PROBLEM TO BE SOLVED: To solve the urgent problem confronting fishing businesses every year in which damage amounts to several billion yen in the fishing industry due to recent deaths of yellow tails and the like caused by red tide, and countermeasures against the problem have not been developed.SOLUTION: Provided is a method for completely preventing red tide invasion most effectively, by which high-grade fishes such as yellow tails bred in a net cage arranged near the surface of the sea are prevented from dying due to the invasion of the highly virulent red tide. In the method, copper ions which are effective in fish diseases are used, because the copper ions are also effective in annihilating the red tide, and a pair of electrodes for a metal wire net made of titanium platinum plating is installed in a cooling sea water pipe, for applying electrical shock to cell membranes of the red tide and immediately annihilating the same, and the power is applied thereto together with a copper ion generation device, copper ion discharge water that has thereby annihilated the red tide is discharged into the sea, and also the annihilation of the red tide is carried out while towing the copper electrode net and running to and from a fishing ground, and dissolved oxygen is supplemented from a lower surface of the fish-farming net cage to revive the dying fishes, and a buoy, which has copper electrodes suspended therefrom and is equipped with a solar battery, is provided outside the net cage for preventing the red tide from invading into the net cage.
