Performance Of Antarctic Krill Midwater Trawl

Antarctic krill(Euphausia superba) is the largest single biological resources on the earth. Many countries have paid more attentions to the Antarctic krill fishery because of its large biomass and potential. China began to be involved in the Antarctic krill fishery in 2009 when two large scale factory trawlers from 2 Chinese fishing enterprisers went down to the Southern Ocean for exploratory fishing. At the beginning, large mesh size trawl net was used that redesigned based on Chilean Jack Mackerel trawl or introduced with factory trawls. The fishing efficiency was not satisfactory due to mismatching between the trawl and otter boards, and the total catch was much lower than those of Norway, Korea and Japan fishing fleets. In 2010, Chinese fishing vessel introduced a small mesh size specialized krill trawl from Korea, and fishing operation indicated that catch was also not satisfactory because of the limitation of the opening of mouth. This was changed in 2012 when Liaoning Province Dalian Ocean Fishery Group CO introduced the specialized krill trawler “Fukuei Maru”(Chinese name “Fu Rong Hai”) from Japan and used the Japanese krill trawl on board the fishing vessel, and the catch increased significantly since 2013. Therefore, the study on the performance of krill trawl is essential.The performance of the trawl contains many indexes, such as stability, sweeping performance, selectivity and so on. This paper will choose rapidity, openting performance, economy and hydrodynamic performance as the main indexes of fishing performance. Model test has been used very necessary to understand the performance of Antarctic krill trawl and which factors could influence on. The results could help us to understand difference of performance of different trawl, and how the ratio of buoyancy and sinking force, bridle and leg influenced on performance of Antarctic krill trawl, and also as an important reference for the measured data of capture and the computer simulation.3 Antarctic krill trawls were chosen as prototype net, 2 of the 3 were introduced from Japan and Korea by Chinese fishing enterprisers, another one has been used by Chile. Firstly, analysis of characteristics of Antarctic krill trawls based on the references to well understand advantages and disadvantages of different Antarctic krill trawls be used by different coutries; secondly, according to the model test rules and 3 drawings of Antarctic krill trawl we chose to calculate the main parameters of model net, and then made model nets; thirdly, pseudo-scale ratio was be used in the model test, and performance of 3 Antarctic krill trawls could be obtained, and then analyzed how the ratio of buoyancy and sinking force, bridle and leg influenced on performance of Antarctic krill trawl, based on the prototype trawl from Japan; then based on the prototype trawl from Japan, the relationship between drag, height of mouth and buoyancy and sinking force changed in the same proportion could be obtained and by this ratio of drag and the height of a net mouth to fit the performance index of prototype trawls to verify the accuracy of results obtained by pseudo-scale ratio; finally, according to the test results to optimize Antarctic krill trawl from Japan and made the drawing of optimazed trawl. The main conclusions of this paper were as follows:(1) In amount of Antarctic krill trawl, large mesh trawl due to a less twine area had better rapidity, economy and hydrodynamic performance compared with other trawls. Because of the huge nets and specifications, it was easy to obtain a large horizatal and virtical opening, but due to the influence of length of warps and performance of otterboard, opening performance did not match to its specification. Due to the small mesh size of small mesh trawl to make smaller space to escape. Due to the performance of otterboard and specification of trawl were matched better, also could make a better horizatal and virtical opening. But small mesh trawls had huge twine area and also double-net body structure. Therefore, the rapidity of small mesh size trawl was worse than large mesh trawl, resulting in economy and hydrodynamic performance were worse than large mesh Antarctic krill trawl.(2) Model test laws and limites of application were introduced in this paper. The the main parameters of model net were calculated based on the Tauti’s law, and then produced model net.(3) Analysis the performance of different Antarctic krill trawls by model test. Pseudo-velocity ratio was 3, so pseudo-scale ratio was 9. According to the pseudo-velocity ratio, the mean towing speeds of the model net ranged from 0.345m/s to 0.685m/s with the interval 0.085m/s. The ratio of the distance between two lower wing ends to lead line length(L/S) ranged from 0.45-0.55 with the interval of 0.05, The 3 repeated experiments were carried out under each condition, and the average values were converted to the parameters of the prototype net. The results showed that: the towing speed had significant influence on drag, opening performance( height, vertical opening ratto, sweeping section area and net opening ratio), economy(energy consumption coefficient), hydrodynamic performance and mean attack angle(P<0.05); L/S has no significant influence on the above parameters(P>0.05); the comprehensive performance of Net C( used by Chile) was better than the other two(Net A: introduced from Japan; Net B: introduced from Korea), rapidity was also better than the other two. Net A had a best opening performance, and then Net C was better than that of Net B. Maintain to stable in the water during the tank experiments for model nets, body of nets were streamlined and smooth when nets were towed in the tank, back-end of Net C is relatively obvious; according to Takeo KOYAMA and Hu Fuxiang’s trawl drag estimation formulas, Antarctic krill trawl drag estimation formulas could be expressed as: when n≠2,1.6690.086 A d R CLV a=(Net A); 0.0651.678 B d R CLV a=(Net B); 1.5350.030 C d R CLV a=(Net C); When n=2,20.076 A d R CLV a= and 0.328 21(0.209)2A eR ρR SV-=(Net A);20.057 B d R CLV a= and 0.318 21(0.191)2B eR ρR SV-=(Net A); 0.0252 C d R CLV a= and 0.460 21(0.202)2c eR ρR SV-=(Net A).(4) Based on the Net A, analyzed how the ratio of buoyancy and sinking force, bridle and leg influenced on performance of Antarctic krill trawl by orthogonal test. Set ratio of buoyancy and sinking force, bridle and leg are: A(0.9, 1.2, 1.5), B(2.00 m, 2.25 m, 2.50 m) and C(2.00 m, 2.25 m, 2.50 m); according to the orthogonal experiment table, tests were conducted in 5 mean towing speeds and 4 L/S levels, The 2 repeated experiments were carried out under each condition, and the average values were converted to the parameters of the prototype net. The results were as follows: 3 factors had significant influence on drag, height and energy consumption coefficient(PA>0.05, PB>0.05, PC>0.05). In view of energy consumption coefficient could reflect the relationship between drag and opening performance comprehensively, according to the variance analysis results of energy consumption coefficient, when L/S=0.4, leg> bridle> ratio of buoyancy and sinking force; when L/S=0.5, bridle>leg> ratio of buoyancy and sinking force; when L/S=0.55, bridle>ratio of buoyancy and sinking force>leg; when L/S=0.55, bridle>ratio of buoyancy and sinking force>leg. In different L/S, the optimal combination correspondingly are: A2B2C1, buoyancy was 816 g, the ratio of buoyancyand sinking force was 1.2, bridle was 2.25 m, leg was 2.00 m; A2B2C3: the buoyancy was 816 g, the the ratio of buoyancy and sinking force was 1.2, bridle was 2.25 m, leg was 2.50 m; A2B2C2: buoyancy was 816 g, ratio of buoyancy and sinking force was 1.2, bridle was 2.25 m, leg was 2.25 m; A2B2C2: buoyancy was 816 g, the ratio of buoyancy and sinking force was 1.2, bridle was 2.25 m, leg was 2.25 m. Performance adjustment of Antarctic krill trawls should appropriate to increase ratio of buoyancy and sinking force, discrease length of bridle, increasing length of leg.(5) Based on the Net A, analyzed how the inner net influenced on performance of Antarctic krill trawl by model test. Tests were conducted in 5 mean towing speeds and 4 L/S levels, The 3 repeated experiments were carried out under each condition. Take down 6th-8th panel of inner net successively(each panel accounted for 6.2%, 5.5% and 5.4% of total twine area of the trawl), according to the ratio of twine area of inner net to fitting performance of prototype net with only single body, then compare with the results of prototype net obtained by model net with material of PE. The results were as follows: the twine area of inner net had significant influence on all the performance indexes(P<0.01). Without inner net, the largest descrease of drag was 41.33%; the height and vertical opening ratio maxi increased 19.11%, sweaping section area and net opening ratio increases as the same as height of mouth, energy consumption coefficient and hydrodynamic performance improved significantly. Compared with Net C, except the drag, other performance were better than Net C. Antarctic krill trawl should focus on the single body structure of the trawl in the future.(6) The curve of relationship between drag, height of mouth and towing speed could be obtained when buoyancy and sinking force changed in same ratio at same time with pseudo-velocity ratio. According actual towing speeds and formulas of curve to calculate performance indexes, and then fitting the indexes of prototype net based on the changed ratio of buoyancy and sinking force. The buoyancy and sinking force were 25%, 30%and 20%. Also make model test with model net which produced by the material as same as prototype trawl. Results were obtained in different model net to verify the accuracy of results by pseudo-velocity ratio. Tests were conducted in 5 mean towing speeds and 4 L/S levels, The 3 repeated experiments were carried out under each condition. The results were as follows: buoyancy and sinking force change in same ratio at same time had no significant influence on drag(P>0.05), but had significant influence on height of mouth(P<0.05). Compare with results of performance ofprototype net obtained by pseudo-velocity ratio and Two groups of results of performance of the prototype net in this chapter, there were errors. The results of performance corresponding to the maximum error was less than 15%, the mean error was below 7%.(7) Based on the Net A, and according to the relevant test results to optimize Net A. The main scale Net A remains unchanged, but without inner net. Mesh size were increased from wing to 5th section of body. After adjustment, total twine area was descreased from 964.51 m2 to 486.90 m2, twine area of outer net was descreased 153.29 m2. The drag was further descreased; the height of mouth was increased. Then make drawing of opitimzed trawl.A systematically analysis of the characteristics of Antarctic krill trawl had been done in this study. It had an important significance in improving performance, reducing energy consumption, improving the capture efficiency and optimizatal designing of Antarctic krill trawl. I hope to provide the necessary parameters for the development of Antarctic krill trawl.