Lepeophtheirus salmonis progresses through 10 developmental stages: nauplius (×2), copepodid (infective), chalimus (×4, attached by frontal filament), pre-adult (×2), and adult. The adult stage — particularly gravid (egg-bearing) females — causes the greatest welfare harm. Development rate is temperature-dependent: cycles complete in 7-17 days at typical sea temperatures.
Copepodids attach to salmon and migrate to feeding sites on the head, back, and fins. Chalimi are immobile, attached by frontal filament. Pre-adults and adults feed on mucus, skin, and blood, creating lesions. Feeding creates inflammatory responses, secondary infections, and osmoregulatory disruption at heavy infestation levels.
Salmon mount immune responses to sea lice including mucus hypersecretion, inflammatory cell recruitment, and melanocyte activation. However, sea lice actively suppress host immune responses through secreted factors. Previous exposure does not create reliable protective immunity, complicating vaccine development. Stress from other sources (high density, disease) impairs the host response to sea lice.
Sea lice development and reproduction are faster at higher temperatures. Climate change is warming sea temperatures in major salmon producing regions (Norway, Scotland, Chile), accelerating lice development and increasing treatment frequency requirements. This creates both increasing welfare challenges and greater antibiotic/pesticide use pressure.
Wrasse (Labridae) and lumpfish (Cyclopterus lumpus) consume sea lice from salmon skin. They are deployed at ratios of 1:50-100 salmon. Cleaner fish provide chemical-free control, but their own welfare is a growing concern: wrasse welfare in aquaculture requires appropriate habitat, hiding structures, and species-appropriate nutrition. Over-reliance on cleaner fish in poor conditions generates welfare problems.
Resistance to emamectin benzoate (slice) is widespread in Norwegian and Scottish aquaculture. Resistance to hydrogen peroxide and azamethiphos is emerging. Resistance monitoring programs and treatment rotation strategies preserve efficacy. New biological control approaches (selective breeding for louse resistance, genome editing, vaccines) are active research areas.