Herring fish have a secret nocturnal habit of squeezing gas bubbles out of their anal pores, producing distinctive noises like someone blowing raspberries, a quirky new study by Canadian and U.K. researchers has revealed.

The flatulent fish are known to respond to underwater sounds but their apparently unique sound production method is a scientific surprise, report a team led by Dr Ben Wilson, from the University of British Columbia, Vancouver, in today's issue of the Royal Society's Biology Letters.

"We know they have excellent hearing but little about what they actually use it for," says Wilson. "It turns out that herring make unusual farting sounds at night."

The report describes experiments in which wild Pacific and Atlantic herring were caught off the Canadian and Scottish coasts then transferred to laboratory tanks. Sound and video recordings were made to assess if and when the fish made sounds and how they responded to changing conditions and the presence or absence of predators.

Apart from revealing their sound production for the first time, the most significant finding was that an individual herring produces more such noises when it is in company with other herrings, suggesting that the sounds play a role in social communication. Infrared video recordings at night showed that the bubbles emerge in fine streams, accompanied by characteristic "raspberry" sound.

"Pacific herring produce distinctive bursts of pulses, termed Fast Repetitive Tick (FRT) sounds," the scientists write in their report.

The sounds consisted of "trains" of rapid broadband pulses lasting from less than a second to almost eight seconds. Most were produced at night, regardless of whether the fish were well-fed or hungry, and the fish could make them even when they did not have direct access to the air.

"Digestive gas or gulped-air transfer to the swim bladder, therefore, does not appear to be responsible for FRT sound generation," the researchers say. "Atlantic herring also produce FRT sounds and video analysis showed an association with bubble expulsion from the anal duct region (i.e. from the gut or swim bladder). Sound production by such means has not previously been described."

The scientists concede that the FRTs may have no purpose and are simply incidental to an expulsion of gas from their swim bladders - although it seems unlikely that they would do so mainly at night if it were simply a means to control buoyancy - but the fact that they make them more often when school densities increase implies that sound may be more important for herring than previously thought.

"The presence of high-frequency components and absence of low frequencies makes these sounds unusual when compared with other fish sounds," report the scientists.

The FRTs are near or above the known auditory range of most predatory fish but well within the detection capabilities of marine mammals: herring are important prey for many whales and seals.

The discovery opens up new possibilities for locating and studying herring through sound monitoring, the researchers note. As well, if herring do communicate with each other by sound it will mean that the potential impacts on them of human-generated sounds at sea will need to be re-evaluated, say Wilson and team.

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