Spawning by dominant coral species on the Great Barrier Reef collapsed after the back-to-back mass bleaching events, stoking fresh fears about the reefs' biodiversity and resilience.

Not only did the amount of new baby corals crash - falling as much as 95 per cent in some parts of the north - the composition also shifted, according to research published Thursday in Nature.

With fewer adult corals after bleaching, spawning levels collapse, new research shows. Credit:Chris Simpson

The researchers examined adult stock and larval recruitment at 15 sites scattered along the world's biggest network of reefs in 2018, and compared the levels before the unprecedented consecutive mass coral bleaching events in the two previous summers.

“We were shocked at the extent of the decrease," said Terry Hughes, the paper's lead author. Recruitment fell by an average of 89 per cent, with the north and central regions worse hit.