Fish make music! It could be the key to healing degraded coral reefs

Do fish bay at the moon? The answer to that question may also point to a way to protect the ocean's damaged coral reefs.

That's a vital goal for the approximately one billion people – most of them in low and middle income countries – who depend on coral reefs. These complex ecosystems are, of course, a breeding ground for fish that are a major source of protein and income. But because reefs provide a barrier between the ocean and land, they also offer crucial protection against the rising sea levels and violent storms wrought by climate change.

Now an intriguing effort is underway to study and protect the reefs. NPR spoke with one of the leaders, Aran Mooney, a marine biologist at Woods Hole Oceanographic Institution in Falmouth, Massachusetts. He's part of a network of scientists who've set up underwater microphones across the planet to essentially eavesdrop on marine life.

"It's just really striking what we can learn without actually visually observing," says Mooney. "Just by listening — quiet listening — we can observe what the animals are doing out there in the ocean."

One of their coolest findings is just how many fish live by the lunar cycle – ramping up the sounds they make depending on the phase of the moon.

Some are loudest when the moon has waned. Take these long thin fish called "cusk eels" recorded off the coast of Cape Cod. They're strumming their muscles against their swim bladders – that's the organ that helps them float – like a bass drum.

Why do this during the new moon? One clue may lie in the fact that the noise they're making is almost certainly a mating call. The fish equivalent of putting on a Barry White record.

"Yeah," says Mooney chuckling. "It's probably a lot of males trying to entice the females into spawning with them, because when the eggs and the sperm are released into the water they're going to get dispersed pretty quickly. So it has to be an extremely coordinated event."

And what better time, he adds, than when it's too dark for predators to swoop in and eat the eggs? "These predators can't see, but the sound is traveling really well," says Mooney. "So it's a way to hide from the predators, but at the same time communicate with each other."

Other fish are noisiest when the moon is full. These tiny ones were recorded by other scientists in the network, off the coast of Southern India. The engine-like chugging the fish are making is the sound of their swim bladders vibrating, possibly as they're eating a kind of plankton that glistens in the moon's rays.

"So eating animals that are associated with light?" posits Mooney.

The international group of scientists is racing to record these soundscapes at reefs and other ocean habitats threatened by climate change and pollution.

Consider this coral reef off the U.S. Virgin Islands recorded in 2013, when it was thriving. Snapping shrimp pop bubbles. Whales and fish call out.

A year ago, the scientists recorded a reef in the same area that had been degraded by pollution run-off from nearby coastal communities. This time most of the sounds were gone.

"It's going to be hard for you to hear," says Mooney. "It's just going to be quieter."

Though officials have now put environmental protections on that reef, it's too late: The animals have long departed – starting with the tiny larvae that are needed to build up new coral.

But Mooney and his collaborators have started an experiment: Setting up underwater speakers to broadcast their recordings of the old, healthy reef from 2013 in hopes of luring back the coral larvae.

Mooney explains that these tiny jelly-fish like animals get released from healthy reefs and then float for a while in the sea looking for a place to settle. "They're not Olympic swimmers, but they are swimmers," says Mooney. "A healthy habitat is super important for them because that's going to be their permanent location for the rest of their life. Once they attach themselves at the bottom, there's no chance of moving."

To the scientists' delight the effort seems to be working. Compared to a degraded reef where they're not playing sounds, says Mooney, "the reef that we're acoustically enhancing, we get more coral settlement." Specifically, about two to three times as much settlement.

It will take a few more years to see if, as the coral gets re-established, more fish return as well. But Mooney says the results so far suggest an encouraging possibility: All these recordings that the scientists are making don't have to be one more memento of a vanishing world. They could be a key to restoring it.