Parrotfish thrive in the wake of severe coral bleaching and may help repair reefs, according to an AIMS study that examined two separate coral reef systems in two different oceans — the Great Barrier Reef (GBR) in the western Pacific and the Chagos Archipelago in the Indian Ocean - that had the same response to global heat events.

Bleaching is coral’s stress reaction to prolonged exposure to higher sea surface temperatures. When bleaching reduces coral cover on reefs, it creates large areas of newly barren surfaces which are immediately colonised by a layer of microalgae and cyanobacteria, or ‘scunge’.

Parrotfish are able to scrape this scunge from the dead coral, using their beak-like teeth that give the fish its name. The microorganisms provide an abundant source of nutritious food for the parrotfish, and removing the scunge gives corals the best chance to recolonise the available surface.

Parrotfish populations were between two and eight times greater in areas damaged by coral bleaching. Individual parrotfish were also about 20 per cent larger than in unbleached sections. In comparison, the abundance of almost every other species of fish was in sharp decline in the bleached areas surveyed.

The study concluded that a feedback mechanism regulates coral and parrotfish abundances such that when reefs are damaged, parrotfish numbers swell which, in turn, reduces the amount of scunge, helping coral to recolonise and repair the damaged reef. As coral cover increases, parrotfish numbers decline dramatically.

The fact that plump parrotfish were found in large numbers on both the GBR and the Chagos Archipelago - two coral reef systems 8000 kilometres apart that were subjected to widespread pantropical bleaching in 2015-2017 following global heat events - indicates the feedback loop is an inherent part of reef ecology and a general phenomena, not an isolated incident or caused by local factors.

These results demonstrate that the scale, magnitude, and severity of recent thermal events is causing spatially disparate fish communities to respond in synchrony to extreme environmental occurrences.

The study also found their actions can maintain reef health and help reefs recover from the impacts of mass coral bleaching due to marine heatwaves. Because of this important ecological role, parrotfish have been described as ‘ecosystem engineers’ of reef systems.

This study, published in Global Change Biology, was conducted in collaboration with James Cook University, the University of Auckland and the University of Lancaster, with funding from the Bertarelli Foundation. An Altmetric Attention Score of 878 placed it in the top 5% of all research outputs tracked by Altmetric in terms of media and social media coverage and was the 3rd highest ever Altmetric score for a paper published by Global Change Biology.

While the metrics reflect that the work was timely and important it indicates the public’s interest in understanding the outcomes of coral bleaching better. It also demonstrates the impact of AIMS’ research through a broader look at the magnitude of bleaching, coral mortality, or other coral-focused outcomes, by considering feedbacks throughout the wider ecosystem. This research is recognition of AIMS’ international pre-eminence which is aligned to our Strategy (year-on-year improvement in science excellence, remaining in the top three marine science research institutions in the world).