Fish comes in wide range of sizes. Big fish are called groupers that make friends with divers. We have heard of salt water fishes and fresh water fishes. There are also some species of fishes that migrate between fresh water and salt water. Such fishes are called diadromous species meaning they can live both in salt and fresh water.
Depending on how they migrate, they can be classified into many categories by the zoologists. Three important ones include amphidromous, anadromous and catadromous species.
Anadromous fish like salmon etc are born in fresh water, and then migrate to sea for development. After they grow into adults they migrate from the sea into fresh water to release or deposit eggs. Catadromous varieties are born in salt water and then migrate to fresh water for growth and development. From there they migrate back to sea water to spawn. American eel is a catadromous variety.
Amphidromous fish simply migrate between fresh water and salt water definitely not for the purpose of breeding. They normally migrate for food or as refuge. There are wide varieties of amphidromous species across the globe. These species live, breed and spawn in fresh water but larvae must go to sea for development. Then after a few months or years of development, they return to the fresh water. Fresh water shrimps, torrent fish are amphidromous variety. Around one-fifth of diadromous species are amphidromous.