Effects Of Cascade Hydropower Development In The Main Stream Of The Upper Reaches Of The Yangtze River On Fish Communities

The upper reaches of the Yangtze Rive is the area with the most abundant freshwater fish species and the highest fish biodiversity in China.However,with the rapid development of national economy in recent years,human interference to the original natural water system structure of the Yangtze River Basin is also gradually increasing.Especially,the effects of cascade hydropower projects on the natural structures of the river is of utmost obvious.For a long time,there has been insufficient quantitative researches on the impact of cascade hydropower development on aquatic organisms,especially on fish community structures,in the upper reaches of the Yangtze River.This results in difficult to effectively support the effective implementation of water ecological protection measures in the Yangtze River Basin.In this study,taking the fish species distributed in the mainstem and tributaries of the upper reaches of the Yangtze River as the research object,the fish and habitat samplings were conducted in some representative sections of the upper reaches of the Yangtze River.Based on the results of samplings,the effects of alterations of natural hydrological regimes,barrier effect,and shrinkage of suitable habitat area resulting from the construction of hydropower stations in the main stream of the upper Yangtze River on the fish community structures in different habitat types of river sections(natural flowing reach below dams,natural flowing river section between two dams,and the riverine zone,transitional zone,and lacustrine zone in a reservoir)were estimated firstly;secondly,the ecological effects and influence patterns of cascade hydropower development on the fish community structures in the main stream of the upper reaches of the Yangtze River were explored;finally,by integrating other human disturbances,the combined impact effects and relative importance of different human activities,such as cascade hydropower development,over-fishing and river sand-mining,on fish community structures were analyzed.The results of this study would be expected to provide theoretical and technical supports for fish resource protection in the upper Yangtze River.The main results were as follows:(1)Cluster analysis showed that fish community structures of Yichang section below the Gezhouba Dam and Three Gorges Dam from 1999 to 2016 could be classified into five clustering groups at a statistically significant similarity level of51.76%(global R=0.889,p=0.001,n=999 permutations).The years of these five clusters corresponded to different stages of the Three Gorges reservoir(TGR)impoundments(pre-impoundment stage,135-m impoundment stage,156-m impoundment stage,172-175 m impoundment stage,and 175-m impoundment stage).With the rise of the water level of the TGR,the relative abundances of fish species which mainly from the reaches upstream of the Three Gorges Dam,such as Coreius guichenoti,Rhinogobio ventralis,and Jinshaia sinensis,decreased gradually,whereas that of many eurytopic or carnivorous fish species increased gradually.The inter-annual variation of fish community structures in Yichang section during the study period was mainly determined by the decline of water sediment concentration,the change of water temperature rhythm,and the change of fishing intensity in the study area,and the water level rise in the TGR.Of which,the decrease of water sediment concentration in the study area and the rise of the water level in the TGR were the two most key factors.(2)Based on the investigation of fish resources and habitat conditions performed in the riverine,transitional and lacustrine zones of the TGR during the period of2011-2018,the relative roles of the construction of upstream cascade dams(Xiangjiaba and Xiluodu dams)and the longitudinal gradient in environmental conditions in the spatial and temporal variations in fish assemblages in a downstream reservoir(TGR)were investigated.The results showed that : the fish community structures of the four sampling sites in the TGR could be divided into four statistically different main clustering groups along the river longitudinal gradient direction from2011 to 2018(global R=0.846,p=0.001,999 permutations),and each main clustering group could be further classified into two small statistically significant clustering groups(all p<0.05).During the survey period,the river sections in the lacustrine and transition zones were dominated by the eurytopic and limnophilic species which adapted to the static and slow-flowing waters,while that in the riverine zone was mainly inhabited by the rheophilic species which adapted to the fast-flowing waters.Fish assemblages in the upper area(the riverine and transitional zones)of the TGR were more vulnerable to upstream dam construction than that in the lower area(the lacustrine zone).The longitudinal environmental gradient among sites in the TGR was more important in determining the spatial and temporal distributions of fish assemblages in the reservoir than the combined effects caused by the upstream dams and reservoirs in the lower reaches of Jinsha River.(3)Based on development of a new fish-based index of biotic integrity(F-IBI),and abiotic-biotic data relationship models,the impacts of cascade hydropower development on the change of fish community structure in the sections between Sigu and Fuling in the upper reaches of Yangtze River were evaluated.The results showed that: the differences in fish species compositions between the natural reach between two adjacent dams and transitional region in reservoir were mainly caused by the difference in the species numbers of zoobenthivore fishes.The endemic fish species,as well as rheophilic,zoobenthivore and phytobenthivore fish species,were most easily affected by reservoir cascades.In a reservoir cascade,compared to that in the transitional zone,the free-flowing stretch between reservoirs had been suffered greater influences by the impoundment and operation of cascade reservoirs.The longitudinal position of cascade reservoirs and the proximity of the downstream reservoir and the upstream dam had utmost important effects on fish species composition.F-IBI could effectively identify the cumulative effects of the reservoir cascades on fish community structures.(4)Based on the relative abundance data of different fish species,and related habitat data in the lower reaches of Heishui River in 2006,2008 and 2010-2017,the effects of the impoundment of reservoirs Xiangjiaba(XJBR)and Xiluodu(XLDR)in the mainstream of the lower reaches of Jinsha River,fishing activity,instream gravel and sand mining,the natural flow regime,water temperature,and precipitation on the temporal changes of fish community structure in the lower reaches of Heishui River were analyzed.The results showed that the temporal variation of fish community structure in the lower reaches of Heishui river was mainly affected by instream gravel and sand mining,mainstream reservoir impoundment,and seasonal flow regime.