Sea lice treatment in salmon farming involves thermal, chemical, and mechanical methods that cause significant acute welfare impacts on millions of fish annually.
Sea lice treatment creates a welfare dilemma: untreated lice cause chronic suffering, but treatments themselves cause acute welfare harms. Thermal lice treatments at imprecise temperatures cause fin damage, cataracts, and internal injuries in a proportion of treated fish. Mechanical delousing causes scale loss and haemorrhage. Chemical treatments cause stress responses lasting hours. Each delousing event is a welfare-costly intervention repeated multiple times per production cycle. Cleaner fish — wrasse and lumpsuckers added to pens to eat lice — reduce treatment frequency but introduce new welfare concerns: cleaner fish have high mortality and significant welfare challenges in salmon pen environments. Reducing sea lice through site fallowing, lower stocking densities, and selective breeding for resistance is the most welfare-positive long-term approach.