Since this issue comes up so frequently on Reef Central, some additional information might be useful. An aggregation of fish is the general term for any collection of fish that have gathered together in some locality. Fish aggregations can be structured or unstructured. 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.
If, in addition, the aggregation comes together in an interactive, social way, they are said to be shoaling. 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 behavior such as swimming, so as to remain close to the other fish in the group. Shoaling groups can include fish of disparate sizes and can including mixed-species subgroups.
If, as a further addition, the shoal becomes more tightly organized, with the fish synchronizing their swimming so they all move at the same speed and in the same direction, then the fish are said to be schooling. Schooling fish are usually of the same species and the same age/size. Fish schools move with the individual members precisely spaced from each other. The schools undertake complicated maneuvers, as though the schools as a whole have minds of their own.
Shoaling is a special case of aggregating, and schooling is a special case of shoaling. While schooling and shoaling mean different things within biology, they are often treated as synonyms by non-specialists, with speakers of British English tending to use "shoaling" to describe any grouping of fish, while speakers of American English tend to use "schooling" just as loosely.[1] The intricacies of schooling are far from fully understood, especially the swimming and feeding energetics. 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. Schooling also has disadvantages, such as excretion buildup in the breathing media and oxygen and food depletion. The way the fish array in the school probably gives energy saving advantages, though this is controversial.
Fish can be obligate or facultative shoalers. Obligate shoalers, such as tunas, herrings and anchovy, spend all of their time shoaling or schooling, and become agitated if separated from the group. Facultative shoalers, such as Atlantic cod, saiths and some carangids, shoal only some of the time, perhaps for reproductive purposes.
Shoaling fish can shift into a disciplined and coordinated school, then shift back to an amorphous shoal within seconds. Such shifts are triggered by changes of activity from feeding, resting, traveling or avoiding predators.
