Assessment of long-term implementation of sea lice prevention technologies: efficiency in reducing infestations and impact on fish welfare

Salmon lice prevention strategies are steering towards passive implementation, and this study aimed to monitor these strategies in commercial cages over time, to determine the efficiency of these approaches and their effect on welfare. Four strategies were tested at a commercial scale at the Centre for Aquaculture Competence (CAC; funded by CAC and FHF), over a 13-month period. The additive effect of multiple treatments was established in 12 cages, which were assigned to a prevention strategy of either cleaner fish only (A group), cleaner fish and functional feed (B group), the previous two factors plus deep attractant lights and submerged feeding (C group), or the previous three factors plus a lice skirt (D group). Environmental profiles and school swimming depth were monitored throughout the study period, and sampling events occurred every 3 –5 weeks to assess the infestation and welfare status of salmon. The rate of infestation fluctuated with season, however the group with all prevention strategies (D) maintained a lower rate of new infestations compared to the groups with in-cage prevention (A and B groups). However, even with strong differences in new infestations and swimming behaviour, the level of mobile lice was similar among all groups, thus incurring a similar frequency of delousing events. This is potentially due to unrepresentative sampling of salmon or reduced cleaner fish efficiency in treatment cages. There was no overall effect of these prevention strategies on welfare status of salmon during the study period. Thus, the tested prevention strategies have promising potential for commercial implementation; however, improved and consistent efficiency is likely to be achieved with flexible operation that changes with specific environmental conditions