Abstract
This work aimed to investigate the distribution and occurrence of fish species along the São Francisco Falso Braço Norte (SFFBN) River basin, a tributary of the Middle Paraná River basin now artificially connected to the Upper Paraná ecoregion, to evaluate how such connectivity may affect the biogeographic distribution and regional composition of freshwater fish species. Quarterly samplings were conducted between August 2022 and May 2023 at 12 sampling points distributed along the longitudinal axis of the basin. The fish were collected using sieves, casts, seines and gillnets. The results suggest that, although the SFFBN River basin is currently integrated into the Upper Paraná ecoregion and strongly influenced by the dynamics of the Itaipu Reservoir, its fish community still harbours species likely linked to the biogeographic history of the Middle Paraná River basin. These species, particularly in the middle and upper stretches of the SFFBN River basin, appear to have persisted without being directly affected by the damming. Therefore, monitoring the fish community in Itaipu's tributaries is essential for understanding the impacts of ongoing ichthyofaunal homogenization between the Lower and Upper Paraná ecoregions.