diadromous

Diadromous is a term used in ichthyology to describe fish that migrate between saltwater and freshwater during their life cycles. Diadromous species rely on the contrasting habitats for different life stages and often use estuaries or river mouths as productive transition zones.

There are three main forms: anadromous, catadromous, and amphidromous. Anadromous species are born in freshwater, migrate to the sea to grow, and return to freshwater to spawn; examples include the Atlantic salmon (Salmo salar) and some trout such as steelhead (Oncorhynchus mykiss). Catadromous species are born in saltwater, grow in rivers or lakes, and migrate back to the sea to spawn, with the European eel (Anguilla anguilla) and American eel (Anguilla rostrata) as classic examples. Amphidromous species migrate between marine and freshwater during their life cycles but not specifically for spawning; many amphidromous fishes are gobies and other tropical species that move between habitats as juveniles or adults.

Geographic patterns vary, but diadromy is common in regions with extensive river systems and estuaries. Ecologically, diadromous life histories connect marine and freshwater ecosystems and influence nutrient cycling, predator–prey dynamics, and genetic connectivity. Human activities that disrupt migratory routes—such as dams, water withdrawals, pollution, habitat loss, and climate change—pose serious risks to diadromous populations. Management often focuses on preserving or restoring migratory corridors, improving fish passage, and maintaining estuarine habitat quality.

The term diadromous encompasses the two primary categories (anadromy and catadromy) and the broader amphidromy, reflecting diverse strategies by which species exploit both saltwater and freshwater environments.