Dams and water diversions block fish migration routes, preventing reproductive completion and causing population declines with welfare implications for individual fish.
Fish passage welfare involves both the individual experience of fish unable to complete natural migration and the population-level consequences of blocked reproduction. Salmon and other migratory fish have powerful reproductive drives that are physiologically triggered - a salmon unable to reach spawning habitat is in a state of thwarted drive that has physiological consequences even before the obvious outcome of death without reproduction. The ecological welfare consequence is population collapse, with fewer individuals across subsequent generations experiencing any wild existence at all. Dam removal programs have demonstrated that restoring river connectivity produces rapid population recovery - fish populations respond within years when passage is restored. The welfare and conservation case for dam removal and better fish passage is among the most clearly supported in freshwater ecosystem management.