Coral reefs are often referred to as the ‘rainforests of the ocean’ due to the richness of biodiversity they harbour and the vital role they play in the functioning of marine ecosystems. Globally, reefs support over 500 million people
through tourism and fishing, providing a livelihood and an essential source of protein, whilst also producing multiple medicinal compounds used to treat cancer, HIV, and cardiovascular diseases.
Reefs act as physical barriers which impede coastal erosion. As the intensity of tropical storms is increasing with climate change, this is becoming a very important factor, with 275 million people living within 30 kilometres of a reef. In
addition to this, reefs provide a habitat for around 25% of all marine species, harbouring many species which comprise the base of important marine food webs, and are a nursery ground for the larvae of a multitude of organisms.
John Brewer Reef, Great Barrier Reef, Australia. | Matt Curnock / Ocean Image Bank
Coral reefs are sensitive to damage through human disturbances, pollution, warming sea temperatures, and ocean acidification, all of which can cause them to bleach. In recent years, the impact of anthropogenic noise pollution on reefs has
been extensively studied, particularly that of motorboats and other marine vehicles.
The noise generated by motor use in marine environments can crowd out the natural sounds produced by a reef environment. It is also directly impacting the behaviour of many reef species, from corals and molluscs to larger fish.
Many fish species lay their eggs on the reef and the larvae swim out into the pelagic zone to feed and grow, as there are less predators in the open ocean and it is a safer environment than on the reef. After this initial growth period, the
juvenile fish then swim back towards the reefs, where they will continue to grow and breed. Healthy fish populations help to maintain a functioning reef ecosystem, and their populations rely on the return of the juvenile fish.
Healthy coral reefs have unique and characteristic soundscapes, interlinked with a multitude of marine processes which scientists are only just beginning to understand. The sounds are produced by many different species and organisms and
provide information on the predation, mating and suitability of the reef as potential habitat.
A study carried out in the Caribbean has proven that coral larvae are impacted by noise pollution. The larvae begin life in the water column, and use signals and sounds produced by the reef ecosystem to find a suitable habitat, in which to
settle and establish. Noise pollution is masking these important sound cues, leading to many coral larvae failing to settle and therefore dying.
In the study, a choice-chamber was used that offered larvae habitats of different conditions, specifically, varying levels of noise pollution. Healthy reef acoustics were found to be most attractive to the coral larvae, whilst heavy noise
pollution was much less so, indicating the impact anthropogenic disturbance may be having on the recovery of coral reefs.
Healthy coral reef in Siaba Kecil, Komodo, Indonesia, showing a diversity of fish and coral species. | Beth Watson / Ocean Image Bank
The effects are not confined to corals, but extend to many other marine species. Researchers on the Great Barrier Reef have found that the juvenile fish use these sounds to locate suitable reefs to settle in, however, in areas where the
reef soundscape is being interrupted and crowded out by anthropogenic noise pollution (such as marine traffic, fishing practices, oil rig construction and coastal activities), the acoustic waves produced by reef ecosystems become inaudible
or unrecognisable to the juvenile fish.
In addition to this, degraded reefs, such as those which have suffered bleaching events or are heavily polluted, sound and smell less attractive to the juvenile fish. These reefs are therefore experiencing dwindling fish populations, which
in turn is impacting entire ecosystems.
In a recent study carried out by the Australian Institute of Marine Science, researchers investigated the effect of broadcasting the soundscape of a healthy reef on the directional movement of juvenile fish. Reef acoustics characteristic of
a healthy, functional reef were played underwater from loudspeakers placed within experimentally ‘degraded’ reefs, a method termed acoustic enrichment.
After two months (November and December, the natural fish recruitment period), the fish populations from the acoustically enriched reefs were compared with control reefs. The acoustically enriched reefs had twice as many fish as the control
reefs, and 50% higher overall species diversity than the control reefs.
These results suggest that acoustic enrichment could be a potential tool in the restoration and regeneration of coral reef ecosystems, helping repopulate reefs with their former fish communities. This method may also only need to be used
temporarily, as once a certain number of fish populate a reef, the reef soundscape becomes more attractive to other species, initiating further recruitment of organisms to the reef and enriching the species diversity and overall functioning
of the ecosystem.
A bleached and degraded coral reef in the Great Barrier Reef in 2017. | The Ocean Agency / Ocean Image Bank
However, acoustic enrichment of degraded reefs does not remedy the root of the problem. Anthropogenic noise pollution needs to be reduced if coral reefs are to recover to their former health. Shipping vessel traffic across the Great Barrier
Reef is estimated to be increasing by 5% annually, and over 90,000 motorboats are used recreationally across the reef every year—and this number is ever-increasing, predicted to rise by 250% in the next two decades.
Distant sources of noise pollution, such as oil drilling, gas exploration methods and low-frequency sonar technology, are also detrimental. Restoring coral reefs to optimum health requires a multi-factorial approach, with global sea
temperature and pH levels of utmost importance.
If sea temperatures continue to rise and the ocean continues to acidify, acoustic enrichment will be a futile effort. Chemical and sediment pollution, disturbance by snorkelling and other leisure activities, and destructive fishing
practices must all be addressed and reduced to allow full reef recovery.
Despite the detrimental impacts caused by underwater noise pollution, there is very limited legislation in place to protect marine ecosystems. With tourism and anthropogenic marine disturbance increasing, the first step to reducing the
impact of noise pollution on coral reefs is to fund research in this area, which will facilitate the creation of appropriate laws and legislation to protect reef organisms.
The combination of reduced noise pollution and acoustic enrichment practices will likely produce a significant increase in reef health, with visible results in a relatively short time, compared with efforts to mitigate global ocean
temperature rise and acidification. These, on the other hand, are long-term and ultimately more important issues.
Featured Image: Shaun Wolfe | Ocean Image Bank
Ferrier-Pagès C., Leal M., Calado R., et al. (2021) Noise pollution on coral reefs?—A yet underestimated threat to coral reef communities. Marine Pollution Bulletin. Volume 165, page
112129.
Gordon T., Radford A., Davidson I., et al. (2019) Acoustic enrichment can enhance fish community development on degraded coral reef habitat. Nature Communications. Volume 10, article
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Peck M., Tapilatu R., Kurniati E. and Rosado C. (2021) Rapid coral reef assessment using 3D modelling and acoustics: acoustic indices correlate to fish abundance, diversity and environmental indicators in West Papua,
Indonesia. PeerJ. Volume 9.
Reef Resilience Network (2022) Value of Reefs: Reef Resilience. The Nature Conservancy. Available at: https://reefresilience.org/value-of-reefs/ [Accessed February 11th, 2022]
