Sea lice are the most significant welfare and production challenge in salmon aquaculture, and the methods used to control them have welfare implications for both salmon and cleaner fish.
Sea lice cause direct welfare harm to salmon through lesion formation, physiological stress, and secondary infection. High lice burdens cause severe skin damage and systemic disease. Treatments to control lice each carry welfare trade-offs: chemical bath treatments cause acute stress through handling and confinement; thermal or mechanical treatments cause physical injury and stress; biological control through cleaner fish introduces welfare challenges for the cleaner fish themselves. The development of lice resistance to chemical treatments has increased treatment frequency and welfare impact. Coordinated area management and reduced stocking densities reduce lice transmission and treatment need.