The Great Barrier Reef's corals have been halved over the past 25 years, with scientists identifying climate change and bleaching as culprits.
They say the reduction in the reef's size has compromised its ability to recover and have urged immediate action to sharply decrease greenhouse gas emissions.
Their new study shows the reef's number of small, medium and large corals has declined by more than 50 per cent since the mid-1990s.
While coral cover declines have been heavily publicised, natural shifts in the size of colonies are rarely measured over large distances.
Lead author Andy Dietzel, of ARC Centre of Excellence for Coral Reef Studies, said their broader study was important to grasp the corals' demography and reproduction capacity.
The researchers assessed coral communities and their colony sizes along the 2300km length of the reef from 1995 to 2017.
They found coral colony abundances on reef crests and slopes have declined sharply across all sizes and types compared to historical baselines.
Declines were observed in both shallow and deep water and were especially pronounced along the reef's northern and central regions.
"These were the worst affected by record-breaking temperatures that triggered mass bleaching in 2016 and 2017," co-author Professor Terry Hughes said in a statement.
The southern part of the reef was also exposed to record-breaking temperatures in early 2020 and the experts say climate change is driving an increase in reef disturbances such as marine heatwaves.
"There is no time to lose - we must sharply decrease greenhouse gas emissions ASAP," the authors said.
The decline isn't just hurting the reef, with lost branching and table-shaped corals leaving less habitat for fish and other inhabitants.
Dr Dietzel says a major implication of coral size is its effect on survival and breeding.
"A vibrant coral population has millions of small, baby corals, as well as many large ones - the big mamas who produce most of the larvae," he said.
"Our results show the ability of the Great Barrier Reef to recover -its resilience - is compromised compared to the past, because there are fewer babies, and fewer large breeding adults."
The study authors are calling for more concrete demographic data to establish how coral populations are changing and whether or not they can recover between disturbances.
"We used to think the Great Barrier Reef is protected by its sheer size," Dr Dietzel said.
"But our results show that even the world's largest and relatively well-protected reef system is increasingly compromised and in decline."
Australian Associated Press
