Influences Of Environmental Factors On The Spawning Stock-recruitment Relationship Of Portunus Trituberculatus In The Northern Zhejiang Sea Area

The Portunus trituberctdatus is a kind of crab with high economic value.The meat is delicious and nutritious,which is welcomed by consumers.Since the 1990 s,due to greater market demand and blind fishing,P.trituberculatus has been over-exploited,its output has fluctuated significantly,and began to show characteristics such as small individuals,early sexual maturity,and single age structure.The utilization and alteration of P.trituberculatus resources have aroused wide concern,and governments and enterprises have taken timely protective measures to increase and release them.Although recruitment has rebounded in recent years,the biomass base of the P.trituberculatus is still fragile.In 2017,the Ministry of Agriculture and Rural Affairs implemented a quota fishing management system(TAC)for P.trituberculatus in Zhejiang Province,and establishing a pilot quota fishery in Northern Zhejiang fishing grounds to limit P.trituberculatus fishing and restore marine productivity.In addition,the natural environment in which P.trituberculatus live is complex and may be influenced by marine environmental factors from larval development to adult recruitment.To study the relationship between changes in P.trituberculatus resources and marine environmental factors,and to understand the variation regularity of P.trituberculatus,it is helpful to formulate timely fishing strategies and protection measures.To ensure the development of reasonable fishing strategy and protection measures,to achieve sustainable development of fishery resources.Therefore,it is an important and necessary work to evaluate and predict the resources of P.trituberculatus under the influence of environmental factors.Based on the traditional Ricker model,the stock and recruitment relationship(S-R)of P.trituberculatus was studied,and the effect of marine environmental factors on recruitment was discussed.Firstly,based on the correlation analysis of the environmental factors and the recruitment data of P.trituberculatus from 2001 to 2014,the more significant influencing factors were screened,and the S-R model of P.trituberculatus with environmental impact was established.The marine environmental factors mainly include red tide area(AORT),El Nino index(EINI),from sea level(SLH),sea surface salinity(SSS),typhoon landing frequency(TYP)and sea surface temperature(SST).Besides the traditional Ricker model,two other Ricker-type S-R models were built: Ricker model with ln-linear environmental impact(Ricker-type 2)and Ricker model with ln-quadratic polynomial environmental impact(Ricker-type 3).Secondly,the data from 2015 to 2017 are used for verification,and the AIC information criterion(Akaike information criterion)and significance test are used as the test criteria to select the optimal model.The optimal model is used to predict the different combinations of a variety of environmental factors,and the uncertainty of environmental factors and recruitment are analyzed.Considering the influence of various environmental factors and the occurrence of extreme cases,the recruitment amount affected by different environmental combinations at a given stock and recruitment relationship level is estimated.Finally,in order to ensure the accuracy of the prediction results,we determine the distribution of the original environmental factors,assuming five distributions(weibull distribution,gamma distribution,normal distribution,exponential distribution and lognormal distribution),select the best distribution of each environmental factor by comparing AIC,BIC and maximum likelihood,and use the distribution function to simulate the environmental data(repeated 10000 times).At the same time,because the stock of P.trituberculatus is also variable in the natural environment,it is assumed that the stock is normal distribution,and the stock of normal distribution is randomly generated by sd=0.882.The environmental data and stock data were substituted into the optimal model(Model 2)to simulate the changing combination of environmental factors and predict the recruitment.Results showed that AORT,SLH,SSS,and TYP had great influences on the recruitment of P.trituberculatus.When SSS reached 29 and 31,recruitment decreased from 20.7×103 to 8.3×103 million individuals.In this case,recruitment declined,whereas AORT and TYP increased.Analysis of the S-R model showed the Akaike information criterion(AIC)value of the traditional Ricker model was 14.619,which remarkably decreased after the addition of the environmental factors.The results showed that the Ricker model with environmental factors can better describe the relationship of the stock and recruitment relationship(S-R)of P.trituberculatus.Different numbers of environmental factors were added to the Ricker model,and the best result was obtained when four factors were added to the model together.Four factors were the best choice by added different quantity of environmental factors in Ricker-type model.Moreover,Ricker-type 2 model was better than Ricker-type 3 model.The results of the analysis showed that the probability of occurrence of recruitment between 10×103 million tails and 15×103 million tails is the highest,and the probability is about 39.56%.The estimation of the situation in which the four environmental factors are in the optimal and worst combination at the same time(extreme environment)showed that if the recruitment is less than 6.394×103 million at a given probability level,it is possible to encounter a very bad environment.Similarly,if the recruitment is more than 28.305×103 million tails,it may be in a better environmental combination.At a given average spawning stock level,the probability of arbitrary recruitment between extreme environments is predicted,and the probabilities of the worst and best environmental combinations are 2.02% and 1.92%,respectively.The purpose of this paper is to analyze the relationship between natural resource recruitment and marine environmental factors of P.trituberculatus in the northern waters of Zhejiang Province,and to establish a prediction model of recruitment amount of P.trituberculatus.The results have a certain guiding significance for the resource management and sustainable utilization of P.trituberculatus in the northern waters of Zhejiang Province.