Efficiency and welfare impact of long-term simultaneous in situ management strategies for salmon louse reduction in commercial sea cages

The Atlantic salmon (Salmo salar) aquaculture industry is faced with an obstacle in sustainability with increasing production, which is the control and prevention of the ectoparasitic salmon louse Lepeophtheirus salmonis. Lice prevention management is steering towards passive applications, and this study aimed to monitor multiple 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 over a 13-month period, covering a large proportion of a standard production cycle. The additive effect of multiple treatments was established in 12 cages, which were assigned to a prevention strategy of either: cleaner fish only, cleaner fish and functional feed, the previous two factors plus deep attractant lights and submerged feeding, or the previous three factors plus a lice skirt. Environmental profiles and school swimming depth were monitored throughout the study period, and sampling events occurred every 2–6 weeks to assess the infestation and welfare status of salmon. The rate of infestation fluctuated with season; however, the group with all prevention strategies maintained a lower rate of new infestations compared to the groups with cleaner fish or functional feed only. Cages with deep lights and feeding influenced the school swimming depth, with these groups generally swimming deeper; this meant that these cages also swam ~6 m deeper than the halocline when pooled over time. However, even with strong differences in new infestations and vertical distribution, the level of mobile lice was similar among all groups, thus incurring a similar frequency of delousing events. There was no effect of these prevention strategies on overall welfare status of salmon. This study shows the promise of utilising multiple lice prevention approaches and highlights the interaction between environment and infestation pressure.