Eavesdropping on fish could help us keep better tabs on underwater worlds

Ashlee Lillis’ interview subjects don’t talk. But they have plenty to say through grunts, growls, clicks and other odd noises.

“I put my hydrophone — which is our underwater microphone — on the end of a long pole and keep sticking it into fish faces,” says Lillis, a marine ecologist who leads Sound Ocean Science, an organization based in Gqeberha, South Africa, focused on international marine research and conservation.

Lillis is among a vanguard of researchers around the world cataloging fish sounds, aiming to put a species name to each underwater call. Eavesdropping on — and understanding — all that chatter is a powerful way to reveal under-the-surface secrets. The idea isn’t novel: Whale songs have long been used to track cetacean behavior and migrations. But now researchers want to tap into the far broader symphony of fish sounds.

It’s a whale of a task. For instance, take ray-finned fishes. More than 34,000 species make up this largest group of bony fish, including salmon, eels, herring and the like. But only about 1,000 of these species have been documented in published research to make a ruckus. Those fish span about 130 families, but a 2022 analysis estimates that 175 families — representing nearly 85 percent of all species in the ray-finned group — could have soniferous species that communicate via sound.

Fish clamor can reveal a lot: Is there the hubbub of biodiversity? Has an invasive fish species moved in? When do conservationists need to put up a “do not disturb” sign to ensure that human activity doesn’t disrupt mating season?

Sound recordings can complement more traditional ways of monitoring, such as catching fish to examine abundance and health. But beyond deepening our understanding of a largely hidden realm, there’s an urgency to the task. “We need to log these sounds as quickly as we can before climate change and anthropogenic stressors affect the aquatic world” even more than they already have, says acoustician Miles Parsons of the Australian Institute of Marine Science in Perth. Warming waters can push fish to new places and even alter their sounds, he notes. Listening in can help track those shifts. “From a science point of view, the world is changing very quickly.”

Sounds fishy

Humans have known for millennia that fish are noisy creatures; many age-old common names like drum and croaker come from the fish’s distinctive cries.

The Greek philosopher Aristotle pondered fish grunts and squeaks back in the fourth century B.C. Even then, he noted that these animals’ “voices” aren’t generated in the traditional sense. Instead, fish may rub or click their bony structures together, contract certain muscles to drum the gas-filled swim bladder or vibrate stretched tendons in fins like a stringed instrument. Some fish even expel air out of their rear ends, aptly named fast repetitive tick, or FRT, sounds. These numerous ways to produce sounds evolved independently about 33 times in ray-finned fishes, a 2022 study in Ichthyology & Herpetology suggested.

Fish “probably have the greatest diversity of sound-producing mechanisms across the tree of life,” says marine ecologist Audrey Looby of the University of Florida Nature Coast Biological Station in Cedar Key. Many known fish sounds are within human hearing range, but they’re relatively quiet and occur in an environment where we’re not suited to hear well.

Fish hear each other thanks to tiny stones in their heads, which move in response to sound vibrations, triggering signals to the brain. It’s similar to how human hearing works. The animals also have specialized sensory cells running down their bodies that detect movement, including sound waves, in the water.

Fish sounds can serve different purposes. Some are distress signals, warning others of danger or attempting to scare away a predator — noises well-known to fishers, Looby says. “A lot of the fish that you catch will make grunting sounds when they’re caught.”

Other cries indicate aggression, produced when fish mark territory or get into fights. Some of the most ear-piercing noises deal with reproduction. For instance, certain male plainfin midshipman (Porichthys notatus), which live off the North American west coast, advertise their reproductive quality to picky females through an incessant foghorn-like hum (SN: 10/29/16, p. 4).

“They’re vibrating their swim bladder like crazy,” says marine community ecologist Kieran Cox of Simon Fraser University in Burnaby, Canada. When he worked on a project involving the species, the males’ eerie hum would surround him during late nights in the dark fish tank–filled lab. The louder the mating call, Cox says, the greater the number of females drawn in to lay eggs in the male’s cavelike nest for him to fertilize and guard.

Still, there’s an ocean (and lake, river and pond) full of fish sounds with unknown sources and purposes. Simply noting the variety of sounds, even if the specific sources are mysterious, can give glimpses into biodiversity and ecosystem health. But the lure of learning more intimate details of fish lives is spawning new efforts to solve those mysteries.

Spying on soniferous species

The modern field of underwater bioacoustics had a fraught start. During World War II, the ocean served as a battlefield, where submarines kept an ear out for enemy vessels.

“The technicians that were listening on the submarines were hearing all these weird things,” says marine bioacoustician Michelle Schärer-Umpierre of HJR Reefscaping, an environmental consulting group based in Cabo Rojo, Puerto Rico. “They thought it was enemy warships, but they were really animals.”

After the war ended, the U.S. Navy hired marine biologist Marie Poland Fish to investigate. She began cataloging the sounds of fish and other sea creatures both in their natural habitats and in laboratory tanks at the University of Rhode Island’s campus in Narragansett. More than two decades later, in 1970, she published a book — accompanied by audio recordings — with colleague William Mowbray containing analyses of 153 fish species’ sounds out of 220 described species. By the end of her career, Fish had recorded and examined the sounds of more than 300 marine species.

But that’s merely a drop in the bucket.

Other researchers, including Schärer-Umpierre, have resumed that work. For almost 20 years, Schärer-Umpierre has been studying the sounds made by groupers, an assortment of large-mouthed, stout-bodied fishes. She figured out what behaviors certain noises match up with. Using passive acoustic monitoring, she now uses that link to track what the fish are doing in the wild. Listening to their underwater calls helps her spy on these fish without disturbing them.

Most of the time, groupers are solitary animals whose sole sounds are alarm calls produced when in danger. But at specific times of the year, many species travel long distances — up to hundreds of kilometers — to tropical waters to reproduce. That includes to the Caribbean, where Schärer-Umpierre studies the fish. The massive, sometimes monthslong gatherings that happen there result in a symphony of diverse grouper sounds associated with reproduction.

Most of the ruckus comes from males. Male red hind groupers (Epinephelus guttatus), for example, make distinct noises when fighting over territory, courting females and preparing to release sperm to fertilize eggs. The latter sound consists of nonstop singing for a few hours on nights around the full moon, Schärer-Umpierre says. Females make just one noise. “It’s a very short grunt that doesn’t really ring very well.”

Groupers are both ecologically and commercially important in the Caribbean, and sex-specific sounds can help fishery managers better understand what’s going on during aggregations, as well as the ratio of males to females. Poor weather can prevent visual surveys, and determining the timing of breeding seasons often relies on cutting open females to assess egg development.

The sounds can also signal when the groupers are easy targets, leading to overfishing: Many species are so focused on reproduction that they don’t swim away when danger is near, Schärer-Umpierre explains.

To protect groupers during this vulnerable period, the Caribbean Fishery Management Council, CFMC, forbids fishing certain species during breeding. The red hind’s closed season lasts from December through February, and it was first implemented in 1993 based on the fish’s physical characteristics measured throughout multiple breeding seasons. But more recent passive acoustic monitoring off of western Puerto Rico has shown that this period doesn’t quite align with the species’s reproductive schedule, Schärer-Umpierre reported at a meeting of the management council in 2021.

“We have never seen in Puerto Rico, since we started doing this in 2007, that the red hind aggregated during the month of December,” she says. The fish have, however, remained gathered in early March. Using that data, she’s urging the CFMC, which creates regional fishery management plans for approval by the U.S. Department of Commerce, to shift the species’s closed fishing season.

Tracking a fish interloper

Fish sounds aren’t just good for tracking species we want to protect; they can help solve other environmental challenges, too.

In 2003, ichthyologist Rodney Rountree, then at the University of Massachusetts Dartmouth, was presenting preliminary data at a scientific meeting in San Antonio. When he played a mystery sound recorded in the Hudson River in New York, it struck a chord with a fellow researcher. The listener suspected it was a type of drum, belonging to a collection of fishes known for their deep rumble.

The suggestion surprised Rountree; he wasn’t aware of any drum species living in the freshwater area where the sound was recorded. But he later found documentation that the freshwater drum (Aplodinotus grunniens, whose species name means “grunting” in Latin) had made its way into the river.

“It’s going to change the dynamics of the Hudson River tremendously because it is a very successful river species,” Rountree thought at the time.

Scientists weren’t sure how established the drums were in the river system, or how they got there. Some speculated that the drums came from the Great Lakes’ native populations. Rountree eyed another potential source: Lake Champlain, which lies north of the Hudson River. Perhaps, he thought, the roughly 100-kilometer-long Champlain Canal connecting the two bodies of water might be a fish passageway. As a source, it had largely been ignored, Rountree says, possibly because the Great Lakes’ drums were more well-known and the Erie Canal is a prominent connector to the Hudson River.

In July 2010, he drove south along Lake Champlain and the Champlain Canal for three days, stopping roughly every 10 kilometers to assess the soundscape, the cacophony of noises below the water’s surface. “I would drop the hydrophone into the water and listen for a few minutes — and brr, brr, brr or silence was out there,” Rountree says.

Based on those recordings, he and a colleague reported in 2017 in Biological Invasions that Lake Champlain’s drum population probably played a large part in the migration into the Hudson River. The project also showed how quickly and easily such passive sound recordings could be used to snoop on species.

“I did it based on just my free time in a few days,” says Rountree, who is now primarily an independent bioacoustics consultant known as the Fish Listener based in Waquoit, Mass.

Catching fish, rather than listening for them, is one of the main traditional ways to survey an aquatic environment. Scientists can make direct measurements on fish, though it sometimes requires killing the animals. More technologically advanced approaches include using sonar to estimate biomass or tagging the animals to track their locations.

Such operations can be time-consuming and costly. And, while useful, many represent “a snapshot at one point in time,” says acoustician Xavier Mouy of the National Oceanic and Atmospheric Administration’s Northeast Fisheries Science Center in Woods Hole, Mass.

Passive acoustic monitoring, on the flip side, can record sounds for months at a time, and it’s become easier and more accessible in recent years. Simple underwater recording devices can cost as little as $135 while more advanced versions start at around $3,000.

“We’ve got a situation where now we can very easily collect terabytes and terabytes of data,” says marine biologist Tim Lamont of Lancaster University in England. “So there’s all this data that we’ve never been able to get before.”

A deep dive into these underwater symphonies will allow scientists to learn more about changes in fish populations over time and across broad areas, adding a new dimension to traditional survey methods. Scientists expect that data will help track large-scale shifts induced by climate change and other human influences.

For instance, Schärer-Umpierre suspects that the Caribbean’s warming waters are pushing the red hind’s aggregation period even later, as the fish prefer to spawn when the sea cools to around 26.5° Celsius. And a 2023 study in PeerJ suggests that increasing water temperatures and ocean acidification will enhance how sound travels in some ocean regions worldwide, making ship noise there up to seven decibels louder by the end of the century.

Such human-caused discord can harm a fish’s ability to find food, reproduce, escape predators and even induce hearing loss, according to an analysis that Cox and colleagues published in 2018 in Global Change Biology (SN: 3/14/20, p. 13). Listening in may help scientists document some of these changes and push for policies to limit noise pollution. Canada, for instance, is developing the Ocean Noise Strategy as a road map for addressing the problem.

Modern mysteries to solve

Efforts to build repositories of underwater animal sounds are growing. In 2021, Looby, Cox, Rountree and others joined forces to launch the first library of every known fish that produces sound, according to published research. Today, the library documents more than 1,000 soniferous fish and holds more than 1,200 audio recordings.

In 2022, scientists sounded the call for a global library of underwater biological sounds to detail the ruckus made by every underwater species, from fish and mammals to invertebrates. The scientists behind this effort, known as GLUBS, also hope to create a data repository for soundscape recordings, a community science platform where anyone can upload underwater animal sounds, and an AI-based system to detect different calls from these recordings.

Researchers are now collecting so much data that “the old manual techniques that people used to use to look for specific calls just aren’t appropriate anymore,” says Parsons, who is a leader of the GLUBS effort. Some scientists and companies have already developed algorithms that can detect specific sounds in large soundscape recordings. Parsons hopes to have GLUBS up and running in two years, depending on funding.

Still, much of what’s recorded leaves people puzzled. “In acoustics, you come across hundreds and hundreds of mystery sounds,” says Jill Munger, who until recently worked as a marine acoustic analyst at Conservation Metrics, a company in Santa Cruz, Calif., that develops wildlife monitoring tools.

New innovations to simplify matching sounds with species in the wild may also provide crucial clues. Mouy and colleagues, for instance, have developed easy-to-assemble audio-video arrays. A large version consists of an open house-shaped structure built from PVC pipes, measuring 2 meters wide, 2 meters long and 3 meters high, that fish and other critters can swim through. Six hydrophones sit at different locations, and their simultaneous recordings can help pinpoint the source of a sound produced inside the structure. Two video cameras point inward to identify the noisemaker. The devices can record two weeks of data at a time.

When testing the array and two smaller versions off the coast of British Columbia, Mouy recorded the sounds of a lingcod (Ophiodon elongatus), a species not previously documented in research to be soniferous. How the fish, which doesn’t have a swim bladder, produces its pulsating grunt remains unknown.

Such great unknowns can both frustrate and delight. It was Lillis’ annoyance with the lack of research on fish sounds that led her down the “shrimp hole” of “interviewing” fish, she says, which she started doing a little over a year ago. She’s currently testing various eavesdropping techniques, including her own version of one of Mouy’s arrays, to match species to their calls.

Meanwhile, Munger, who’s now at the University of New Hampshire in Durham, has been obsessed with a bugling sound heard near both Hawaii and the Palmyra Atoll farther south. She and colleagues dubbed the noise “cascading saw” for its shape when graphed as pitch over time, which resembles sawlike teeth that rapidly tumble downward. People have been weighing in, whittling away possible species. But the call’s origin still eludes Munger.

“It’s definitely a mystery,” she says. “Everybody loves a good mystery.”