Scientists mistook the underwater rumble for a train - but it was sturgeons
In the Hudson River, researchers recorded a strange low rumble. At first, they thought they were hearing muffled trains underwater. But later it turned out that the sound did not come from the shore or from bridges, but from Atlantic sturgeons during spawning.
This is the first confirmed description of sounds associated with the spawning of Atlantic sturgeon. The scientists used underwater microphones - hydrophones - and passive acoustic monitoring: the equipment simply listened to the river for long periods of time without disturbing the fish.
The discovery is not only important in itself. The Atlantic sturgeon is a rare species that suffered greatly from fishing in the 19th and 20th centuries. If scientists can learn to find spawning grounds by sound, it will help to better protect the parts of the river where the fish spawn.
Details
The Atlantic sturgeon is a large, ancient fish that spends part of its life in the ocean and comes up rivers to spawn in the spring. In the Hudson, these fish can reach large sizes and live for decades, but they reproduce slowly: it takes many years for females to return to spawn for the first time. So populations have a hard time recovering from overfishing.
In the new study, scientists recorded a characteristic low-frequency sound associated with sturgeon spawning behaviour. According to the study description, the signal had a frequency of about 44 Hz and coincided with the presence of labelled adults during the breeding season.
The researchers speculate that this low hum may arise from the males' movements during egg fertilisation and swim bladder resonance. But this should not be understood as a "conversation" of sturgeons or a deciphered language of the fish. It is more correct to say this: scientists have described for the first time the acoustic signal that is associated with Atlantic sturgeon spawning.
The team is now placing additional underwater microphones in areas where sturgeons may spawn, but where these locations have not yet been confirmed or have insufficient protection. This method is convenient because it doesn't require constantly trapping fish, placing nets or physically interfering with their behaviour.
Why it matters
The Hudson remains a key habitat for Atlantic sturgeon, but the species' numbers are far from historic levels. Cornell cites an estimate: before the heyday of commercial fishing in the late 19th century, about 6,000 sturgeon could come up the river, but now fewer than 700.
To protect such species, it is important to know not only how many fish are left, but also where exactly they spawn. If the spawning site is unknown, it is more difficult to protect it from construction, shipping, pollution and other factors.
Sound could be a new tool. Underwater microphones can "eavesdrop" on a river and record fish activity where it is difficult to see. The study authors believe that passive acoustic monitoring can help determine the spatial and seasonal distribution of spawning, assess habitat utilisation and support conservation management.
Background
Atlantic sturgeon were once an important part of the Hudson River ecosystem. But it was actively fished for meat and caviar. Females were particularly vulnerable: they are large, take a long time to mature, and their eggs could make up a significant portion of their body weight. After heavy fishing pressure, the species was given conservation status, but recovery is slow.
Passive acoustic monitoring has long been used in marine research and in the study of birds, whales and other animals. In freshwater systems like the Hudson, this approach is less commonly used. That's why the work with sturgeon is important: It shows that rivers can also be studied through their soundscape.
Researchers are also using the recordings in educational projects: the sounds of the Hudson help show schoolchildren and the general public that an almost invisible world exists beneath the surface of a familiar river - with ships, trains, currents and rare ancient fish.
Source
Rebecca E. Cohen et al., "Sounds of Atlantic sturgeon spawning: first description and opportunities for riverine endangered species conservation with passive acoustic monitoring", Endangered Species Research, 2025.