Sea lice infestation is the most significant welfare and environmental problem in Atlantic salmon aquaculture. These parasitic copepods cause tissue damage, immune suppression, and stress in farmed fish while threatening wild salmon populations through transmission.
Sea lice infestation in farmed salmon represents a welfare problem at significant scale: global salmon aquaculture produces approximately 2.7 million tonnes annually, with sea lice infestation affecting the majority of fish to some degree. Fish welfare science has established that salmon possess the neurological architecture to experience pain and stress, and the tissue damage, immune suppression, and behavioural changes associated with sea lice infestation are consistent with ongoing welfare compromise. Treatment interventions introduce their own welfare costs: thermal delousing subjects fish to brief but intensely stressful hot water exposure; hydrogen peroxide treatments cause acute oxidative stress; and the mechanical stress of all treatment procedures is significant. The use of cleaner fish creates a second welfare challenge — wrasse and lumpfish deployed in sea cages to consume lice from salmon often have very high mortality rates in the cage environment, raising the question of whether one welfare problem is being addressed by creating another. Genuinely resolving salmon sea lice welfare requires either closed-containment systems that exclude lice, or biological control methods with credible welfare standards for cleaner fish.