Scientists chart a 400-mile zone of coral devastation at the Great Barrier Reef

The results confirm an overall picture suggesting that the northern part of the reef has seen devastating losses. Photo / 123rf

Australian researchers who have been surveying the damage at the Great Barrier Reef from an unprecedented coral bleaching event earlier this year released a new map showing a large zone of intense coral death as well as several regions that escaped largely unscathed.

The results confirm an overall picture suggesting that the northern part of the reef, which scientists say was once its most "pristine," has seen devastating losses. This is a region extending some 700km northward from Port Douglas, the researchers say, and comprises about 30 per cent of the reef overall (it is 2300km in its entirety). And the researchers say it saw an average coral mortality of 67 per cent (the range was actually 47 to 83 per cent at disparate reefs that comprise the larger northern section of the Great Barrier Reef).

But the new analysis also contained some good news in that a lengthy offshore part of the northern reef appeared to fare better than its coastal side. Here, average coral death was only 26 per cent.

"Here we are, eight months later, and we're still measuring slowly accumulating levels of mortality," said Terry Hughes, who directs the ARC Centre of Excellence for Coral Reef Studies at James Cook University, which released the map.

"The map tells the story that the barrier reef has been turned into a giant jigsaw puzzle in terms of those four zones," Hughes said. He added that "there is a bit of a silver lining in terms of the southern half being okay, and there was also a northern region offshore that had significant mortality but not as high as the red region".

Indeed, coral mortality is at only 6 and 1 per cent in the central and southern parts of the reef, respectively. The reef's tourism industry is worth A$5 billion per year.

The original coral bleaching event occurred when an oceanic "heat wave" featuring temperatures 1 degree Celsius warmer than normal hit the reef in March, which is summer in the southern hemisphere. The warming was tied to the extremely strong global El Nino event of 2015-2016, but scientists also attributed the anomalous heat to climate change.

Coral bleaching occurs when corals, subjected to stresses like these warm temperatures, eject symbiotic algae that they rely upon to survive and then lose their colour, turning white. The Great Barrier Reef also bleached in 1998 and 2002, but not as extensively.