Sea lice are external parasites that cause significant pain, tissue damage, and immune stress in farmed Atlantic salmon, representing one of the most serious and persistent welfare challenges in salmon aquaculture.
Sea lice cause genuine suffering through direct tissue damage — attachment sites become inflamed and ulcerated, providing entry points for secondary bacterial infection. Thermal delousing involves exposing fish to water at 34 degrees Celsius for approximately 30 seconds, causing thermal stress and mortality in weak fish. Mechanical delousing using jets or brushes causes physical trauma and scale loss. Even chemical treatments stress fish through handling. Every treatment approach currently available carries welfare costs for the fish being treated, making prevention through site selection, lower stocking densities, and barriers (submersible cages, snorkel barriers) the most welfare-positive strategy. The welfare of wrasse cleaner fish — often housed in inadequate conditions and dying at high rates — adds a further welfare dimension to biological control approaches.