Analysis Of Shape Characteristics Of Antarctic Krill Mid-water Trawl Based On Sea Trials And Model Tests

Antarctic krill belongs to crustacean zooplankton,which plays an important role in the Southern ocean ecosystem.It is the largest single species of known resources on the earth at present.At the same time,it is rich in a variety of nutrients needed by human beings.With its great value of development and utilization,krill continues to attract the attention of all countries in the world.According to the long-term production practice in many countries,the single-ship middle trawling is the main way to catch Antarctic krill.China started commercial exploration of Antarctic krill resources for the first time in the 2009/10 fishing season,and subsequently introduced a variety of small-scale and easy-to-operate small-mesh krill trawls into production,compared to the large-mesh trawls previously used,the fishing efficiency was slightly improved.Trawling shape is an intuitive performance of trawling performance.Through the analysis of trawl shape,we can effectively grasp the resistance change of nets,adjust selectivity,improve fishing efficiency,and ensure the safety of operation.From March to October 2018,the author went to Antarctica with China's Antarctic krill fishing boats to implement the development and utilization project of Antarctic marine biological resources of the Ministry of Agriculture and Rural Affairs.Through the first-line investigation,it is believed that there are some problems in trawling production of Antarctic krill fishery in China:(1)There are many factors that affect the shape of trawls,including fishing operations,marine environment and catch,etc.In what way does the shape of each part of the trawl change during production?(2)Restricted by relevant regulations,Antarctic krill fishing vessels are only equipped with a wireless network position meter that measures the height of the mouth opening.How to estimate the overall trawl shape during production?(3)There is no reference standard during the trawling production of krill,which operation form is the best?The author collected the data of point depth,fishing operation,marine environment and other data using the krill trawl on the "Fu Ronghai" ship through the actual measurement at sea.Through research and analysis,the changing trend of the shape of the trawl under various factors and the importance of each influencing factor were obtained.According to the measured data,a method for calculating the middle trawl shape based on the height of the trawl mouth opening is proposed to solve the shape monitoring problem.Subsequently,a model net of the prototype trawl net used by the"Longteng" trawler was used in the model test to analyze the changes in the resistance and shape of the net under abnormal conditions,and put forward the reference standard of trawl form.The main conclusions are as follows:(1)According to the sea measurement analysis of the Antarctic krill trawl used by the "Fu Ronghai" ship,the depth range of the headline is 145.82?302.27 m,and the depth difference between the headline and the section 3-4 net body connection is-0.2 to 8.02 m,and the depth difference between the headline and the upper midpoint of the cod-end mouth is 6.49 to 30.16 m.Warp length,towing speed,wind speed and water layer velocity at 150 m depth have significant effects on the depth of the headline(P<0.05);The headline depth is positively correlated with the warp length and water layer velocity at 150 m depth;There is a negative correlation between the headline depth and towing speed;The headline depth decreases first and then increases with the increase of wind speed.Towing speed,water layer velocity at 200 m depth and the wave height 2 m have significant effects on the depth difference between the headline and the section 3-4 net body connection(P<0.05);The depth difference between the headline and the section 3-4 net body connection has a tendency to increase first and then decrease with the towing speed and water layer velocity at 200 m;When the towing speed is 2.6 kn and the water layer velocity at 200 m is 0.3 kn.The catch is positively correlated with the depth difference between the headline and upper midpoint of the cod-end mouth.The importance of the factors that affect the status of trawling operations are in order:warp length,towing speed,wind speed,wave height,and water flow velocity.(2)The whole trawl shape is regarded as an elliptical cone,and the headline and footline shapes are set as parabola(y=ax2).According to the height data of net mouth obtained by wireless network position meter,combined with the mesh shrink length and belly line of each part of the net,the calculation method can be used to find the shape of each part of the net,the angle between the net wing and the towing direction of the fishing boat,and the headline and horizontal angle etc.The results of the calculation are verified by the measured data in the first chapter,and it is concluded that this method is suitable for the shape estimation when the catch is empty or small.(3)The ratio of the horizontal expansion of the trawl sleeve end to the length of the footline(L/S)and the towing speed significantly affect the resistance of the trawl and the expansion of the mouth.With the increase of L/S and towing speed,the drag of trawl decreases.When the towing speed is 2.5-3.5 kn,the drag increase rate of each expansion group decreases with the increase of horizontal expansion and towing speed.When the towing speed is 5 kn and the L/S is 0.35-0.55,the drag of the trawl is basically equal.The results show that the drag of the trawl is more significantly affected by the towing speed.At L/S 0.17-0.27,there is drag transition change.The height of the mouth opening decreases with increasing towing speed and L/S.When the towing speed is 2.5-3.5 kn,the rate of decrease of the trawl mouth height of each expansion group increases with the increase of the drag rate.The variation of the depth difference between the headline and the section 3-4 net body connection with the towing speed and L/S is similar to the height of the trawl mouth opening,but the rate of decrease of the depth difference is prone to fluctuation.The rate of decrease in the average depth difference of each horizontal expansion group increased with flow rate,then increased and then stabilized.When L/S is 0.27-0.35,When L/S is 0.27-0.35,there is a transitional change between the height and depth of the trawl.(4)Under the condition of L/S=0.45,the energy consumption coefficient and net area of the trawl are basically kept optimal within the towing speed range of the test setting,and the change rate of the depth difference at the connection point between the upper and the third and fourth sections of the network body is more gentle than that of other expansion groups.When the towing speed is 2.5 kn,the trawl shape is the optimal shape.When the towing speed is 3.0 kn,it can be used as the reference standard of the trawl operation shape,and the shape can be adjusted based on this.