Anadromes

Anadromes, or anadromous organisms, are species that spend most of their adult life in saltwater but migrate into freshwater to spawn. The term comes from Greek roots meaning “upstream running” and is used to distinguish these species from catadromous ones, which migrate from freshwater to the sea to reproduce. While the word can apply to various groups, it is most commonly used for fish.

The best-known anadromous fishes are those in the salmon family (Salmonidae) and related species such as striped bass and certain lampreys. In a typical anadromous life cycle, adults migrate from the ocean into rivers or estuaries to spawn. After spawning, eggs hatch into larvae that grow in freshwater before returning to the sea as juveniles. Lifecycles vary by species: many salmon die after their first spawning (semelparity), while others, such as some steelhead and trout, may spawn multiple times (iteroparity).

Geographically, anadromous species occur in temperate regions of North America, Europe, and northern Asia, where rivers provide access to the ocean. Their migrations make them particularly vulnerable to barriers such as dams, water withdrawals, and habitat degradation. Climate change and overfishing further threaten their populations by altering migratory cues, water temperatures, and riverine habitat quality.

Conservation measures focus on restoring migratory pathways and habitat, including fish ladders, dam removals, improved passage around barriers, and careful management of harvests. Hatcheries are sometimes used to support populations but must be managed to avoid genetic and ecological impacts. Anadromes represent a distinct ecological strategy tied to river-ocean connectivity and can be indicators of freshwater and marine ecosystem health.