The number of new corals on the Great Barrier Reef crashed by 89% after the climate change-induced mass bleaching of 2016 and 2017.

Scientists have measured how many adult corals survived along the length of the world’s largest reef system and how many new corals they produced in 2018 in the aftermath of severe heat stress and coral mortality.

The results, published in Nature, show not only a dramatic reduction in new coral recruitment compared with historic levels, but also a change in the types of coral species produced.

The paper’s lead author, coral scientist Terry Hughes from the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said the results paint an uncertain picture for the reef in years to come if further bleaching events occur before corals have time to sufficiently recover – which typically takes a decade.

Researcher collecting populated recruitment tiles at southern end of Great Barrier Reef Photograph: Margaux Hein/ARC CoE for Coral Reef Studies

“We’ve told the story of coral dying, we’ve told the story of some being winners and losers. Now we’ve got the next phase where species have a chance to recover,” Hughes said.

“But what we’re seeing is that it’s happening a lot slower because we only have 10% of the babies.”

Across the entire reef the scientists measured an 89% drop in new corals, but that average includes a small increase in new coral in the southern-most sections of the reef, which were less affected by bleaching.

The amount of baby coral declined by an even higher percentage in the northern sections.

Hughes said replenishment of corals around Lizard Island dropped 98% last year on what has been seen historically. He said it was now faring slightly better, but the rate of new coral growth is still only 4%.

The researchers also found that the mix of species that make up the pool of coral babies has shifted dramatically. Acropora, the branching and table corals that are the reef’s dominant species, declined by 93%.

On a healthy reef, acropora typically make up two thirds of the corals and provide the nooks and crannies for fish and other species.

“If you change the mix of babies, you change the mix that they grow up to be,” Hughes said.

“We’re not saying the Barrier Reef is doomed, but it is on a new trajectory. The way it’s connected, the mix of species, it’s all changing.”

The cause of the collapse in coral recruitment is the death of adult coral stock.

For it to recover, the reef will need more mature coral, something that Hughes says will not happen overnight.

“We’re not saying it’s a permanent crush. But I’m pretty damn sure it’s going to be slow,” he said.

“The main concern is it won’t be a sustained recovery because the timeline of it – a decade – is almost certainly going to include one or two future bleaching events.”

