Pack, elementary population, herd, ecological race, geographical race (subspecies). Their formation is an adaptation aimed at ensuring the existence of a species under certain conditions. Abiotic conditions in most cases have the same effect on both an individual and a group. Biotic factors affect an individual and a group of individuals differently.

There are four levels of social behaviour in fish. In addition to individual fish, there are schools in which fish are randomly distributed in a disorderly mass, such as cod on feeding grounds. Then there are schools, or shoals, where the fish line up in strict order at a certain distance from each other, giving them enough space to swim. And there are schools, where the fish are gathered together in a dense mass so that their bodies touch.

Pelagic fish, especially herring, are the most common shoals. A large shoal behaves like a single organism. Shoals have no leader or group of leaders. Often the fish swimming at the front begin to lag behind and their place is taken by others from neighbouring rows. When the shoal changes direction, the fish from the flank are in front, the leading edge becomes the flank. This manoeuvre is performed with great precision, even by a huge school of a million fish. A moving school of fish is made up of individuals of the same species and size with the same muscular strength; an inhomogeneous group could not move as a monolithic mass. In a number of species, the shoal has a protective value. A school of fish at a greater distance will detect danger and in many cases be better protected from predators. A school of fish at a greater distance will notice fishing gear and avoid it more effectively, making it easier to escape from a trap. Schooling is also important for finding food. Schooling behaviour is an adaptive trait of animals. It is best studied in fish, which is explained by the practical need for the knowledge required for fishing.