Pharmaceutical pollution influences river-to-sea migration in Atlantic salmon (Salmo salar)

Despite the growing threat of pharmaceutical pollution, we lack an understanding of whether and how such pollutants influence animal behavior in the wild. Using laboratory- and field-based experiments across multiple years in Atlantic salmon (Salmo salar; n = 730), we show that the globally detected anxiolytic pollutant clobazam accumulates in the brain of exposed fish and influences river-to-sea migration success. Clobazam exposure increased the speed with which fish passed through two hydropower dams along their migration route, resulting in more clobazam-exposed fish reaching the sea compared with controls. We argue that such effects may arise from altered shoaling behavior in fish exposed to clobazam. Drug-induced behavioral changes are expected to have wide-ranging consequences for the ecology and evolution of wild populations.