93 per cent of Great Barrier Reef hit by coral bleaching

Australia's iconic Great Barrier Reef is suffering its worst coral bleaching in recorded history with 93 per cent of the World Heritage site affected due to rising water temperatures, scientists said today.
Researchers unveiled the full extent of the coral bleaching that is unfolding on the world's largest coral reef system, off the coast of Queensland.
The final results of extensive aerial and underwater surveys show that 93 per cent of the reef has been affected. It is a mixed picture of very severe, moderate and little damage that changes dramatically from north to south along the 2,300km length of the Reef, researchers said.

On the west coast of Australia, researchers are also discovering large-scale bleaching caused by elevated temperatures on both sides of the Australian continent.
"We've never seen anything like this scale of bleaching before. In the northern Great Barrier Reef, it's like 10 cyclones have come ashore all at once," said Professor Terry Hughes, convenor of the National Coral Bleaching Taskforce that is documenting and studying the event.
"Towards the southern end, most of the reefs have minor to moderate bleaching and should soon recover," said Hughes.

"We have now flown over 911 individual reefs in a helicopter and light plane, to map out the extent and severity of bleaching along the full 2,300km length of the Great Barrier Reef. Of all the reefs we surveyed, only 7 per cent (68 reefs) have escaped bleaching entirely.

"At the other end of the spectrum, between 60 and 100 per cent of corals are severely bleached on 316 reefs, nearly all in the northern half of the Reef," he said.
Underwater, teams of scientific divers have confirmed the accuracy of the aerial surveys, and are continuing to measure the ongoing impact of the bleaching, researchers said.
"The bleaching is extreme in the 1,000km region north of Port Douglas all the way up to the northern Torres Strait between Australia and Papua New Guinea," said Andrew Baird from the ARC Centre of Excellence for Coral Reef Studies.
"Tragically, this is the most remote part of the Reef, and its remoteness has protected it from most human pressures: but not climate change.

"North of Port Douglas, we're already measuring average of close to 50 per cent mortality of bleached corals. At some reefs, the final death toll is likely to exceed 90 per cent.
"When bleaching is this severe it affects almost all coral species, including old, slow-growing corals that once lost will take decades or longer to return," said Baird.
More modest bleaching is now reaching its peak in a 6,00km central band of the Great Barrier Reef, between Cairns and Mackay.
Mass coral bleaching occurs when heightened sea temperatures due to global warming cause corals to expel tiny photosynthetic algae, called 'zooxanthellae'.

The loss of these colourful algae causes the corals to turn translucent and white, and 'bleach'. Mildly bleached corals can recover if the temperature drops and zooxanthellae are able to recolonise them, otherwise the coral may die.