Stock Assessment Of Pennahia Macrocephalus And Trachurus Japonicus In Beibu Gulf Based On Length Frequency Method

As an important part of the South China Sea,the Beibu Gulf contains rich fishery resources.Among them,Pennahia macrocephalus gourd and Trachurus japonicus,as the dominant species in this sea area,have important economic status.In the production of fishery resources in the Beibu Gulf,the annual output of the genus Baigu is about30,000 tons,and the output of the genus Baigu accounts for about 30%of the production of the genus Baigu.It was not until the beginning of the 21st century that the P.macrocephalus was taken seriously by most scholars.For example,in 2007,the study found that the winter P.macrocephalus accounted for more than 60%of the total catch.In 2016,the national T.japonicus production was 39,712 tons,of which the T.japonicus production in the northern South China Sea was about 25,000 tons,accounting for more than 60%of the country’s total output that year.P.macrocephalus and T.japonicus rank the top three in the composition of dominant fish species in the Beibu Gulf,and have important economic value.Scientific assessment of the resource status of P.macrocephalus and T.japonicus is an important basis for ensuring the sustainable development of this fishery resource.To this end,this article is based on the survey data of the fishery resources of the Beibu Gulf P.macrocephalus in a total of 18 voyages from 2006 to 2014 and a total of 30voyages in the Beibu Gulf T.japonicus from 2006 to 2018.Based on the length-based Bayesian biomass estimation method（LBB）,estimate the changes in the inter-annual population parameters of P.macrocephalus and T.japonicus,and grasp the development of P.macrocephalus and T.japonicus stockes,It also discusses the changes of population parameters under different group distances,and the differences in fishery resource assessment results of different group distances,to provide a basis for the scientific management of the two resources.The main research conclusions are as follows:（1）The resource status of P.macrocephalus in the Beibu Gulf.The length-based Bayesian biomass estimation method（LBB）is used to estimate the asymptotic body length（L_∞）,relative natural mortality（M/k）,and relative fishing mortality（F/k）,the optimal body length-at-first-capture(L_copt),exploitation rate（E）and the relative biomass development index（B/B₀）.The results showed that the average progressive body length of P.macrocephalus in the Beibu Gulf from 2006 to 2014 was 265 mm,the optimal average body length was 157 mm,and the relative mortality rates M/k,F/k and Z/k were 1.39,3.58 and 4.96,respectively;the opening body length showed an upward trend,while the progressive body length showed a downward trend.At present,the development rate of P.macrocephalus is relatively high,the development rate E is0.58.The relative biomass development index（B/B₀）is less than 0.5,indicating that the current resources of P.macrocephalus in the Beibu Gulf are over-exploited.（2）Estimating the population parameters of T.japonicus in the Beibu Gulf based on LBB.Through the estimation of the population parameter values of different group distances,it is found that the average values of L_∞,M/k,F/k,Z/k and L_copt of Beibu Gulf T.japonicus are 248 mm,1.38,6.92,8.28 and 156 mm,respectively;Secondly,the range of the initial sexual maturity length is 159（157～162）mm.In order to more rationally develop and utilize T.japonicus stock,it is recommended that the minimum length of T.japonicus is 159 mm;In order to further grasp the biomass development status of T.japonicus,it is estimated that the relative biomass B/B₀ of T.japonicus in the Beibu Gulf is lower than 0.5,and E is greater than 0.5,indicating that the biomass of T.japonicus in the Beibu Gulf is low and in an over-exploitation state;The results of population parameters estimated by the reconstruction length frequency of different group distances are different,and the fluctuations are more obvious with the increase of group distance.（3）In order to better manage and utilize the resources of P.macrocephalus and T.japonicus in the Beibu Gulf,this article analyzes the problems faced by the use of fishery resources in the Beibu Gulf,and proposes measures that can be taken to achieve sustainable development,as well as management systems such as fishing bans and value-added releases.It is recommended that the fishery government departments in the future step up to improve the fishery management system and combine the market economy to transform the fishery economic development mode and adjust the industrial structure.