How scientists are using fish music to protect coral reefs

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Do fish bay at the moon? As NPR's Nurith Aizenman reports, the answer to that question may also point to a way to protect the ocean's damaged ecosystems.

NURITH AIZENMAN, BYLINE: Aran Mooney is a marine biologist at Woods Hole Oceanographic Institution in Falmouth, Mass. He's part of a network of scientists who set up underwater microphones across the planet to eavesdrop on marine life.

ARAN MOONEY: It's just really striking what we can learn without actually visually observing. Just by sort of listening, kind of quiet listening, we can observe what the animals are doing out there in the ocean.

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AIZENMAN: 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...

(SOUNDBITE OF CUSK-EEL CLICKING)

AIZENMAN: ...Strumming their muscles like a bass drum.

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

AIZENMAN: What better time than when it's too dark for predators to swoop in and eat the eggs?

MOONEY: These predators can't see, but the sound is traveling really well. So it's a way to hide from the predators but, at the same time, communicate to each other.

AIZENMAN: Other fish are noisiest when the moon is full. These tiny ones were recorded off the coast of southern India...

(SOUNDBITE OF WATER SPLASHING)

AIZENMAN: ...Vibrating their swim bladders - that's the organ that helps them float - possibly as they're eating a kind of plankton that glistens in the moon's rays.

MOONEY: So eating animals that are associated with light.

AIZENMAN: This international group of scientists is racing to record these soundscapes at 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.

(SOUNDBITE OF BUBBLES POPPING)

AIZENMAN: Snapping shrimp pop bubbles.

(SOUNDBITE OF BUBBLES POPPING)

AIZENMAN: Whales and fish call out.

(SOUNDBITE OF WHALE CALLING)

AIZENMAN: And here's a recording taken a year ago at a reef in the same area that's been degraded.

MOONEY: And it's going to be hard for you to hear.

(SOUNDBITE OF WATER FLOWING)

MOONEY: It's just going to be quieter.

AIZENMAN: But Mooney has started an experiment - setting up underwater speakers to broadcast those recordings of the old, healthy reef in hopes of luring back the tiny larvae needed to build up new coral. Compared to a degraded reef where they're not playing sounds...

MOONEY: The reef that we're acoustically enhancing - we get more coral settlement.

AIZENMAN: In other words, all these recordings don't have to be one more memento of a vanishing world. They could be a key to restoring it. Nurith Aizenman, NPR News. Transcript provided by NPR, Copyright NPR.