A value that shows the proportion or percentage of fish of a given age group in relation to the total population in a body of water or to the total number of fish in the catch.

The number of each fish species does not remain constant, its value is influenced by many factors: deterioration or improvement of spawning conditions, epizootics, reduction or expansion of the food supply. The study of the patterns of fish population dynamics is a necessary theoretical basis for the proper organisation of fishing.