The hydrodynamic resistance of small pot traps has been conducted in order to establish some basic information. The specific objectives of the study was to measure the hydrodynamic force and estimate the critical setting condition for traps. Five types of traps with different materials were used in the experiment: a netted semi-cylinder shape, a wire semi-cylinder shape, a heart shape, a box shape, and a cylinder shape. The hydrodynamic force of each trap was measured in a flume tank. Flow speeds in the flume tank were 0.1, 0.2, 0.3, 0.4, and 0.5 m/s. Attack angles for this study were 0, 15, 30, 45, 60, 75, and 90 degrees. At an attack angle of 0 degrees the main axis of the trap was parallel to the water flow and at 90 degrees it was vertical. The values of the hydrodynamic drag coefficient varied with traps: netted semi-cylinder shape, 2.75-5.96; wire semi-cylinder shape, 2.81-4.49; heart shape, 2.77-3.66; box shape, 2.39-2.97; and for cylinder shape, 3.57-3.67. The flow speed (0.5 m/s) was effective to set the netted semi-cylinder, wire semi-cylinder, box, and cylinder shaped traps. The same flow speed applied to the heart-shaped trap was only effective to a maximum of 30 degrees attack angles and below.
The authors propose a new bycatch reduction device for trawling with a simply constructed TREND (trawl flow regulative ecological friendly netting device). TREND is a cylindershaped netting device with a rectangular-shaped mesh. We examined a selective mechanism from the viewpoint of the flow inside and around TREND. A series of model experiments was conducted and the flow speeds were measured at a flume tank. The ratio of flow speed decreased at points close to the netting and increased slightly at other points in all sections. The angles of tuft at the top netting in the rear area were almost twice the value in the front sections. As the exhaustion of flow and reduction of speed occurred simultaneously, these tendencies were remarkable at higher speed. This flow towards the outside of TREND was considered to be acting as a bycatch reduction device for trawling.
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