Mechanisms Of Fish Community Construction And Midstream Fish Biodiversity In Ganjiang River

Ganjiang River is the longest river in Jiangxi Province,which has abundant fish resources and plays an important role in the fish diversity and protection and restoration of biological-integrity in Poyang Lake and even the Yangtze River Basin.In recent decades,due to the gradual increase of construction of hydro-junction in the main stream and tributaries of Ganjiang River,water pollution,sand mining and overfishing,etc.,the water ecosystem and fish diversity in Ganjiang River Basin have changed.With the implementation of the ten-year fishing ban,it is an important foundation for fish resources protection and scientific management to study the changes and mechanisms of fish community and diversity in Ganjiang River.In this study,the catches data from 2016 to 2018 in the Ganjiang River,Community phylogenetic method was used to explore the fish community structure and its construction mechanism in Ganjiang River.At the same time,from October 2020 to November 2021,the fish resources were investigated at Xiajiang-Xingan section in the middle reaches of Ganjiang River,the fish resources and the genetic diversity of Hypophthalmichthys molitrix and Hypophthalmichthys nobilis was analyzed.To provide scientific reference for the evaluation,management and rational development and utilization of germplasm resources.The main results are as follows:1.According to historical and current data,there are 180 fish species in Ganjiang River,Cypriniforme fish is the greatest(127),accounted for 70.56% of the total number of fish species.In recent years,exotic species have been found,such as Labeo rohita,Cirrhinus cirrhosus,Oreochromis mossambicus,Lepomis gibbosus,Gambusia affinis and Micropterus salmoides.2.Based on the Net Relatedness Index,the mechanism of fish community structure in Ganjiang River was analyzed.On the scale of sampling points,12 of the29 sampling points showed that environmental filtration led community assembly,while the other 17 sampling points showed that stochastic process led community assembly,and stochastic process(contribution rate 58.62%)led fish community assembly in Ganjiang River.On the scale of interval analysis,the middle reaches show that environmental filtration plays a leading role in community assembly.In the upstream and downstream,the stochastic process dominated the community assembly,and the stochastic process(66.67%)dominated the fish community assembly in Ganjiang River.The phylogenetic diversity of the lower reaches of Ganjiang River is higher,It is suggested that the fish diversity should be protected in the lower reaches.3.Two hydro-junction have been built in the Xiajiang-Xingan section of the middle reaches of Ganjiang River in recent ten years,This section of the river is also the spawning ground of four major fishes,and it is an important fishery habitat.60 species were collected in this section of the river,belonging to 44 genera,14 families,5 orders.Cyprinidae fish is the greatest(41),accounted for 68.33% of total number of fish species.The dominant species were Carassius auratus,Squalidus argentatus,Hypophthalmichthys nobilis,and Hemiculter leucisculus.The catch weight proportion is mainly composed of Hypophthalmichthys nobilis,Carassius auratus,Ctenopharyngodon idella and Xenocypris davidi,accounted for 12.30%,10.50%,7.65% and 6.48% respectively;The Quantity proportion is mainly composed of Carassius auratus,Squalidus argentatus,Pseudorasbora parva and Hemiculter leucisculus,accounted for 19.10%,12.28%,10.49% and 10.00% respectively.The dominant body length of the catch is 0-10 cm,accounting for 54.11%.ABC curve analysis shows that the community is dominated by fast-growing and small-sized fish,which indicates that the community is seriously disturbed and in an unstable state.Compared with the history,the fish community structure has no obvious change.4.Based on molecular markers of mitochondrial D-loop region,genetic diversity of Hypophthalmichthys molitrix and Hypophthalmichthys nobilis after stock enhancement in Xiajiang-Xingan section of the middle reaches of Ganjiang River were analyzed.The results showed that 33 haplotypes were detected in 95 samples of Hypophthalmichthys molitrix,and the haplotype diversity(Hd)and nucleotide diversity(Pi)were 0.921 and 0.040,respectively.Among 82 samples of Hypophthalmichthys nobilis,18 haplotypes were detected,and Hd and Pi were 0.840 and 0.009,respectively.Both Hypophthalmichthys molitrix and Hypophthalmichthys nobilis showed high Hd and Pi levels,and their genetic diversity is rich,which is higher than that of existing studies of wild Hypophthalmichthys molitrix in Ganjiang River,It indicated that the stock enhancement may not cause the decrease of genetic diversity of wild Hypophthalmichthys molitrix population in this section.