movements that result in an alternation between two or more separate habitats and which occur with a regular periodicity, involving a large proportion of the population.

Potadromous   migrations entirely within freshwater

(Oceanodromous  migrations entirely within the sea)

Diadromous   migrations between freshwater and the sea - three sub-classes:

1   Anadromous   S → F

2   Catadromous   F → S

3   Amphidromous   S ↔ F