Fish assemblage distribution along free-flowing rivers entering a reservoir cascade

Reservoir cascades have cumulative effects on fish assemblages that can be less pronounced with the presence of incoming free-flowing tributaries. However, it is still poorly understood how the effects of cascade reservoirs on fish communities are driven. This study aimed to assess the fish assemblage distribution along two long free-flowing rivers that enter the first two reservoirs in cascade within the upper Grande River basin. Fishes were collected bi-monthly throughout 1 year along this cascade and were evaluated by composition and richness of native and non-native species. The transitional areas between the rivers and the first reservoir presented the highest richness. An increase of non-native species was observed from the uppermost (Rivers) to the downmost (lower reservoir) region. Although the seasonality did not influence fish composition in the Camargos-Itutinga cascade system, discharge-related variables (water level, transparency, and temperature), together with the position along the gradient, were the main drivers of the variation on fish assemblage’s structure. The results reinforce the role of cascade systems in the increase of non-native species, as well as the importance of free-flowing rivers in structuring the fish fauna in such conditions.