Sea lice are the most costly health problem in farmed Atlantic salmon. The treatments used to control them — chemical, biological, and mechanical — all carry significant welfare impacts.
Sea lice treatment represents one of the clearest welfare trade-off dilemmas in aquaculture: untreated infestations cause chronic suffering, but most available treatments cause acute suffering. Bath treatments using hydrogen peroxide or organophosphates require crowding fish into treatment vessels and cause significant stress responses including scale loss, oxygen depletion, and behavioral disturbance. Mechanical thermal treatments cause significant tissue damage. Cleaner fish, used as biological control, face their own welfare challenges — they are crowded, handled frequently, and often provided with poor feeding and shelter conditions. Breeding programs for sea lice-resistant salmon strains offer a longer-term welfare-positive solution that avoids treatment altogether. Welfare assessments must account for treatment frequency across an entire production cycle.