Analysis Of The Biology And Stock Assessment For Blue Marlin (Makaira Nigricans) In The Western And Central Pacific

Blue marlin（Makaira nigricans）is one of the common by-catch species in tuna longline fishery.With the continuous increase of the global fishing amount of blue Marlin,this study provides a reference for understanding the current stock status of blue Marlin in the Central and Western Pacific and for sustainable management of the stock.Based on the observer data recorded by Chinese longline fishing fleet in the Central and western Pacific from October 2010 to March 2019,the catch characteristics of billfish by-catch,biology and spatial and temporal distribution characteristics of blue Marlin stock abundance in the central and western Pacific were studied.Based on the database data of Western and Central Pacific Fisheries Commission（WCPFC）,the CMSY model was used to analyze the stock status of Blue Marlin.The research content and main results of this paper are as follows:1.Catch characteristics and analysis of billfish in tuna longline fishery in the Central and western PacificIn this study,the catch and discard data of billfish in the observer data of longline fishing in the Central and Western Pacific from 2010 to 2019 were matched,collated and analyzed with fleet information.With latitude and longitude 5°×5°as spatial statistical unit,classification summary and calculation were carried out.The catch composition,catch rate,discard composition,discard degree and reason of billfish were analyzed.Catch analysis initially identified 6 species,which included Makaira nigricans,Tetrapturus Audax,Kajikia Audax,Istiompax Indica,Istiophorus Platypterus and Kajikia Albida.The concurrent fishing range of billfish is 40°N～40°S,150°E～120°W.At the same time,the catch rate of billfish was close to0.36 fish per thousand hooks,and the total discard rate was about 2.05%.The catch rate of swordfish was different among different target fleets.The comparison of the mandibular fork length of billfish discards showed that the average length of mandibular fork was 163.4 cm,the minimum was 33.5 cm,the median was 160.0 cm,the maximum was 253.0cm,the upper quartile was 182.0 cm,and the lower quartile was 150.0cm.A summary of the causes of billfish discarding showed that the common causes were small size and body damage.2.Preliminary biological characteristics and spatial and temporal distribution characteristics of West and Central Pacific Blue MarlinAccording to the analysis of blue marlin data captured in the observer data of Chinese longline fishing fleet in the Central and Western Pacific From 2010 to 2019,it is found that the majority of blue marlin with fishing range of 15°N～15°S.The sex ratio of blue marlin is 1:2.2,and the distribution range of male and female is basically the same.The relationship between the jaw-fork length and the total weight of The Sino-Western Pacific Blue Marlin is a power function,and the function expression is as follows:L=9.84×10-6 W2.9711,（n=63,R2=0.9549）;The results of covariance analysis（ANOVA）showed that the relationship between the mandibular fork length and body weight of Blue Marlin was significantly different（P<0.05）.In terms of feeding grade,the feeding grade of Blue Marlin is mainly 0 andⅠ,and the probability of feeding grade 0 andⅠaccounts for 20.4%and 40.4%,respectively.The F test and T test showed that there was no significant difference in feeding grade of blue Marlin（P>0.05）.The range of blue Marlin’s capture position is 40°S～40°S,150°E～120°W,and the higher Catch Per Unit Effort（CPUE）is mainly concentrated in 15°N～10°S,160°E～175°W.Summer is the most concentrated and the average CPUE is the largest,followed by winter,and the average CPUE of blue marlin is the lowest in spring.3.Influence factor analysis of blue Marlin fishing rate based on Tweedie-GAM modelThe effects of latitude,longitude,year,month,SST,SEA level anomaly,chlorophyll A concentration and sea state on CPUE（Catch per unit effort）of blue Marlin were analyzed by using Tweedie-GAM model.The final interpretation rate of the model was 25.8%.The cumulative deviation interpretation rate of the model from high to low is SLA,BF,month,latitude,SST,longitude and Chl-a.Four environmental factors,STT,SLA,Chla and BF,accounted for about 28.6%of the total explanatory rate,and the spatial-temporal factors accounted for about 71.3%.The analysis of spatio-temporal factors shows that longitude has no significant effect on CPUE of blue marlin,while latitude has the greatest effect on CPUE.In addition,blue marlin CPUE presents seasonal changes.The CPUE of spring blue marlin showed a downward trend,while the CPUE of summer blue Marlin showed an upward trend.The analysis of environmental factors shows that the influence of SST on THE CPUE of blue Marlin can be divided into two stages,and the extreme value appears at 15℃and 30℃.Sea level anomaly（SLA）had the greatest influence on CPUE of blue Marlin,and the overall trend was downward.The CPUE and chlorophyll a concentration of blue marlin were directly proportional.The influence of sea condition（BF）on the CPUE of blue Marlin can be divided into three stages,the CPUE is in a significant rising stage at 0-2,relatively stable at 2-7,and in the second rising stage at 8-9.4.Analysis of the stock status of West and Western Pacific Blue Marlin based on CMSY modelBased on data-limit method,CMSY model was used to analyze and judge the stock status of Blue Marlin.The model results showed that R=0.57,K=131000t,B=65600t,MSY=18600t,F_MSY=0.28.Management analysis based on the CMSY model shows that the B/B_MSYvalue in 2020 is 0.91,and the F/F_MSYvalue is 0.75,both less than 1.The biomass of Blue Marlin in the Central and Western Pacific has been lower than B_MSYlevel.Sensitivity analysis results showed that Bend/k and r had the greatest influence on B/B_MSYresults,so it should be carefully considered when setting Bend/k and R.Based on the evaluation results of the CMSY model,this paper recommends reducing current fishing mortality.In this study,the differences of by-catch rate,composition,and discard of billfish were investigated from the aspects of by-catch and discard.Taking blue Marlin as an example,the differences in the basic biological characteristics and spatial and temporal distribution of blue Marlin were analyzed,and the effects of various influencing factors on the CPUE change of blue Marlin were analyzed.Finally,the stock status of blue Marlin in the Central and western Pacific were evaluated.The results showed that the biomass of Blue Marlin in the Central and western Pacific was still lower than the B_MSYlevel in 2020.It is of positive significance to the development of Tuna longline fishery in China.The CMSY method used in this paper provides a reference for the application of data deficiency method in billfish,and also provides a reference for the study of stock distribution and conservation of blue Marlin in the Central and western Pacific.