Sea lice — particularly Lepeophtheirus salmonis and Caligus elongatus — represent the greatest welfare and production challenge in Atlantic salmon aquaculture. Infestations cause severe tissue damage, stress, and mortality, and their management through chemical treatments creates additional welfare costs.
Lepeophtheirus salmonis is host-specific to salmonids and particularly damaging in its adult stages. Adult females and males attach to the salmon's skin, mucus, and sub-dermal tissue, feeding on skin cells, mucus, and blood. Lesions develop at attachment sites, particularly on the head, back, and flank. Severe infestations cause large, open wounds that allow secondary bacterial infection, disrupt osmoregulation, and induce prolonged stress responses with elevated cortisol affecting immune function, growth, and reproduction.
Sea lice welfare scoring uses standardised counting protocols for lice at different life stages (chalimus, pre-adult, adult). Industry thresholds — typically 0.5 adult female lice per fish in Norway — trigger mandatory treatment. However, welfare research suggests that even sub-threshold lice burdens at chronically maintained levels cause measurable behavioural changes: increased surface-scraping, reduced feed intake, and altered shoaling behaviour, all indicating ongoing discomfort. Welfare-based thresholds may need to be lower than production-based thresholds.
Bath treatments involve crowding fish and exposing them to chemical agents including hydrogen peroxide, azamethiphos, or deltamethrin. The crowding and chemical exposure process itself causes acute stress — cortisol spikes, surface-seeking, and gasping behaviour. Hydrogen peroxide at high concentrations causes cataracts, which are visible welfare markers that have driven reconsideration of treatment protocols in Norway.
Thermolicers pump fish through a brief exposure to warm water (>34°C for seconds) that dislodges and kills lice. Welfare concerns include: post-treatment skin damage and scale loss, cardiac injuries documented at autopsy, and stress from crowding and pumping. Mortality during treatment varies between operations and is a key welfare metric.
Cleaner fish — wrasse species and lumpfish — are used in salmon pens to pick lice from salmon skin. Welfare concerns have shifted to the cleaner fish themselves: high mortality rates from disease, predation, and inappropriate conditions have raised serious welfare questions about cleaner fish welfare standards. Wrasse sourced from the wild have additional welfare concerns during transport and capture.
Snorkel barriers, deep light systems, and submersible feeding systems exploit lice's preference for surface water to keep salmon deeper and reduce larval lice infestations. Coordinated sea lice management at bay level reduces larval pressure. Closed containment systems eliminate sea lice risk entirely but require high capital investment.
Selective breeding programmes have identified heritable variation in sea lice resistance in Atlantic salmon. Breeding for improved resistance while maintaining other production traits is a long-term solution that could reduce the welfare costs of both infestation and treatment.
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