How is sound used to study coral reefs?
Coral reefs are noisy environments due to an abundance of sounds, including those from wind, waves, and various marine animals (for more information see What are common underwater sounds?). Reef sounds vary on a daily basis, by moon phases, and by seasons. Human-generated sounds are increasingly becoming part of many marine soundscapes, especially in coastal environments. Acoustic characterization and monitoring of underwater soundscapes have become a major interest because of the importance of sound to many marine species.
The life history of many coral reef fishes (and invertebrates) includes a pelagic larval stage that metamorphoses to the juvenile stage. Variations in the acoustic signature of different reef habitats may provide information about habitat quality and help larvae and juveniles differentiate amongst reefs. Several studies suggest that the coral reef fish and invertebrate larvae may use a combination of visual, chemical, and acoustic cues to recognize and orient to suitable reef habitat on which they can settle and develop into adults. Studies have shown that larvae of some species may be attracted to certain reef sounds and avoid others.
The distance over which larvae can detect reef sounds may depend on their hearing ability. Studies suggest the lower frequency band of reef noise enhances larval settlement. Sounds associated with fish vocalizations and moving water are predominately low frequency. Because many fishes and marine invertebrates respond primarily to particle motion, it is likely to be an important acoustic cue for settling larvae.
Coral reefs face many threats, such as pollution, ocean warming, acidification, coastal development, and overfishing. To keep a reef healthy, a constant supply of larvae needs to return to the reef and replace the animals that leave the system or die. An understanding of natural reef soundscapes is needed in order to determine how changes in reef structure and soundscape impact larval recruitment and to investigate the potential impact of sounds contributed by human activities.
A growing focus of acoustics is to evaluate underwater soundscapes to determine community features, identify differences among habitats, and monitor change. Soundscape analysis has proven useful with terrestrial habitats in determining habitat structure and species diversity. By analyzing underwater recordings of various coral reefs in the Pacific Ocean, marine biologists have found clear differences from reef to reef. A healthy and diverse reef produces more sounds than degraded reefs. Studies comparing the acoustic features of marine protected areas vs. non-protected areas found that protected sites present a significantly higher ambient sound pressure level and are more acoustically complex than those that are not protected. Reef-generated sound contains a wealth of information. Passive acoustic monitoring is one way to monitor reef health, characterize ecological changes, and evaluate the efficacy of marine protected areas.
Soundscape ecology is a growing area of research. Although data exist for many locations, much remains unknown. Underwater acoustic indices continue to be developed and tested. For coral reef soundscapes, further investigations are needed to identify vocal species and determine acoustic communities. It is also important to understand patterns of reef health and recovery after disturbance.
Additional Links on DOSITS
- Science of Sound: What are common underwater sounds?
- Animals and Sound: Why is sound important to marine animals?
- Technology Gallery: Archival Marine Acoustic Recording Units (ARUs)
- Indeck, Katherine L., Simard, P., Gowans, S., Lowerre-Barbieri, S., and Mann, D.A. (2015) A Severe Red Tide (Tampa Bay, 2005) Causes an Anomalous Decrease in Biological Sound. Royal Science Open Society. 2,150337. https://10.1098/rsos.150337.
- Lillis, A., Bohnenstiehl, D., Peters, J.W., and Eggleston, D. (2016) Variation in Habitat Soundscape Characteristics Influences Settlement of a Reef-Building Coral. PeerJ. 4, e2557. https://doi.org/10.7717/peerj.2557.