Sea lice are the primary welfare challenge in marine salmon farming, causing skin damage, stress, and immune suppression that affects fish welfare throughout the infestation.
Sea lice create chronic welfare harm that permeates the biology of affected fish. The parasites physically damage skin — the primary barrier to osmoregulation — causing lesions that range from irritation through to deep wounds reaching subcutaneous tissue in heavy infestations. The behavioral responses to lice — increased swimming speed, surface aggression, and appetite suppression — indicate that lice cause genuine distress beyond the mechanical tissue damage. Chronic lice infestation compromises immune function, increasing susceptibility to secondary infections that add further welfare burden. Treatment itself creates welfare costs — chemical bath treatments require crowding and handling stress, while in-feed medications require the fish to eat medicated pellets that some affected fish will not consume. The integrated pest management approach — combining biological control, laser treatment, strategic chemical treatment, and selective breeding for lice resistance — represents the most welfare-positive management strategy.