Mechanical and thermal sea lice treatments in salmon farming cause significant welfare harm. Understanding treatment welfare helps producers and regulators select the least harmful approaches.
Sea lice treatment welfare involves choosing the lesser of multiple evils — both the infestation and available treatments cause harm. Thermolicer treatment that causes cataracts (impairing sight for life) and scale loss (compromising the protective barrier) causes significant long-term welfare harm in surviving fish. Freshwater treatment causes less trauma to the fish than thermal or mechanical methods and should be preferred where effective. The welfare case for prevention over treatment is overwhelming: preventive management reducing lice loads through site selection, snorkel barriers, and lower stocking density avoids both the infestation welfare cost and the treatment welfare cost. Genetic breeding for lice resistance offers the long-term possibility of eliminating lice without either infestation or treatment harm.