Catadromous
Living in fresh water but going down to the sea to spawn, as freshwater eels do.
Also: catadromy
Catadromous fish are the mirror of anadromous ones. The European eel lives for years in rivers and lakes, then swims thousands of kilometres to the Sargasso Sea in the western Atlantic to spawn and die. Its young drift back to Europe as transparent glass eels. The American and Japanese eels make similar journeys, and the Indian mottled eel and the barramundi, which spawns in estuaries and coastal water, also move downstream to breed. Glass eels are caught and sold to farms in very large numbers.