Aluminium in acidic river water causes mortality of farmed Atlantic Salmon (Salmo salar L.) in Norwegian fjords

Acute mortality of Atlantic salmon has been described from fjord-based fish farms in Western Norway. Mortality is often related to snowmelt and heavy rainfall in the catchment areas during the winter. Increased freshwater runoff reduces the surface water salinity from >20 to <10, while water temperature is reduced from 8 to 3 °C. Recent studies have suggested that aluminium transported by acid rivers to the fjords during these episodes can be the cause for mortality. During the present study, we documented an increased deposition of aluminium (Al) on gills of Atlantic salmon (from <10 to >200 μg g−1 dry weight). The increases in gill Al were related to increased discharge episodes where acidic, Al-rich freshwater elevated the surface water concentrations of Al from <20 to >70 μg Al l−1. Increased mobility of reactive Al (Ala) and increased Al accumulation on gills during flood episodes was the probable cause of the massive salmon mortality.