Atlantic herring (Clupea harengus) are an abundant fish, occurring on both sides of the Atlantic. They play a central role in helping to transfer energy from species lower in the marine food web to the larger fish that prey on them. Herring feed on small zooplankton, such as copepods and amphipods, krill, and fish larvae [2, 3]. Herring shoals attract predators, which include cetaceans [4-6], seabirds (e.g. gannets) , gadoids (cod, hake etc.) [6-8] and humans. Haddock, sand eels and cod also eat the eggs (and larvae) of herring .
Atlantic herring form large schools. These are highly synchronised and fast-moving, allowing the fish to make sophisticated escape maneuvers . Schooling behaviour can start as early as the larval stage [10-12]. It decreases the risk of predation to individual fish by improved predator detection and deterrence .
Schools migrating to spawning locations are densely packed and fast moving. The maturity of individuals in schools can vary from pre-spawning stage to resting stage at the same spawning site . Single spawning schools have been observed splitting into pelagic (mostly in mid-water or upper layers of the ocean) and demersal (mostly in deeper layers of the ocean) groups . After spawning, feeding becomes the priority, and herring form smaller, less dense schools near the surface [13-15].
Herring can hear: their “ears” have thin air-filled tubes that project from the swim bladder to air chambers, connected with the inner ear . There is evidence that they can hear the ultrasonic clicks emitted by cetaceans . They can also hear sounds made by other herring . They produce distinctive bursts of pulses called Fast Repetitive Tick (FRT) sounds, aka FaRTs . The discoverers of the herring FRTs, including researchers from the Scottish Association for Marine Science, were awarded an Ig Nobel award in Biology. FRTs are thought to be produced by gas released from the swim bladder via the anal duct. As herring also shoal in the dark, it is possible that FRTs are used as “contact calls” between individuals within schools. The number of FRT’s released increases with school size and may be important to their social behaviour . Whilst FRT’s can’t be heard by most predatory fish (2 kHz), but cetaceans can locate FRTing herring through echolocation.
Herring FRTs can also be detected by submarines, which almost led to a serious diplomatic crisis in 1982 when the Swedish Navy thought Soviet nuclear submarines were in Swedish waters. However, after a decade of fruitless pursuits, Sweden discovered it wasn’t the noise of submarines, but of large shoals of FRTing herring that they were recording.
Banner image: Shoaling herring. © Andy Jackson, SubSeaTV; reproduced with permission. Herring drawing: © Carl-Werner Schmidt-L
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