A fish delousing system (100) for sea-based fish farms (FF) which comprises a holding tank (101), a water supply system (102) connectable to a water source and to the tank (101) and adapted for supplying fresh water (FW) to the tank (101), and a fish transport system (103) for moving euryhaline fish (EF) from the fish farms (FF) into the tank (101) for a delousing treatment. The water supply system (102) comprises a reverse osmosis desalination unit (104), which is capable of converting sea water (SW) from the water source (WS) to fresh water (FW) for the delousing treatment and is powered by one or several electric power production units. The desalination unit (104) is located in the vicinity of the fish farms (FF) or the power production units and the water source (WS) is sea water (SW) in the vicinity of the fish farms (FF) or the power production units.
