Almost all fish undertake migrations of varying lengths during their lives!
The reason for this lies in the changing demands fish place on their habitat over the course of their lives.
For a newly hatched fry, for example, oxygen-rich waters with zooplankton that is suitable for its size are ideal. As an adult fish, habitats further downstream offer a much better food source, and the risk of being eaten by other fish also decreases as it grows larger.
Thus, most fish have a reason to undertake migrations of varying lengths during their lifetime. Accordingly, almost all fish species can be described as migratory fish.
These migrations are vital for the preservation of the species.
Long-distance migratory fish move from freshwater to the sea and vice versa during their lifetime.
Fish species that live in the sea and reproduce in freshwater (anadromous migratory fish) include salmon, sea trout (Salmo trutta forma maris), various sturgeon species (Acipenser ssp.), shad, lamprey, and many more.
The European eel (Anguilla anguilla), on the other hand, spawns in the sea (Sargasso Sea in the Bermuda Triangle) and spends its growth phase in freshwater (catadromous migratory fish).