Effects of temporal, geographical and environmental factors on salmon lice (Lepeophtheirus salmonis) levels of Atlantic salmon (Salmo salar) in Ireland.

The salmon louse (Lepeophtheirus salmonis) poses a major challenge for Atlantic salmon (Salmo salar) aquaculture in the northern hemisphere. These ectoparasites feed on the blood, tissue, and mucus of their fish host, causing skin damage, increasing susceptibility to secondary infections, and, in severe cases, leading to fish mortality. The economic impact on aquaculture is substantial due to treatment and management costs. Effective management strategies rely on a comprehensive understanding of infection dynamics. To investigate these dynamics, we analysed a 30-year dataset on sea lice abundance from Atlantic salmon farms in Ireland, collected by the Marine Institute through the National Sea Lice Monitoring Programme. Using generalised linear mixed-effects models, we detected temporal fluctuations, with an overall decline in lice abundance. Seasonal variation was evident, with the lowest abundance in spring and the highest in autumn. Sea lice abundance was influenced by farm exposure, represented by bay mouth width, bay length, number of active farms per bay and dissolved oxygen levels. By identifying key factors driving infection patterns, this study provides valuable insights for predicting high-risk periods and optimising management strategies. These findings will inform strategies for salmon lice control, contributing to improved fish health and sustainability in aquaculture.