环境友好型虾夷扇贝捕捞网具优化设计与试验

目前中国虾夷扇贝网具存在捕捞效果差、海底生境影响严重等问题，该研究基于水下视频监控技术，设计了水压板导流与随动式惊扰相结合的拖曳网架结构。采用FLUENT软件对改造后捕捞网具的水下拖曳水动力特性进行分析得出，在拖曳水深在30~50 m、拖曳速度为1.8 m/s、网口高度为350 mm时，采用22.5°～30°负迎角弧式水压板，可减轻顶部湍流对底栖鱼类的兼捕影响，且随动式惊扰装置满足作业要求，此时拖曳稳定绳长约为156.80～261.34 m、稳定角度为11.03°、稳定拖曳力为2275.83 N。通过海上捕捞对比试验得出，在拖曳速度为1.5、1.8和2.0 m/s时，改造后捕捞网具与生产用捕捞网具在捕捞量方面无显著差异（P>0.5），在碎贝量和底栖鱼类兼捕量方面显著降低（P<0.5）。单网平均捕捞量的偏差率分别为6.30%、0.59%和5.55%，单网平均碎贝量的偏差率分别为90.63%、84.78%、85.29%，单网底栖鱼类平均兼捕量的偏差率分别为78.57%、81.25%、84.85%。且在拖曳速度为1.8 m/s时，与生产用网具相比，单网平均捕捞量偏差率最低，单网平均碎贝量的偏差率最低，底栖鱼类平均兼捕量显著减少，符合环境友好型捕捞网具设计要求。

Optimizing sustainable fishing gear in environmentally friendly fishing for patinopecten yessoensis

Patinopecten yessoensis have been one of the most valuable shellfish in the North Yellow Sea. Trawling has also been one of the most widely used harvestings. However, the bycatch of benthic fish and the impact of the trawling process on the bottom ecology have posed a great threat to the sustainable development of the scallop farming industry. Eco-friendly fishing has been a high demand in recent years. It is very necessary to optimize the structure of the net gear (the disturbing device and the trawling structure) and the speed of the trawling net in the combination with the bottom of the seabed. In this study, an environment-friendly fishing optimization and transformation scheme was proposed to evaluate the performance of the disturbing device and the trawl structure for the three scallop fishing nets, such as the dredge, cutting, and hydrodynamic types. The underwater video surveillance technology was also used to clarify the causes of the problems (leakage capture, broken, and choking mud) in the underwater operation of scallop fishing nets. The gravity mesh head, torsion spring type elastic teeth/multi-strand wire rope type elastic teeth, and flexible tail trawl were transformed into the adjustable hydraulic plate mesh head structure, follow-up the disturbing device, and flexible connection of rigid tail trawl net structure. As such, there was a significant decrease in the missed catch rate, broken shell rate, sediment content, and bottom-dwelling fish bycatch. The Ansys-Fluent software was utilized to simulate the three hydraulic plates (plate, positive angle of attack arc, and negative angle of attack arc type) at four towing speeds of 1, 1.5, 2, and 2.5 m/s. The hydrodynamic characteristics demonstrated that the drag resistance of three hydraulic pressure plates increased with the increase of the outflow angle and the drag speed. There was less resistance of the positive angle of attack arc type and the negative angle of attack arc type hydraulic pressure plate, compared with the flat type. The negative lift generated by the hydraulic plate increased with the increase of the drag speed. Once the outflow angle was 22.5°, the negative lift generated by the negative angle of the attack arc hydraulic plate was smaller than those of the flat plate type and the positive angle of the attack arc. When the angles were 30° and 45°, the negative lift generated by the negative angle of attack arc type hydraulic plate was greater than those of the flat plate type and the positive angle of attack arc type. The maximum negative lift was achieved for the best solution to the shape of the negative angle of the attack arc hydraulic plate. Furthermore, the mesh head and the disturbing device were in the best working condition, when the towing speed was between 1.5 and 2 m/s. When the towing speed was 1.5 m/s and the towing water depth was 30-50 m, the optimal combination of parameters was achieved as follows: the towing critical rope length was about 34.87-58.12 m, the stable rope length was about 156.80-261.34 m, the critical angle was about 59.35°, and the critical drag force was 3 848.16 N, the stability angle was 11.03°, and the stable drag force was 1 998.85 N. When the towing speed was 2 m/s and the towing water depth was 30-50 m, the optimal combination of parameters was: the critical towing rope length was about 35.20-58.67 m, the stable rope length was about 156.80-261.34 m, the critical angle was about 58.45°, the critical drag force was 5 222.04 N, the stability angle was 11.03°, and the stable drag force was 2 783.93 N. When the optimal towed speed was 1.8 m/s and the towed water depth was 30-50 m, the optimal combination of parameters was: the length of the towed boundary rope was about 35.02-58.36 m, the stable rope length was about 156.80-261.34 m, the critical angle was about 58.95°, the critical drag force was 4 148.41 N, the stability angle was 11.03°, and the stable drag force was 2 275.83 N. The scallops were expected to smoothly enter the net bag when the height of the net opening was 350 mm. As such, there was a reduced introduction of benthic fish into the net bag by the water flow, which fully met the requirements for the limiting bycatch capacity of benthic fish. A comparison experiment of sea fishing demonstrated that there was no significant difference between the modified fishing nets and the fishing nets for production (P>0.5), but a significant decrease was found in the amount of shellfish and benthic fish (P<0.5), when the towing speed was 1.5, 1.8 and 2.0 m/s. The deviation rates of the average catch of single netting were 6.30%, 0.59%, and 5.55%, respectively, the deviation rates of the average shellfish yield of single netting were 90.63%, 84.78%, and 85.29%, respectively, and the deviation rates of the average concurrent catch of single netting benthic fish were 78.57%, 81.25%, and 84.85%, respectively. There was the lowest deviation rate of the average catch per net and the average shellfish per net, as well as the significantly reduced average concurrent catch of benthic fish when the towing speed was 1.8 m/s. Consequently, the optimal combination of structure and process parameters was achieved for the environmentally friendly fishing dredge and trawling. The finding can also provide promising theoretical support for the sustainable production of scallop fishing.