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Why do fish swim in circles?

Fish often swim in circles when the school wants to stay put: due to wintering, before spawning or forced to by predators. The understanding of this behaviour may provide us with important general insights regarding interactions between predators and prey. But how are decisions made when one million fish swim together and act as one unit?

Fish often swim in circles when the school wants to stay put: due to wintering, before spawning or forced to by predators. The understanding of this behaviour may provide us with important general insights regarding  interactions between predators and prey. But how are decisions made when one million fish swim together and act as one unit?

By Gro Nilsson and Rune Vabø


Fish schools often swim in circles when attacked by predators. Photo: BBC

The most dominating advantage of schooling is understood to be protection against enemies. An extreme but common response of schools attacked by predators is the formation of a circle. This behaviour is peculiar and has therefore often been seen as an artefact by scientists, even though it is very common across several species and ecosystems. We believe it must have a functional and evolutionary basis, where the circling group behaviour has benefits to the individual fish.

Predators may take advantage of the collective behavior

Sometimes it may seem stupid of the fish to swim in a circle. Both killer whales and baleen whales often force the schooling fish to the surface and take advantage of the packed circling fish. While a baleen whale may eat the whole school in mouthful, the killer whales and other predators eat the fish one by one until there are none left.

The question is whether this is some kind of evolutionary arms race between predators and prey. Is it the predators that shape the schools? Or is it the schooling fish that benefits from the behaviour?

Millions of small decisions

A fish school can be understood as a leaderless, self-organised system, where the dynamics of the school emerges from mutual local interactions between individual fish simultaneously across the entire school. When there are millions of fish there are also millions of small decisions and responses every second, each decision being based on internal motivation, the configuration of nearby fish but, importantly, also by imitation of the responses of neighbouring fish. This process results in what can be observed from the outside: the collective behaviour of the school.

A collective decision of a school originates from this type of self-organising process, not as a democratic agreement among individuals, but as an emergent outcome of all the interactions and responses within the school. This principle is not very different from what happens when humans with mutual interactions in a community collectively decide to speak the same dialect or engage in the same cultural practices.

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Marine Research News No 1. 2007