A healthy marine ecosystem is an orchestra of sounds — the rhythmic humming of fish calling to each other, crabs scuttling along reefs and sea grass rustling in the currents. But when an ecosystem is dying, not much life is there to make sounds.
Mounting research shows that the sounds of a healthy ocean habitat may be a pivotal tool in bringing unhealthy marine ecosystems back to life. Numerous research teams studying soundscape diversity in the ocean over the past few decades have found this to be the case. Just like how big cities draw more people, a bustling part of the ocean seems to draw more marine life. If marine animals hear a healthy-sounding environment, they’re inclined to move in — even if the habitat has been destroyed.
What happened off Lizard Island in the Great Barrier Reef is an example.
A group of marine biologists studying ocean soundscapes regularly visited it because it’s a protected area that was once teeming with life. But in 2016, weeks of a heat wave led to an enormous coral bleaching event that wiped out much of the reef. After the event, sounds of the reef had diminished significantly, says Steve Simpson, professor of marine biology and global change at the University of Bristol. That quiet matched the desolate ghost town the reefs had become.
“When you swam around, it looked like a black and white movie with the odd painted fish [standing out with] its bright colors, because it used to be camouflaged in a world of color,” Simpson says.
Simpson’s team decided to see whether they could lure larval fish back to the almost barren reefs off Lizard Island using old sound records of the marine habitat back when it was full of life. They also built up some of the broken reefs into piles to create more shelter for the fish. Sure enough, twice as many fish took up residence near speakers playing the old soundscape recordings than the areas of the reef that had no acoustic enhancement.
“If we did that on a bigger scale, we started to think, well, maybe sound could be a tool that we use to actually accelerate recovery,” Simpson says.
Sound is a vital part of marine animals’ ability to navigate and survive in their environment. In the early stages of life, they use sound to determine which habitat is the best place to call home.
“Sound travels very far underwater without being lost to things like currents, making it a long-distance cue,” says Brittany Williams, a researcher at the University of Adelaide’s Southern Seas Ecology Lab in Australia. It’s much more useful than sight since water often obscures anything at a distance, as researchers such as Simpson quickly learned while traversing the Great Barrier Reef.
A marine ecosystem’s health can be assessed by its soundscape diversity, or how complex and productive it is.
“Soundscape diversity can be thought of as the phonic richness of an environment, or the number and loudness of biotic sounds,” says David Eggleston, director of the Center for Marine Sciences and Technology at North Carolina State University. Eggleston contributed to soundscape studies that demonstrate how oyster larvae are drawn to acoustically enhanced marine soundscapes, just as larval coral reef fish are.
Hydrophones are used to record marine soundscapes. Researchers then parse the soundscape diversity metrics within them.
“For example, we can calculate the number of snaps per minute in an ecosystem, to determine whether snapping shrimp crackle fills the ecosystem,” Williams says.
Scientists haven’t been closely listening to ocean soundscapes for long in retrospect, so researchers such as Williams are pioneers in the field. They have heard creatures that are rarely seen on diving expeditions. Some sounds are remarkable — one fish, Simpson recalls, sounded like a man laughing loudly across a bar. Other sounds are subtler, such as the rhythmic thumping that comes from an oyster toadfish’s swim bladder.
When the underwater chatter in a previously declining environment grows more boisterous, it is a clear indicator of an ecosystem healing. So with improving underwater listening and recording technology, soundscape assessments will probably become integral to showing the complete picture of a restoration project’s success. By listening to the soundscape, Simpson’s team could tell that marine life had returned in spades to a reef off Indonesia that had been nearly destroyed by blast fishing, thanks to a prolific reef recovery project there.
As the catalogue of underwater sounds grows, scientists are even learning which sounds may help bolster the restoration efforts of a specific ecosystem. “We call it ‘Reef DJ,’ where we learn how to kind of mix the right track to get the recovery to happen in a particular place in the right way,” Simpson says.
But the process involves a lot of trial and error. Williams says using targeted sounds may attract some species while it repels others, or attract two conflicting species. “For example, a sound that attracts larval oysters will be no good if it also attracts predators that eat these larvae,” she says.
Thanks to advancements in artificial intelligence, however, researchers are no longer alone in dissecting this growing soundscape catalogue. “We’ve got a team in London now developing artificial intelligence algorithms, machine-learning algorithms, to classify these sounds so that we can be listening to the ocean in real time,” Simpson says.
He says there soon may be public systems that can broadcast these real-time records globally so that anyone can hear them.
This could be useful to restoration efforts that often rely on collaborations with scientists across specialties. It could even help scientists detect ocean habitats that are just beginning to decline so restoration efforts can be deployed before the ecosystems are in real trouble. And if more civilians connect with ocean soundscapes via these public broadcast systems, the struggling ecosystems may receive more support.
As climate change continues to trigger heat-fueled bleaching events and severe storms, reefs around the world will struggle to survive. But with the help of tools such as underwater soundscape enhancement and real-time listening and parsing technology, scientists can better protect and restore what remains.
“If we can actively maintain enough healthy coral reef environments around the world in places [where] they’re least susceptible to either global or local threats, then that gives us the broodstock of the future, which will be able to repopulate areas that may lose reefs over the next few decades,” Simpson says.
