What Are Diadromous Fishes?

Posted: 09-May-2005; Updated: 24-Apr-2009

• Share
• Facebook
• Email
• Print

A school of alewives make their way upriver to spawn. (Photo: Byron Young)

• Long Island South Shore Estuary Page

Diadromous fishes are species that use both marine and freshwater habitats during their life cycle.  Species can be anadromous, living primarily at sea but migrating up rivers to spawn, or catadromous, living primarily in lakes, ponds and rivers but migrating out to sea to spawn.

The anadromous strategy is far more common, with the suite of Pacific salmon species being perhaps the best-known examples.  But the Atlantic coast is not lacking in anadromous fishes.  Large rivers in Maine support the last remaining wild spawning runs of East coast salmon, the Atlantic salmon (Salmo salar), in the United States.

Rivers throughout the Northeast support runs of alewives (Alosa pseudoharengus), along with its close cousins the American shad (Alosa sapidissima), blueback herring (Alosa aestivalis) and hickory shad (Alosa mediocris), as well as the Atlantic sturgeon (Acipenser oxyrinchus) and rainbow smelt (Osmerus mordax). 

The anadromous striped bass (Morone saxatalis) abounds in estuaries along the coast, though its spawning runs locally are confined to the Hudson River estuary.  Furthermore, sea-run strains of the native brook trout (Salvelinus fontinalis) and non-native brown trout (Salmo trutta) and rainbow trout (Onchorhynchus mykiss) exist alongside resident freshwater populations in the Northeast.

Although the catadromous strategy is less common, the Atlantic coast is home to one major catadromous species, the American eel (Anguilla rostrata). Juveniles of this species migrate up rivers to grow and mature, before migrating out to the Sargasso Sea to spawn with eels from along the coast.  Sadly (for the eels, at least), they, like Pacific salmon, die after their one chance to spawn.

What about Long Island?

Larger diadromous species are naturally absent from Long Island, including Atlantic salmon, American shad and Atlantic sturgeon. But South Shore tributaries are important habitats for several diadromous species: Alewives and blueback herring, collectively known as "river herring,"; and American eel and rainbow smelt still make their way up rivers along the South Shore to reach feeding, spawning and nursery grounds.  Sea-run brook, brown and rainbow trout also make spawning runs in South Shore tributaries.