Hydropower dams kill millions of juvenile salmon and other migratory fish annually through turbine strike, pressure changes, and altered river conditions.
The welfare of migratory fish in hydropower river systems involves both direct mortality during turbine passage and chronic stress from altered river conditions. Juvenile salmon that survive turbine passage may experience barotrauma — injury from rapid pressure changes — that compromises their survival and welfare in subsequent days. The cumulative effect of multiple dam passages on a single migratory journey is severe. Fish-friendly turbine technology and effective bypass systems exist and have been demonstrated to dramatically reduce these harms, but implementation is slow due to cost and regulatory complexity. Spill operations that route water over spillways rather than through turbines offer an immediate welfare benefit during migration seasons, though at electricity generation cost.