Study On Stock Assessment Of Winter-spawning Cohort Of Todarodes Pacificus Based On The Environmental Factors

Japanese flying squid,Todarodes pacificus,is an important marine economic cephalopod in the world,and it is also one of the important fishing objects of pelagic squid fishery in China.It is an annual species with short life cycle.It can be divided into winter spawning cohort,autumn spawning cohort and summer spawning cohort.Among them,winter spawning cohort is one of the most important fishing objects.Japanese flying squid is an annual species with short life cycle,which is characterized by death after spawning.The marine environment has a great impact on its resource recruitment.However,the impact of abiotic factors on resource changes is often ignored in the traditional resource assessment and management,especially in the early life history stage,which will increase the uncertainty and risk of resource assessment results.Therefore,based on the scientific hypothesis that environmental factors have an impact on the intrinsic natural growth rate（r）and the maximum environmental capacity（K）,this paper intends to carry out the fishery resources assessment research and management strategy evaluation on the winter spawning cohort of Japanese flying squid based on environmental factors,which will provide a scientific basis for the sustainable utilization and management of this species.Based on the fishery production data of the winter spawning cohort of Japanese flying squid from Japan fisheries agency and the environmental data obtained from NOAA ocean watch database of the United States,this paper uses the catch per unit effort（CPUE）,the sea surface temperature（SST）of spawning ground and feeding ground,the environmental factors affecting CPUE is analyzed and the SST distribution during the continuous high value years of CPUE and the continuous low value years of CPUE is also discussed,which will benefit to find out the environmental mechanism affecting the resource abundance of the winter spawning cohort of Japanese flying squid.At the same time,based on Schaefer model and Bayesian statistical method,four residual yield models based on environmental factors and non-environmental factors are constructed respectively to evaluate the winter spawning cohort of Japanese flying squid,and the following main research results are obtained:（1）Distribution of SST in the spawning and feeding grounds affecting the resource abundance of winter spawning cohort of Japanese flying squid.The results showed that the regional distribution of significant correlation between SST in the habitat area and CPUE was relatively concentrated.The correlation between SST and CPUE in January was significantly higher than that in February and March,and the correlation between SST and CPUE in February was higher than that in January and March,but the significant correlation points in January were relatively concentrated（P value was0.0028）,which is mainly distributed in the sea areas of 25°N-40°N and 125°E-145°E,the optimum SST range is 18-26℃.The distribution of sea areas significantly related to SST and CPUE in the feeding ground in June and July is similar,but the significant correlation areas in July are concentrated（P value is 0.0048）,which are mainly distributed in the sea areas of 27°N-39.5°N and 125°E-144.5°E,and the optimal range of SST is 18-28℃.The results show that the SST of 25°N-40°N and 125°E-145°E in the spawning grounds in January and 27°N-39.5°N and125°E-144.5°E in the feeding grounds in July are the environmental factors affecting the resource abundance of winter spawning cohort of Japanese flying squid.（2）Influence mechanism of the environmental factors in the spawning ground on resource abundance of winter spawning cohort of Japanese flying squid.The research shows that the significantly related sea areas of the spawning ground in January are mainly distributed in the front of the Tsushima warm current.In February,the significantly related sea areas are mainly distributed in the main axis and branch front of the Kuroshio,and in March,the significantly related sea areas are mainly concentrated in the front of the main axis for Kuroshio.The paired sample difference t-test of SST in different years of continuous high and low values of CPUE showed that there were significant differences in SST values in the same month in different years（P<0.01）,but there was no significant difference in mean value of SST in different years（P>0.05）.It is concluded that the strength of Kuroshio affects its change of spawning ground of winter spawning cohort of Japanese flying squid,which in turn affects the resource abundance of winter spawning cohort of Japanese flying squid.（3）Influence mechanism of environment in the feeding ground on resource abundance of winter spawning cohort of Japanese flying squid.The research shows that the sea areas significantly related to SST and CPUE in the feeding ground in July are mainly distributed at the intersection of the front and first branch of the Kuroshio in the waters of north of the North Pacific and south of Hokkaido.It is found that the annual mean value of SST in continuous high-value years of CPUE is lower than that in continuous low-value years of CPUE.The sea area temperature value in the feeding ground of Japanese flying squid varies greatly in different years,but the suitability SST changes little.The SST in the feeding ground is negatively correlated with CPUE.The increase of SST in the feeding ground does not necessarily lead to the increase of resources of Japanese flying squid.（4）Parameter setting of resource evaluation model for winter spawning cohort of Japanese flying squid.According to the basic information on catch and biology of squid and Bayesian statistical principle,the approximate distribution range of r,K and q is determined.The research shows that the three parameters obey uniform distribution,and the ranges are[0.5,2.1,[10×10⁴,150×10⁴ and[1×10^-5,5×10^-5,respectively.The results of Markov chain Monte Carlo（MCMC）method show that the prior distribution and posterior distribution of the three model parameters are quite different,indicating that the fishery data can provide enough available information under the assumption of prior distribution,which can be used to establish the surplus production model.（5）Stock assessment of winter spawning cohort of Japanese flying squid based on environmental factors.Based on the parameters of Schaefer model,combining the intrinsic natural growth rate（r）and maximum environmental capacity（K）affecting the population change of winter spawning cohort of Japanese flying squid with the suitable SST range of spawning ground,four surplus production models are constructed respectively,that is,the surplus production model（SP）without considering environmental factors,the surplus production model（Sr-EDSP）which modifies the intrinsic natural growth rate based on the suitable SST area of the spawning ground,the surplus production model（S_k-EDSP）which modifies the maximum environmental capacity based on the suitable SST area of the spawning ground,and the surplus production model（S_r-S_k-EDSP）which modifies the intrinsic natural growth rate and the maximum environmental capacity based on the suitable SST area of the spawning ground.The research shows that the deviation information criterion（DIC）value of the stock assessment model（Sr-EDSP,S_k-EDSP,S_r-S_k-EDSP）with environmental factor SST is less than that of the traditional surplus production model（SP）.The DIC value of S_r-S_k-EDSP is the smallest（132.6）,indicating that the model can better simulate the change of fishery resources.The research shows that at present,the fishing mortality F_t of Japanese flying squid is generally lower than that of F_msy,and the maximum sustainable yield MSY is generally lower than the biomass B_MSY at MSY level.The research shows that the resources of winter spawning cohort of Japanese flying squid are not overfished at present,and their resources are greatly affected by the environment.It is suggested that environmental factors must be considered in the stock assessment and fishery management of winter spawning cohort of Japanese flying squid.