Mike Emslie, the program director at AIMS, said while there were some encouraging signs of recovery picked up in the survey, the long-term trends of the reef’s health remain mostly downward.
“We’re now seeing climate-induced disruptions that are not only becoming more frequent but are affecting a greater area of the Great Barrier Reef,” Dr Emslie said.
While corals can rebound from events such as cyclones and even bleaching, it can take years to decades. “The reef is not getting that time,” he said.
Corals rely on algae known as zooxanthellae to provide the bulk of their energy and much of their vibrant colour. When exposed to sustained abnormal heat, corals begin to expel the algae, leading to mass bleaching, with many of them dying.
As global heating pushes up background temperatures, it no longer takes a strong El Nino year –when tropical temperatures tend to be particularly warm – to push corals beyond their tolerance levels.
Compared with the highest levels of hard coral cover during the 35 years of surveys, the northern section of the Great Barrier Reef had lost about half and the central and south about one-third, prior to the last bleaching event, Dr Emslie said.
One positive finding was the number and size of coral trout have continued to increase in marine reserves in which fishing is prohibited. The total trout biomass was 122 per cent greater inside the reserves compared with reefs open to fishing, the report noted.
Terry Hughes, the director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said “the reef is really struggling to show any meaningful recovery [since the previous bleaching bouts] and now it’s bleached again”.
“We’re all anticipating a drop in coral cover in the south” once next season’s survey is done, Professor Hughes said.
He noted Thursday’s announcement by Deb Frecklington, the head of Queensland’s Liberal National Party, that her party would roll back native vegetation controls if elected in October’s state election to encourage jobs and investment for farmers.
“We had already seen only limited progress in meeting the water quality targets Reef 2050 Plan,” Professor Hughes said.
Allowing farmers to clear more land would likely increase soil and nutrient run-off into the reef region, potentially providing another pulse for the crown of thorns starfish that are also a threat to corals.
“The coastal reef water quality is important for the capacity of corals to rebound from a bleaching event,” he said.
Federal Environment Minister Sussan Ley said the government was “very conscious of the work that needs to be done and which is being done under the $2.7 billion Reef 2050 plan including crown of thorns star fish control, water quality improvements, our increased on water surveillance activities and partnerships with traditional owners and reef communities”.
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.