Catadromy

Catadromy is a migratory pattern in which most individuals spend the majority of their lives in freshwater or estuarine environments and migrate to marine waters to spawn. This is the opposite of anadromy, where fish live in marine environments and migrate into freshwater to reproduce.

The best-known catadromous fishes are true eels of the family Anguillidae. They hatch in coastal or open-ocean waters, and their larvae, called leptocephali, drift with currents until they metamorphose into glass eels. After entering freshwater or brackish habitats, they grow into yellow eels, and upon reaching maturity they become silver eels and migrate back to the sea to spawn, often traveling long distances and crossing ocean basins.

Prominent catadromous species include the European eel (Anguilla anguilla), the American eel (Anguilla rostrata), and the Japanese eel (Anguilla japonica). These species depend on intact river systems, estuaries, and coastal habitats to complete their life cycles and may show strong homing to natal streams.

Human activities threaten catadromous populations through barriers to migration (dams and culverts), habitat degradation, pollution, and overexploitation. Conservation approaches focus on maintaining and restoring migratory routes, regulating fishing, and protecting critical habitats around rivers and coasts.