Study On The Trophic Characteristics And Downward Effects Of Fish Communities In Xiaojiang River,the Tributary Of The Three Gorges Reservoir

After the impaction of the Three Gorges Reservoir,the eutrophication of the tributaries of the reservoir intensified,and the water bloom occurred frequently.At the same time,the fish community structure changed fundamentally.As a typical zooplankton feeding fish,the Neosalanx taihuensis population expanded rapidly,and became an important economic fish in the reservoir area.In order to explore the characteristics of the trophic structure of the main fish communities in Xiaojiang River.We analyzed the composition of the fish communities,the trophic levels,niche widths and the trophic relationships among the main fish species by combining fish resource survey and stable isotope technique in dry season(July)and wet season(December).Based on the feeding selectivity and population dynamics of N.taihuensis,we analyzed the effects of the downward effect of its population growth on zooplankton community structure and cyanobacterial bloom outbreaks.It could provide theoretical basis for the management of fish resources in Xiaojiang River and the prevention and control of cyanobacteria blooms.The main results are as follows:(1)In 2019,a total of 58 species of fish were collected in Xiaojiang River,which belong to 47 genera and 13 families of 6 orders.The species composition is mainly cypio and Bagridae,accounting for 62.07% and 10.34% of the total species,respectively.According to the IRI,the dominant species in dry season were Coilia brachygnathus,Culter alburnus,Cyprinus carpio,Pelteobagrus nitidus and Hemiculter bleekeri,in wet season the dominant species were Cyprinus carpio and Hemiculter bleekeri.(2)There were three nutrient levels in Xiaojiang River in different hydrological periods,and the length of nutrient levels in wet season(TL=3.72)was longer than that in dry season(TL=3.42).The interspecific relationship of Xiaojiang River fishes showed that the niche overlap between fishes in dry season was significantly higher than that in wet season(P<0.05),and the trophic niche overlap between N.taihuensis and C.brachygnathus was the highest in dry season(0.56)and wet season(0.47),respectively,indicating that C.brachygnathus was the main food resource competitor of N.taihuensis.(3)N.taihuensis mainly feeds on copepods,cladocerans,and less on phytoplankton and rotifers.N.taihuensis has strong selectivity to large zooplankton with a wet weight of more than 0.045 mg,and occasionally feeds on small zooplankton such as copepods.(4)From March to June(spring breeding season of N.taihuensis),the biomass of N.taihuensis increased rapidly,and maintained a high level of biomass in May and June.During this period,the zooplankton community structure was significantly miniaturized,and the biomass decreased from 28.93 mg/L to 1.13 mg/L.Density increased from 21173.39 ind./L to 26808.23 ind./L.In July,when the fishing ban in Xiaojiang River ended,the biomass of N.taihuensis began to decrease sharply,while the density and biomass of copepod and cladocera increased rapidly,and in August(density: 151.98 ind./L;biomass was 3.39 mg/L),which was the highest in the whole year.There was a strong correlation between the biomass of N.taihuensis and zooplankton community structure.Furthermore,the results of this study showed that the population of N.taihuensis increased rapidly in March and April,and maintained a high level of biomass in May and June.During this period,due to the strong selectivity of N.taihuensis to cladoptera and copepods,the biomass of small river zooplankton decreased and miniaturized,and the grazing pressure on algae decreased.Therefore,the changes of population resources and feeding intensity of N.taihuensis have changed the community structure characteristics of phytoplankton,and may be one of the important reasons for the cyanobacterial blooms.