Sea lice are the most significant welfare problem in Atlantic salmon farming, causing painful skin lesions, anaemia, and secondary infections — and treatments themselves carry welfare costs.
Sea lice represent a welfare problem with no perfect solution in open net pen aquaculture. The parasites cause chronic pain and tissue damage; the treatments cause acute stress and sometimes mortality. This welfare trap is inherent to open net pen production and partially resolvable only through system change — transition to closed or semi-closed containment. Within open net pen systems, the welfare-best approach combines preventive measures (cleaner fish, snorkel barriers, deep lighting to keep fish below surface lice concentrations) with treatment triggered at the lowest practical threshold using the least harmful effective method. For consumers, this dynamic reinforces the welfare case for choosing salmon from RAS or certified semi-closed systems where sea lice are not a management challenge.