Diadromous fish are fish that migrate between freshwater and saltwater. The migration patterns differ for each species and have seasonal and lifecycle variations. Only one percent of all fish in the world are diadromous. Some diadromous fish migrate great distances, while others migrate much shorter distances. In either case, these fish undergo physiological changes that allow them to survive as they migrate from freshwater to saltwater or vice versa. There are several types of diadromous fish, differing in their specific migration patterns.
Anadromous fish spend most of their adult lives in salt water, and migrate to freshwater rivers and lakes to reproduce. East Coast anadromous fish species include alewife, striped bass, Atlantic salmon, and shortnose sturgeon. West Coast anadromous species include five salmon species, steelhead, white sturgeon, and American shad (not native to the West Coast). Once the eggs of an anadromous fish hatch, the juvenile fish spend varying lengths of time in freshwater before migrating to saltwater, where they mature. The fish eventually return to freshwater to spawn. Some anadromous fish die after spawning (as with most salmon species), while others make the journey several times in their life. About half of all diadromous fish in the world are anadromous.
Catadromous fish spend most of their adult lives in freshwater, and migrate to saltwater to spawn. Juvenile fish migrate back upstream where they stay until maturing into adults, at which time the cycle starts again. The only catadromous species in the United States is the American eel. A fascinating aspect of the American eel's life history is that they migrate thousands of miles to spawn in the Sargasso Sea, located in the mid-Atlantic Ocean, south of Bermuda and north of the Bahamas. American eels do not eat once they leave the freshwater. Having spent so much energy to migrate and spawn, they die soon after. About one quarter of all diadromous fish in the world are catadromous. Amphidromous species move between estuaries and coastal rivers and streams, usually associated with the search for food and/or refuge rather than the need to reproduce. Amphidromous fish can spawn in either freshwater or in a marine environment. About one fifth of all diadromous fish are amphidromous.
NOAA National Oceanic & Atmospheric Administration