The aftermath of the Great Barrier Reefs worst coral bleaching event ever recorded has been revealed by scientists. Pictures and video taken after the extreme underwater heatwave of 2015/2016 show the large-scale devastation, particularly in the northern third of the Reef.

Researchers from James Cook University took the footage and images in March and April – the height of the bleaching event. They have since returned to the 83 reefs to assess the damage and recovery. Their results show many of the reefs that did not die during the bleaching event have succumbed to disease.

Greg Torda whose team recently returned from re-surveying the reefs, said: Millions of corals in the north of the Great Barrier Reef died quickly from heat stress in March and since then, many more have died more slowly.

Six months after the peak bleaching, the corals now have either regained their algal symbionts and survived, or they have slowly starved to death without the nutrition the algae provide to them. On the reefs we surveyed close to Lizard Island, the amount of live coral covering the reef has fallen from around 40% in March, to under 5% now.

Andrew Hoey, who is currently working from Lizard Island Research Station, added: In March, we measured a lot of heavily bleached branching corals that were still alive, but we didnt see many survivors this week. On top of that, snails that eat live coral are congregating on the survivors, and the weakened corals are more prone to disease. A lot of the survivors are in poor shape.

Their survey showed coral reefs in the southern half of the Great Barrier Reef were not as badly affected and remain in good condition. Andrew Baird, who led the re-surveys in the central part of the Great Barrier Reef, said: There is still close to 40% coral cover at most reefs in the central Great Barrier Reef, and the corals that were moderately bleached last summer have nearly all regained their normal colour.

The survey results follow widespread media attention to an obituary about the Great Barrier Reef, the result of which led many to think it had been declared dead. This is not the case, but the latest survey results highlight the danger it is in. The final death toll of the bleaching event is expected after all the surveys are carried out in mid-November.