Global climate change is impacting the world’s coral reef ecosystems. Rapid ocean warming, and associated coral bleaching events, as well as increasing potential for impacts from tropical cyclones are causing significant damage to coral reefs. To better understand and mitigate these impacts, much work is being conducted in coral reef research, management, and conservation.
A scuba diver conducts an ecological assessment at a bleached coral site in the waters off Jarvis Island, Pacific Remote Islands Marine National Monument. Image credit: NOAA.
Passive acoustics has become an important coral-reef monitoring tool. Coral reefs are noisy underwater environments with abundant sounds from wind, waves, and a diversity of marine animals. Scientists and managers use the underwater sounds produced by reef-associated organisms to better understand reef ecosystems and monitor reef health. Research shows healthy coral reefs differ acoustically from those that have been damaged and scientists have started to develop automated systems to discriminate sounds produced by healthy and degraded reefs
Larval fishes and invertebrates use a variety of visual, chemical, and acoustic cues to detect and select suitable reef habitat.Since degraded reefs differ acoustically from healthy reefs, scientists hypothesize that the functional role underwater sound plays in facilitating larval orientation and settlement may be impacted. In a field-based, playback study in Australia, degraded reef sounds were found to be less attractive than healthy reef sounds. A 40% reduction in the settlement of juvenile fishes occurred at sites with underwater playback of degraded reef sounds. Robust fish populations are critical to coral reefs as many fishes excrete key nutrients for coral development, limit macroalgae growth, and create new, clean substrate on which larval corals can settle. As such, reductions in fish recruitment may impede the successful recovery of degraded coral reef sites.
Scientists have found the soundscapes of restored coral reefs to have similar levels of “acoustic richness” (diversity of sounds recorded) to that of healthy reef environments. This “acoustic recovery” suggests that restored reefs have the potential to attract a variety of reef animals, helping restored reefs become more self-sustaining (the restored soundscape attracts more animals that maintain the reef and, in turn, produce more sound, resulting in a positive feedback loop). To enhance and/or accelerate community development at restored coral reef sites, scientists are using “acoustic enrichment”, underwater playbacks of reef sounds, to attract fishes and other organisms to restored reefs. In a laboratory study, Williams et al. 2021 found flat oyster larvae to have a higher settlement response in the presence of restored reef sound playback when compared to degraded reef sounds or the “no sound” control treatments. Australia’s flat oyster is a key reef-building organism targeted for restoration efforts. Acoustically enriched reefs at a field site off Australia’s Lizard Island attracted two times as many fishes as the control (no playback, and “dummy loudspeaker” with no sound), with a 50% increase in the diversity of fishes present. The fishes not only returned to the acoustically enriched sites but remained on site and set up territories, even with the reefs in a state of recovery. Also important, acoustic enrichment increased the abundance of juvenile fishes across all major trophic levels, with significantly more herbivores, omnivores, planktivores, etc. on acoustically enhanced reefs than at unmanipulated sites. A functionally diverse fish assemblage bolsters reef health and resilience.
Passive acoustic monitoring has become a valuable tool for tracking coral reef recovery. It enables scientists and managers to expand their monitoring efforts beyond that of solely measuring coral growth to programs that capture broad ecological data on the communities that comprise reef habitats. In addition, acoustic enrichment is an innovative technique that complements the suite of tools used in coral reef restoration. Additional research is needed to understand applications of the technique across different reef habitats and geographical locations, as well as potential effects of on adult fishes, other reef organisms, and ecosystem processes.
- NPR, December 2021, Hear the new sounds that show life returning in these coral reefs.
- The Washington Post, February 2022, Sounds of a healthy ocean can bring degraded marine ecosystems back to life.
- The Guardian, May 2022, Click, clack and pop: sounds indicate health of coral reefs, study finds.
- Chris Morgan/Into the Wild [podcast], May 2022, Coral reefs: a biological symphony being silenced.
- World Economic Forum, June 2022, Want to save the world’s coral reefs? Just listen to them, say scientists.
- Ocean Biodiversity Listening Project: https://sites.google.com/view/marine-ecoacoustics/projects/biodiversity-listening-project
- Mars Coral Reef Restoration Project: buildingcoral.com
- Freeman, L., & Freeman, S. (2016). Rapidly obtained ecosystem indicators from coral reef soundscapes. Marine Ecology Progress Series, 561, 69–82. https://doi.org/10.3354/meps11938.
- Freeman, S. E., Freeman, L. A., Giorli, G., & Haas, A. F. (2018). Photosynthesis by marine algae produces sound, contributing to the daytime soundscape on coral reefs. PLOS ONE, 13(10), e0201766. https://doi.org/10.1371/journal.pone.0201766.
- Williams, B. R., McAfee, D., & Connell, S. D. (2021). Repairing recruitment processes with sound technology to accelerate habitat restoration. Ecological Applications, 31(6). https://doi.org/10.1002/eap.2386.
Williams, B. R., McAfee, D., & Connell, S. D. (2022). Oyster larvae swim along gradients of sound. Journal of Applied Ecology
(7), 1815–1824. https://doi.org/10.1111/1365-2664.14188