Sea lice are the most significant parasite welfare challenge in Atlantic salmon aquaculture, with management involving trade-offs between lice control efficacy and the welfare costs of treatment methods.
Lepeophtheirus salmonis (salmon louse) causes progressive skin damage through feeding on mucus, skin, and blood. At low infestations, welfare impact is limited; as counts rise, skin erosion, secondary bacterial infections, reduced feeding, stress responses, and in severe cases, fatal haemorrhage occur. Salmon experiencing high lice burdens show elevated cortisol, altered behaviour, reduced growth, and impaired immune function—a chronic welfare drain that reduces lifetime welfare quality significantly if uncontrolled.
Wrasse (lumpfish and wrasse species) are deployed as cleaner fish, removing lice from salmon through direct picking behaviour. The welfare of cleaner fish themselves—often reared intensively or captured wild—deserves attention alongside their welfare contribution. Appropriate wrasse housing (complexity, shelter, substrate), adequate feeding (wrasse require supplementary food beyond lice alone), and monitoring of wrasse welfare status is required. Wrasse mortality rates in salmon farms have been concerning, prompting welfare standard development.
Hydrolicer and Thermolicer systems use water jets or warm water to remove lice. These treatments cause handling stress and, in the case of Thermolicer, can cause thermal damage to salmon at poorly controlled temperatures. Post-treatment monitoring for injuries and mortality is essential. Welfare-conscious operational protocols—fish in good condition before treatment, controlled temperature and flow rates, adequate recovery periods—reduce treatment-related welfare impacts.
Organophosphates and hydrogen peroxide bath treatments are effective against lice but carry direct welfare costs. Hydrogen peroxide causes gill irritation and respiratory distress in treated fish. Organophosphates affect neuromuscular function and can cause mortality at excessive doses. Treatment welfare is minimised through strict adherence to dose protocols, monitoring of fish behaviour during treatment, and immediate termination if acute distress responses are observed.