"Herring and other clupeids such as pilchards and sardines have a sophisticated auditory system," said Batty. "This is made even more sensitive by a gas-filled sac near the inner ear which acts to amplify sound pressure."
Clupeid fish, like herring, anchovies, and sprats, can detect sound frequencies up to around 40 kilohertz, way beyond the hearing range of most other fish. (The normal range of human hearing is 20 to 20,000 kilohertz.) So a method of nighttime communication using pulses of air would be extremely useful. It would enable herring to maintain contact after dark, but without giving their position away to predatory fish.
While unusual, other marine fish are known to communicate using sound. For instance, male cod make a noise to attract females when they breed. But Batty adds: "These are produced using the swim bladder, which vibrates to create a kind of drumming sound. However, the method we found hasn't been noticed before."
The researchers say further studies into how herring produce such sounds could help fishermen in locating shoals. Pacific and Atlantic herring are both important commercial species in the Northern Hemisphere.
Furthermore, given the herring's sensitivity to underwater sounds, and the likelihood they use them to communicate, there are concerns about the possible impacts of noise pollution. For example, engine noise from shipping or seismic guns used for oil surveys could all interfere with the fish's hearing.
Similarly, herring-eating dolphins and whales, which can pick up high frequency sounds, may use FRTs as a foraging clue. Consequently, noise pollution may seriously impair their effectiveness as hunters, researchers say.
"There are pods of killer whales that specialize in feeding on herring," Batty said. "The fear is they won't be able to pick up the sounds the herring are making."
It might seem an amusing idea to us that herring communicate using farts. But for herring and the mammals that prey on them, FRTs may signal safetyor the next meal.
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