Study on size selection system of fish in trawl fishing

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In test tank (100 × 5 × 1.5m) the model experiments of Sort-X were carried out with different towing speed using model trawl net. Fluid resistance of Sort-X was 3% to 4% of all net. In the experiment using a one fifth scale model it was observed that No.3 grid frame was sticked fast to bottom net. To keep them apart the No.3 grid canvas was replaced by minnow net. As it required time to replace No.1 and 2 grids by different bar spacing grids, new technique of bar spacing control was developed. From Aug.26 to Sep. 1, 1998 the selectivity experiment of the Sort-X built as a trial was carried out in the Okhotsk by using T/S Shinyomaru. In the experiment two types of Sort-X were used ; one was fixed bar spacing and another was trial Sort-X in which bar spacing was changed from 2.5cm to 4.5cm easily. To save fish escaping from grid spacing the cover nets were set on No.1 and 2 grids, respectively. The numbers of flat fish caught in a cod end and in two cover nets were counted, respectively ; at the same time fish length, height and weight were measured. The catch of cover nets was 10% to 25% of all catch ; this deviation was considered to be the effect of trial Sort-X grid space. The master curve of grid selectivity in the trial Sort-X was computed in which 50% selective fish height wa 3.2cm, 4.0cm and 4.8cm at grid spacing of 4cm, 5cm and 6cm, respectively. The weight of trial Sort-X was so heavy as twice of usual one that it was necessary to add many floats and shape of net was deformed.