Coral reef fish running out of time to recover at Great Barrier Reef islands, research warns

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A study has found concerning new evidence of long-term declines in coral reef fish around highly visited inshore islands of the Great Barrier Reef.

Scientists surveyed reefs at 100 sites around the Palm Islands, Magnetic Island, Whitsunday Islands, and Keppel Islands over a 14 year period, revealing fish communities are struggling to cope with increasingly frequent disturbances including coral bleaching, floods and cyclones.

Lead author, Dr. Daniela Ceccarelli, previously of James Cook University and now an Australian Institute of Marine Science marine ecologist, said these fringing reefs around inshore islands suffer from intense human pressure due to their proximity to the coast.

"Reefs further from the coast have generally shown fish to be quite stable in the face of environmental disturbances, but it's a different story for the inshore fish communities," she said.

"These inshore fish habitats have endured greater cumulative and intense local-scale pressures, with too little time to recover between stress events.

"It's possible that we are witnessing a step change in coral and reef fish community dynamics in the face of increasingly frequent disturbances."

The study, published in the journal Global Change Biology, is part of a reef monitoring program now led by JCU TropWATER, around popular inshore islands, which are important for recreational fishing, tourism and local communities.

The study found fish populations across these islands dropped by 33%-72%, and the numbers of fish species fell by 41%–75%, depending on the location.

Dr. Maya Srinivasan, a coral reef ecologist at JCU and co-author of the study, said these were substantial and concerning declines in fish abundance and diversity at these sites.

"In the Keppel Islands we observed a dramatic drop after major flood events in 2011 and 2013, with fish abundance falling to nearly one-tenth of its previous levels—and although many sites recovered, the worst hit sites didn't and are now covered in algae with very little coral," she said.

In the Whitsunday Islands, Cyclone Debbie in 2017 caused a steep drop in fish abundance.

"Cyclone Debbie's intense impact on coral cover and physical damage to reef structures reduced available habitat, especially for coral-dependent species. This loss has made it challenging for fish populations to maintain stability in these areas."

Dr. Srinivasan said marine park zones where fishing is banned improved the picture to some degree, providing a buffer for the species that are usually targeted by fishers.

"However, we found that No-Take Marine Reserves are unlikely to mitigate the impacts of an increasingly chaotic climate for all fish species in the long term," she said.

Most types of fish in the study showed a decline in population density, except for species like herbivorous damselfish and parrotfish.

"These species increased in numbers because they feed on algae, which is more abundant on degraded reefs. We get winners and losers in this age of human impact, but there tends to be more species at the losing end of the scale," Dr. Srinivasan said.

Dr. Ceccarelli said ecological communities are naturally subject to cycles of disturbances such as cyclones, heat waves and floods, followed by periods of recovery.

"These disturbances play a major role in maintaining complex seascapes and promoting species diversity. But in the present-day period, known as the Anthropocene, the nature of these disturbances is changing because of human activities such as habitat destruction and increasingly chaotic fluctuations in the weather caused by climate change," said Dr. Ceccarelli.

"This adds to the mounting evidence that without global action on reducing emissions, protected areas and fisheries management alone are not sufficient to safeguard coral reef fish."

More information: Daniela M. Ceccarelli et al, Regional‐scale disturbances drive long‐term decline of inshore coral reef fish assemblages in the Great Barrier Reef Marine Park, Global Change Biology (2024). DOI: 10.1111/gcb.17506

Journal information: Global Change Biology

Provided by James Cook University