Biologists have linked a mysterious, underwater farting sound to bubbles coming out of a herring’s anus. No fish had been known to emit sound from its anus nor to be capable of producing such a high-pitched noise.

“It sounds just like a high-pitched raspberry,” says Ben Wilson of the University of British Columbia in Vancouver, Canada. Wilson and his colleagues cannot be sure why herring make this sound, but initial research suggests that it might explain the puzzle of how shoals keep together after dark.

“Surprising and interesting” is how aquatic acoustic specialist Dennis Higgs, of the University of Windsor in Ontario, describes the discovery. It is the first case of a fish potentially using high frequency for communication, he believes.

Arthur Popper, an aquatic bio-acoustic specialist at the University of Maryland, US, is also intrigued. “I’d not have thought of it, but fish do very strange and diverse things,” he says
Grunts and buzzes

Fish are known to call out to potential mates with low “grunts and buzzes”, produced by wobbling a balloon of air called the swim bladder located in the abdomen. The swim bladder inflates and deflates to adjust the fish’s buoyancy.

The biologists initially assumed that the swim bladder was also producing the high-pitched sound they had detected. But then they noticed that a stream of bubbles expelled from the fish’s anus corresponded exactly with the timing of the noise. So a more likely cause was air escaping from the swim bladder through the anus.

It was at this point that the team named the noise Fast Repetitive Tick (FRT). But Wilson points that, unlike a human fart, the sounds are probably not caused by digestive gases because the number of sounds does not change when the fish are fed.

The researchers also tested whether the fish were farting from fear, perhaps to sound an alarm. But when they exposed fish to a shark scent, there was again no change in the number of FRTs.
Night waves

Finally, three observations persuaded the researchers that the FRT is most likely produced for communication. Firstly, when more herring are in a tank, the researchers record more FRTs per fish.

Secondly, the herring are only noisy after dark, indicating that the sounds might allow the fish to locate one another when they cannot be seen. Thirdly, the biologists know that herrings can hear sounds of this frequency, while most fish cannot. This would allow them to communicate by FRT without alerting predators to their presence.

Wilson emphasises that at present this idea is just a theory. But the discovery is still useful, he says. Herring might be tracked by their FRTs, in the same way that whales and dolphins are monitored by their high-pitched squeals. Fishermen might even exploit this to locate shoals.

There may even be a conservation issue. Some experts believe human-generated sounds can damage underwater mammals. Now it seems underwater noise might disrupt fish too.

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