lagoonal areas of the northern Nyinggulu Reef, including popular tourist sites such as Turquoise Bay, had died.

How it happened

In March, in the middle of the bleaching period, our surveys of corals in the northern Ningaloo Reef lagoon showed up to 90% of the coral had bleached. Bleaching doesn’t automatically mean death, but in a heatwave this severe we feared a large dieback of coral.

In partnership with the Minderoo Exmouth Research Laboratory, my PhD student David Juszkiewicz and I returned to Ningaloo to resurvey the reef in October, 6 months after we last surveyed it mid-bleaching. We revisited eight sites spanning a 40 kilometre section of the northern lagoon, from the Osprey Sanctuary Zone to Tantabiddi Sanctuary Zone.

Within this area, we found between 52% and 71% of corals had died. Of the more than 1,600 individual corals counted and identified in March, only about 600 remained alive by the end of October. Most of the corals that were bleached in March did not survive.

Superheated ocean

Globally, the ocean is the hottest it has ever been, and has been increasing in temperature year on year for the past decade. The 2024–25 heatwave was not normal, nor part of a natural cycle.

Such a prolonged heating event – where the entire water column down to 300 metres deep was superheated – has never been recorded in Australia. For corals, which have a relatively narrow range of thermal tolerance limits, the consequences have been dire.

Our finding of an average of 61% coral mortality in the lagoon habitats of Ningaloo Reef is confronting, but fortunately slightly lower than the predicted 80% mortality rate.

Which coral survived?

Some resilient species, such as Veron’s tube coral (Echinopora ashmorensis) and lesser knob coral (Cyphastrea microphthalma), have persisted, occasionally in high abundance. But populations of other previously dominant species such as the staghorn corals (Acropora tenuis, Acropora millepora, Acropora spicifera) and thin birdsnest coral (Seriatopora hystrix) have been decimated.

These types of branching corals punch above their weight in providing a habitat for marine animals. Their intricate network of branches offer space and shelter for a range of fish, crustaceans, molluscs, worms, tiny echinoderms such as starfish and more.

When these corals die, their branches quickly become infested with organisms like sponges, and the surfaces become overgrown with algae. Together, these new inhabitants erode and eventually flatten the coral skeleton, severely diminishing its value as habitat.

Not only has this heatwave had a direct impact on the coral at Ningaloo Reef, but it also likely had an indirect effect on the fauna that lives within the coral. So far, no data is available to quantify or substantiate this.