Characteristics Of Set Gillnet Fishery And Resource Assessment Techniques For Portunus Trituberculatus

The set gillnet has the characteristics of low cost,high selectivity,easy to use,clean and environmentally friendly.It is one of the important fishing gears for fishing Portunus trituberculatus in the East China Sea.It is also the main operation method in the pilot waters of Zhejiang Province’s quota fishing management system.In addition,set gillnets are also used for fishery resource monitoring and estimation of fishery resources.Based on the catch of set gillnets,scientific and reasonable assessment of the resources of P.trituberculatus can not only reduce the cost of assessment,but also be a useful supplement to the investigation of conventional bottom trawl resources.Based on the fishing method of set gillnets,the biological characteristics of P.trituberculatus and the hydrological characteristics of the sea area,the effective fishing range of the set gillnets is simulated based on the ocean current speedv₀,the moving speedv₁ of P.trituberculatus and the length of the set gillnets(According to the angleθformed by the interaction between the movement direction of P.trituberculatus and the current flow direction and the sea area enclosed by the set gillnet as the effective fishing range of the set gillnet,an algorithm model was established.Four sampling survey methods are simulaited based on computer（including fixed station sampling,simple random sampling,stratified fixed station sampling and stratified random sampling）.Among them,fixed station sampling and simple random sampling are each set 16 and 25 sampling stations,while the stratified fixed station sampling and stratified random sampling are each set 16stations.In addition,considering the differences in resource density in the four seasons,the ocean current flow rates in different seasons,and the number of gillnets（50,80,and 150 set gillnets were selected for comparison and simulation analysis）,a total of 72sets of simulation experiments were performed,and each set was repeated 1000 times.Using relative estimate error（REE）and relative bias（RB）to evaluate the accuracy and precision of the simulation results,the conclusions are as follows:In terms of the sampling method:fixed station sampling with the increase of the number of set gillnets,the REE and RB values gradually decrease.In the random sampling scheme,the REE and RB values change in a smaller range.When the number of gillnets and the number of sampling stations are the same,the effect of stratified sampling is better than that of non-stratified sampling,and stratified random sampling has the best effect in this resource evaluation simulation experiment,lower resource density and higher ocean current speed made the REE value relatively small,and results in the better results.In the case of a certain survey site,stratified sampling is superior to the fixed station and simple random sampling,and the result of resource estimation is more accurate.In terms of the number of sampling stations:for the fixed station sampling and stratified fixed station sampling methods,the results of 25 sampling sites are better than those of the number of 16 sampling sites,showing a trend that the more the number of sites,the more accurate the fishery resource survey results.For the random sampling and stratified random sampling methods,the error values of the scenario of 25 sampling stations are smaller than those of the number of 16 sampling stations,but the error range is not greatly different.It shows that the survey results of random sampling methods tend to be stable,and the influence of the number of sampling stations is not obvious.In terms of the number of set gillnets:Besides the scenarios of the spring fixed station sampling,whose REE and RB values decreased with the increase in the number of set gillnets,other sampling scenario all appeared with the trends of the RB value changes from positive to negative with the number increases,and there is a certain deviation in the estimation results.Therefore,when assessing fishery resources based on set gillnets,the larger the number of nets may not give better results.It must be comprehensively considered based on the structure and size of the actual sampled species,seasonal temporal and spatial distribution characteristics,as well as the environmental characteristics of the sampling sea area.In terms of the seasonal characteristics:P.trituberculatus has the characteristics of seasonal temporal and spatial distribution,and seasonal factors have a certain impact on the accuracy of the survey of P.trituberculatus resources.In seasons with high resource density,a stratified sampling design is suitable.The resource density of P.trituberculatus in autumn is high and unevenly distributed,and the ocean current is the slowest among the four seasons.Therefore,the error in the estimation results of resources in autumn is also the largest.The resource density of P.trituberculatus in winter is low,and the error in the estimation results of winter resources is smaller than those in other seasons.The estimation results are closest to the"true value"among all sampling plans.The use of set gillnets for fishery resource surveys can reduce the damage of fishing gears to the ocean bottom,and plays a positive role in maintaining the marine ecosystem and promoting the sustainable development of fishery resources.However,as a passive fishing gear,set gillnets have strong selectivity,and have certain requirements for the selection of sampling methods and the number of sampling stations.It may overestimate or underestimate the abundance of resources.Therefore,it is necessary to comprehensively consider the impact of other possible factors when application.