Study On Hydrodynamic Characteristic And Sinking Performance Of Tuna Purse Seine

The tuna purse seine is one of the most advanced modes of fishing operation for surrounding nets.A circumferential casting motion attempt to surround the fish school,and then hauling the purse line to close the net bottom based on the aggregation characteristic of pelagic fish.The basic techniques of surrounding net were initially applied to small species such as mackerel,sardines and anchovy,and later extended to surface tuna fisheries.The target species of tuna purse seine include skipjack?Katsuwonus pelamis?and yellowfin tuna?Thunnus albacares?,etc.As the previous fishing technology was based on relatively rich fishery resources,low energy costs and fishing on the basis of less environmental impact,but with the continuous development and utilization of fishery resources,fishing gear is now more concerned about the protection of the ecological environment,as well as energy cost savings,which led to changes in fishing gear design concept.Purse seine development into the gear performance optimization stage,which is mainly driven by fisheries management,so that people are not only concerned about the efficient purse seine,more to pay attention to ecological protection and energy-saving optimization of the net.The main contents of purse seine performance include the sinking performance of the gear,hydrodynamic characteristic and the gear shape.In order to improve the fishing performance of the purse seine and realize the optimization of the performance of the gear,this study investigate the effects of the attributes of the net and its fishing parameters optimization on gear performance based on hydrodynamic characteristic and sinking performance of tuna purse seine.The optimization parameters include net structure,net attributes,fishing parameters,set pattern,etc.But there are still some problems in the process of net modifications,1)What are the main operating indicators and influencing factors of purse seine? 2)What are the differences in hydrodynamic characteristics among netting made of different materials? 3)What are the hydrodynamic characteristics of the netting under lower attack angle? 4)How to evaluate the impact of optimization parameters on the performance of the net? 5)How to evaluate the fishing performance of the optimized net? The above mentioned problems are some of the main reasons for the uncertainty of optimization performance program of Chinese tuna purse seine.Therefore,this study analyzed the factors that affect the performance of the purse seine based on sea trials results,and studied the hydrodynamic characteristics of netting made of emerging materials to understand the effect of netting materials,knot types and attack angles on hydrodynamic characteristics of netting.We quantified the effect of the leadline weight,the setting speed on the sinking performance of purse seine,and determined the difference in sinking performance between different netting panels with different materials.Finally,the optimization program of tuna purse was proposed,and the optimized performance of purse seine was evaluated by flume tank test.The results showed that:?1?In term of gear performance,the sinking speed and setting speed of the purse seine are the key factors affecting successful rate of capture.The successful rate of capture decreased as fish swimming speed increased.In addition to surface migratory fish,there was no catch possible when the fish inhabit at deeper water and the fish swimming speed was greater than 1.5 m/s.The effect of setting net position on successful rate of capture cannot be ignore.When the gear scale is larger,it must cooperate with the operating conditions and the dynamic performance of the fish to determine the fishing operation,thereby increasing the successful rate of capture.?2?In this research,we measured sinking performance of 88 purse seine sets on free-swimming skipjack?Katsuwonus pelamis?schools in the western and central Pacific Ocean from October 2006 to May 2013.A purse seine set is considered to have a better sinking performance if its purse line reaches a greater depth in a short time period – It has higher sinking speed and sinking depth.A generalized additive model?GAM?was used to analyze sinking depth and sinking speed under different sea conditions?current speed and direction at different layers?,gear design?length-height ratio,towing line length and purse line length?,and operational methods?shooting duration and setting speed?.The results in this study showed that current speed at 120 m depth was the most important environmental factor affecting sinking depth of the purse seine.The sinking performance of a purse seine with a lower length-height ratio was better than a seine with a higher ratio.Sinking depth was strongly associated with length-height ratio,shooting duration and purse line length.Sinking speed was related to length-height ratio,shooting duration,current speed at 60 m,purse line length and towing line length.The sinking performance models obtained from this study can be used to predict sinking behavior in relation to operational and environmental conditions,which is essential for the success of tuna purse seine operation on free-swimming tuna schools.?3?In this study,the hydrodynamic coefficient of six knotless PE netting panels with different solidity ratios were assessed in a flume tank under various attack angles of netting from 0°?parallel to flow?to 90°?perpendicular to flow?and current speeds from 40 cm/s to 130 cm/s.It was found that the drag coefficient was related to Reynolds number,solidity ratio and attack angle of netting.The solidity ratio was positively related with drag coefficient for netting panel perpendicular to flow,whereas when setting the netting panel parallel to the flow the opposite result was obtained.For netting panels placed at an angle to the flow,the lift coefficient reached the maximum at an attack angle of 50° and then decreased as the attack angle further increased.The solidity ratio had a dual influence on drag coefficient of inclined netting panels.Nonlinear regression on the data gathered produced the following formulae for knotless PE netting panels with different solidity ratios: C_D90=5.568?^0.292Re^-0.125?500< Re <5300?C_D0=0.127?^-0.534Re^-0.040?500< Re <5300?C_D?=C_D90sin?+(C_D0-1.775?sin?)cos²?C_L?=0.251?^-0.481C_D90sin?cos??4?This study aims to investigate hydrodynamic characteristics of nylon netting of different knot types and twine characteristics and under different attack angles and flow velocities.It was found that the hydrodynamic coefficient of the netting panels was related to Reynolds number,solidity ratio,attack angle,knot type and twine construction.The solidity ratio was found to positively correlate with drag coefficient when the netting was normal to the flow?CD90?,but not the drag coefficient when the netting was parallel to the flow?CD0?.For netting panel inclined to the flow,the inclined drag coefficient increased as solidity ratio decreased at attack angle was less than 50°,but increased as solidity increased at attack angles from 50° to 90°;the lift coefficient increased with the attack angle,reached the culminating point at an attack angle of 50°,and subsequently decreased.We found that the drag generated by knot accounted for 15-25% of total drag,and the knotted netting with higher solidity ratio exhibited a greater CD0,but it was not the case for the knotless netting.Compared to knotless polyethylene?PE?netting,the drag coefficients of knotless PA netting were dominant at higher Reynolds number?Re>2200?.Nonlinear regression on the data gathered produced the following formulae for knotless PA netting panels with different solidity ratios:C_D90=1.691?^0.114Re^-0.011?400< Re <3600??C_D90=19.371R_h^-0.643+1.139C_D0=0.172?^-0.407Re^-0.031?500< Re <5500?C_D?=C_D90sin?+(C_D0-0.889?sin?)cos²?C_L?=0.38?^-0.162C_D90sin?cos??5?The smaller attack angle motion of netting is the most common situation in purse seine and trawl.Using the self-designed experimental setup,the smaller attack angle test of netting was carried out in a flume tank.It was found that the frame was very stable at attack angles from 0° to 20°,the ratio of frame drag to total drag was less than 20%,indicating the frame satisfied the basic requirements for experiment.The drag coefficient was strongly associated with Reynolds number,solidity ratio and attack angel.The drag coefficient increased as attack angle increased,but decreased as solidity ratio and Reynolds number increased.We found that the drag generated by knot accounted for 22.1% of total drag.Based on experimental data,the drag coefficient formula for smaller attack angles motion of netting was proposed:C_D=0.172?^-0.407Re^-0.031(1+sin?^0.9051.822Which can be applied to the theoretical calculation and numerical simulation of net at smaller attack angel from 0° to 20°.We also corrected the model law for model net of purse seine using above mentioned formula.?6?Three different materials of netting were employed to experiment in a flume tank,in order to understand the differences in the sinking characteristics of the netting with different materials.It was found that the sinking depth increased as sinking time increased and until a maximum depth was reached.The increased of the weight of the net had a significant improvement in the sinking characteristics of the netting,in which the polyethylene netting is the largest increased,followed by nylon and polyester netting.There was a negative correlation between the solidity ratio and the average sinking speed of netting.There were significant differences in the sinking characteristics of different materials of netting.The results showed that Polyester netting had the best sinking characteristics,followed by nylon and polyethylene netting.The results show that the sinking characteristics of the knotted netting was better than that of knotless netting.?7?Gear performance evaluation for tuna purse seine using both full-scale sea trials and model testing was designed in the study.A model tuna purse seine of 1:20 scale was constructed and tested in Lake Qiandao,China in 2013;and the field observations of its full-scale net in operation was conducted on board a typical Chinese tuna vessels during 2006;the physical model testing data were then assessed to determine the model's ability to predict main performance parameters of full-scale net at sea.A generalized linear model?GLM?was employed to standardize and compare sinking performance between the model net and its prototype.It was found that increase in leadline weight or setting speed of net was conductive to improved sinking performance.The leadline weight was the most important controllable factor affecting sinking performance of the purse seine,the sinking speed in wing end position was higher in comparison to that in other position of model net.Though there is likely no fully ideal model net which can be used to accurately predict full-scale at-sea performance,our sea trials and physical model testing showed consistent results,indicating our model testing method can be used as an important complement method to successfully evaluate the purse seine performance at sea.?8?According the results of foundation test,using the formula of smaller attack angle to modify model law for purse seine,and then a model net was constructed and tested in a flume tank.In this study,two modified model nets of tuna purse seine were employed to execute a series of experiments in a specialized purse seine test tank,in order to obtain the variations of moving and drifting distance,spatial geometry,purse line tension and sinking performance of purse seine under different shooting patterns and current speeds.Gear modifications projects: Modified model net A: We modified the model net from main body sections of 25 millimeter mesh to 30 millimeter mesh;Modified model net B: We modified the model net from 1/3 of main body sections?in lower part of net body?of 30 millimeter mesh to 45 millimeter mesh.The results showed that:?1?Compared to back set and front set,the cross set was dominant at spatial geometry and sinking behavior.?2?Front set was considered as the optimal strategy of setting the net judged from the purse line tension,as it provides the least tension.Purse line tension increased continuously until the conclusion of pursing and the tension exerting at bunt was more than that at wing end.?3?Larger mesh nets increase the net sinking speed and decrease pursing time.We modified the model net from 1/3 of main body sections of 30 millimeter mesh to 45 millimeter mesh,which decreased the purse seine of the net by 9.26%.?4?The least moving and drifting distances occurred in front set,larger mesh nets decreased the net drifting distance due to it provide lower net forces.Our gear modifications projects for tuna purse seine were feasible judged from energy-saving gear,gear configuration and sinking characteristic,which were considered as the main direction of gear performance optimization for Chinese purse seine.