Development of the Fish Cage Installed in Variable Depths

Abstract

Aquaculture is expected to play a key role in the increase in fishery production in the future. Because of environmental and coastal utilization issues, however, it is necessary for us to operate cultivation in exposed area for more fishery production, where the fish cage and its mooring system face severe environmental condition such as typhoon. In that area the fish cage should be submerged to protect it from high waves since the influence of waves is reduced in deeper water. Nevertheless, the technologies for open ocean aquaculture are still in the phase of being developed. For example, in general operating systems, a fish cage is floated or submerged on the basis of the framework which is fixed by buoys and anchors. In the exposed area, however, buoys are broken by waves, resulting in the high maintenance cost. Our research aims at creating the new mooring system of a fish cage. The fish cage is moored without the framework in various depths. Buoys can be submerged so that the maintenance cost of the mooring system must be reduced. The feasibility of the mooring system is examined by smaller-scale water tank testing and simple numerical analysis. First the behavior of the mooring system in still water was examined by both numerical analysis and tank model testing. As a result, the fish cage can be submerged at an arbitrary depth only by changing the ratio of the buoyancy to the weight of the fish cage. Then, using a new model of a fish cage, towing test was carried out to estimate the drag force. The drag force is proportional to the 1.5 powered velocity of water current, which is different from the second powered velocity usually used in numerical analysis. This discrepancy may be attributed to the lack of measured data. It should be also noted change in the attacking angle of water current can influence the drag force. Finally, towing test of the mooring system was carried out at the various towing speeds to examine the change in the geometrical formation of the system. At the higher towing speed fish cage is more submerged due to the increase of drag force. In addition, the fish cage begins inclining just after towing. The drag force must vary since the projected area changes due to the inclination of the fish cage and to the deformation of the net. However, the forces on the fish cage are balanced again so that the geometrical formation becomes stable. As future studies, the experimental result will be compared with the analytical one, in which the effect of drag force will be added to the balance of forces on the fish cage. The water tank has a wave maker, so waves will be added to the water current since wave-induced forces work on the fish cage submerged near water surface.