Scientists record fish sounds to unlock secrets of coral reef ecosystems. Photo / Getty Images
The cow goes moo. The duck goes quack. The dog goes woof. And the fish goes ... what, exactly?
Toddlers aren’t the only ones asking this question.
Scientists are eavesdropping on fish to research and record the strange and subtle sounds they make underwater.
So far, they have captured a cacophony of thumps, honks, burps and grunts.
And they hope that deciphering these sounds will not only improve our understanding of these aquatic creatures but also provide data needed to monitor and protect the health of coral reefs and other marine ecosystems.
Fish sounds are “less elaborate than birdsong, they’re not as enchanting as humpback whale song”, said Aaron Rice, a Cornell marine biologist who has helped build one of the world’s largest collections of natural fish sounds.
“They’re acoustically less sophisticated, but they are significantly more diverse.”
Reports of fish sounds are ancient.
In the 4th century BC, Aristotle recognised that fish emit “inarticulate sounds and squeaks” despite lacking a windpipe.
During the Cold War, United States Navy biologists catalogued the clicks, creaks and croaks that fish make to distinguish them from enemy submarines.
Plummeting costs and technological advancements in underwater microphones and video cameras now allow scientists to record more fish sounds than ever.
“The field has really continued to explode in recent years,” said Audrey Looby, a University of Victoria fish ecologist who specialises in fish sounds.
When a birder hears a tweet, it’s pretty easy to tell which individual avian is belting the tune. Simply find the one opening its beak and puffing its chest.
Fish tend to make sounds quietly and inconspicuously. And underwater, it is much more difficult to detect the direction any sound is coming from.
“When you’re on a reef, not only can’t you hear, but even if you’re very quiet and you can hear, you can’t localise,” said Marc Dantzker, a bioacoustician who founded the FishEye Collaborative, a non-profit aiming to use passive acoustic monitoring to improve marine conservation. Rice is the organisation’s chief scientific adviser.
For the past three years, Dantzker and Rice have deployed 360-degree cameras in reefs off the coasts of Hawaii’s Big Island and the Dutch Caribbean island of Curacao.
The cameras are unmanned, so as not to disturb the fish. And they are paired with four microphones, picking up slight variations that help determine the source of a sound.
Most landbound vertebrates – including birds, frogs and people – push air in and out of their lungs to vocalise and communicate. Fish, without the luxury of air, have instead evolved a huge array of other techniques to make a ruckus.
Triggerfish, like those near Hawaii, have been reported to slap their pectoral fins on specialised scales and membranes.
In the Caribbean, glasseye snappers rattle their swim bladders, gas-filled organs that fish use to change their buoyancy. The raps from these chatty, nocturnal fish may be a form of social interaction.
Blackbar soldierfish use special sonic muscles stretching from the back of their heads to their ribs, which pulse to vibrate the ribs and the swim bladders beneath.
These solitary, nocturnal fish hide in crevices during the day and chase away soldierfish that encroach on their hideouts.
Fish make sound for the same reasons terrestrial animals do: to find and keep mates and resources.
“Birds do the same thing,” Rice said. “Humans, ultimately, do the same things.”
Rice and Dantzker’s team watched hours of underwater footage to identify the unique prattle, trill, and chirp made by 46 species in Curacao and another 31 in Hawaii.
The results from Curacao were published on Friday in the journal Methods in Ecology and Evolution.
Looby, who was not involved in the study, said the recording technique is “the most cutting edge” in the field. “They’re the furthest along development of being able to record fish sounds in the wild.”
There is still more work to do to improve the process. It took two days of sitting in front of a computer to annotate an hour of footage, Dantzker and Rice said.
They hope artificial intelligence will be able to do this tedious work faster, but it can’t yet.
The manual associations their team is currently making between the pictures and sounds of fish will be training data for a future AI program.
The pair continue to record, with plans to go to Indonesia and return to Curacao, and they have developed a second-generation recording device, with 10 cameras and 20 hydrophones.
“For me, it’s one of these inflection points in my career,” said Rice, who has worked on birds and whales. “Going forward, I don’t really want to do anything else.”
Many existing methods for assessing ocean biodiversity are destructive (dragging nets or running electricity through water to incapacitate and count animals) or labour-intensive (diving and counting marine life along a set length of seafloor).
Dantzker and Rice hope that with a large enough library of fish noises, conservationists will be able to deploy underwater acoustic monitors to keep tabs on rare species, track the spread of invasive aquatic pests and learn when fish mate to help decide when to open fishing seasons – all more cheaply and non-invasively than traditional methods.
“We just put it on the seafloor, and we go away, the fish get to do their thing and just through that simple act of recording we’re able to get footage that nobody has ever seen before,” Rice said.
The combination of the 360-degree camera and underwater microphones is “a revolutionary way to identify which fish are producing sounds we record on coral reefs,” said Miles Parsons, a senior research scientist at the Australian Institute of Marine Science.
He was not involved with the study but said he was “very keen to use this technique on coral reefs in Australia”.
Conservationists are seeking all the help they can get to protect coral ecosystems, which millions of people depend on to guard coastlines from storms and provide fish for protein.
The overwhelming majority of coral reefs are at risk of disappearing for good because of rising temperatures and increasing acidification of the ocean, according to United Nations scientists.
“We don’t have the luxury of just doing science for the sake of science,” Rice said.
“There is this global and societal need to try to do everything that we can to bring tech to solving ocean conservation problems.”
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