栅型兼捕减少装置的规格和倾角对网囊水动力及形态的影响

Effects of size and grid angle of sorting grid on the hydrodynamic performance and shape of trawl codend

为减少副渔获物的兼捕,提高拖网渔具选择性,栅型兼捕减少装置已在底拖网渔业中得到广泛应用。兼捕减少装置水动力变化影响着鱼类接触后的行为反应,从而决定着装置的分隔效率和选择性效果。实验通过动水槽模型探究3种不同规格的栅栏(Grid-0.3栅、Grid-0.4栅、Grid-0.5栅)分别在倾角30°、45°和60°,3种状态下对网囊网目张开程度、空间形态和阻力的影响效果。结果发现,(1)无栅栏网囊的网目平均张开角度22°±5°。设置Grid-0.4栅和Grid-0.5栅网囊的网目平均张开角度较无栅栏网囊分别增大了3.3%和18.8%;而设置Grid-0.3栅网囊的网目平均张开角度减少了9.2%。(2)Grid-0.4栅对网囊形态影响程度最小,贴合网囊流线型;Grid-0.3栅和Grid-0.5栅使网囊发生扭曲形变。(3)设置Grid-0.3栅的网囊,不同倾角下的阻力差异较小,与无栅栏网囊的阻力相近。以不同倾角设置Grid-0.4栅和Grid-0.5栅的网囊,平均网囊阻力分别比无栅栏网囊的阻力高12.7%和24.4%。除Grid-0.3-A-30°栅外,有模拟渔获物条件下,网囊阻力均随栅栏倾角和规格的增大而增加。本研究对我国底拖网选择性研究和对保护近底层渔业资源具有重要意义。

Reducing trawl bycatch rates on non-target species is a hot topic in fisheries science.The sorting grid is widely used in trawl fisheries to reduce bycatch and improve trawl selectivity.The change in hydrodynamic force of the bycatch reduction device influences the behavioral response of fish after contact,determining the device's separation efficiency and selectivity.Therefore,understanding the hydrodynamic performance of the bycatch reduction device is critical for improving the selectivity of trawl gear.This study investigated the effects of grid sizes(Grid-0.3,Grid-0.4 and Grid-0.5)on the mesh opening degree,spatial geometry,and drag of codend under three different grid angles(A-30°,A-45°and A-60°)using flume tank experiment.The results showed that:(1)The average mesh opening angle of codend without grid was 22°±5°.The average mesh opening angle of the codend with Grid-0.4 and Grid-0.5 was 3.3%and 18.8%respectively,greater than that of the codend without grid,while the mesh opening angle of the codend with Grid-0.3 was 9.2%lower than that of the codend without grid.(2)Grid-0.4 has the least influence on the codend shape and was streamlined to accommodate it,while Grid-0.3 and Grid-0.5 distorted the codend.(3)The differences in the drag of the codend with Grid-0.3 at different grid angles were small with an average similar to that of the codend without grid.The average drag of codend with Grid-0.4 and Grid-0.5 at different grid angles was 12.7%and 24.4%greater than that of the codend without grid,respectively.Except for the codend with Grid-0.3 at A-30°,the codend drag increased with increasing grid angle and size.The findings could have a significant impact on the research on bottom trawl selectivity and the protec-tion of demersal fishery resources in China,particularly research on devices that can effectively release bycatch and juvenile fish.