Analysis Of Fish Community Structure And Keystone Species In The Offshore Waters Of Southern Zhejiang

Located in the middle and south of the East China Sea,the coastal area of southern Zhejiang belongs to the warm temperate zone.The coastal area is always under the dual influence of the low-salt Zhejiang coastal current and the high-temperature and high-salt Taiwan Warm Current.The nutrient salts and biological bait are abundant and the fishery resources are rich.In recent years,with the increase fishing intensity and habitat degradation,China’s offshore waters fishery resources appear serious recession phenomenon,in great changes of fish community structure,the important economic fishes,such as small yellow croaker showed a trend of obvious recession,fishing objects tend to be miniaturization,although the national system to a certain extent,ease the implementation of moratorium,this kind of situation,but the situation is still not optimistic,So research on fish is still necessary.Based on the survey data of offshore fishery resources in southern Zhejiang province from May 2020 to January 2021,the relationship between fish species and dominant species composition,diversity and community structure and environmental factors was analyzed by using the methods of relative importance index,species diversity index and canonical correspondence analysis.The results showed that a total of 133 species were caught in the coastal waters of southern Zhejiang province,belonging to 67 families and 18 orders.Perciformes were the most abundant（67species）,accounting for 50.4%of the total,followed by Alewives（14 species）and flatfishes（8 species）,accounting for 10.5%and 6.0%of the total,respectively.The dominant species in all four seasons were Benthosema Pterotum and Trichiurus lepturus.There was a significant seasonal difference in total net catch and total net catch,which showed a significant decreasing trend from spring to winter,with the highest value in spring and the lowest value in winter.In summer,Shannon-Wiener diversity index（H’）was positively correlated with salinity,Pielou species evenness index（J’）was positively correlated with salinity,and Margalef species richness index（D）was negatively correlated with water depth.Canonical correspondence analysis（CCA）showed that the main environmental factors affecting fish communities were water temperature,water depth and salinity.This study analyzed the spatial and temporal characteristics of fish community structure and the influence of environmental factors on the dynamic distribution of community structure in the coastal waters of southern Zhejiang province,aiming to provide scientific basis for the protection of fishery resources in this area.Based on the bottom-trawl fishery survey data in summer in the offshore waters of southern Zhejiang from 2016 to 2018,the characteristics of fish community structure in summer were studied and grouped.The typical and divergent species of fish community structure were screened based on the similarity within groups and the difference between groups.The key species in different groups were analyzed through the typical and divergent species,and the influence of the key species on the stability of community structure was explored by ABC curve.The results showed that when the similarity level was between 45%and 50%,the MDS stress coefficients of fish communities were all less than 0.2,and the clustering grouping and fitting degree of each year were good,and fish communities could be divided into northern and southern populations.Typical and divergent species of the northern and southern communities are Trichiurus lepturus.Hairtail was selected as the key species of communities in the south of Zhejiang.ABC curve showed that the southern community was severely disturbed in the summer of 2016-2018,while the northern community was severely disturbed in 2016 and 2018,and remained stable in 2017.After the removal of key species,the statistical value W of each year decreased,suggesting that the removal of key species of hairtail would aggravate the community instability.Considering the characteristics of community structure,it is inferred that hairtail is the key species of summer fish community structure in the coastal waters of southern Zhejiang,which has a great influence on the stability of community structure.Based on the analysis of feeding habits of the main fish collected from the bottom trawl in the south of Zhejiang province and the data of feeding habits in the early stage,the food web topology of the fish community in the south of Zhejiang province was constructed based on the interspecific nutrition relationship,and the key species of the fish community in this area were determined by using the topological network indexes and KPP calculation.The results showed that the gastric contents of 34 fish species from the coastal waters of southern Zhejiang were analyzed,and 276 pairs of nutritive linkage numbers were formed.Trichiurus lepturus had the highest D,D_in,CC,BC and IC values in topological food web,indicating that it had the largest number of feeding related species and had the strongest ability to control information exchange,which played a key role in maintaining the stability of community structure.The D_out,TI¹ and TI⁹ values of Benthosema Pterotum were the highest,indicating that the number of prey species was the largest,indicating that Benthosema Pterotum had the strongest diffusion ability of species information.The K and K_tof hairtail were the highest,which had the greatest influence on energy flow and information transfer in the community,and the descending control effect was the largest.The K_b value was the largest,which indicated that the up-regulation effect on the community was the largest.The stability of fish community structure and interspecific information transmission of hairtail were determined by KPP calculation.In this study,11 topological key parameters were calculated and KPP operation was used to screen out the key species of community structure.It was found that hairtail(D,D_in,CC,BC,IC,K,and K_t)and Benthosema pterotum(D_out,TI¹,TI⁹,and K_b)played key roles in the stability of community structure,and hairtail was the key predator.The Benthosema pterotum is the key prey.