When researchers heard a low, rumbling thunder beneath the surface of the Hudson River, they first thought it was the muffled sound of distant trains. But a closer listen revealed something far more profound: the spawning calls of Atlantic sturgeon, an iconic and endangered species that has been making this ancient sound for millions of years.
For the first time, a team led by Maija Niemistö, science educator with the New York State Water Resources Institute, and Rebecca Cohen, a postdoctoral researcher at the Cornell Lab of Ornithology, has confirmed and documented these underwater vocalizations using hydrophones—underwater microphones that passively record sound over extended periods. The discovery, published in Endangered Species Research in 2025, represents a breakthrough in freshwater bioacoustics and offers a powerful new tool for protecting one of North America's most vulnerable fish.
"It's almost that you feel it more than you hear it," Niemistö explained. "You can hear these chirps and squirts and bubbles underwater, but this is a different experience entirely. These are ancient fish, and the thunder—it's almost like you're brought back in time, because they've been making this sound, communicating with each other, for millions of years."
The collaboration, involving the New York State Water Resources Institute, the New York State Department of Environmental Conservation, Cornell's Department of Natural Resources and the Environment, and the Hudson River National Estuary Research Reserve, placed hydrophones in known and suspected spawning grounds throughout the river. The thunder, Cohen said, is most likely caused by the thrashing of males—and the resonance of their swim bladders—as they fertilize eggs.
The context for this discovery is urgent. An estimated 6,000 Atlantic sturgeon came up the Hudson River to spawn before the commercial fishing era in the late 1800s. Today, that number has plummeted to fewer than 700—making the Hudson home to the largest remaining Atlantic sturgeon population on the Atlantic coast. These massive fish, which can reach 14 feet and weigh 800 pounds, were nearly erased from existence by overfishing that targeted them for their caviar. A single female could yield 20 percent of her substantial body weight in eggs, making her an extremely lucrative catch. The fish's slow reproduction—females can take up to 20 years to mature before returning to spawn—meant populations collapsed with devastating speed and have never recovered, even after nearly 30 years of legal protection.
Now, acoustic monitoring offers a noninvasive way to locate spawning grounds that are unknown or unprotected. By listening for the distinctive thunder of spawning males, researchers can identify new areas that need protection and develop better strategies for recovery. The technique, long used in marine and terrestrial research, has rarely been applied in freshwater settings—making this project a notable methodological advance.
"They've been an integral part of the ecosystem for millions and millions of years," Cohen said. "Trying to support and hold on to this remnant population is the best bet for maintaining the Hudson River ecosystem that functions in the way we all benefit from." As the state of New York works to safeguard these ancient fish, the sound of sturgeon thunder now serves as both a scientific tool and a reminder of the hidden life thriving in one of America's most important rivers.