| In order to restore the resources of Sepiella japonica,Zhejiang began to try to increase the breeding and release of S.japonica in 2007 and carried out large-scale proliferation and release.After a long period of large-scale proliferation and release,by 2018,the amount of S.japonica released in Zhejiang Province exceeded 600 million fertilized eggs,and the restoration of this resource is of great interest to scholars and the community.After a long period of proliferation and release,scholars began to pay attention to the impact of release on S.japonica.At present,a large number of scholars have carried out relevant research on S.japonica.However,after a long period of proliferation and release,there is no systematic study on the resource status of S.japonica.Therefore,it is of great significance to study and analyze the resource distribution,morphological differences and reproductive biology of S.japonica in Zhejiang coastal waters.In order to supplement the research data on the resource status of S.japonica in Zhejiang coastal waters,this study intends to use the fixed-point survey of fishery resources,the monitoring survey data of conventional fishery resources and historical data to systematically study the fishery utilization status and resource status,the relationship between resource density and environmental factors,the morphological characteristics of spring and autumn,and the reproductive habits of spring and autumn in Zhejiang coastal waters,so as to lay a foundation for the subsequent research on the resource supplement mechanism and fishery resource management of S.japonica.In order to grasp the dynamic and biological characteristics of S.japonica resources in Zhejiang coastal waters in recent years,and provide basic data for its resource recovery and subsequent proliferation and release.It provides scientific basis for the rational utilization and management of fishery resources of S.japonica in the future.(1)According to the survey data of trawl fishery in Zhejiang near the seabed in April,August,November 2020 and January 2021,the spatial and temporal distribution characteristics of the biomass resource density of S.japonica in this sea area were analyzed.The results showed that the distribution of S.japonica resources showed obvious seasonal characteristics.It was distributed in the coastal waters in April and August,and it swam out to the sea in November and January.The highest resource density was 1182.03 kg·km-2 in April,and the highest density was 2.049×104 ind.·km-2 in August.The station occurrence rates in January,April,August and November were 4.3%,31.3%,12.1%and 13.0%,respectively.The average resource densities of S.japonica were 23.16 kg·km-2,30.30 kg·km-2,22.69 kg·km-2 and 29.02 kg·km-2,respectively.The average mantissa densities are 1.7×102 ind..km-2,4.6×102 ind.·km-2,1.36×103 ind.·km-2 and 4.0×102 ind.·km-2,respectively.The model fitting results showed that the water depth,bottom water temperature and dissolved oxygen had significant effects on the distribution of S.japonica resources.The emergence of S.japonica showed a positive effect in the water depth of 20～60 m,the bottom temperature of 15～30℃ and the dissolved oxygen of 7～11 mg/L.(2)The independent sample T test,principal component analysis and discriminant analysis were used to study the morphological differences between the spring and autumn populations of S.japonica in Zhejiang near-seabed trawl fishery survey samples in spring(March-April)and autumn(October-November)of 2022.The results showed that there were significant differences in mantle length/total length,mantle width/total length,head length/total length,tentacle length/total length and tentacle length/total length between spring and autumn populations.Two principal components reflecting morphological characteristics were constructed by principal component analysis,and the cumulative contribution rate was 75.31%.The contribution rate of principal component 1 was 10.13%,and the contribution rate of principal component 2 was 65.18%.The comprehensive discriminant accuracy of 11 morphological indexes in discriminant analysis was 81.3%,and the discriminant rate of cross validation was 80.7%.Studies have shown that there are some differences between the spring and autumn populations of S.japonica,and the differences are mainly reflected in the brachiopods.In the same range,the brachiopods of the autumn population are larger than those of the spring population.(3)The reproductive biological characteristics of S.japonica in spring and autumn were compared and analyzed based on the survey data of near-sea bottom trawl fishery in Zhejiang Province in spring(March-April)and autumn(October-November).The results show.The average mantle length and body weight of the spring population were 105.80 mm and 159.91 g,respectively.The average mantle length and body weight of the female were 105.79 mm and 162.24 g,respectively.In autumn,the average mantle length was 70.01 mm,the average body weight was 56.20 g,the average mantle length of female was 74.90 mm,and the average body weight was 66.77 g.The gonadal development of the samples was statistically analyzed.The female population in the spring population accounted for 55%,and the female population in the autumn population accounted for 59%.Among them,the Ⅴ stage of S.japonica in spring accounted for the largest proportion,accounting for 38%of the total number of spring samples.In autumn,the gonadal development maturity of S.japonica was mainly in stage Ⅳ and stage Ⅴ,accounting for 13%and 27%of the total number of autumn samples,respectively,indicating that spring was the main breeding population and autumn was the secondary breeding population.There were more individuals with stage Ⅴ gonadal maturity in spring than in autumn.There were a few individuals with stage Ⅵ gonadal maturity in spring,and some individuals with stage Ⅱ gonadal maturity in autumn.According to the mass ratio method,the average absolute fecundity in spring was 755.The average absolute fecundity in autumn was 353,and the highest absolute fecundity was about 6 times that of the lowest absolute fecundity.The fecundity of female reproductive individuals in spring was much higher than that of the autumn population,which was 2.14 times that of the autumn population and one third of the absolute fecundity in the 1980s.At present,S.japonica is still in the decline stage compared with the historical level.In summary,this paper clarifies that the distribution of S.maindroni resources in Zhejiang coastal waters has obvious seasonal characteristics in recent years.,and it is verified that there is a trend of swimming to the sea in autumn and winter.The amount of resources in spring is the largest,and the amount of resources in autumn is second to that in spring.Through comparative analysis,it was found that there were some differences in the morphology of spring and autumn populations,and there were great differences in fecundity.The fecundity of autumn population was smaller than that of spring population,and the morphological index and fecundity of autumn population were close to those of artificial breeding population.It was concluded that the autumn population in natural sea areas was more from release.The spring group was the main breeding group,and the autumn group was the breeding group with lower fertility than the spring group.Based on the results of this study,relevant resource conservation suggestions for the protection of the spawning population of S.japonica and the improvement of the germplasm of released seedlings were proposed to provide reference for its resource restoration. |
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