网目尺寸对金枪鱼围网沉降性能及网具形态的影响

根据以往的研究结果和海上网具性能实测,并在听取生产第一线船长意见的基础上,把上海开创远洋渔业有限公司现行金枪鱼围网主网衣下部约三分之一网衣的网目尺寸放大50%,即从260 mm增大到390 mm,以探讨改进围网渔具的沉降性能。实验应用田内准则换算并制作成模型网,在日本下关西日本日网公司专业围网动水槽进行模型试验,比较了侧流、背流、顺流(流速8 cm/s,相当实物网0.20 m/s)3种相对流向的放网模式下放网以及无流(0 cm/s)海况下放网,网目尺寸改变前后围网的沉降性能、网具形态和闭合效果。结果显示,主网衣下部网目尺寸从30 mm (对照网,相当于实物网网目尺寸260 mm)放大到45 mm (试验网,相当于实物网网目尺寸390 mm),网具的沉降深度和沉降速度分别提高1.6%和1.1%;试验网的收绞时间比对照网缩短14.3%,且能保持较好的形态;有流时的3种放网模式下,试验网和对照网浮子纲在放网结束时刻的初始包围面积基本相同;在收绞结束时,试验网浮子纲包围面积为对照网的1.33倍,绞纲结束前6 s,试验网沉子纲包围面积为对照网的1.79倍,说明增大网目尺寸有利于网具包围面积的增大,改善了被围鱼群的活动空间。

Influence of mesh size on sinking performanceand the shape of tuna purse seine

The net performance of tuna purse seine is influenced by mesh size, leadline weight, flow velocity and other factors. Most of the purse seine fishing vessels in China are used vessels. The age of the vessels is high and the equipment is old. The settling performance of the nets is hardly improved by increasing the weights of the leadline. In order to understand the effect of mesh size on the sinking performance, enclosed effectiveness and net shape, the authors based on our previous research results, net performance measurement at sea and the opinion of the captain, enlarged the mesh size from 260 mm to 390 mm (equivalent to 50%) of the low one third of the main body of the tuna purse seine currently used by the Shanghai Ocean Fishery Co., Ltd. Two model nets with different mesh sizes in the main body were made by the Tauti rule and the comparative tests were conducted through three shooting patterns at 8 cm/s and one at still water in a professional purse seine test tank of Nichimo Co., Ltd. of Shimonoseki Research Institute. The results show that:both the sinking depth and sinking speed of the experimental net with bigger mesh size (45 mm) are greater than that of the control net with smaller mesh size (30 mm)(Sinking depth increased by an average of 1.6%, and sinking speed increased by an average of 1.1%), indicating that increasing the mesh size can improve the sinking performance of purse seine. The pursing time of the experimental net was 14.3% less than that of the control net, and experimental net also kept a good form. The opening areas surrounded by the floatline at the end of shooting are almost identical for both the experimental net and control net; however, at the end of pursing, the opening area surrounded by floatline of experimental net was 1.33 times that of the control net and the opening area of the leadlline was 1.79 times that of the control net, indicating that the bigger mesh netting can provide bigger areas for fish school swimming.