In 2019, scientists mounted speakers playing a healthy reef's sounds over dead coral and watched fish move back in: an experiment now being built into a tool to help restore dying reefs

Picture your favorite beach town after the shops close for the night. No music drifting out of restaurants, no chatter on the sidewalk, barely any life. Even if the buildings are all still standing, something about it feels wrong, and you would not want to stick around. Marine biologists say young fish feel something similar when they swim past a coral reef that has died out. The reef might still be there, but it has gone quiet, and quiet reefs do not get picked as a place to call home.

According to NOAA Fisheries, warming oceans, pollution, and other pressures have wiped out 30 to 50 percent of coral reefs worldwide, and reef scientists have been looking for tools to help these ecosystems recover more quickly for years. An idea that sounded pretty simple: blast the sounds of a healthy reef through an underwater speaker, over a dead one, and see if fish come back on their own.

A dead reef sounds like nothing at all
Healthy coral reefs are loud places. Snapping shrimp crackle, fish grunt and chirp, and the entire habitat vibrates with activity. Degraded reefs, however, go almost silent. According to the 2019 study, ‘Acoustic enrichment can enhance fish community development on degraded coral reef habitat,’ published in Nature Communications, young fish drifting through the open ocean in their larval stage use that noise and smell to detect and orient toward a reef to settle. When a reef goes quiet, fewer fish arrive to rebuild it, which keeps it degraded, which keeps it quiet.

Marine biologist Tim Gordon, then at the University of Exeter, and a team of colleagues set out to test whether that loop could be broken. The research was a six-week field experiment conducted on Australia’s northern Great Barrier Reef, in a lagoon that had experienced extreme mass coral bleaching in the two years before the study.

The setup: 33 patches of dead coral rubble
The team built 33 small patch reefs from dead coral rubble, all similar in size and shape, and placed them in open sand near a natural reef, according to the Nature Communications study. Each patch was randomly assigned to one of three groups: some were provided with a real underwater loudspeaker, which played recordings of a healthy reef overnight, some with a dummy speaker rig that produced no sound to rule out fish simply being attracted to the equipment itself, and some with no speaker at all. The “acoustic enrichment” reefs played back real reef recordings for 40 nights, roughly at the same time of night as when the sounds were originally recorded.

Twice the fish, and a more varied crowd
The results were hard to argue against. According to the study, after 40 days, there were roughly twice as many juvenile damselfish on the acoustically enriched reefs compared with the two silent groups, and that gap opened up early and held throughout. When researchers dismantled the enriched reefs at the end of the experiment to count the fish inside, the pattern held across the entire community, not just one species.

The study revealed significantly greater numbers of herbivores, omnivores, planktivores, invertivores, and piscivores on the enriched reefs compared to the unmanipulated reefs. Each reef was monitored by a SCUBA diver staying at least 1 meter away to reduce disturbance, and the whole community was counted only at the end by dismantling the patch reefs piece by piece. The paper says corallivores were essentially absent, found on fewer than 0.25% of all fish and on only two of the 33 reefs, so they were excluded from the analysis. Overall fish abundance was about twice as high, and the enriched reefs supported about 50 percent more species than the silent ones.

The paper reported that the two silent groups, one with no speaker and one with a dummy speaker, turned out to be very similar to each other. This is the important detail because it suggests the extra fish were responding to sound itself, and not some other side effect of having equipment on the reef.

From one experiment to an actual restoration tool
Gordon’s team was careful to frame acoustic enrichment as a promising early finding, not a fix in itself. The approach is supposed to supplement, not replace, active habitat restoration and conservation measures, as a speaker cannot regrow coral by itself, the study noted.

But that caution hasn’t stopped the idea from going ahead. Since then, Woods Hole Oceanographic Institution researchers in Massachusetts have extended the same approach to coral itself, not just fish. A 2024 study led by marine scientist Nadège Aoki and published in Royal Society Open Science found that coral larvae exposed to playback of a healthier reef soundscape settled onto a degraded reef site in the U.S. Virgin Islands at significantly higher rates than larvae on reefs with no added sound. According to the study, the team used a custom solar-powered acoustic playback system to broadcast recordings from a reef about a decade ago, when its soundscape was richer, onto the degraded site nearby.

What this means for reefs closer to home
The attraction of acoustic enrichment for American reef systems, such as those in Florida and the U.S. Virgin Islands, both of which have suffered their own bleaching events and storm damage, is that a speaker system is relatively low-tech and inexpensive compared with coral transplanting or large-scale nursery work. No one involved in either study is suggesting that sound alone can rebuild a reef. So far, the research has shown something narrower, but still striking: giving a damaged reef its voice back really seems to attract life to it, whether it's young fish looking for a place to live or coral larvae looking for a place to settle, and that head start is something restoration efforts have lacked.