What term describes fish that hatch in freshwater, live at sea, and return to freshwater to spawn? The answer is anadromous.
Anadromous fish move from freshwater toward the sea as they mature, then return upstream to reproduce. Salmon are the classic example: eggs hatch in rivers, juveniles develop there, and adults spend much of their feeding and growth phase in the ocean before migrating back to spawning grounds.
The word comes from Greek roots meaning “running upward,” referring to the movement up rivers. This is one form of diadromy, the broader term for migrations between freshwater and saltwater. Anadromous species include many salmon and trout, striped bass, sturgeons, sea lampreys, and some smelts.
A common mix-up is catadromous. Catadromous fish, such as freshwater eels, do the reverse: they grow mainly in freshwater and migrate to the sea to spawn. Fish that migrate between the two environments for reasons other than breeding are called amphidromous.