Spatial changes of fish assemblages in relation to filling stages of the Three Gorges Reservoir, China

To study how fish assemblages changed at different fillings stages, we compared the present fish assemblages (2015-2017; after the third filling in 2009) with those from periods prior to impoundment (1975), after the first closure (1997-2000), after the first filling (2005-2006), and after the second filling (2006-2009) in the upper, middle and lower sections of the Three Gorges Reservoir, the largest in the world. Our results demonstrated that species richness was higher in the upper section than in the lower and middle sections, whereas the overall richness was lower than that prior to damming. The present assemblages were different among sections associated with their specific habitats; i.e., the proportion of rheophilic fishes decreased, whereas those of limnophilic and eurytopic fishes increased from the upper to the lower sections. Temporal comparisons suggested significant declines in proportions of rheophilic and migratory fishes, whereas small-bodied and eurytopic fishes increased after the impoundment. Fish biomass (CPUE) was greater in the upper section than in the lower and middle sections, and significantly increased after the third filling in the upper section. Overall, our results indicated that the Three Gorges Dam significantly impacted fish assemblages, and the effects of impoundment were magnified by the increased filling scales. The fish assemblages in the upper section were less impacted, demonstrating the high conservation value offered by the upper section to the fish species inhabiting the upper reaches of the Yangtze River.