Stock Assessment And Management Strategy Evaluation Of Swordfish (Xiphias Gladius) In The Western And Central North Pacific Ocean

As an eminently migratory fish species,swordfish（Xiphias gladius）is an important division of the Western and Central North Pacific Ocean fish stock that constitutes high economic value.It is terrifically important to assess the stock in order to achieve the sustainable utilization of the swordfish and ecosystem conservation in the Pacific Ocean.Its conservation and management has been the focus of global fisheries management organizations.Through in-depth study of its resource status and reasonable management strategies,scientific support is provided for its related research.Based on the data provided by the International Scientific Committee for Tuna-like Species in the North Pacific Ocean（ISC）website,this paper evaluates the status of swordfish resources considering various uncertainties,and calculates the relative mortality rate suitable for the stability of swordfish populations,so as to provide management strategies for the sustainable development of swordfish.The main research contents are as follows:（1）The AIM（An Index Method）model is a linear model for calculating the relative fishing mortality（F）,which is one of the important parameters in fisheries resource assessment models.Based on the catch per unit fishing effort（CPUE）in the North Pacific Ocean from 1951-2018,Japan and Taiwan,China,the AIM model was used to study the stable and suitable F for the status of swordfish populations in the west-central North Pacific Ocean.The results showed that the F（Japan）value was 1.15,which was similar to the SS3 model results in 2019,and the catch prediction showed that F=1.15 was more suitable for the stability of the swordfish stock.The F（Taiwan,China）value was 4.28,which deviated significantly by comparing the SS3 results in2019,and the catch prediction results indicated that the sustainable development of the swordfish stock was seriously damaged.the AIM model yielded the most suitable F value of 1.15 for the swordfish stock in the west-central North Pacific Ocean.（2）Based on catch data from 1951 to 2018 and Catch Per Unit Effort（CPUE）from 1975 to 2016,a Bayesian state-space surplus production model was developed to evaluate the dynamics of the stock of swordfish in this area.Sensitivity analysis on CPUE sequences of different time series affecting the assessment results was then conducted.The estimation of F/F_MSY and B/B_MSYis sensitive to different CPUE time series.Carrying capacity K and intrinsic growth rate r is 161904t and 0.45 respectively.The maximum sustainable yield is 22612t,with a probability of 100%to bring the stock to a healthy condition.When Total Allowable Catch（TAC）increases to the range of8000t and 20000t,the stock size will stay above B_MSY for the next 10 years.When TAC moves to 22000t,the stock biomass will decline to B_MSY in 2028.The results show that the stock will not be overfished whereas there could be a possibility of overfishing in the future.（3）Based on the stock assessment result of the International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean（ISC）with catch data from 1952 to 2018,operating models were created in which stock dynamics of swordfish in the Western and Central North Pacific Ocean were defined by setting a prior parameter.Nine alternative management procedures,Delay–Difference Stock Assessment（DD）,Delay–Difference Stock Assessment 4010（DD4010）,Geromont and Butterworth Constant Catch（CC1）,SBT simple MP（SBT1）,Geroment and Butterworth index slope Harvest Control Rule（GB＿slope）,Index Confidence Interval（ICI）,Index Confidence Interval 2（ICI2）,Surplus production based catch-limit modifier（SPmod）,and Mean Catch Depletion（MCD）were chosen to be tested in our simulation circulation with 48 simulations of 50 years yield projection.Through the priors of the time series and trade-offs on management procedures,the impact on stock status with the implementation of management procedures was intuitively represented by generating Kobe plots,and ICI was concluded as the best management strategy.Following this,total allowable catch（TAC）calculation was carried out using two time series abundance index data from Chinese Taipei and Japan.TAC results were calculated under the implementation of 4 output-control management procedures（MCD,ICI,ICI2,and SPmod）.TAC results of ICI were established as 10 404.48（6678.51～18 743.22）with a 50%confidence interval.Subsequently,sensitivity analysis on each parameter within ICI was conducted,wherein the results demonstrated that it is sensitive to both catch and abundance index data.