The aftermath of the Great Barrier Reef's 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 didn't 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.

coral bleaching great barrier reef
A healthy Rib Reef (near Townsville) at risk from Crown-of-thorns, October 2016Tane Sinclair-Taylor
coral bleaching great barrier reef
Crown-of-thorns attached to healthy coral. Photo taken between Mackay and Townsville, October 2016Tane Sinclair-Taylor
coral bleaching great barrier reef
Dead corals still provide habitat to fish but will soon crumble away. Yonge reef (Lizard Island region), October 2016Greg Torda
coral bleaching great barrier reef
Extensive bleaching of Acropora corals on the reef crest of North Direction Island April 13 2016Andrew Hoey
coral bleaching great barrier reef
Extreme mortality of bleached corals on the reef crest of North Direction Island, October 19 2016Andrew Hoey
coral bleaching great barrier reef
Scientists survey Rib Reef, off Townsville where coral has escaped bleaching damageJames Cook University
coral bleaching great barrier reef
Maxwell Reef after multiple disturbances, two cyclones and a severe bleaching event. (Lizard Island region), October 2016Greg Torda
coral bleaching great barrier reef
Reef fish that do not directly depend on live corals are still around – but how long before the dead skeletons erode and deprive them of their habitat? No Name reef (Lizard Island region), October 2016Greg Torda
coral bleaching great barrier reef
Surviving corals now experience increased pressure from predators like Drupella snails. Yonge reef (Lizard Island region), October 2016.Greg Torda
coral bleaching great barrier reef
This giant clam used to sit in a colourful field of corals before March 2016 – now she is alone on the reef slope. No Name reef (Lizard Island region), October 2016Greg Torda
coral bleaching great barrier reef
Typical partial mortality of a coral colony. The top half died of heat and light stress, the bottom part survived and is regrowing. Yonge reef (Lizard Island region), October 2016Greg Torda