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.

Schooling by predators increases the probability of their detecting a school of prey, just as shoaling in planktivores (fish that feed on plankton, the very small plants and animals in the water) increases the probability of detecting suitable patches of plankton. The reason for this is the presence of many searching eyes over a large area. Fish in shoals share information by monitoring each other’s behavior. Feeding behavior in one fish quickly stimulates food-searching behavior in others.?For planktivores, however, this advantage may be decreased by having to share the patch with many other fish: fish in the rear of a school are likely to encounter much lower densities of plankton than those in the front as well as higher concentrations of waste products from their schoolmates.

For fish that shoal throughout their lifetimes, little energy has to be expended in finding a mate when spawning time comes. Presumably, shoaling also allows fish to closely synchronize their reproductive cycles (through behavioral and hormonal cues). In addition, for fish that migrate long distances to breed, schooling may increase the accuracy of finding the way home, because the average direction sought by fish in a school is likely to be a better choice than that chosen by an individual migrating alone. For tunas, with habitual migration routes, this means that groups of individuals may stay together for long periods.

Shoaling reduces predation risk in two main interacting ways: by confusing predators and by providing safety in numbers. When predators attack, large shoals are particularly advantageous because of the reduced probability that any one individual will be eaten; in any given attack, a smaller percentage of a large shoal will be eaten compared to a small shoal. The confusion effect is based on the common observation that shoaling fish tend to get eaten mostly when they have been separated from the shoal; many attack strategies of predators can thus be best interpreted as efforts to break up shoals so that individuals can be picked off. Shoaling fish are the same size and silvery, so it is difficult for a visually oriented predator to pick an individual out of a mass of twisting, flashing fish and then have enough time to grab its prey before it disappears into the shoal. Shoaling fishes also are fairly uniform in size and appearance, a further factor confusing predators. Individuals that stand out (for example, because of heavy parasite infestations) are quickly picked off. Even when shoals are made of several species, individual fish are similar in size, although the species may remain segregated within the school. Thus, it has been noted that banded killifish, golden shiner, and white sucker shoal together, but that the killifish occupy the top of the shoal, the shiners the middle, and the suckers the bottom. These shoal positions reflect the preferred feeding areas of the three species (surface, mid-water, and bottom, respectively).

1.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.