Scientists Learn to Hear What Fish Are Saying

On the fringes of the Gulf Stream, off the east coast of Florida, the sea is very deep and very blue. I hold tight to the railing on the fly deck of the dive boat as it rolls sharply from side to side, and look down into water that’s a thicker, denser color than I’ve ever seen.

For a moment I imagine that if I leaned over the side and dipped my hand in the water, it would come out coated in blue, like paint. Golden fragments of seaweed float by, escapees, perhaps, from the Sargasso Sea’s swirling gyre in the Atlantic Ocean. I would be content to stay on deck, watching the sea’s colors go by, but there are deeper things for me to see. I pull on my dive gear and jump in. Beneath the waterline, as I kick downward, the colors lose their intensity and slowly fade away.

Sitting on the sandy seabed at 100 feet is a shipwreck. It’s a tanker that was seized in 1989 after U.S. customs found it stuffed with marijuana, and was then deliberately scuttled and sunk to create a new underwater habitat. I aim for the deck that’s become fuzzy with a halo of seaweeds, corals and other soft creatures, and hunker down behind the railing at the back of the ship in a quiet spot away from the current.

Dark shadows lurk nearby in a hatch in the tanker’s superstructure. Before I see the animals inside, I hear them, or rather I feel them push pressure pulses into the water that resonate through my body. The bass notes are probably around 50 or 60 hertz, the lower notes on a pipe organ. Another boom and I notice the wreck is vibrating. Then a fish reveals itself, a goliath grouper. It looks as if it were carved from a great chunk of granite; it may well weigh as much as a grizzly bear.

Since the wreck was installed on the seabed, the goliaths have adopted it as a seasonal home where they congregate in the summer months to mate. There are far fewer of these fish across the midwest Atlantic than there once were, though. Not so long ago, their meat was canned for dog food and their carcasses used to smuggle drugs into the U.S. For decades, they’ve been a favorite of sports fishermen who love to reel them in, hold them up for photos, then throw them back into the sea, already dead. A 2009 study measured the goliath grouper’s historic decline using archives of trophy photographs taken by sport fishermen. In the 1950s, the catch of goliaths often outweighed the human passengers on board a sport fishing boat; their numbers had already been decimated by the late ’70s.

Following prohibition on hunting them in U.S. waters in the 1990s, goliaths seem to be doing a little better, at least in eastern Florida. If you venture underwater at the right time of year, there’s a good chance you’ll find a crowd of these giants and hear the deepest fish voices in all the oceans. It’s not clear exactly what meaning lies behind these sonorous calls — a warning, perhaps, or a male showing off to females — but there’s little doubt that these big fish are indeed talking to each other.

It would be easy to assume fish are silent and unhearing creatures. They don’t have ears, at least not ones that stick out of their heads. And the sounds of the sea stay trapped below: Most sound waves don’t pierce the waterline, but instead bounce back down into the depths.

Fish certainly do make and hear sounds, but it’s taken a long time for people to realize just how sonic an aquatic world can be, partly because we ourselves are not well adapted to hearing when our ears are full of water. Normally, airborne sound waves travel down a canal toward our inner ears, making our eardrums vibrate. But when that canal is flooded, it dampens the quivering membranes, muffling the sound.

The handful of noisy fish that have been known since antiquity are ones that protest loudly when they’re taken out of water and dangled in fresh air. Aristotle wrote about fish that call like cuckoos, grunt or make piping sounds; there were also, he said, some sharks that squeak.

Another difficulty in hearing fish sounds underwater is the fact that normally, in air, the slight delay between sound waves reaching our left and right ears tells our brains where the sound is coming from. Traveling so much faster in water, sound waves hit both ears almost simultaneously, making it hard to pinpoint the source.

In between my noisy breaths when I’m scuba diving, there’s often a diffuse cloud of sound all around me. It takes something as loud and obvious as a booming goliath grouper to give me a good idea of what’s going on. All in all, human ears are not good at picking up and distinguishing the sounds of fish. To make sense of underwater noise, to fully appreciate how talkative fish can be, we need special recording devices to do the listening for us.

In December 1963, a woman with short curly hair sat behind the wheel of a gray Chevrolet sports wagon as she drove north from Rhode Island, along America’s eastern shore toward Maine. The car was packed with gadgets; there were banks of waterproof microphones, spools of cables hundreds of meters long, two-way radios and walkie-talkies, battery packs and generators, a collapsible aquarium tank made of canvas, and an aluminum boat strapped to the roof. This was a fast-response mobile listening station, on a mission to find noisy fish. The driver’s name, it just so happened, was Marie Poland Fish. She was usually known as Bobbie.

As director of a research lab at the University of Rhode Island, Bobbie’s work was funded by the U.S. Navy. Back then, the military was keen to know what sounds fish make.

Historically, mariners have reported eerie sounds at sea. Moans, thumps and clanking of chains made many think their ships were haunted. This clamor became a major problem in World War II, when the hydrophones of underwater listening stations could no longer detect the distant whir of ship and submarine propellers. Submariners described all sorts of unidentifiable noises: mild beeping, croaking and hammering, whistling and mewing, coal rolling down a metal chute and the tapping of a stick being dragged along a picket fence. At times, the racket even drowned out the biggest battleships, disabling an important part of wartime surveillance.

Following initial investigations, it became clear that some of the noise came down to waves, wind and tides — but animals were chiefly to blame. Fish were so noisy they triggered underwater bombs, which were supposed to detonate only at the sounds and vibrations of a nearby submarine. There was obvious strategic advantage to be gained from knowing more about the hubbub of sea life, including when and where it was noisiest. That’s where Bobbie Fish came in.

When the war finished, and for the next 20 years, she set out to record and identify these unseen sound-makers, most of them fish. Using hydrophones developed as part of the war effort, she fixed long-term listening stations in rivers and bays to gather ambient sounds of the underwater world. Between 1959 and 1967, a research boat went out every week into Narragansett Bay, off the coast of Rhode Island, and brought back fish to Bobbie’s lab, where she recorded their voices.

In 1970, she co-wrote Sounds of Western North Atlantic Fishes, a book filled with spectrograms that showed the shape and texture of fish sounds. Some of the spectrograms came from the fish Bobbie recorded in Maine’s Boothbay Harbor, like a pollock that was lowered into the canvas tank and made thumping sounds when it was handled; its spectrogram shows repeated smears of sound, like a comb dragged through paint. Another Boothbay fish was the grubby, whose spectrogram has two clean lines, one lower- and one higher-pitched, both lasting for four seconds, then repeating for two seconds more. The book also features the voice of an ocean sunfish that was found just outside Narragansett Bay and held in a sea pen. It made rasping grunts like a pig, which became louder and more frequent the more it was handled. A goliath grouper in Puerto Rico let off a tremendous boom whenever it was prodded, producing a spectrogram that looks like a series of short strokes of a soft paintbrush; another in the Bahamas stayed quiet, although it did, on one occasion, almost swallow the hydrophone in its enormous mouth.

These findings helped Navy personnel tune out the sounds of fish and once more tune in to the sounds of their enemies. Bobbie had shown it’s not just a few fish species that are noisy, but hundreds of them.

Indeed, fish gnash their teeth to make rasping sounds. Coral reef-dwellers called grunts get their name from the grunting sounds they make by grinding their second set of teeth together at the back of their throats. Porcupine fish rub their toothless jaw bones together, making a sound like a rusty hinge. Sculpins use muscles to rattle their pectoral girdle. The list goes on and on.

Since Bobbie’s work, biologists have, for the most part, continued to focus on the sounds individual fish make and hear. Gradually, though, a new approach is emerging. More people are beginning to listen to the entire aquatic symphony.

The world is bathed in light from the sun, and it’s also bathed in sound. Underwater, this soundscape may at first seem like a disorderly din, but there’s more to it than that. Off the coast of Western Australia, a series of waterproof microphones have recorded distinct dawn and dusk choruses, lasting for hours at a time. These are the sounds of thousands of fish, calling to each other, fighting, flirting, mating and eating at those most active times of day. There is structure in this noisy world.

In the cool, fish-rich rocky reefs off New Zealand’s North Island, another set of listening devices revealed that different habitats have their own particular sounds and a unique acoustic signature. By listening, it’s possible to tell apart a rocky reef covered in seaweed from one inhabited by sea urchins; as they graze and scrape the rocks with their teeth, the urchins’ shells resonate like bells.

Much remains unknown about how fish listen to these ambient sounds. It could be that they try to tune it out so they can hear each other, like having a conversation at a loud party. But there are clues that the backdrop of noise matters to them, that fish listen in and extract useful information from the sonic miscellany.

Nocturnal sounds may be especially important. In shallow tropical seas, many fish are on the move between day and night. During the day, some hide and rest in patches of coral reef or among mangrove tree roots. Then, as night falls, they swim to nearby seagrass meadows to feed. Most make their move when it’s dark in the hope they’ll go unseen by the most dangerous predators, the bigger fish that hunt by sight. Similarly, newborn fish spend their first days and weeks in open water, again to avoid the reef’s many hungry mouths. In time, the young ones’ muscles and fins become strong enough to push against tides and currents. Only then do they turn around and begin a long swim back home. They are guided at night by a built-in magnetic compass and during the day by a celestial compass, pinpointing the position of the tropical sun beaming down on the water.

As they get closer, the young fish zero in on their native habitat, following their noses and also their ears, listening for the sounds that could act as beacons guiding the traveling fish through the dark.

To investigate this idea, Craig Radford from the University of Auckland in New Zealand led a research team that built small, identical piles of coral rubble, spaced out across shallow waters around Lizard Island on Australia’s Great Barrier Reef. Through underwater speakers suspended over each rubble pile, researchers played back soundtracks recorded in different habitats. The morning after a noisy night, Radford and his team counted the fish that had arrived on each rubble pile and found that some did indeed seem to be lured by the sounds of certain habitats.

Young damselfish headed for rubble piles that sounded like a fringing reef (dominated by the popping and cracking of pistol shrimp as they snapped their claws) and young bream were drawn to the piles that sounded like an open lagoon. Far fewer fish were enticed by the sound of silence, played back to them in the control rubble piles. It’s early days, but it seems likely that fish can distinguish between the sounds of different places underwater, and follow their ears to the spot they most want to be.

These habitat soundscapes are subtly composed. Recent studies are revealing that, far from this being an impromptu free-for-all, fish don’t simply yell and shout however and whenever they want: They fit their voices together like an orchestra of instruments in a melodic musical score.

One such study took place off the KwaZulu Natal coast of South Africa, in the Indian Ocean, a short way south of the Mozambique border. Just off shore, steep canyons carve into the seabed. About 330 feet down, in a cave where coelacanths live, a team of European researchers led by Laëtitia Ruppé wedged a small recording device into a crevice in the wall. After two months, the team fetched the device and listened to the sounds of the cave dwellers.

South African biologists inside mini-submarines previously had visited caves in the area. They’d seen hundreds of fish species living down there, including sound-making groupers, soldierfish and toadfish. So it was perhaps no surprise when the cave recordings played back thousands of noises, many of them fish voices. But what was surprising was the patterns those voices made.

Taking the most obvious voices and plotting them on spectrograms like the ones in Bobbie Fish’s book, Ruppé’s team found that, at night, fish were acoustically avoiding one another. In two dimensions, pitch and time, each voice occupied its own space on the spectrograph, like pieces of a sonic jigsaw — different fish called at different times or different pitches, building up distinct layers of sound.

There were deep, isolated booms, low and long tones and clear, coarse pulses, pops, grunts and high-pitched whistles. The species awake during the day produced more jumbled sounds, perhaps because they could see one another and combine their calls with gestures; when they call, they can swim and flick their fins in eye-catching ways, like shouting to a friend on the other side of a busy room and waving at the same time to catch their attention. In the dark of night, when fish can’t see each other, it matters more if they have overlapping, clashing calls. Nocturnal species make sure their voices don’t drown each other out.

These fish are partitioning sound in the same way they divide up many other aspects of their ecosystem. Within a community, species evolve to eat different foods and they split up the physical space they occupy. Now it’s becoming clear that species also set out and establish their own vocal territories.

The ecology of sound is still a relatively new idea, and so far has mostly been applied to terrestrial ecosystems. There are various birds, insects and frogs that similarly divide up their soundscapes and avoid masking each other’s calls. Studies on land also point to the problems that unfold for these vocal species when the world becomes noisier with human sounds. Traffic makes it difficult for birds to hear each other and they can miss important messages, particularly during mating times.

It’s too early to say whether fish will suffer as we fill the oceans with our human sounds, from shipping traffic, seismic surveys, underwater sonar and thousands of off-shore oil and gas platforms. Marine mammals are the focus of most investigations into underwater noise pollution. Fish studies are few and far between. But chances are, there are many fish out there whose lives are shaped by sound — fish that are doing their best to talk and make themselves heard in the clamor of an increasingly noisy world.