downstream migration
Technical term in biology/ecology for the migration of aquatic animals downstream, typically to spawn or as part of their life cycle. Often contrasted with 遡河回遊 (upstream migration).
Eels are known as fish that undertake downstream migration.
Refers to upstream migration (e.g., salmon returning to rivers to spawn), the opposite direction of 降河回遊.
Compound of 降河 (descending a river) and 回遊 (migration, especially of fish).