Shoaling Behavior
A shoal is any group of fish that remain together for social reasons. A shoal may be a school (a coordinated group with synchronized movements) at some times and a less organized mass at others. Shoaling is of considerable interest because of its prevalence, and various hypotheses have been put forward to explain it: increased efficiency of movement in water (hydrodynamic efficiency), increased efficiency of food finding, reproductive success, and reduced risk of predation.
The idea that shoaling increases the efficiency of swimming applies mainly to schooling. This idea is very appealing, both because of the regular spacing that seems to characterize fish in schools and because fish in shoals tend to be uniform in size. To gain hydrodynamic advantages, however, each fish must maintain rather precise positioning within a school to use the hydrodynamic lift created by its neighbors. By and large, measurements of fish positions within schools in the laboratory have not found this to be true. Nevertheless, the regular spacing of fish in schools observed in the wild indicates that being in a school probably does have a hydrodynamic advantage- -at least for fish that are behind other fish. Some scientists argue that the leadership of a school is constantly changing, because while being immersed in a school incurs hydrodynamic advantages to individuals, leaders of schools are first to find food, which is also advantageous.
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