Fish that migrate from the sea into fresh water to spawn are called anadromous.
The word combines Greek roots meaning “up” and “running,” describing movement upstream from the ocean into rivers or other freshwater habitats. Salmon are the classic example: they hatch in freshwater, spend much of their adult lives at sea, then return—often to their natal river—to reproduce. Striped bass, sea lampreys, shad and some sturgeons are also anadromous.
Anadromous is one form of diadromous migration, the broad category for regular movement between salt water and fresh water. The opposite pattern is catadromous: freshwater eels grow in rivers and lakes but migrate to the sea to spawn. Oceanodromous fish migrate only within marine waters, while potamodromous fish remain within freshwater systems.
Not every salmon population follows exactly the same route, and introduced salmon in the Great Lakes can become potamodromous because they migrate entirely within fresh water. That distinction is why “anadromous” describes a life-history pattern, not simply any fish swimming upstream.