A flock (shoal) is a temporary grouping of fish that exhibits unity of behavior. Packs are formed by individuals of similar biological condition and age. The pack way of life is characteristic mainly of plankton-eating fish, others unite only in the initial period of life or during hunting (carps, predators).

A herd (population) is a local self-reproducing grouping of fish of the same species of different ages, which permanently inhabits a certain area of a water body. A herd has specific breeding, feeding and wintering grounds. Fish from different local herds have some morphoecological features.