What term describes fish migrations that occur wholly within the sea?

Fish migrations that occur wholly within the sea are described as oceanodromous. Oceanodromous fish remain marine throughout the migration, moving between feeding, spawning, nursery or seasonal habitats without entering fresh water.

The category includes many marine fishes whose movements may span large distances. Tuna, for example, can migrate north and south through the ocean as temperatures and prey availability change. Marine forage fish also shift among spawning, feeding and nursery grounds, often following ocean currents and seasonal concentrations of plankton.

Oceanodromous is easily confused with potamodromous, which refers to migrations wholly within fresh water. Anadromous fish, such as salmon, migrate from the sea into fresh water to spawn, while catadromous fish, such as eels, generally move from fresh water to the sea for reproduction. All of those terms describe different patterns of habitat use.

The word belongs to the same Greek-derived vocabulary used in fish-migration biology. “Oceanodromous” combines ocean-related terminology with a root meaning running or course. It describes where the migration occurs, not necessarily why the fish move or how far they travel.