Weirs, dams, culverts, and other structures block fish migration routes, causing physiological stress, failed spawning attempts, and reduced survival in migratory species like salmon and trout.
Migration is a fundamental biological drive in salmonid fish — they are physiologically programmed to return to their natal rivers to spawn. Barriers that prevent this create severe frustration and welfare harm: fish repeatedly attempt to leap over weirs, sustaining injuries and exhausting energy reserves they cannot replenish. Fish that fail to reach spawning grounds die without completing their biological imperative, representing both a welfare and conservation failure. Even fish passes, designed to mitigate barrier impacts, cause welfare costs through energy expenditure and exposure to predators during passage. Complete barrier removal is the most welfare-positive intervention and is being pursued increasingly through river restoration programmes across Europe and North America.