Sea lice infestations are one of the most significant welfare and environmental problems in Atlantic salmon aquaculture, and the treatments used to control them carry their own welfare costs that must be weighed against the harm of untreated infestation.
Sea lice welfare in salmon aquaculture illustrates how the cumulative welfare burden of a production system requires assessment not only of the primary welfare problem but of the treatments used to address it. Lice infestations cause chronic skin damage, pain from open lesions, and physiological stress from compromised osmoregulation. Untreated heavy infestations cause significant welfare harm. However, the treatments deployed to control lice impose their own acute welfare costs: thermal delousing with warm water causes thermal shock, chemical bath treatments cause acute toxicological stress, and mechanical brush systems cause physical trauma. The optimal welfare outcome requires lice prevention that reduces the need for treatment, combined with the least harmful treatment options when intervention is necessary. Biological control through cleaner fish requires those fish to be maintained under appropriate welfare conditions, adding another welfare consideration to the system.