Long-term study of reservoir cascade in south-eastern Brazil reveals spatio-temporal gradient in fish assemblages

In reservoir cascades, effects on fish assemblages are expected to strengthen over time and transfer from one reservoir to an adjacent one. To test this, fish-assemblage data from 23 years of monitoring in the Araguari reservoir cascade system, upper Parana River basin, were analysed. The results showed a clear reduction in richness of native and migratory fish species and an increase of non-native species, following reservoir formation. Migratory species richness was higher in reservoirs that presented habitats similar to lotic stretches or tributaries upstream of the impounded area. There was a clear tendency for native species to decline and non-native fish species to increase, in a downstream direction. Fish assemblages became increasingly dissimilar as reservoirs became more distant from each other (longitudinal gradient) and were dominated by small and medium-sized species. Alongside longitudinal position, reservoir area, age and the presence of herbivorous non-native fish were found to be important predictors of variation in fish-assemblage structure. Results from the present study help clarify the potential accumulated impacts of reservoir cascades on fish diversity, which must be carefully considered in river-basin inventories for hydropower plants, and reinforce the importance of long-term monitoring, considering longitudinal and lateral dimensions of the basin.