Herring communicate with each other by forcibly expelling gas from the anal area, producing bubbles and a high-pitched sound. The researchers call this sound production an FRT (Fast Repetitive Tick). They did have another word in mind when they created the term, however.
Both the Atlantic herring (Clupea harengus) and the Pacific herring (Clupea pallasii) produce FRTs. The fish gulp air from the water surface and then store it in the swim bladder. During the night, in darkness and when surrounded by other herring, air is released through the anal duct. The gas that is emitted isn’t made from the digestion of food, since captive herring produce the sounds whether or not they have been fed.
Herring have a good sense of hearing. The purpose of the FRT sounds may be to ensure that the fish stay close together.
Herring are among the more spectacular schooling fish. They aggregate together in huge numbers. The largest schools are often formed during migrations by merging with smaller schools. “Chains” of schools one hundred kilometres long have been observed of mullet migrating in the Caspian Sea. Radakov estimated herring schools in the North Atlantic can occupy up to 4.8 cubic kilometres with fish densities between 0.5 and 1.0 fish/cubic metre, totalling about three billion fish in a single school.These schools move along coastlines and traverse the open oceans. Herring schools in general have very precise arrangements which allow the school to maintain relatively constant cruising speeds. Herrings have excellent hearing, and their schools react very rapidly to a predator. The herrings keep a certain distance from a moving scuba diver or a cruising predator like a killer whale, forming a vacuole which looks like a doughnut from a spotter plane.