Sea lice are a major health threat to farmed Atlantic salmon. Treatment methods — including thermal and chemical approaches — have significant welfare implications for both salmon and wild fish.
Sea lice management represents a welfare dilemma where the disease causes welfare harm and all current treatment methods also cause welfare harm — the question is which approach minimises net suffering. Untreated sea lice at high burdens cause severe damage: open lesions, secondary infections, and physiological compromise. Thermal delousing causes acute thermal stress and fin damage. Mechanical methods cause physical injury. Chemical treatments may be welfare-preferred for fish but have environmental risks for wild crustaceans. Cleaner fish — wrasse and lumpfish that eat sea lice — offer a more natural, lower-welfare-cost biological control approach, though cleaner fish welfare requirements must themselves be met. Prevention through lower stocking densities, closed containment systems, and strategic site placement reduces sea lice pressure and therefore treatment frequency — the most welfare-beneficial approach.