Study on size selection system of fish in trawl fishing

在100 × 5 × 1.5m的试验水池中，利用模型拖网进行了Sort-X在不同拖速下的模型试验。Sort-X的流体阻力为所有网的3%至4%。在使用五分之一比例模型的实验中，观察到3号网格框架牢固地粘在底部网上。为了保持它们分开，3号网格画布被小鱼网取代。针对1、2号网架采用不同间距的钢筋代替需要一定时间的问题，提出了一种新的钢筋间距控制技术。1998年8月26日至9月1日，使用T/S Shinyomaru在鄂霍次克进行了作为试验建造的Sort-X的选择性试验。试验中采用了两种类型的Sort-X，一种是固定栅间距，另一种是试验Sort-X，其中栅间距可以从2.5cm变化到4.5cm。为防止鱼类从格距中逃逸，在1号格和2号格分别设置了覆盖网。分别对单网和双网的平口鱼数量进行计数，同时测量鱼的体长、体高和体重。覆盖网的渔获量为总渔获量的10%至25%;这一偏差被认为是试验Sort-X网格间距的影响。计算了试验Sort-X中网格选择性的主曲线，其中网格间距为4cm、5cm和6cm时，50%选择性鱼高分别为3.2cm、4.0cm和4.8cm。试验用Sort-X的重量是常规Sort-X的两倍，需要增加许多浮体，网形变形。

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.