anadromie

Anadromy is a migratory life history in which certain fish are born in freshwater, migrate to the ocean to grow, and return to freshwater to spawn. The term contrasts with catadromy, in which species spawn in marine waters and mature in freshwater.

In a typical anadromous cycle, juvenile fish hatch in rivers or streams and drift downstream to estuaries or the sea. There they grow and accumulate energy reserves, often for years, before returning to their natal freshwater systems to reproduce. During the transition between fresh and saltwater, fish undergo physiological changes to cope with different salinities, a process known as smoltification in many species. Many anadromous fish display homing behavior, returning to the river of their birth to spawn. After spawning, some species die, while others may survive and reproduce again.

Notable examples include the Atlantic salmon (Salmo salar) and various Pacific salmon species (Oncorhynchus spp.), such as chinook, coho, sockeye, and pink salmon. Other anadromous species include the American shad (Alosa sapidissima) and striped bass (Morone saxatilis). Some populations of other species exhibit partial or facultative anadromy, where individuals may migrate or remain resident depending on environmental conditions.

Ecology and human impact: Anadromous migrations connect riverine and marine ecosystems, influencing nutrient cycles and food webs. They face threats from barriers like dams and culverts, habitat loss, pollution, overfishing, and climate change. Management responses often include installation of fish ladders or bypass systems, hatchery programs, river restoration, and regulations on harvest to protect declining populations.