Aquaculture and environmental drivers of salmon lice infestation and body condition in sea trout

Infestation of sea trout Salmo trutta L. by salmon lice Lepeophtheirus salmonis is associated with increased mortality risk and possible sub-lethal effects. Separating anthropogenic causes of infestation from background ecological variability has proved difficult. A unique 25 yr dataset was collated comprising lice counts from >20000 sea trout sampled from 94 separate river and lake systems in Ireland and Scotland at varying distances from marine salmon farms. Statistical models were developed to explore the potential effects of distance to a salmon farm, rainfall and ambient temperature on sea trout lice infestation and body condition (weight at length). These models indicated that sea trout captured closer to salmon farms had significantly higher levels of lice infestation, and that this effect was exacerbated in warmer years. Sea trout sampled closer to salmon farms also had significantly reduced weight at length (impaired condition), with the strongest impact in dry years. The study dataset covers a broad geographic area over multiple years, and accounts for variability in temperature and rainfall. Our results imply a rather general impact of salmon farming on lice infestation and body condition of sea trout. This finding has implications for current lice control management strategies, coastal zone planning, recovery of sea trout stocks in aquaculture areas and the scale of aquaculture-free zones