Sea lice treatments in salmon farming — including thermal, mechanical, and chemical methods — carry significant welfare costs for farmed salmon that must be weighed against the welfare harm of untreated infestation.
Sea lice treatment represents one of the most welfare-significant aspects of salmon aquaculture because it combines the welfare harm of infestation with the welfare harm of treatment — creating a double burden on individual fish. Untreated lice cause progressive tissue damage, immune compromise, and in severe infestations, death through osmotic failure. Treatment mitigates infestation welfare harm but introduces its own stressors: thermal delousing causes measurable heat stress and disorientation; mechanical treatments cause physical trauma; chemical treatments involve pharmacological stress and mortality in a proportion of fish. The welfare ideal is prevention — reducing lice infestation through site selection, reduced stocking density, synchronized harvesting, or closed containment — rather than repeated treatment of established infestations. Welfare-positive treatment selection considers both efficacy and the stress burden of each approach, with lower-stress methods preferred where equally effective.