It turns out that herring fart. It's not digestive gas, though. Scientists call it a Fast Repetitive Tick: a FRT.
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.
