Physiology of Finfish and Shellfish (2+1)

2.4.9. Osmoregulation in migratory fishes

Migratory fish are hyperosmotic to their environment during some periods and hypoosmotic at other times.

To complete their life cycle, amphihaline fishes show remarkable adaptation in being able to migrate from the sea to freshwater bodies or vice versa. They have to survive in 2 different environments for the sake of their progeny. There are 2 distinct types of amphihaline fish. One lives in seawater, grows there and after becoming reproductively mature starts migrating to breed in riverine or land locked freshwater. The young born in freshwater migrate into seawater and live there until they are in turn mature and ready to spawn in freshwater. These fish eg. Salmon and hilsa are known as amphihaline potemotocous.

Another type of amphihaline fish which live in riverine or lake water, when reproductively mature are catadromous. They migrate into the sea to breed there. These kinds of fish are known as amphihaline thalassotocous. Eg., eel.