Researchers record the first known noises from this prehistoric fish—and they’re loud  

With rows of armor plates, spine-tipped fins, and whisker-like sensors covered in tastebuds, Atlantic sturgeon look remarkably like their ancestors that swam alongside dinosaurs 100 million years ago. 

Now, these ancient fish have finally broken their silence. 

A team of scientists recently recorded Atlantic sturgeon congregating at a breeding mecca in the Hudson River. As they sifted through the audio, the researchers heard thousands of low-frequency rumbles that sounded like a reverberating thunderclap. These sounds, described in the journal Endangered Species Research, are the first known noises produced by Atlantic sturgeon.

Listening for these sounds, which the team dubbed “thunders,” could be crucial for conserving these fish, which are endangered across much of their range. “It's very exciting that this sound is closely tied to reproduction,” says Rebecca Cohen, a researcher who studies bioacoustics at the Cornell Lab of Ornithology and the lead author of the new paper. “Better understanding this crucial activity is what's going to drive population recovery for a depleted species like this.”

Elusive giants

Atlantic sturgeon are found from the Gulf of St. Lawrence to Florida and reach 14 feet long. Yet despite their wide range and prodigious size, these fish are evasive. Many aspects of their life history, such as their coastal migration routes and how often they spawn, remain poorly understood. These gaps hamper efforts to conserve the fish, which were decimated in recent centuries by habitat loss and overfishing. Today, all Atlantic sturgeon populations in the United States are listed as endangered or threatened. 

To protect these mammoth fish, scientists and wildlife managers have focused on protecting their freshwater breeding grounds. Like salmon, Atlantic sturgeon spend much of their adult lives in the ocean but return sporadically to spawn in the rivers where they hatched. Female sturgeon deposit up to two million unfertilized eggs along the riverbed, driving male sturgeons into a tizzy as they rush to release their sperm.

Many sturgeon living along the northeastern United States swim more than 80 miles up the Hudson River to breed along a stretch of river near Hyde Park, New York. This area is thought to be the site of the largest spawning aggregation of Atlantic sturgeon in the country.

Tuning in to the environment

To study the sturgeon population congregating near Hyde Park, most scientists have deployed nets or outfitted the fish with acoustic trackers. However, physically handling these protected fish can cause significant stress for the animal.

Cohen’s team instead utilized passive acoustic monitoring. This less invasive approach involves setting up recording devices throughout an environment and documenting local species based on sounds. In recent years, acoustic monitoring has been increasingly used to track aquatic species, including oceanic fish, marine mammals, and even crustaceans. However, the approach has rarely been utilized in freshwater ecosystems because relatively little is known about the sounds that fish make in rivers and lakes.

During the spring and summer of 2021, the team deployed underwater recording devices attached to cement blocks around the sturgeon breeding ground. Over multiple months, the team captured 31 days of audio.

Ready to rumble

As the team combed through the recorded audio, Cohen filtered out the clamor caused by nearby trains and the loud “thuds” of freshwater drums, a nonnative species of fish. This helped them pinpoint recurring rumble-like noises that peaked at a frequency of 44 hertz, which is towards the lower end of humans’ hearing capabilities. To Cohen, these noises sounded like the metallic bang of a timpani kettledrum. “You can hear pulses like someone is striking a drum and this continuous rumble around each pulse,” she says. 

The thunder-like sounds were nonexistent in April, before adult sturgeons reached the breeding ground. But during the peak of the spawning season in June and early July, the noises caused a cacophony. In just over a month, the team recorded nearly 7,700 individual rumbles. 

To confirm that the Atlantic sturgeon were making these sounds, the team recorded several captive sturgeon living in a tank at a National Fish Hatchery in South Carolina. The resulting sounds echoed the noises produced at Hyde Park. 

Decoding the thunder

Dennis Higgs, a biologist at the University of Windsor who was not involved with the new paper, thinks the sturgeon may communicate with one another using the thunders, which are more than just auditory signals. He notes that spawning lake sturgeon (which Higgs discovered make a similar thunder-like sound) caused the water to ripple as the fish released a vibrating rumble. “The females likely hear and feel the water, telling them the male is ready to spawn,” Higgs says. 

To determine how Atlantic sturgeon actually process these thunders, more research is needed to gauge the fish’s hearing capabilities. But it is clear that these fish are sensitive to noise, according to Arthur Popper, a biologist who studies bioacoustics in fish at the University of Maryland and was not involved in the paper. Popper and his team tracked sturgeons in another part of the Hudson River and found that they avoided noise pollution related to the construction of a major bridge. “It’s clear the animals are capable of hearing something, because we found that the animals swam away during the pile-driving and then came back” after the work was done, he says.

According to Cohen, tuning out anthropogenic noise to listen in on sturgeon will help researchers and wildlife managers better pinpoint optimal locations and environmental conditions for spawning. It may also help scientists gauge the sizes of sturgeon spawning populations.

These sounds can also be a clue to the state of an entire river ecosystem, Higgs says. Because sturgeon require clean water to spawn, a thundering river is likely a healthy river.