Sea lice are the primary disease challenge in Atlantic salmon aquaculture, and the treatments used to control them — chemical baths, freshwater flushing, and mechanical removal — cause significant welfare harms of their own.
Sea lice treatment in salmon farming creates a welfare paradox: the lice themselves cause suffering through tissue damage and immune stress, but the treatments used to control them are also harmful. Chemical baths expose stressed, crowded fish to toxic compounds — hydrogen peroxide at effective concentrations causes oxygen depletion and acute respiratory distress. Mechanical removal devices cause scale loss, hemorrhage, and secondary infection at the point of louse attachment. Freshwater bathing is effective but requires extensive crowding and pumping. Cleaner fish used as biological controls are often caught wild, transported in poor conditions, and face high mortality in salmon pens — adding a second species welfare concern to the system. The fundamental solution requires reducing stocking densities that allow lice to spread rapidly — a structural change the industry has been slow to adopt.