The number and biomass of fish of each species and individual populations do not remain constant, but are in dynamics, changing over the course of the season, year and longer periods.

Fish numbers dynamics:

1) changes in the number of fish;

2) fluctuations in the total number of fish within the same systematic category or range.

The main accounting relationships for population dynamics are: births, migration, deaths, and emigration.

Numbers are influenced by three dynamic indicator functions:

Birth rate or recruitment (recruitment means reaching a certain size or reproductive stage);

growth rate (this is a measure of the growth of individuals in size and length);

mortality (this includes fishing mortality and natural mortality).

A population cycle occurs in which populations rise and fall over a predictable period of time. There are some species whose populations have fairly predictable patterns of change, although the full causes of population cycles are one of the major unresolved ecological problems. There are a number of factors that influence changes in abundance, such as food availability, predators, disease, and climate.