Fish like salmon, trout, and eel are migratory fish, which means their bodies are well adapted to any environment.

They migrate through coastal wetlands, rivers and estuarine which have different degrees of salinity in the water. These species can tolerate any level of salinity, and are referred to as ‘eurythaline’. While some salmon and trout move from salt water to fresh water, others live their entire lives in freshwater.

Salmon and trout are known to travel many thousands of miles out in water, but to return to the same place they were spawned when it is time for them to spawn.

Eels are born in salt water, but migrate and spend most of their lives in freshwater.

All migratory fish have to be able to quickly adjust to the salinity of their water, which means that their bodies must be able to lower their salt concentration when in salt water, to make it lower than their surroundings, and increase this concentration when in freshwater.

The tolerance in salinity varies according to the stage of the fish’s life.