Hydroelectric dams block migratory fish from accessing spawning grounds, causing population declines and welfare harms to individual fish attempting to navigate impassable barriers.
Migratory fish attempting to pass dams undergo prolonged physical exertion and physiological stress that reduces immune function and spawning success. Fish that cannot pass barriers spend extended energy reserves in repeated futile upstream attempts before dying without spawning. The welfare of downstream migrants, particularly eel and juvenile salmon, is affected by turbine passage injury and mortality. Even fish ladders that successfully pass some individuals cause significant physical challenge: fish navigating ladders show elevated stress hormones and fin damage. Dam removal programmes in the US and Europe have demonstrated that river restoration rapidly improves both fish welfare and population recovery.