What a Healthy Reef Sounds Like

In the first half of 2024, a marine heatwave sat over this part of the ocean for 11 weeks. Reef-building corals live in partnership with microscopic algae that feed them, and when the water stays too warm for too long, the partnership breaks down: the coral expels its algae and turns bone white, and if the heat persists, it starves. About 70 percent of the coral on Merrow’s south rim died. The north rim, washed by cooler water rising from the depths, lost less than a fifth. The station’s recorders caught the difference as it happened. By the end of 2025, the south reef at dawn was about eight decibels quieter, in the low frequencies where fish call, than before the heat, and the number of distinct calls the team could pick out had fallen by more than half.1

Who is making the noise

The crackle, which dominates almost every recording ever made on a shallow tropical reef, comes from snapping shrimp, animals about the length of a thumb joint that live in crevices in their millions. One of a snapping shrimp’s claws is outsized, and it snaps shut so fast that it fires a jet of water quick enough to tear open a bubble in its wake. The sound is not the claw meeting itself. It is that bubble collapsing.2 Add up every shrimp on a reef and you get the frying, which a hydrophone can pick up kilometers away.

The fish are lower and more sporadic. Many reef fish make sound by drumming special muscles against their swim bladders, the gas-filled sacs that keep them buoyant; others grind their teeth or snap their jaws. Damselfish chirp and pop when they court and quarrel. Groupers boom. Parrotfish, biting chunks off dead coral, make a crunch Lalanne swears he can hear through the hull. Much of this swells into choruses at dawn and dusk, when fish are busiest and most likely to be eaten, and those are the hours Sarre’s recorders are built around.

None of this would matter to restoration if fish were merely noisy. What matters is who is listening. Most reef fish begin life not on the reef but in open water. Their parents release eggs into the current, and the larvae that hatch drift and grow for weeks—a month or two, for many species—far from any coral. Then, at about a centimeter long, they must find a reef and settle on it, usually under cover of darkness and often around the new moon, when fewer predators can see them arrive. A larva at that stage is a stronger swimmer than it looks, and it is weighing up cues: the smell of the water, the light, and sound.

Experiments going back two decades have shown that settling larvae swim toward recordings of reef noise and away from silence, and that the sound of a healthy reef draws more of them than the sound of a degraded one. That raised an obvious, slightly mad idea, which researchers now call acoustic enrichment. If a dying reef has gone quiet, and quiet reefs attract fewer young fish, what happens if you put a loudspeaker on it and play it the sound of a healthy one?

Twenty-four piles of rubble

In October 2025, Sarre and Lalanne built two dozen small patches of dead coral rubble on the south rim, each about a meter and a half across, spaced 30 meters apart over sand. On eight they set underwater loudspeakers, each wired to a battery in a sealed drum and playing the north rim’s dawn chorus, recorded in 2019, on a loop from dusk to dawn.3 Eight got identical speakers that stayed silent, to rule out the possibility that fish were simply curious about a new object. Eight got nothing. Then, every week for 10 weeks, Lalanne and two volunteer divers swam the patches with slates and counted every newly settled fish.

The speakers worked, in the narrow sense. Over those 10 weeks, the patches that played reef sound gained 2.3 times as many young fish as the silent ones, and 41 percent more species.^* Most of the newcomers were damselfish and cardinalfish, small, common, and quick to settle, but there were juvenile surgeonfish and rabbitfish too, grazers that crop the seaweed that would otherwise smother the next generation of coral. “When I saw the third week’s numbers I went and sat on the jetty for a while,” Sarre says. “It is a very strange feeling, being able to call fish.”

First aid

Sarre is careful, in conversation and in print, about what the experiment did not show, and the list is long. It did not show that sound brought new fish into existence. Larvae drawn to the speakers might otherwise have settled on another patch, or on the healthier north rim, so the loudspeakers may have rearranged fish rather than added them. It did not show that the fish stayed. When the divers went back in March 2026, three months after the speakers were switched off, the gap had narrowed: the sound patches held 1.6 times as many fish as the silent ones, not 2.3. And it showed nothing at all about the coral.

Petra Vinter, a reef-fish ecologist at the University of Corran who read the results before they were submitted for publication, thinks the work is careful and the idea worth testing, and worries about how it will be heard. She puts it plainly: “A loudspeaker is a dinner bell. It is not the dinner.” If young fish are lured onto a reef with too little shelter and too many mouths, more of them may be eaten than if they had been left to find their own way. Ecologists call that an ecological trap, and the March figures, Vinter notes, are at least consistent with one. Small groupers, it turned out, had found the speaker patches too.

There is a subtler problem, about hearing itself. Sound underwater is two things at once: a pressure wave, which is what a hydrophone measures, and a tiny back-and-forth jostling of the water, which is what a fish’s ear mostly feels, through small stones called otoliths that rock on beds of sensory hair cells. A hydrophone is a far more sensitive instrument than a larval ear, and the jostling a fish can feel is strongest within a few meters of whatever made the sound. A recording that seems rich and full through Sarre’s headphones may be, to a larva a few hundred meters off, almost nothing. Nobody knows how far a settling fish can really hear a reef, and for a loudspeaker the size of a dinner plate the honest answer is: not very far.

Sarre does not dispute any of it. Her own hopes for the technique are modest: that it might help reefs where coral is already being replanted, by drawing in the grazing fish those young corals need during the few critical months before the reef can make its own noise again. It cannot cool the water. “If you ever hear me on the radio saying we can save reefs with speakers, come and find me,” she says. “The reef went quiet because the ocean got too hot. Everything else is first aid.”

On my last morning on the atoll, Lalanne took the boat out to the south rim before dawn to change the batteries on a new set of speakers, part of a second, larger trial that runs until the end of the year. He let me hang over the side in a mask while he worked. In the gray light, above one of the rubble patches, a knot of damselfish no longer than my fingernail hovered beside a dark disc of plastic that was playing, very quietly, the sound of the north rim in 2019. The fish could not know it was a recording. Neither could the reef. For now, the south rim of Merrow Atoll is listening to a memory of what it used to be. Whether it will ever sound like that again depends on things no speaker can play: cooler water, and time.