From the Latin immigrans, immigrantis, immigrant.

Immigrants are organisms that have not previously inhabited a given water body. A distinction is made between active immigrants, which actively penetrate new water bodies on their own, and passive immigrants, which enter water bodies by drifting currents, wind or humans.

Immigrants, species, genera, families and other groups of fish that originated and developed in another area and entered the area much later. Immigrants are usually named by the place from which they moved (Siberian in Europe, South American in North America, etc.) and by the time of immigration. In some cases, the time of immigration and its direction are determined by direct observation or historical data (the process of species dispersal, especially in recent decades, is very intensive). However, in most cases these questions are solved on the basis of geological and paleontological data, analysis of relationships (systematic position), and habitat character.