Stock Assessment Of Chub Mackerel In The Northwest Pacific Ocean

Chub Mackerel（Scomber japonicus）is widely distributed in the Indian Ocean and the Pacific Ocean.It is an important offshore and pelagic fishing target species in the North Pacific.In order to maintain and manage Chub mackerel resources reasonably and effectively,it is necessary to accurately understand its life history process,population dynamics and fishery management.In this study,we describe the length-at-age,weight-at-length,relative condition factor relationships,spatiotemporal heterogeneity and compare estimated growth parameter values to those reported from other regions based on survey data collected from 2016-2020 of the Northwest Pacific Ocean;This study uses the catches,effort,population life history parameters,and fishery selectivity data from the NPFC database,stock assessment meeting reports and other reference,and uses 5 models to conduct stock assessment,and analyze various uncertain factors that affect the results of stock assessment.In this study,Chub mackerel in the Northwest Pacific Ocean was studied by using traditional methods and Management Strategy Evaluation（MSE）.And the optimal Harvest Control Rules,（HCR）and Management procedure（MP）were found respectively.The results showed that:（1）The von Bertalanffy growth equation is selected as the best growth model for Chub mackerel of high Sea near Japan by Akaike Information Criterion and Bayesian Information criterion.However,due to the lack of samples of large individuals and small individuals,the fitting of growth parameters is not very ideal.The growth parameters of Chub mackerel in the Zhejiang coastal area are estimated by the ELEFANⅠmethod in Fi SATⅡ,comparing the results of previous studies,it is found that the Chub mackerel in the Zhejiang coastal area has a smeller L_∞.Also,growth heterogeneities of Chub mackerel were studied based on survey data collected from 2016-2020 of the Northwest Pacific Ocean,including the relationship between fork length and age,fork length and body weight,relative fatness,and its spatial and temporal heterogeneity.In this study,the fork length range was 40-294mm,body weight range was 0.8-311.8g and the fork length-weight relationship was W=(1.41×10^-6)×L^3.37.Seven linear mixed-effects models（LMEM）were were used to analyse the heterogeneity of fork length-body weight relationships of Chub mackerel.AIC and root mean squared error（RMSE）values showed that a random effect with both year,season（spring,summer and autumn）and with a performed the best fit for the fork length-weight relationship for Chub mackerel.Condition factors relative to reference years relative that the resources is not as before of Chub mackerel in recent years.In addition,relative condition factors varied among years,seasons and regions.（2）Used 5 models Namely,the Catch MSY（CMSY）model,a Bayesian state-space implementation of the Schaefer production model（BSM）,a stochastic surplus production model in continuous time（SPICT）,the Bayesian state space model JABBA（Just Another Bayesian Biomass Assessment）and JABBA-Select model that adds selectivity and life history parameters based on JABBA to conduct stock assessment,the results indicate that Chub mackerel stock might be slightly be overfished as indicated by CMSY(B₂₀₂₀/B_MSY=0.98,F₂₀₂0/F_MSY=1.12),BSM(B₂₀₂₀/B_MSY=0.97,F₂₀₂₀/F_MSY=1.21)and the base case run for the JABBA-Select(SB₂₀₂₀/SB_MSY=0.99,H₂₀₂₀/H_MSY=0.99)models.The results of the models SPi CT(B₂₀₂₀/B_MSY=2.30,F₂₀₂₀/FMSY=0.31)and JABBA(B₂₀₂₀/B_MSY=1.40,F₂₀₂₀/F_MSY=0.62)showed that the state of this stock may be healthy.Changes in the catch time series did not affect CMSY results but did affect BSM.The present study shows that prior information for BSM and SPi CT models is very important in order to present reliable results on the stock status.The results of JABBA-Select showed that different selectivity coefficients can affect the stock status of a species,as observed in the present study.According to various factors such as goodness of fit and posterior distribution,JABBA is the optimal model in this study,and according to its resulting resources are in a healthy state.Taken together,the base cases of the five stock assessment models used in the present study indicated that the current state of the Chub mackerel stock in the Northwest Pacific might be approaching optimal harvests.However,as a precautionary measure,preferences would be directed towards maintaining or slightly reducing the fishing effort for sustainable harvest of this fish stock,while laying more emphasis on accurately estimating prior input parameters for use in assessment models.（3）In the control rules for catch,the future resource dynamics of the stock are modelled according to the relative best-fit model JABBA in the previous chapter and the alternative representative JABBA-Select,with four HCRs,using 0.8-1.4 times the2020 catch,MSY,B×F_MSYand SB×H_MSY（for JABBA and JABBA-Select respectively）and the historical maximum catch as the total allowable catch,respectively.The results showed the best HCR for both HCR＿3(B×F_MSYor SB×H_MSY).MSE by DLMtool software,according to the requirements,12 kinds of MPs with good effects were selected.After further screening for the three most effective strategies based on their advantages:DAAC and SBT2.The TAC obtained from SBT2 was the most appropriate for conservation and maximisation of stock use,and was also close to the traditional estimates and recent yields.Robustness analysis of the 12 MPs that were more effective in conservation management showed that the catchability factor had the most significant effect on conservation management effectiveness.A sensitivity analysis of the two most effective MPs showed that DAAC was less affected by fluctuations in life history characteristics and stock abundance indices and more affected by stock depletion rates and average annual catches;SBT2was less affected by fluctuations in supplemental abundance indices but more affected by average annual catches.At this stage,HCR＿3 is the optimum HCR and SBT2 is the optimum MP to enable both conservation and maximum use of the resource.This study targeted Chub mackerel fishery in the Northwest Pacific,and completed life history characteristics,stock states and evaluate the performance of alternative management procedures.This study provides a scientific basis for the decision-making of fishery conservation and management.