Every coral reef has its own soundscape. The pops, grunts, crackles and croaks made by marine creatures create signature soundtracks to each reef.
Scientists are racing against time to uncover what clues the sounds reveal about ways to maintain healthy reefs — and how to help reefs that are in serious danger.
Ocean temperatures have been rising for decades. Last year those temperatures reached new extremes, prompting America's National Oceanic and Atmospheric Administration (NOAA) to declare a global mass bleaching event for the fourth time.
Experimental research in Australia wants to use reef sounds, and an underwater boombox, to help.
What makes reef soundscapes unique
For over a decade, researchers have turned to underwater sounds to evaluate the health of marine ecosystems.
Sounds can travel much further than light under water — up to thousands of kilometres — and marine creatures are reliant on sound cues to navigate their watery world.
"A healthy habitat has a rich, complex soundscape," explains Miles Parsons, an acoustic scientist with the Australian Institute of Marine Sciences (AIMS).
For example, a fish "choruses" predominantly for feeding or spawning, he says. "And those choruses can change in their timing depending on what that particular species is responding to."
Fish, crabs, shrimp and other invertebrates on the reef also make noise as they forage, hunt, feed, groom and mate.
Dr Parsons likens an underwater area to a human neighbourhood. "A bustling suburb is a healthy suburb," he says.
Conversely, a degraded reef with no structure and no animals is quiet.
Dr Parson says part of the complex work acoustic scientists are tasked with is understanding what sounds can tell them about how marine communities are functioning — and how they interact with factors in their environment such as the sun, moon, tide, temperature and salinity, all of which can impact the soundscape.
How scientists use sound to help reefs
Since 2021, Dr Parsons and other acoustic scientists from AIMS have been conducting acoustic enrichment research in the Ningaloo Reef off the coast of Western Australia and at the Great Barrier Reef's Lizard Island.
Their experiment, called the Reef Song Project, builds on previous research and involves broadcasting sounds of healthy reefs to attract fish to settle on degraded reef patches.
"We go out to the reef and load [a set of big live speakers] down with snorkellers [to] put them on the seabed," Rohan Brooker, who leads the project, told ABC Radio Sydney.
Every night, scientists play seasonally-appropriate recordings of healthy reefs through these speakers.
"[We go out] every new moon in the summer period where the fishes are coming in large numbers," Dr Brooker says.
"We're trying to bring in the baby fishes, with hopefully a flow-on effect to the corals, in the sense that those fishes will be able to provide nutrients and other things that will be able to help corals grow."
Fish play an important role in coral growth, shaping them and providing nutrients — food for baby corals.
"The sound of healthy reefs is used by these fishes [to decide] where they want to settle," Dr Parsons says.
More fish and healthier coral contribute to a more biodiverse reef, and biodiversity means more resilience to environmental changes.
'An important complementary method'
It's too soon to say exactly what impact the Reef Song Project is having; the project still has a year to go, and climate change has thrown up some additional challenges.
Scientists are in the process of measuring the impact of the mass-bleaching event in the Great Barrier Reef, while, for now, the Ningaloo reefs are expected to escape the worst of the mass-bleaching.
But Dr Parsons is hopeful the project will have a positive impact.
"There's certainly been some studies, particularly in the Philippines and Malaysia, where restoration projects tracked how the soundscape changes [and found] the sounds of the reef get healthier as the restoration brings back the health of the reef," he says.
A similar experiment off the Virgin Islands found promising signs that acoustic enrichment efforts could increase the reef's biodiversity.
Dr Parsons acknowledges that what his team is doing might not be a silver bullet to restore the health of Australian coral reefs, but he hopes acoustic monitoring and enrichment can help boost their health.
"It's an important complementary method to understand how aquatic animals are responding to a changing environment," he says.