Water Tank and Field Tests on the Performance of a Submergible Fish Cage for Farming Silver Salmon

Silver salmon is usually cultured around the eastern coast of Tohoku Region in Japan. The smolt of silver salmon begins to be cultured in a cage from November. Currently, the aquaculture of silver salmon is terminated until the end of July due to high water temperature since silver salmon will die in a few days if the diurnal minimum water temperature becomes 21 or 22°C. Live salmon cannot be obtained around August because wild salmon is captured from September or October. In the present study, a submergible cage using flexible tubes is proposed to farm silver salmon in deeper and cooler waters in August. The cage was submerged and floated up by ejecting air from and injecting air into the flexible tubes, respectively. The flexible tubes were inserted into the polyethylene pipes. First, water tank test using the 1/3.64 scaled model was carried out in the Ocean Engineering Basin, Institute of Industrial Science, The University of Tokyo. The cage was submerged or floated up, changing the methods of air ejection or injection. The cage was submerged and floated up faster in case of two vents to increase the rate of air flow than in case of only one vent on flexible tubes. The submersion was also faster if the length of an injection tube between the vents and air compressor was shortened to reduce the pressure loss. However, the maximum inclination angle of the cage was determined by the diameter of the cage and the submerged depth, not depending on the methods of air injection or ejection. Similar results were observed in the field test. Consequently, silver salmon could be farmed in deeper and cooler waters until the middle of August. The inclination of the cage was not the problem for silver salmon, while it may have unfavorable effects on the other species through the deformation of netting. Hence, the methods to reduce the inclination of the cage in submerging or floating up operation should be considered and validated in field test in the future.