Sea lice infestations and their treatments are major welfare challenges in Atlantic salmon aquaculture, with both infestation and treatment causing significant harm to fish.
Sea lice cause chronic pain through tissue erosion and immune suppression that impairs fish health and healing capacity. Chemical treatments cause acute stress responses - hydrogen peroxide baths involve exposure to caustic conditions. Mechanical treatments including thermal delousing can cause thermal shock and mortality. The welfare ideal is prevention over treatment: reduced stocking densities, site fallowing, and closed-containment systems that eliminate sea lice reduce both infestation suffering and treatment stress. Welfare considerations should drive treatment selection alongside efficacy.