Sea lice infestation causes significant welfare harm in farmed salmon; treatment methods themselves carry welfare costs that require careful trade-off analysis.
Sea lice management in salmon aquaculture presents a welfare paradox: the lice themselves cause harm, but treatments impose their own welfare costs on the fish being treated. Mechanical and chemical de-lousing procedures involve crowding, handling, and chemical or thermal stress that cause acute distress and sometimes injury. Cleaner fish used for biological control often experience poor welfare themselves. The welfare-optimal strategy focuses on prevention — site selection, coordinated fallowing, timing management — that reduces lice burden without treatment. When treatment is necessary, the least stressful effective option should be chosen with recovery monitoring following any mechanical procedure.