Sea lice (Lepeophtheirus salmonis and Caligus elongatus) are ectoparasites causing the most significant disease welfare problem in Atlantic salmon aquaculture. Treatment methods — from chemical treatments to mechanical removal — all carry welfare implications for both salmon and non-target species.
Sea lice at heavy infestation levels cause visible tissue destruction around the head, dorsal, and caudal regions of salmon. The osmotic disruption caused by lice damage compromises salmon osmoregulation. Chemical bath treatments create acute stress from crowding and chemical exposure. Thermal delousing causes corneal cataracts in treated fish that permanently impair vision and feeding ability. Mechanical removal using high-pressure water is associated with scale loss and injury. Snorkel-based cage designs that prevent lice from reaching the surface, where salmon typically aggregate, offer a passive welfare-positive prevention approach. Integrated pest management combining multiple low-impact methods achieves lower lice burdens with lower cumulative welfare cost per treatment event.