Seasonal fish distribution along undammed rivers to reservoir cascades

Abstract

Reservoir cascades have cumulative effects on fish species, which may be less pronounced in the presence of free-flowing tributaries. However, it is still poorly understood how these effects are seasonally processed. Through this perspective, this study aimed to assess the seasonal distribution of fish assemblages within a system composed of two reservoirs in cascade, located in the Upper Grande River basin, Brazil, and its two undammed tributaries. The dissertation was organized in two articles: the first considered the entire fish assemblage and the second focused on migratory species. The species were collected bi-monthly during a one-year period along the system, assessing composition, richness, biomass, and abundance of native and non-native species. Furthermore, for migratory species, the presence of fingerlings and juveniles in the system was recorded, and the gonadal maturation stage was evaluated. The transition areas between rivers and reservoir presented the higher values of total species richness, and along the cascade, an increase in non-native species was observed. The results also show the role of upstream free flowing rivers in fish fauna structure and in maintaining the richness of native species, especially migratory species. It was possible to notice that the transition areas do not seem to have a significant role in the recruitment of migratory species, since the recruitment probably depends on the undammed rivers and their associated floodplain lagoons. We therefore highlight the importance of conserving these rivers and the integrity of their floodplains for maintaining regional fish diversity in reservoir cascade systems.