Development of new fish-based indices of biotic integrity for estimating the effects of cascade reservoirs on fish assemblages in the upper Yangtze River, China

Abstract

In recent decades, dams worldwide are increasingly being constructed in a row along a single river or basin, thus forming reservoir cascades, and in turn producing cumulative ecological effects along these reservoir cascades. Use of multimetric indices (MMI) based on fish assemblages to assess the ecological health status of rivers and lakes have also been developed extensively. However, there are no studies using MMI for identifying the cumulative effects of reservoir cascades. The aim of this study was to develop a new fish-based index of biotic integrity (F-IBI) that can effectively identify the cumulative effects of reservoir cascades on fish assemblages in two important habitats (the free-flowing reach between reservoirs and the transition zone in the reservoir). Fish assemblages from 14 sites were sampled along the cascade reservoirs in the upper Yangtze River, China. First, through screening for redundancy, precision, and responsiveness of the candidate metrics, a new F-IBI based on ecological trait information of fish species composition was developed to estimate the ecological status of all sites. F-IBI scores exhibited an obviously downward trend from upstream to downstream in a reservoir cascade, and the transition zones in the reservoir displayed significantly lower F-IBI scores than the free-flowing reaches between reservoirs. Secondly, using random forest models, it was shown that the F-IBI can effectively identify the cumulative effects of the reservoir cascades on fish assemblages. Furthermore, we also demonstrated metric-specific responses to different stressors, particularly the multiple reservoir cascades, which showed: 1) endemic fish species, as well as rheophilic, zoobenthivore and phytobenthivore fish species, are easily affected by reservoir cascades, and 2) the longitudinal position of cascade reservoirs and the proximity of the downstream reservoir and upstream dam have very important effects on fish species composition. It was suggested that targeted fish conservation with the integration of habitat and biotic information should be given more attention in the free-flowing reaches between reservoirs.