拖速和曳纲长度对南极磷虾中层拖网网位的影响

利用2015年2—7月随青岛远洋捕捞有限公司南极磷虾(Euphausia superba)大型中层拖网渔船"明开轮"赴南极南舍得兰群岛水域执行农业部南极海洋生物开发利用项目期间收集的网位(D)、网口高度(H)、曳纲投放长度(L)和拖速(V)等数据,分析了拖速和曳纲长度对拖网网位和网口高度的影响。本研究中,网口高度定义为网口上下纲深度之差;网位定义为网口中心位置水深。根据渔船作业习惯和虾群群体特点,曳纲投放长度范围138~258 m,每档间隔20 m。拖速1~3 kn,间隔0.5 kn。结果表明:(1)网口高度变化范围为13.6~24.1 m,网位水深变化范围为50~70 m;(2)作业过程中,南极磷虾拖网网位变化主要由曳纲长度决定,曳纲从138 m开始投放时,每增加20 m,网位平均下降深度约1.9 m,网口高度平均减小1.1 m,曳纲长度对网位和网口高度均产生极为显著的影响(P0.01);(3)拖速由1.0 kn变化至3.0 kn时,网位平均上升速率约2.9 m/kn,网口高度平均减小速率为2 m/kn,平均降低19.8%,拖速对网位和网口高度的影响显著(P0.05);(4)不同曳纲长度时的各拖速区间内的平均网位变化速率呈先减小后逐渐增大的规律。本研究结果不仅可为南极磷虾渔船船长根据虾群的群体大小、水层变化及其与网具的位置关系,适时调整曳纲长度和拖速,使网位到达预设水层,实现瞄准捕捞提供参考,还可为研究南极磷虾拖网网具性能的优化以及南极磷虾拖网网具设计的改进提供基础资料。

Effects of towing speed and warp length on the net position of Antarctic krill mid-trawl

Based on the on-sea measurements on net position and net mouth height (vertical opening) of the 400 m × 143.44 m (headline 61 m) of Antarctic krill trawl used by the "Mingkai" krill trawler operated at South Shetland Island from February to July 2015, this paper analyzed the effects of towing speed and warp length on the net po-sition and net mouth height. The net position and net mouth height were measured by DR-1050 depth instruments and the corresponding towing speed and warp length during trawling were also recorded. In this study, the net mouth height is defined as the difference in water depth between the headline and footrope, while the net position is defined as the water depth of the central position of the net mouth. According to the usual operation practice of the fishing vessel and characteristics of the krill aggregation, the range of warp length increased from 138 m to 258 m, with 20-m intervals, and tow speed increased from 1 kn to 3 kn with 0.5-kn intervals. The main results are as follow: (1) The net mouth height of the krill trawl studied ranged from 13.6 m to 24.1 m, while the net position ranged from 50 m to 70 m deep. (2) During trawling, net position was controlled mainly by the length of warp. For each additional 20 m from 138 m of the warp length, the net position decreased by 1.9 m on average, and the net mouth height reduced by approximately 1.1 m on average. Warp length showed significant effects on both the net position and net mouth height (P<0.01). (3) When towing speed increased from 1.0 kn to 3.0 kn, average rise rate of the net position reached 2.9 m/kn with the variation of warp length changing from 138 m to 258 m. When tow-ing speed increased from 1.0 kn to 3.0 kn, the vertical opening of net mouth average reduction rate reached 2 m/kn, accounting for 19.8% on average. Towing speed had a significant impact on the net position and net mouth open-ing (P<0.05). (4) Analysis of the relationship between rate of net position change and towing speed expression showed that average rate of the net position change decreased first and then increased with different ranges of towing speed under different warp lengths. This result can not only provide useful reference for the Antarctic krill fishing vessel captain who can, according to the size or water layer of the krill aggregation and its relation with the location of the net, timely adjust the warp length and towing speed and control the net at the water depth for ac-curate fishing, but also provide basic information for Antarctic krill trawl model experiments and independent design of the Antarctic krill trawl net.