Songs of the Sea: the sounds of recovering coral reefs

Environment | Oceans


By Bethan Henderson, Freelance Writer

Published February 15th, 2022

Whoops, knocks, purrs, croaks and laughs are all sounds that emanate from the creatures that live in the restored coral reefs of Indonesia. These sounds are a part of healthy, functioning reef ecosystems and can tell us about how they function and their diversity.


Coral reefs are incredibly valuable ecosystems—home to 25% of all marine species. Not only are they beautiful and essential habitats with high levels of biodiversity, but also humans are also highly reliant on them. Reefs provide $375 billion in ecosystem services. These include being huge attractions in the diving and tourism industry, producing large amounts of oxygen through photosynthesis (hence their nickname the ‘rainforests of the sea’). They support the livelihoods of six million fisherpeople and protect 100 million homes by acting as a buffer for waves and storms.


Fish on a healthy coral reef | Nariman Mosharraf / Unsplash

However, the world has lost half of its coral reef cover in just the last 30 years. This is down to a myriad of anthropogenic factors including destructive fishing methods; for example, dynamite/blast fishing, water pollution, coastal development, climate change-induced coral bleaching and sediment, and nutrient runoff from deforestation, agriculture and construction.


Mars is a company trying to restore coral reefs in Indonesia with their Mars Assisted Reef Restoration System (MARRS). The company uses Reef Stars—hexagonal structures coated with resin, coral sand and coral fragments—encouraging the settlement of coral and other wildlife.


These Reef Stars act as a platform for coral growth and eventually become engulfed by coral, creating a novel habitat for fish, invertebrates and more coral species. This kickstarts natural ecological processes and helps to rebuild and revive the reef ecosystem.


To determine the effectiveness of the MARRS approach, researchers recorded the soundscapes of degraded, healthy and the MARRS restored reefs in the Badi and Contosua islands of Indonesia.


Most studies on restored reefs only measure coral cover, which may not be a good indicator of the health, diversity, or functioning of the ecosystem. Moreover, visual studies exclude the diversity of nocturnal or cryptic (camouflaged) marine species. Whereas analysing the biotic sounds made by reef creatures can give a more holistic idea of the recovery of the healthy fish and invertebrate populations required for functioning reef ecosystems.


‘Most studies on restored reefs only measure coral cover, which may not be a good indicator of the health, diversity, or functioning of the ecosystem’.


A loud and diverse soundscape does not just indicate a busy and healthy reef, full of soniferous fish and invertebrates, but it also helps the recruitment and settlement of more reef organisms. Many marine species leave and return to reefs at different stages of their life cycles. Larval fish, for example, use sound as an orientation cue and use it to discriminate between reefs when they seek one to settle at.


A loud and diverse soundscape, therefore, attracts more organisms, helping maintain and increase diversity in reef populations. This means acoustic recovery is an important part of the overall ecosystem recovery process.


Coral reef restoration structure in Gili Islands, Indonesia. | Marionpinta / Wikimedia Commons

Many novel sounds were heard in the study on the MARRS restored coral reefs. The biophonic sound types recorded were characterised as scrapes, knocks, purrs, raspberries, croaks, growls, grunts, foghorns, whoops and laughs. Each sound had specific descriptions, spectrograms and wavelengths. For example, a scrape was a crunching sound, when grazer species bite hard substrates on the reef.


The number of these distinct sound types heard in a sample indicates phonic richness. Seven of the ten sound types were heard 50% more on the healthy and restored reefs than the degraded reefs. This meant that the phonic richness was significantly higher on the healthy and restored reefs than the degraded reefs, with no significant difference between the healthy and restored soundscapes. This indicates that MARRS restoration was effective in restoring healthy soundscapes and functioning ecological processes.


Professor Steve Simpson, one of the study's biologists from The University of Bristol, states ‘We have a lot still to learn about what they [reef sounds] all mean and the animals that are making them. But for now, it’s amazing to be able to hear the ecosystem come back to life.’


Dr Tim Lamont of The University of Exeter and the lead author of the study adds, ‘Restoration projects can be successful at growing coral, but that’s only part of the ecosystem. This study provides exciting evidence that restoration really works for the other reef creatures tooーby listening to the reefs we’ve documented the return of a diverse range of animals.’


‘MARRS restoration was effective in restoring healthy soundscapes and functioning ecological processes’.


Coral restoration is being undertaken in many other ways across the globe. Oceanus is an organisation that constructs coral nurseries to propagate coral, and transplants genetically healthy donor coral to restoration sites, increasing reef resilience to stressors through enhanced genetic diversity.


The Coral Nurture Program uses stewardship based management of economically important areas of the Great Barrier Reef, protecting existing coral, planting new coral and safeguarding coral reproduction and nurseries.


Waveforms and spectrograms of the biophonic sound types heard on reefs in Badi and Contosua islands in Indonesia | Lamont et al. / Journal of Applied Ecology 2021

While restoration can alleviate the damage done to coral reef habitats, the underlying anthropogenic stressors problems causing coral degradation still need to be addressed. Planting more coral is pointless if climate change causes ocean temperatures to continue to rise beyond the thermal tolerance of coral, survival is impossible. Similarly, if laws against harmful fishing practises are not established and enforced, reefs will continue to suffer at a rate that restoration (which is slow and resource-consuming) will not be able to keep pace with.


As said by Dr Lamont: ‘conditions for reefs will get more and more hostile and eventually restoration will become impossible.’


Although the results of soundscape studies provide a glimmer of hope that restoration is working, this does not alleviate the monumental challenge of tackling these wider issues.


Featured image: Gregory Piper | Ocean Image Bank


Lamont T. et al. (2021) The sound of recovery: Coral reef restoration success is detectable in the soundscape. Journal of Applied Ecology. Volume 00, pages 1-15.

MARS Coral Reef Restoration (2021) ‘Our Solution’. BuildingCoral. Available at: https://buildingcoral.com/our-solution https://buildingcoral.com/our-solution [Accessed February 4th, 2022]

Oceanus (2022) ‘Coral Reef Restoration Program’. Oceanus. Available at: https://oceanus.org.mx/en/coral-reef-restoration-program/ [Accessed February 8th, 2022]

The Coral Nurture Program (2022) ‘A new approach for the Great Barrier Reef.’ The Coral Nurture Program. Available at: https://www.coralnurtureprogram.org/ [Accessed February 8th, 2022]

University of Bristol (2021) ‘Newly discovered fish songs demonstrate reef restoration success.’ University of Bristol. Available at: https://www.bristol.ac.uk/news/2021/december/coral-reef-restoration-project.html [Accessed February 4th, 2022]



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