Coral reefs are noisy with snapping shrimp and calling fish. Healthy reefs are louder than degraded ones, and scientists now use sound to assess reef health.
The underwater orchestra
A healthy coral reef is a surprisingly noisy place. It is not a silent world, it buzzes with a constant chorus of pops, whoops, grunts, and crackles produced by its inhabitants. This biological soundscape is so distinct that scientists can use it as an acoustic way to determine the health of a reef. By deploying underwater microphones, called hydrophones, researchers engage in passive acoustic monitoring, essentially giving the reef a check-up by listening to its heartbeat.
The dominant sound on the reef is the continuous crackle of snapping shrimp, which creates a noise akin to frying food or a campfire. These small crustaceans produce one of the loudest sounds in the ocean by snapping a specialized claw shut so quickly it creates a cavitation bubble. The bubble's collapse generates a powerful shockwave and a loud pop that can exceed 210 decibels. Fish are another major contributor to the reef's chorus. They produce a variety of sounds—grunts, chirps, and pops—often by vibrating their swim bladders. At Lizard Island on the Great Barrier Reef, studies have identified distinct fish choruses that can raise the ambient noise level by 40 decibels. These sounds are not random; they are linked to specific behaviors like mating, territorial disputes, and foraging.
Acoustic enrichment and reef restoration
The variety of a reef's soundscape directly correlates with its health. A biodiverse reef produces a complex and loud symphony, whereas a degraded, bleached reef becomes eerily quiet as the animals that produce the sounds disappear. This acoustic difference has led to an innovative restoration technique known as "acoustic enrichment." Scientists have discovered that free-floating fish and coral larvae use sound to find a healthy reef to settle on. The sound of a healthy reef signals a good home with ample food and shelter.
In experiments conducted on the Great Barrier Reef, researchers placed underwater speakers on patches of dead coral rubble and played recordings of healthy reef sounds. One study found that broadcasting healthy reef sounds doubled the number of juvenile fish that settled on the degraded patches and increased the number of species present by 50%. Another study showed that acoustic enrichment could increase coral larval settlement by up to seven times. Projects like "Reef Song," led by the Australian Institute of Marine Science (AIMS), are investigating how to scale up these techniques to help damaged sections of the Great Barrier Reef recover after bleaching events. By playing the "song of the reef," scientists hope to attract the next generation of corals and fish, kick-starting the natural recovery process.
💡Fun Facts
The snap of a pistol shrimp is created by the collapse of a cavitation bubble, which briefly generates immense pressure and temperatures.
Fish have inner ears, called otoliths, which are dense, bone-like structures that allow them to detect sound vibrations and orientation.
Some fish choruses on the Great Barrier Reef are so loud they can be a major component of the ocean's ambient noise, with sound levels reaching 120 decibels.
Larval fish can be attracted to reef sounds from up to a kilometer away, using the soundscape to navigate the open ocean.
The Great Barrier Reef Marine Park is accessible 24/7, though specific access is determined by individual tour operators' schedules and weather conditions.
Admission
Access to the park itself is free, but all visitors entering via a commercial tour operator are subject to an Environmental Management Charge (EMC), which contributes to the reef's conservation.
Accessibility
Accessibility varies by location and tour operator. Many larger pontoons and island resorts offer some wheelchair-accessible facilities. Most experiences involve boat travel and water activities.