Sea lice infestations are the most significant health and welfare problem in Atlantic salmon aquaculture. Heavy louse burdens cause painful tissue damage and open lesions, and treatments themselves carry welfare and environmental costs.
Sea lice at high burden levels cause genuine physical suffering in farmed salmon — open lesions on the head and back expose underlying tissue, cause persistent irritation and pain, and create entry points for secondary bacterial infection. The fish cannot escape or groom themselves. Treatment procedures including thermal baths (hot water) and mechanical removal cause acute stress and mortality. The environmental spill-over — lice larvae spreading from densely stocked farms to wild salmon migrating through adjacent waters — multiplies welfare harm beyond the farm boundary. Reducing sea louse burden through better farm siting, lower stocking densities, and development of enclosed systems is both a welfare and a conservation imperative.