catadromous

Catadromous is a mode of migration in which mature individuals live in freshwater or estuarine environments and migrate downstream into saltwater to spawn. The term is the reverse of anadromous, where organisms spawn in freshwater after living in the ocean. The name reflects the direction of movement: from inland waters toward the sea for reproduction.

In a typical catadromous life cycle, adults reside in rivers or lakes and migrate downstream to marine or coastal waters to spawn. Eggs hatch into leptocephalus larvae that drift in the open ocean and eventually metamorphose into glass eels, which migrate back into freshwater as elvers and continue to grow as yellow eels. When they reach sexual maturity, some eels become silver eels and again migrate toward the sea to spawn, completing the cycle.

The best-known catadromous group is the freshwater eels of the genus Anguilla, including the European eel (Anguilla anguilla), American eel (Anguilla rostrata), and Asian eels such as Anguilla japonica. These species undertake extensive, transoceanic migrations, with spawning grounds thought to be far from their juvenile habitats, though specific details can vary by species.

Catadromy has significant ecological and conservation implications. Its reliance on intact riverine and coastal habitats, migratory corridors, and stable ocean conditions makes catadromous species vulnerable to barriers like dams, pollution, overfishing, and habitat degradation. Protecting both freshwater and marine environments is essential for the persistence of catadromous populations.