Stock Assessment For Pacific Saury(Cololabis Saira)in The Northwest Pacific Ocean

The Pacific saury is a small pelagic migratory specie in the northwest Pacific Ocean?NPO?.It is widely distributed in the international waters of the NPO expanding from subarctic to subtropical region and has become one of the important fisheries in the NPO because of its abundant resources.Owing to the increasingly commercial,cultural,bioeconomic and ecological values,saury has been widely concerned by many countries and regions.The major countries and island constituencies harvesting Pacific saury includ Japan,China,Chinese Taipei,Russia,South Korea and Vanuatu.The total catch has been increasing since 1990s from 15.8×10⁴ t in 1998,16.0×10⁴ t in 1999 to60.7×10⁴ t in 2008 and 62.1×10⁴ t in 2014.China began fishing of Pacific saury in the high seas in 2003,and since then,this fishery has become one of the most important fisheries for China.With the global development of Pacific saury fisheries,the number of fishing vessels has been increasing,coupled with the impact of the marine environment and climate change,Pacific saury resources fluctuated,and fishing production has undergone annual changes.A precise stock assessment of the Pacific saury will contribute to the sustainable development of its commercial fishery,resource management,and oceanic ecosystem studies.The lack of long-term sequence age structure data and imperfect biological research make it difficult to evaluate the stock status of the Pacific saury using standard stock assessment models.At present,the study of saury resource assessment is in its infancy,the stock assessment mainly depends on the scientific investigation,but it is affected due to its large consumption of human and material resources.The North Pacific Fisheries Commission?NPFC?established the saury Working Group in 2015,and took saury as a priority management species.Stock assessment study of Pacific saury was carried out in 2017 and 2019 based on the Bayesian State-Space model?BSSPM?,and a preliminary understanding of the status of saury was obtained.Since the development of Pacific saury fishery,relevant studies on fishing gear and methods,distribution of fishing grounds,and basic biology of the Pacific saury have been conducted.However,the related research on stock assessment of Pacific saury has not been found to be publicly reported.Based on a full understanding of Pacific saury fishery,we plan to start from the study of basic data of Pacific saury fishery and basic life history information of Pacific saury,and conduct standardization and accurate estimation of input data of the stock assessment model,a change from previous methods using empirical parameters.Then,the Pacific saury stock status in the NPO was evaluated and studied based on the accurate model input data,life history information and different assessment models.At the same time,the impact of environmental factors on the stock assessment was considered to comprehensively evaluate the stock status of the Pacific saury.Actively conducting research on the stock assessment of the Pacific saury fishery can not only provide theoretical support for the effective scientific management and sustainable development of saury fisheries in the NPO,but also meet the NPFC requirements,reflecting China's positive attitude towards compliance as a member state.In this paper,we compare the performance of Generalized Linear Models?GLMs?and Generalized Additive Models?GAMs?using CPUE data collected from Chinese saury fisheries in the NPO from 2003 to 2017?excluding data from Chinese Taipei?and evaluate the influence of spatial,temporal,environmental variables,and vessel length on CPUE;Estimating the intrinsic rate of increase?r?for Pacific saury in the NPO using demographic method and substituting accurate life history data into the model can reduce the uncertainty of the model evaluation results;The Catch-MSY model was used in this study to estimate MSY on the basis of catch data and life history information.During the process,17 scenarios,according to different prior distributions of the paremeter r and paremeter K,were set for sensitivity analysis.Moreover,the influence of different catch time series and different process errors were taken into account;Based on the data of catches from 2003 to 2017,collected by the NPFC and the resource abundance data submitted by China,the stock assessment and risk analysis of alternative management strategies for the Pacific saury were carried out using a Bayesian Schaefer model.During the process,two scenarios,a standard scenario and a sensitivity analysis scenario,were considered;We have attempted to evaluate the stock status of the Pacific saury in the NPO using the environmentally dependent surplus production model for the first time.The impact of environmental factors on the Pacific saury resources was added to make the evaluation process more consistent with the actual fishery situation and reduce the uncertainty,and it can also provide a reference for stock assessment studies of other short life speceies.The main conclusions are as follows.?1?Optimal GLM/GAM models were selected using the Bayesian information criterion?BIC?.Explained Deviance and five-fold bootstrap cross-validation results were used to compare the performance of the two model types.Fitted GLMs accounted for 52.47%of the total model-explained Deviance and R² is 0.4266,while GAMs accounted for 61.9%and R² is 0.4850.Standardized CPUE from GLM and GAM model have similar change trend with the nominal CPUE,and are lower or closer to the nominal CPUE except 2003.There are some differences between GLM standardized CPUE and GAM standardized CPUE,which may be related to assumptions about the relationship between CPUE and explanatory variables.The GAM model results show that high CPUE appears in the waters with a water temperature of 16°C-19°C.In terms of space factor,CPUE gradually increases between 145°E-151°E,a maximum appears at 151°E,and then has a decreasing trend.Predictive performance metrics and five-fold cross-validation results showed that the best GAM performed better than the best GLM.Therefore,we recommend a GAM as the preferred model for standardizing CPUE of Pacific saury in the Northwest Pacific Ocean.The goal of this study was to identify the best method for standardizing Pacific saury CPUE data and improve the quality of future stock assessment for Pacific saury.?2?Even though Bayesian methods can provide statistically rigorous assessments of the biological status of fisheries resources,uninformative data can produce highly imprecise parameter estimates.This can be counteracted with the use of informative Bayesian prior distributions?priors?for model parameters.We develop priors for the intrinsic rate of increase?r?in the Schaefer surplus production model using demographic methods to the Pacific saury in the NPO.The results show that the estimated intrinsic rate of increase under the basecase scenario in this study is 0.5580?0.3674-0.7486?,which is slightly lower than the estimates in several other scenarios.The estimated r for scenarios 1 to 3 are:0.5587?0.3602-0.7604?,0.5611?0.3650-0.7572?,and 0.5591?0.3661-0.7520?.When using the demographic method for estimation,the input model parameters should not be determined as a fixed value but need to consider its uncertainty and set it to a range,which can improve the stability and accuracy of the model results.This study shows that the Euler–Lotka method is feasible to estimate the prior distribution of the r of the Pacific saury in the Northwest Pacific,but there is still some uncertainty.?3?The posterior estimation of K concentrated on a narrow range,whereas r had a broad posterior estimation covering the prior range,and there was a strong negative correlation relationship between ln?r?and ln?K?;the maximum sustainable yield?MSY?increases with an increase in the lower limit of r;the upper limit of K only slightly influences on the distribution of MSY when using the informative priors of parameter r;When the process error is considered,it has a greater impact on the estimation of MSY;The time series of catch data had a limited impact on the assessment results,whereas the results of the model were sensitive to the annual catch in the first and last years;The estimated MSYs of the Pacific saury were 47.37×10⁴ t?41.57×10⁴ t to 53.17×10⁴ t?in scenario S1A and 47.53×10⁴ t?41.79×10⁴ t to 53.27×10⁴ t?in scenario S1B.Given the uncertainty of the Catch-MSY model and achieve sustainable development of the Pacific saury fishery in the NPO,maintaining a management target between 50×10⁴ tCatch-MSY model is a useful choice for estimating the MSY of data-limited species such as the Pacific saury.?4?The posterior distribution of the model parameters r,K,and q under the standard scenario and the sensitivity analysis scenario are significantly different from the prior distribution,indicating that the results of the posterior distribution of the model parameters are less affected by their prior distribution,while the fishery data has a great influence on the posterior distribution probability of the parameters.Compared with the results of the two scenarios,those of the posterior distribution of the three parameters are similar.Under the standard scenario,the MSY is 75.26×10⁴ t.To maintain the MSY,the biomass and fishing mortality should be 240.14×10⁴ t and 0.32,respectively.While,under the sensitivity analysis scenario,the MSY is 70.03×10⁴ t.To maintain the MSY,the biomass and fishing mortality should be 232.53×10⁴ t and 0.31,respectively.Under the two scenarios,the Pacific saury resource was maintained at 200×10⁴ t?470×10⁴ t during 2003-2017,which was higher than B_lim,and the amount of resources has been higher than B_tar since 2006.Fishing mortality rates have been lower than F_tar since 2003,which means that the status of the Pacific saury stock is currently good and it is not under overfishing.If the harvest rate is set below 0.35,then the stock will be protected and the probability of resource collapse will be very low.The harvest rates from 2003 to2017 are all close to 0.35,so there is reason to be optimistic for the potential of this resource.We have concluded that in the future,a harvest rate of 0.35 for Pacific saury should be set,as the best and most sustainable management strategy.?5?The EDSP model with P_f had the smallest Deviance Information Criterion?DIC?value,which implied that this model fitted the data better than other models.In thiswas 226.45×10⁴ t.The fishing mortality rates of the Pacific saury in 2017 were much lower than observed at the MSY level(F_MSY)and fishing mortality observed at target level(F_0.1),and stock biomass of Pacific saury was higher than B_MSY,which suggested that the stock status of Pacific saury stock is at a good level and it is currently immune to overfishing or being overfished.The MSY values of other three models are 69.74×P_S-P_f-EDSP model.From the Kobe plot,we could see that the management reference points in the EDSP model with P_f for the Pacific saury were more conservative than other models.It was also found that fishery management reference points largely depended on optimal spawning and feeding habitat areas.According to this investigation,we considered that MSY should be set between 55 and 65×10⁴ t.According to the results of this study,the effects of environmental factors on the feeding grounds should be considered for the evaluation and management of the stock status of Pacific saury in the NPO.