| This study is based on the data of fishery resources bottom trawl survey in April(spring),July(summer),October(fall)and December(winter)in the Sanmen coastal waters,studied the nekton species composition,dominant species and important species and its seasonal variation,niche,diversity,community distribution and stability,etc.,In order to find out the condition of nekton resources in the Sanmen coastal waters,thus studying the condition of the community and its changing trends,so as to provide a scientific theoretical basis for the application of sustainable use and effective management of nekton resources in this sea area,it also provides basic information for the further study of the Sanmen coastal waters.A total of 103 species of nekton were identified in the four seasons,including 55species in spring,72 species in summer,60 species in autumn and 40 species in winter.According to the group analysis,the number of fish species was the most in the four seasons,followed by shrimp and crab,and the number of mantis shrimps and cephalopods in each season were less.From the taxonomic point of view,the species of decapoda and perciformes were dominant in the nekton composition,and the total species of the two orders accounted for more than 58%of the total species in every season.The mantissa and weight composition of the catch reached the peak in summer,decreased significantly from summer to winter,and recovered slightly in spring.There were 4,6,6 and 2 dominant species of nekton in the four seasons.The mantissa and weight of dominant and important species in each season accounted for more than 50%.Portunus trituberculatus is the common dominant species,Trypauchen vagina,Oratosquilla oratoria,and Charybdis japonica are common dominant species or important species.The replacement rate of dominant species and important species was higher in spring-summer,summer-autumn,autumn-winter,and winter-spring,all above 52.94%.As a result,there are seasonal fluctuations in the species,mantissa,weight distribution and composition of swimming animal communities in the Sanmen coastal waters,and dominant species and important species are frequently replaced,but the dominant species composition is relatively stable and single.The niche breadth of the main swimming animals in the Sanmen coastal waters is large,and the niche breadth between different species in the same season and in different seasons of the same species is large.Based on the results of group average cluster analysis of niche breath and the current situation of niche breath,the main nekton can be divided into three categories:narrow niche species(0<B_i<2.00),medium niche species(2.00?B_i?3.00)and wide niche species(B_i>3.00).The distribution range of wide niche species is relatively large,and the number of distributions in each station is large and uniform,and the number of middle niche species is small or the frequency of occurrence is relatively low or the distribution is uneven,while the the number of narrow niche species is small,and the layout is limited and uneven.The difference in niche overlap between different species in the same season is large,ranging from 0.10 to 0.91 in spring,0.02 to0.93 in summer,0.18 to 0.94 in autumn,and 0.21 to 0.90 in winter.This is mainly due to the difference in ecological value of the environment or resources of the species or the difference in the coincidence rate of the site.There is also a large difference in the niche overlap between the same species in different seasons,such as the species of the Portunus trituberculatus,Palaemon gravieri of the genus Trichosanthes,which has an overlap value of 0.24 in spring and 0.90 in winter,indicating that the niche overlap value is not only affected by the influence of the above factors,and it is also related to its own seasonal physiological and ecological behavior,as well as the time and place of investigation.The species diversity of the nekton community in the Sanmen coastal waters changed greatly in the seasons,and the characteristics of summer,autumn were greater than spring and winter.The richness(D)and shannon-wiener(H')indexes of the berthing stations in spring and winter were both smaller than those of the far offshore stations,and the differences between the stations in summer and autumn were not significant,while the spatial variations of the evenness index(J')were not significant in the four seasons.The G-F diversity index is the highest in summer,followed by winter and autumn,and the lowest in spring,which indicates that the distribution of the summer swimming community is relatively uniform in the inter-family and inter-genus classification elements,and while the single-family species proportion in spring is large,and its contribution to the F index is0,resulting in the small value of the G-F index.The calculated results of species diversity index and G-F diversity index are not completely consistent.Therefore,when measuring and investigating regional biodiversity,multiple indexes can be combined to measure community diversity from multiple taxonomic factors such as family,genus and species.Group average clustering analysis and non-metric multi-dimensional scaling analysis both showed that the spatial heterogeneity of nekton community in the four seasons in the Sanmen coastal waters was low,while the seasonal heterogeneity was strong.All stations in the same season can be grouped together at different levels of similarity,among them,the similarity of spring group was the lowest(34.46%)and the highest in winter(65.43%).There was significant difference between any two groups(P<0.01),and the dissimilarity between any two seasons was above 70.86%.The highest difference rate is the summer and winter group(95.44%).This indicates that the community structure of swimming animals in different seasons has a high similarity,and the community structure in different seasons has significant heterogeneity.Percentage analysis of similarity can also lead to a conclusion that typical species with similar community structure in each season group,such as Portunus trituberculatus,Larimichtys polyactis,Panpus argenteus,Exopalaemon carinicauda,etc.,which to some extent is also the divergence species that cause the difference of community structure between different seasons.The seasonal replacement index of the four seasons was higher,all above 93,and the migration index was not 0,indicating that there was seasonal fluctuation in the nekton community in the Sanmen coastal waters,and the species migration and migration were in a non-dynamic equilibrium state.The results of ABC curve show that the nekton community in the Sanmen coastal waters is in a moderate interference state,and the degree of interference is as follows:summer<spring<winter<autumn.In addition,the seasonal turnover rate of dominant species and important species is also higher,and species turnover is more frequent.Therefore,it shows that the stability of nekton community structure in the Sanmen coastal waters is low. |
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