Sea lice are the primary parasite welfare challenge in salmon farming. Treatment methods range from chemical baths to mechanical and thermal delousing — each with distinct welfare implications for millions of fish.
Sea lice treatment welfare involves balancing the chronic welfare harm of parasite burden against the acute welfare harms of treatment. Untreated lice infestation causes persistent pain from open skin wounds, immune suppression leading to secondary disease, and in severe cases mortality — these welfare harms are not acceptable. However, treatment itself causes acute stress and injury. Hydrogen peroxide baths require crowding fish in enclosed bath units, causing hypoxia and chemical eye irritation at poorly calibrated concentrations. Mechanical delousing causes scale loss, bruising, and fin damage. Thermal delousing is effective but precise temperature control is essential — fish exposed to excessive heat experience thermal burn injury. Welfare-optimal sea lice management uses combinations of preventive approaches (single species sites, fallow periods, deep feeding, snorkel lights) to minimise treatment frequency, and chooses the least-harmful treatment when necessary based on fish condition and lice levels.