What term covers all fish that migrate between the sea and fresh water?

The term covering all fish that migrate between the sea and fresh water is diadromous. It is an umbrella category for fish whose life cycles regularly include both marine and freshwater environments.

Two familiar forms of diadromy are anadromy and catadromy. Anadromous fish, such as salmon, grow largely in the sea and enter fresh water to reproduce. Catadromous fish, such as freshwater eels, generally grow in fresh water and migrate to the sea to spawn. Amphidromous fish also move between the two environments, but their movements are not primarily for breeding.

The classification was formalized in modern fish biology by George S. Myers, who introduced the term “diadromous” in a 1949 paper. The word combines Greek roots associated with movement through or between places.

Diadromous should not be confused with potamodromous, which describes migrations wholly within fresh water, or oceanodromous, which describes migrations wholly within the sea. A fish may also be euryhaline—able to tolerate different salinities—without undertaking a regular diadromous migration.