In 1956, when famed explorer, conservationist, and oceanographer Jacques Cousteau titled his pioneering marine nature documentary ‘The Silent World’ – he could not have been more wrong. We now know oceans are incredibly loud, and almost all marine life, from larvae to blue whales, use sound in some way to make a living. Marine biologists are now using novel bioacoustics technologies to help save coral reefs.
Coral reefs are under threat globally from climate change, overexploitation, and habitat destruction. News headlines featuring mass coral bleaching events are becoming regular reading. Reefs provide essential ecosystem services for many coastal and indigenous communities, as well as being oceanic biodiversity hotspots. Protection and restoration of coral reefs is paramount, and marine conservationists are always looking for novel ways to fight this ongoing battle.
Coral Reef Soundscapes
On land, we are intimately familiar with the soundscapes of different habitats. Imagine standing in a forest, hearing the collective choir of birds, insects and mammals, on an ambient backdrop of wind in trees, creaking branches, and falling water. A coral reef is no different. Marine life on a coral reef sings, buzzes, clicks, purrs and whoops either intentionally (e.g. communication) or as incidental sound (e.g. scraping the algae from a hard rock or shell). This is known as the coral reef soundscape – a rich and diverse chorus reflecting the equally rich and diverse inhabitants of the ecosystem.
Importantly, the soundscape of a coral reef is a product of the organisms that live there. Destructive fishing practices, coral bleaching events, or overfishing, will impact a reefs ability to host a diverse community of marine life. Fewer marine organisms means less sound production, and therefore a quieter and less complex soundscape. Researchers are developing ways to understand the relationship between a reef ecosystem and characteristics of the soundscape it produces.
Lamont et al. (2021) found that they could detect the health of a reef by analysing soundscape recordings, and using this to inform conservation practices. The researchers took manual counts of biotic sounds, as well as calculating an Acoustic Complexity Index (ACI) and sound-pressure levels. In their analysis, these indicators were able to predict if the reef was healthy/restored or degraded. This can be incredibly useful for reef management as another tool to diagnose and manage coral reefs. One way it can be used is to monitor the progress of restoration projects by tracking the change of the soundscape over time, and assessing the progress being made.
Marine Larvae and Phonotaxis
Conservationists aren’t the only ones listening for reef quality. Marine larvae have also been shown to be influenced by the quality of a reef soundscape. Many coral species reproduce by releasing clouds of spawn directly into the water column, called ‘broadcast spawning’. Fertilisation and early development into larvae occurs in this free-floating, free-living stage, until they settle on something solid and develop into their adult forms. Larvae use multiple cues for locating places to settle, and new research is showing the importance of phonotaxis (movement orientated/directed by sound) in this process. Lillis et al. (2018) showed experimentally that coral larval settlement was influenced by the quality of the soundscape, preferring reefs with acoustic environments more similar to healthy reefs.
What this study demonstrates is the cyclical nature of a reef and its soundscape. A healthy reef produces an attractive soundscape that recruits larvae that settle, grow, and later directly or indirectly contribute to the soundscape themselves. This feedback loop is inhibited when a degraded reef has a poor soundscape, as fewer organisms will be recruited to the reef, impeding growth and recovery of the reef.
Acoustic Enrichment
The goal of acoustic enrichment is to help push a reef in the right direction by temporarily restoring the soundscape. Using underwater speakers playing the sounds of healthy reefs, Gordon et al. (2019) successfully demonstrated how acoustic enrichment can benefit a degraded reef. Over a 6-week period, reefs receiving acoustic enrichment developed significantly faster, with higher total abundance and greater species richness than reefs with no enhancement. By mimicking the sound of a healthy reef, they were able to attract and retain more fish and coral species. Acoustic enrichment is now being trialled over more coral management areas with ongoing research on its effectiveness as a restoration tool.
This field of research is still young and there is still much to learn about the role of sound on coral reefs, as well as how to maximise restoration efforts with acoustic enrichment. It is not a cure all, and acoustic enrichment and soundscape monitoring are only parts of larger management strategies. The researchers are keen to point out that while acoustic enhancement boosts recruitment and settlement, there needs to be support and infrastructure for organisms to build on. It’s like inviting an orchestra to come and play in an empty music hall, with no instruments, chairs, stands or sheet music, and not even offering anything to eat or drink. The musicians will either settle on the stage and die, or move on and find an orchestra elsewhere.
Technological developments and innovative new ideas are helping restore and protect coral reefs and marine life all over the world’s oceans, and new bioacoustics innovations are a source of hope and optimism in coral reef conservation. But if we fail to protect our oceans, the world Jacques Cousteau mistakenly described may become a reality, and our oceans may fall silent once again.
References
Gordon, T.A.C., Radford, A.N., Davidson, IK., et al. (2019) Acoustic enrichment can enhance fish community development on degraded coral reef habitat. Nature Communications. 10, 5414
Lamont, T.A.C., Williams, B., Chapius, L., et al. (2021) The sound of recovery: Coral reef restoration success is detectable in the soundscape. Journal of Applied Ecology. 59, 742–756
Lillis, A., Apprill A., Suca, J.J., et al. (2018) Soundscapes influence the settlement of the common Caribbean coral Porites astreoides irrespective of light conditions. Royal Society Open Science. 5, 181358
Lin, T.H., Sinniger, F., Harii, S., et al. (2023) Using Soundscapes to Assess Changes in Coral Reef Social-Ecological Systems. Oceanography. 36, 20-27
Readfearn, G., (2023) ‘Huge’ coral bleaching unfolding across the Americas prompts fears of global tragedy. The Guardian. Available at: https://www.theguardian.com/environment/2023/aug/11/coral-bleaching-central-america. Accessed on: 20/11/2023
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