Bleaching kills 35 per cent of corals in Great Barrier Reef

Mass bleaching has killed 35 per cent of corals on northern and central sections of Australia's iconic Great Barrier Reef, scientists said today, with the current event at the World Heritage Site much more extreme than measured before.
After months of intensive aerial and underwater surveys, researchers from the ARC Centre of Excellence for Coral Reef Studies have released an initial estimate of the death toll from coral bleaching.
The impact, which is still unfolding, changes dramatically from north to south along the 2,300km length of the Reef, researchers said.
"We found on average, that 35 per cent of the corals are now dead or dying on 84 reefs that we surveyed along the northern and central sections of the Great Barrier Reef, between Townsville and Papua New Guinea," said Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University (JCU).

"This year is the third time in 18 years that the Great Barrier Reef has experienced mass bleaching due to global warming, and the current event is much more extreme than we have measured before.
"These three events have all occurred while global temperatures have risen by just one degrees Celsius above the pre-industrial period. We're rapidly running out of time to reduce greenhouse gas emissions," said Hughes.

Coral bleaching occurs when abnormal environmental conditions, like heightened sea temperatures, cause corals to expel tiny photosynthetic algae, called 'zooxanthellae'. Loss of these algae causes the corals to turn white, and 'bleach'.

Bleached corals can recover if the temperature drops and zooxanthellae are able to recolonise them, otherwise the coral may die.
"Fortunately, on reefs south of Cairns, our underwater surveys are also revealing that more than 95 per cent of the corals have survived, and we expect these more mildly bleached corals to regain their normal colour over the next few months," said Dr Mia Hoogenboom, also from JCU.
Although substantially fewer corals have died to the south, the stress from bleaching is likely to temporarily slow down their reproduction and growth rates.
According to the scientists, the reefs further south have escaped damage because water temperatures there were closer to the normal summer conditions.

"It is critically important now to bolster the resilience of the Reef, and to maximise its natural capacity to recover," says Professor John Pandolfi from the ARC Centre of Excellence for Coral Reef Studies at The University of Queensland.
"But the reef is no longer as resilient as it once was, and it's struggling to cope with three bleaching events in just 18 years," said Pandolfi.
"Many coastal reefs in particular are now severely degraded," he said.
The recovery of coral cover is expected to take a decade or longer, but it will take much longer to regain the largest and oldest corals that have died.