Sea lice treatment in salmon farming causes significant welfare stress, with the welfare costs of treatment needing to be weighed against the welfare harm of unchecked parasitic infestation.
Sea lice treatment welfare involves a complex trade-off calculation: the welfare harm of treatment versus the welfare harm of allowing lice to proliferate. Heavy lice infestations cause progressive skin damage, anaemia, impaired osmoregulation, and systemic physiological stress that are clearly welfare-relevant. Without treatment, welfare deteriorates progressively. However, the treatments themselves cause acute stress that can result in significant mortality, particularly in weak or compromised fish. Thermal bath treatments (crowding fish into containers of heated water to kill lice) cause severe crowding stress and temperature shock, with survival rates varying by fish condition, time of year, and precise temperature management. The development of biological lice control using cleaner fish (wrasse and lumpsucker) that eat lice from salmon provides a potential welfare improvement by reducing chemical and thermal treatment frequency. However, cleaner fish welfare is itself a concern: these small fish sourced partly from wild capture or intensive aquaculture are used as biological tools and often have high mortality rates in salmon cage conditions. The welfare of the entire production system, including cleaner fish, requires holistic assessment.