Thermal delousing with cold water: Effects on salmon lice removal and salmon welfare

Warm water thermal treatments dominate delousing operations in Norwegian Atlantic salmon (Salmo salar) aquaculture for the removal of ectoparasitic salmon lice (Lepeophtheirus salmonis). However, treatments can lead to poor welfare outcomes for fish. Reverse thermal delousing by rapidly reducing ambient temperatures to very low treatment temperatures may also prove effective if there is a sufficient safety margin in the tolerances of salmon to cold water exposure such that negative side effects do not emerge. We tested the effects of immediate transfer of salmon from ambient temperature (15 degrees C) to cold water at different temperatures and durations on lice removal and short-term impacts on fish welfare. Treatments of -1 degrees C water for 10 min and 1 degrees C for 240 min treatments reduced mobile lice loads, but created more skin and eye damage than controls. Our results on delousing effect and welfare outcomes require ground-truthing at industry scale, as crowding, pumping, and the passage of fish through delousing units add additional and possibly synergistic lice removal effects and pose further welfare risks.