Sea lice are external parasites that attach to salmon skin and cause welfare harm. Their treatment — through medications, thermal treatment or wrasse cleaner fish — each carries its own welfare costs and benefits.
Sea lice management involves welfare trade-offs at every stage. Uncontrolled lice cause open wounds and anaemia in salmon — significant welfare harm. Chemical treatments (emamectin, azamethiphos) reduce lice load but cause stress during bath treatment. Thermal Lice removal involves brief exposure to hot water that causes pain-like responses measurable in salmon physiology. Wrasse cleaner fish, increasingly used as a biological control, face serious welfare issues of their own: overcrowding, aggression, inappropriate housing and high mortality rates mean cleaner fish welfare is now recognised as a significant problem within the solution to sea lice. Integrated pest management that reduces lice pressure through site selection, fallowing and snorkel barriers reduces overall welfare cost of lice treatment.