A study published in Biology Letters by researchers Ben Wilson, Robert S. Batty, and Lawrence M. Dill determined that herring send messages through bubble emissions from their hinder regions. The fish were found to produce high-frequency sound bursts up to 22 kilohertz. The noise was always accompanied by a fine stream of bubbles.
“In video pictures we can see the bubbles coming out of the anal duct at the same time,” said Robert Batty, senior research scientist at the Scottish Association for Marine Science in Oban. “It sounds very much like someone blowing a high-pitched raspberry.”
Further tests revealed these outbreaks of “flatulence” are not a response to fear or feeding. When high concentrations of shark scent were introduced to the tanks, there was no noticeable increase in bubbles or sound. Similarly, unfed herring maintained the same level of emissions.
It appears that darkness triggers the passing of messages through the passing of gas. Exactly what it is they are saying to one another remains to be seen, however it is a fair bet that it is not, “Pull my finger.”