Study on size selection system of fish in trawl fishing

在试验槽（100 × 5 × 1.5m）中，采用模型拖网对Sort-X进行了不同拖曳速度下的模型试验。Sort-X的流体阻力占总净阻力的3% ~ 4%。在1 / 5比例模型的试验中，观察到3号网架与底网的粘接牢固。为了将它们分开，用小鱼网代替了3号网格帆布。由于用不同的杆距格栅代替1、2格栅需要一定的时间，因此开发了杆距控制新技术。1998年8月26日至9月1日，在鄂霍次克号上用T/S新尧丸进行了试制的Sort-X的选择性试验。实验采用两种类型的Sort-X；一种是固定杆距，另一种是易于将杆距从2.5cm改变为4.5cm的trial Sort-X。为防止鱼群从栅格间距中逃逸，分别在第1栅格和第2栅格设置覆盖网。分别统计用一个鱼头和用两张网捕获的比目鱼的数目；同时测量鱼的长、高、重。覆盖网的渔获量占总渔获量的10%至25%；这种偏差被认为是trial Sort-X网格空间的影响。在网格间距分别为4cm、5cm和6cm时，计算出Sort-X试验的网格选择性主曲线，其中50%的选择鱼高分别为3.2cm、4.0cm和4.8cm。试验用的Sort-X的重量是平时的两倍，必须增加许多浮子，网的形状也变形了。

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.