Underwater Acoustics Encourage Coral Reef Growth

But it wasn’t as easy as hitting “play.” Although the team had a basic concept of how they wanted their playback to function, they faced a series of challenges in engineering an underwater system. Every part—electrical acoustic signals, power system, waterproofing, buoy, and mooring—had to operate seamlessly in the field.

Coral reefs are among the ocean’s most imperiled ecosystems. Since the 1950s, the world has lost half of its coral reefs; the UN recently said that every one of the world’s reefs could be bleached by the end of the century unless we drastically reduce greenhouse-gas emissions. Scientists and engineers are seeking novel ways to restore these fragile habitats, and one intriguing approach looks to underwater soundscapes.

“We know what a healthy reef and a degraded reef sound like,” says Ben Weiss, marine robotics engineer at WHOI. Under the auspices of biologist Aran Mooney, who has been at the forefront of building a global library of underwater biological sounds, and along with colleague Nadège Aoki, the team asked: If you serenade corals with the sounds of a healthy seascape, will they rebuild?

Subsea Soundscapes

Mooney had found previously that a playback of healthy reef soundscapes helped to attract certain families of larval reef fish. But, as Aoki says, the early sound system was “fairly basic.” It consisted of an MP3 player in a waterproof box with small batteries plugged into an amplifier and an underwater speaker on a basic mooring. But it didn’t float particularly well, so it tended to flood in rough seas, and it drained the battery quickly. “You had to go out to the buoy every 12 hours or so,” she says.

To change sound sequences and create needed experimental variability, the user had to upload or select new sound files with each daily deployment. If there was a problem—for example if the speaker turned off—it was difficult to tell without detailed analyses. The user effort was high.

Varying soundscapes is key, both experimentally and to better reflect natural conditions. As Weiss explains, reef soundscapes undergo “a crescendo as the sun sets and rises each day.” Further, fish sound patterns can change daily, weekly, and seasonally. He was tasked with building a more robust and flexible system that could be programmed to play files that matched the day-time and night-time cycles of a healthy reef. “We have a bank of recordings from the entire night, in hour-long chunks. We wanted to ensure that on the hour the right file was playing for…