By Peter Hannam
Ancient corals that have survived centuries of storms off Australia's most cyclone-prone coast have been killed off by two severe marine heatwaves, researchers have found.
Large reefs of staghorn and table corals in a remote region off Western Australia's Pilbara coastline stretching from the World Heritage-listed Ningaloo Marine Park to the Montebello Islands and the Dampier Archipelago died or were severely damaged by unusual heat events in 2010-11 and 2012-13.
"Probably half the total area there has seen quite severe bleaching of some or all of the corals over the past couple of years," said Russ Babcock, CSIRO's lead scientist for the joint survey with the University of Western Australia.
Earlier research had shown some of the coral species were more than 400 years old.
"They are very robust and that's how they can live to be so old," Dr Babcock said. "Very few environmental events can affect them, let alone kill them."
Separate research suggests the reefs may have been hit hard by the first heatwave, leaving them more susceptible when the second one arrived.
Ming Feng, a CSIRO oceanographer, said water temperatures off the WA coast and the western Pacific were now "much higher than in previous decades". The 2010-11 marine heatwave, caused by warm tropical water being drawn much further south than usual, had lifted sea temperatures 5 degrees above normal at their peak.
While the first of the two heatwaves was linked to a powerful La Nina event in the Pacific - the second strongest on record - the background global warming meant corals and other marine species were being made more vulnerable, Dr Feng said.
"As we continue to get warmer, any moderate La Nina event will cause a similar kind of heatwave effect, compared with a more severe event in the past," he said.
The bleached coral discoveries came as the CSIRO and the UWA began research on the region as part of a $12 million, five-year survey project partly funded by environmental offsets from Chevron's $60 billion Gorgon Liquefied Natural Gas project.
Researchers plan to study the health and changes in the ecosystem, including why some reefs to the north managed to survive the heatwaves largely intact.
The work "will tell us which parts of the region are going to be most resilient to these kinds of events", Dr Babcock said.
Very few environmental events can affect them, let alone kill them.
The results will also help authorities when they consider future gas or other industrial projects in the region. "We urgently need to know this information to plan for the next development," he said.
The ''unprecedented survey will inform conservation planning into the future'' and ''allow government and industry to balance the environmental sustainability of the Pilbara with increasing use", WA Environment Minister Albert Jacob said. "The survey and research work will provide a scientific basis for government decision-making in protecting the environment, while supporting the energy and tourism industries that are so important to this region.''