Sea lice infestations are the leading disease challenge in Atlantic salmon aquaculture, causing pain, tissue damage, and immune stress in millions of farmed fish while also threatening wild salmon populations through parasitic spillover.
Sea lice create chronic welfare problems for farmed salmon on two levels. At low to moderate burdens, lice cause continuous tissue irritation and immune stress that affect feeding behavior and growth. At high burdens, deep skin lesions expose raw tissue, which is acutely painful and creates entry points for secondary bacterial infection. Treatment adds further welfare burden: mechanical treatments (brushing, warm water) require fish to be handled and stressed; chemical baths expose fish to potential toxicity. Cleaner wrasse used as biological control are themselves welfare concerns - small fish used as a tool and discarded when no longer effective. The cumulative welfare cost of sea lice management throughout a salmon's production cycle is substantial.