Experimental study on the motion of a flexible hose net used for fish-harvesting in a set-net fishery

Abstract

In a set-net fishery, a leader net and the related nets guide migratory fish into a box chamber net. Many workers have to visit the set-net and haul the box chamber net for harvesting almost every day, while the net-hauling operation in the early morning is dangerous. An automated net-hauling system using a flexible hose net has been proposed as a manpower-saving measure in such operation. The hose net comprises the brick forms of flexible hoses and is installed on the water bottom below the box chamber net. Compressed air is injected from one edge of the hose net to haul the box chamber net gradually, resulting in cornering fish in the other edge. The purpose of the present study is to observe the motion of the hose net and to analyze its formation in water current. Water tank testing was carried out using a 1/100 scale model of a set-net and a hose net. As a result, the middle part of the hose net was steep enough to corner fish in the end of the box chamber net. The two-dimensional formation of the hose net could be approximated by a catenary curve, while a part of the hose net including air inside was likely to have stiffness. The stiffness of the hose including air inside should be taken into account in the following water tank testing. The experimental results will provide the benchmark for the validation of the numerical model, which will be developed in the future. In addition, numerical analysis as well as water tank testing will reveal the tension on the hose net and mooring ropes for the basic design of the hose net in the future. For realizing the prototype of the hose net, special care should be taken for the explosion of air with decreasing pressure in the floating operation and for the leakage of air through the pinholes, which may be generated by sudden change in water pressure on the flexible hoses and their connectors.