The study, published last week in PLOS One by a team of JCU TropWATER, AIMS and GBRMPA scientists, describes declines in corals, damselfishes, and butterflyfishes after Whitsunday Islands reefs were hit by severe Category 4 Cyclone Debbie in 2017.

Researchers found a decrease in both the diversity of fish (species richness) and number of fish (density) immediately following the cyclone.

Over a period of 12 years (2012-2023), with support from the Great Barrier Reef Foundation in 2022 and 2023, researchers surveyed coral and fish communities on fringing coral reefs at 43 separate sites around the Whitsunday Islands as part of our long-term monitoring program.

“Most sites were severely impacted … only seven of the 43 sites we surveyed experienced minimal impact,” said lead author Dr Maya Srinivasan.

“Even six years after Cyclone Debbie, we still found that coral cover remained up to 69 per cent less than it was before.

“Both damselfish and butterflyfish species richness and density were significantly lower than they were pre-cyclone”.

Climate change represents a key threat to the Great Barrier Reef, contributing to coral bleaching as well as increased intensity of cyclones, like Cyclone Debbie.

“When it’s a category one or two cyclone, it'll take out the shallower and branching corals, and usually the more robust corals will be okay,” Dr Srinivasan said.

“But Debbie was so severe, so slow moving, it just sat on the reef for hours … large boulder corals were just snapped off, left upside down.

“When it's that destructive, it takes a long time for reef communities to come back.”

The team surveyed 13 different species of damselfish and butterflyfish to understand how the cyclone impacted fish communities on affected reefs.

“When coral reefs are hit by extreme events like cyclones, fish simply perish from the physical impact. Then survivors dwindle because the loss of coral means losing essential food and shelter,” she said.

“Most fish species we looked at suffered declines of up to 85 per cent. Only one species, the Copperband Butterflyfish, didn’t decline in abundance.

“We found three damselfish species that usually don’t rely on live coral also declined … but then recovered after the cyclone … even for these species there was a direct physical impact of the cyclone”.

Dr Srinivasan noted that cyclones are not new to the Great Barrier Reef and that recovery after such events was still possible.

“We did see some recovery, and the reefs could come back to their original condition … they just need more time,” she said.

“Studies like this help us to understand how the reef will respond to future events and will assist in our efforts to improve reef resilience.”