Distribution of fish by age groups. The age structure is a species-specific adaptation, providing the species, the population to exist under changing conditions of nutrition and reproduction, as well as different intensity of exposure to predators.

The age composition of the stock, the ratio of the relative number of individual age groups, the maximum life expectancy within certain limits are specific to the fish species and constitute its specific characteristic, a specific adaptive trait. Fish with a short life cycle, with a population consisting of a small number of age groups, such as hamsa - Engraulis encrassicholus (L.), sprat - Sprattus sprattus (L.), gambusia - Gambusia affinis Gir., minnows of the genus Gobio and related genera, are adapted to life in conditions of significant and highly fluctuating mortality. Among the fish, there are also species (sturgeons, some catfish, large sharks and some others) whose population consists of several dozen age groups.