Listening to the reef

AFor much of the twentieth century, the underwater world was popularly imagined as a place of silence. Early divers, their ears filled with the hiss of their own breathing equipment, had little reason to think otherwise. Yet anyone who lowers a hydrophone, an underwater microphone, beside a healthy coral reef soon discovers the opposite. The water is full of noise: a constant crackling like fat in a frying pan, punctuated by grunts, pops, whoops and the steady crunch of fish grazing on coral. The continuous crackle that dominates many reef recordings is produced largely by snapping shrimp, small crustaceans that create a loud click by closing an oversized claw at great speed. To a trained listener, a thriving reef is less like a quiet garden than a busy marketplace.

BThat so many reef animals produce and respond to sound is no accident. In water, light is quickly absorbed and scattered, so that even in clear tropical seas visibility rarely extends beyond a few tens of metres, and far less in murky coastal waters or at night. Sound, by contrast, travels more than four times faster in water than in air and can be detected over distances of several kilometres. For animals that need to find a mate, defend a territory or detect an approaching predator, the ability to hear is therefore often more useful than the ability to see. Many fish have evolved specialised organs for producing sound, typically by vibrating muscles attached to the swim bladder, the gas-filled sac that also helps them control their buoyancy.

CSound appears to be especially important at one critical moment in a reef fish's life. Most species begin life as tiny larvae drifting in the open ocean, sometimes far from the reef where their parents spawned. After several weeks, they must return to a reef to settle, and the question of how such small, weak swimmers manage to locate one in a vast and featureless ocean puzzled biologists for decades. Experiments in which loudspeakers playing reef recordings were placed in open water have provided part of the answer: larvae of many species gather around the speakers in far greater numbers than around silent ones. Sound is not their only cue, since the smell of the reef and the position of the sun may also play a part, but it seems to guide them over the greatest distances.

DThis dependence on sound has troubling implications. When corals die, as they do during severe bleaching events caused by unusually warm water, the animals that live among them begin to disappear, and the reef becomes noticeably quieter. Recordings made before and after such events show that the soundscape of a degraded reef is both less loud and less varied. A quieter reef, in turn, may attract fewer young fish, and without new arrivals to graze on algae and replace older generations, the reef's recovery is slowed. The decline of a reef, in other words, can be self-reinforcing: damage leads to silence, and silence makes further damage more likely.

EIf silence discourages young fish, could noise bring them back? In a series of field trials, the marine ecologist Dr Clara Venn and her colleagues placed underwater loudspeakers on patches of dead coral and played recordings of healthy reefs. Over several weeks, these patches attracted noticeably more young fish, and a wider range of species, than similar patches where silent dummy speakers had been installed. Venn is cautious about the results, however. Playing sounds, she points out, cannot make a reef habitable if there is no shelter or food, and it does nothing to address the rising sea temperatures that are the root cause of coral loss. At best, she suggests, acoustic enrichment might help damaged reefs recover more quickly once conditions have improved.

FPerhaps the most immediate application of reef acoustics lies not in restoration but in monitoring. Traditional surveys depend on teams of trained divers counting fish and corals, which is expensive, slow and possible only in good weather. A hydrophone, by contrast, costs relatively little and can be left on the seabed for months, recording day and night. Computer programs trained to recognise patterns in the recordings can then estimate the health of a reef from its sound alone. The method has limitations: the noise of boat engines can mask natural sounds, and researchers still need to check samples of the recordings by ear to make sure the software has not been misled. Nonetheless, many scientists believe that listening may soon become one of the standard ways of keeping track of the world's reefs.