Sea lice infestations are the most significant health and welfare challenge in Atlantic salmon farming, requiring intensive management that itself carries welfare costs.
Sea lice management involves a cascade of welfare trade-offs. Untreated heavy lice burdens cause chronic pain from skin lesions and systemic stress — a clear welfare harm. Treatments each carry welfare costs: mechanical brush treatments cause abrasion injury; thermal treatments cause heat stress and sometimes mortality; bath medications require crowding and out-of-water handling. Cleaner fish used as biological controls face their own welfare concerns — wrasse and lumpfish are often raised in conditions inadequate for their species, experience high mortality in salmon pens, and may themselves be harassed or injured. The resistance problem means that treatments that once achieved adequate control now require more frequent application, increasing cumulative welfare burden. Integrated lice management that combines biological controls, physical barriers, selective breeding for lice resistance, and judicious medication use provides better welfare outcomes than reliance on any single method.