Sea lice infestations are treated in farmed salmon using chemical, biological, and mechanical methods. Each treatment type carries significant acute welfare costs that must be weighed against the chronic welfare harm of untreated infestation.
Sea lice treatment represents a genuine welfare dilemma: both the infestation and the treatments cause suffering, requiring careful cost-benefit assessment. Untreated lice cause chronic lesions, pain, and immune compromise in infested fish. Chemical bath treatments involve crowding fish at very high density, pumping them through treatment vessels, and exposure to chemicals that may cause skin irritation and physiological stress. Warm water treatments (Thermolicer) have been associated with gill damage and mortality when poorly controlled. Freshwater treatments cause osmotic stress. Cleaner fish used in biological control face their own welfare challenges — wrasse are often wild-caught and housed at very high densities with high mortality rates in commercial lice management contexts. The ideal approach is preventing lice through site management, lower stocking densities, and submerged feed stations rather than treating established infestations.