Forage fish compensate for their small size by forming schools. Typical ocean forage fish are small, filter-feeding fish such as herring, anchovies and menhaden. Predators include other larger fish, seabirds and marine mammals. Р В РІР‚СњР В РІР‚РЋorage fish are small fish which are preyed on by larger predators for food. Fast moving schools usually form a wedge shape, while shoals that are feeding tend to become circular. Schools that are travelling can form long thin lines, or squares or ovals or amoeboid shapes. The shape a shoal or school takes depends on the type of fish and what the fish are doing. Shoals are more vulnerable to predator attack. When schooling fish stop to feed, they break ranks and become shoals. Such shifts are triggered by changes of activity from feeding, resting, travelling or avoiding predators. Shoaling fish can shift into a disciplined and coordinated school, then shift back to an amorphous shoal within seconds. Facultative shoalers, such as Atlantic cod, saiths and some carangids, shoal only some of the time, perhaps for reproductive purposes. Obligate shoalers, such as tunas, herrings and anchovy, spend all of their time shoaling or schooling, and become agitated if separated from the group. Here a school of jacks accompany a great barracuda.Р В РІР‚СњР В РІР‚РЋish can be obligate or facultative shoalers. Schools of forage fish often accompany large predator fish. The way the fish array in the school probably gives energy saving advantages, though this is controversial. Schooling also has disadvantages, such as excretion buildup in the breathing media and oxygen and food depletion. Many hypotheses to explain the function of schooling have been suggested, such as better orientation, synchronized hunting, predator confusion and reduced risk of being found.

The intricacies of schooling are far from fully understood, especially the swimming and feeding energetics. The schools undertake complicated manoeuvres, as though the schools have minds of their own. Fish schools move with the individual members precisely spaced from each other. Schooling fish are usually of the same species and the same age/size. If the shoal becomes more tightly organised, with the fish synchronising their swimming so they all move at the same speed and in the same direction, then the fish may be said to be schooling. Shoaling groups can include fish of disparate sizes and can include mixed-species subgroups. Although shoaling fish can relate to each other in a loose way, with each fish swimming and foraging somewhat independently, they are nonetheless aware of the other members of the group as shown by the way they adjust behaviour such as swimming, so as to remain close to the other fish in the group. If, in addition, the aggregation comes together in an interactive, social way, they may be said to be shoaling. An unstructured aggregation might be a group of mixed species and sizes that have gathered randomly near some local resource, such as food or nesting sites. Fish aggregations can be structured or unstructured. Underwater video loop of a school of herrings migrating at high speed to their spawning grounds in the Baltic SeaР В РІР‚СњР В РІР‚С›n aggregation of fish is the general term for any collection of fish that have gathered together in some locality. The oddity effect thus tends to homogenize shoals. This may explain why fish prefer to shoal with individuals that resemble themselves.

The 'oddity effect' posits that any shoal member that stands out in appearance will be preferentially targeted by predators. Generally they prefer larger shoals, shoalmates of their own species, shoalmates similar in size and appearance to themselves, healthy fish, and kin (when recognized). It is also likely that fish benefit from shoal membership through increased hydrodynamic efficiency.Р В РІР‚СњР В РІР‚РЋish use many traits to choose shoalmates. Р В РІР‚СњР В РІР‚РЋish derive many benefits from shoaling behaviour including defence against predators (through better predator detection and by diluting the chance of individual capture), enhanced foraging success, and higher success in finding a mate. About one quarter of fish species shoal all their lives, and about one half shoal for part of their lives. In common usage, the terms are sometimes used rather loosely. In biology, any group of fish that stay together for social reasons are shoaling, and if the group is swimming in the same direction in a coordinated manner, they are schooling. They are all swimming in the same direction in a coordinated way.

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