Anadromous

Anadromous refers to a migratory life history in which individuals are born in freshwater, migrate to the ocean to grow, and return to freshwater to reproduce. It is a form of diadromy; the opposite strategy is catadromy, in which organisms live in freshwater as adults and spawn in the sea. Anadromous fish typically spend most of their adult lives in marine habitats and ascend rivers to breed, sometimes traveling long distances upstream against currents.

The transition between freshwater and seawater involves physiological changes known as smoltification, enabling osmoregulation in seawater. Timing of migration is often cued by river flow, temperature, and day length. Many anadromous species die after spawning (semelparity), but others can spawn multiple times.

Well-known anadromous fishes include the Atlantic salmon (Salmo salar) and various Pacific salmon species (Oncorhynchus spp.), as well as the steelhead trout (Oncorhynchus mykiss). Additional anadromous forms include the American shad (Alosa sapidissima) and certain striped bass populations (Morone saxatilis), which migrate from the sea into rivers to spawn.

Geographically, anadromous life histories are common in temperate regions of North America, Europe, and parts of Asia. Human activities—such as dam construction, habitat alteration, overfishing, and pollution—have reduced access to natal rivers and affected population viability. Conservation efforts include dam removals, fish passage improvements, habitat restoration, and catch management to support migratory routes and spawning grounds.