Evaluating bath treatment effectiveness in the control of sea lice burdens on Atlantic salmon in New Brunswick, Canada

The use of medicinal bath treatment for sea lice is becoming more common, due to increasing resistance to in‐feed treatments with emamectin benzoate. Common treatment modalities in New Brunswick, Canada, include Salmosan administered by tarpaulin or wellboat, and Paramove administered by wellboat. In this study, we assessed the effectiveness of these treatment modalities in the field between 2010 and 2015 using a web‐based sea lice data management system (Fish‐iTrends©). Effectiveness was evaluated for adult female (AF) and for pre‐adult and adult male (PAAM) life stages separately. We also investigated the impact of variability in pretreatment lead and post‐treatment lag time on effectiveness measures. There were 1185 treatment events at 57 farms that uniquely matched our pre‐ and post‐treatment count criteria. The effectiveness of treatment modality was significantly influenced by season, pretreatment level of sea lice and by lead and lag times. In summer, Salmosan administered by tarpaulin had the greatest effectiveness on both AF and PAAM, when pretreatment levels were above 10 sea lice; whereas in autumn, the performance of treatment modalities varied significantly, depending on the pretreatment levels for the life stages. Ignoring the lead or lag time effect generally resulted in an underestimation of treatment effectiveness.