Sea lice are the primary parasite problem in Atlantic salmon net pen farming, and the treatments used to control them — chemical, mechanical, and biological — all carry welfare costs for the salmon themselves. Balancing parasite control against treatment welfare harm is a central challenge in salmon aquaculture.
Sea lice treatment places salmon in a double welfare bind: the lice themselves cause progressive skin damage and stress, while the treatments used to remove them impose their own acute welfare costs. Crowding for treatment, thermal stress in thermolicers, and physical trauma from mechanical removal all cause measurable acute distress — elevated cortisol, scale loss, and mortality in poorly managed treatments. The welfare-optimal approach minimises both lice burden and treatment frequency and intensity: maintaining lice below threshold levels through integrated pest management (IPM) is less acutely stressful than allowing high infestations that require aggressive treatment. Cleaner fish welfare adds another layer: wrasse and lumpfish deployed to eat lice face high mortality in sea pen environments for which they are poorly adapted.