Anadromous is used to describe animals, usually fish, that live in the ocean but travel back up through rivers so they can breed in fresh water.
The word anadromous comes from Greek, and means "running upward." When fish swim from the oceans up to the rivers, it's called a run. The most famous version of this is a salmon run, where salmon, an anadromous species, swim all the way from the ocean up to the creek they hatched in. They lay eggs there so that a new generation of salmon will hatch and complete the cycle all over again.