Coral reefs are one of the most biodiverse ecosystems on Earth. They are multi-functional natural habitats allowing marine species to forge, escape from predators, and reproduce. These vibrant underwater ecosystems are under threat from multiple environmental stressors, including noise pollution. Fortunately, a recent study has shown the possibility to restore coral reefs by playing a certain combination of sound in the proximity of a reef. In this piece, we take a look at what underwater soundscapes are and how they can help restore these precious ecosystems.
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Anthropogenic Noise Pollution in Marine Environments
Soundscapes refer to a sound or a combination of sounds that forms an acoustic environment, an auditory landscape. Underwater soundscapes are often overlooked as human ears are not adapted to underwater environments. Yet, marine animals are highly dependent on sound cues for navigation, communication, foraging, and predator avoidance, which are essential for survival.
Unfortunately, not all underwater sounds have a positive impact on marine animals. This is often the case for human-made noise pollution, which comes from various sources, including maritime transportation, offshore construction and operation, fishing, and even military sonar detection. These underwater noises damage marine communities in multiple ways, including physical damage, behavioral alteration, and physiological change.
More on the topic: The Impact of Noise Pollution on Marine Animals
The Soundscape in Coral Reefs
Healthy coral reefs are naturally noisy where multiple animals like snapping shrimp, soniferous fishes, and also breaking waves creating this underwater soundscape. A 2010 study recognized that larval corals are phonotaxis, meaning they move in response to a sound. Larval coralars are attracted to the natural reef sounds generated by other resident marine organisms, such as fish and crustaceans. Although this mechanism is not fully understood, scientists suggest that the exterior cilia on coral larvae enable them to sense and respond to underwater sound by swimming towards it.
Acoustic Enrichment: A Novel Approach to Coral Reef Restoration
Coral recruitment is a vital survival strategy to drive population recovery, which takes place in four phases: mass spawning of eggs and sperms, dispersion of coral larvae, settlement and metamorphosis from plankton to reef, and early post-settlement growth. While coral settlement is a critical early stage within the recruitment process, acoustic enrichment has been shown to facilitate the coral settling process.
A 2024 study conducted by the Woods Hole Oceanographic Institution – the largest independent oceanographic research institution in the US – demonstrated the potential of coral reef restoration via acoustic enrichment. Scientists installed underwater speakers to shape a soundscape mimicking a healthy reef to encourage coral larval settlement. Settlement rates can be up to seven times higher under acoustic enrichment, potentially enhancing the chance of coral larvae survival.
Another 2019 study conducted at a degraded coral reef, also known as coral rubble, in Australia’s Great Barrier Reef has also shown noticeable improvement in the fish community by playbacking healthy reef-sound. With a 40-day acoustic treatment period, visual survey conducted on the juvenile fish community found a 50% increase in juvenile damselfish community (the most common reef fish species in the Great Barrier Reef) and an overall increase of five other trophic guilds within the food web, demonstrating an effective enrichment result in fish communities.
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The ocean’s soundscape can be highly complex, given the multitude of natural and anthropogenic sound sources. While further research will be needed to explore the applicability to other reef habitats and the possibility to scale up as conservation measures, acoustic enrichment has proved promising in recovering and restoring degraded reef habitat.
