Dams block the migration of fish species that have evolved to travel between ocean and freshwater to complete their life cycles, causing population declines and welfare harms to blocked individuals.
Migratory fish blocked by dams face a specific welfare challenge: they are driven by powerful reproductive instincts to reach spawning grounds that are no longer accessible. Fish pile up below impassable structures in large numbers, competing intensely and experiencing the stress of an urgent biological drive they cannot satisfy. Fish ladders and bypass structures reduce but rarely eliminate this barrier — passage efficiency varies enormously and most structures remain inadequate for all life stages of migratory species. Dam removal, where appropriate, eliminates the welfare harm entirely while restoring the ecological functions that migratory fish provide. The welfare benefit of restored migration is one of the strongest arguments for prioritizing dam removal over technological fixes.