Diadromy

Diadromy is a migratory life-history strategy in fishes in which individuals move between freshwater and marine environments during different stages of their life cycle. The term encompasses patterns in which development or reproduction involves alternating habitats with different salinities, rather than remaining exclusively in one environment.

Three main patterns are recognized: anadromy, catadromy, and amphidromy. In anadromous species, individuals are born in freshwater, migrate to the ocean to grow and feed, and return to freshwater to spawn. In catadromous species, adults mature in freshwater, migrate to the sea to spawn, and the offspring migrate into rivers to grow. Amphidromy refers to species in which larvae or juveniles regularly move between freshwater and saltwater for development or feeding, with reproduction not strictly tied to a particular habitat.

Prominent examples illustrate these patterns. Anadromous fishes include many salmon species (such as Atlantic salmon and Pacific salmon) and other taxa like the sea lamprey, which spend initial life stages in freshwater before moving to the sea. Catadromous fish include the European eel and the American eel, which mature in freshwater before spawning in the sea and producing offspring that migrate back into rivers. Amphidromous species include certain gobies and some mullets, which migrate between environments for growth or feeding without a fixed spawning site.

Ecological and conservation considerations focus on habitat connectivity and water quality. Dams, water abstraction, pollution, and climate change can disrupt migratory routes and salinity regimes, affecting survival and reproduction. Management often emphasizes maintaining connectivity, habitat restoration, and sustainable fisheries to support diadromous populations.