- This article is about swarms in biology. For other uses of the term, see Swarm (disambiguation).
The term swarm (schooling or swarming) is applied to fish, birds and insects and describes a behavior of an aggregation (school) of animals of similar size and body orientation, generally cruising in the same direction.
Shoal (or school) is the collective noun for fish. Animal behaviourists use the term "shoal" for any group of fish, reserving "school" for more closely knit groups of the same species swimming in a highly synchronized and polarized manner.
Fish derive many benefits from shoaling behaviour including defense against predators (by enanced predator detection and diluting the chance of capture), enhanced foraging success, and higher success of finding a mate. It is also likely that fish benefit from shoal membership through increased hydrodynamic efficiency.
One feature of a shoal is the strong resemblance between member fish. Fish use many traits to choose shoal mates including size of shoal, species type, body size, health of shoal members, and kinship.
Fish often choose to be in a shoal that consists of individuals similar in appearance to themselves; the "oddity effect" posits that any shoal member that stands out in appearance will be preferentially targeted by predators. The oddity effect would therefore tend to homogenize shoals.
One puzzling aspect of shoal selection is how a fish can choose to join a shoal of animals of similar appearance, given that it cannot know its own colour. Experiments with zebrafish have shown that shoal preference is a learned ability, not innate. A zebrafish tends to associate with shoals that resemble shoals that it was reared in (that is, a form of imprinting).
Other open questions of shoaling behaviour include determining the direction of shoal movement. In the case of Migratory movement, most members of a shoal seem to know where they are going, but foraging behaviour is more problematic. Animal behaviourist Stephan G. Reebs, writing in the journal Animal Behaviour, argues that shoals of golden shiner (a kind of minnow) were led by a small number of more experienced individuals.
- "One of the most striking behaviours of a school is its synchronization. Hundreds of small fish glide in unison, more like a single organism than a collection of individuals" Hiro-Sato Niwa, Journal of Theoretical Biology, 1996: 181,p 47