The longitudinal gradient prevails over local characteristics in shaping fish species distributions in a large neotropical reservoir

In large reservoirs, spatial gradients are the primary drivers of fish species distributions. However, local characteristics can also influence fish distributions and assemblage structure. We assessed the roles of distance from the dam, water quality characteristics, and physical habitat complexity in determining the richness and abundance of native and non-native fish species in Furnas Reservoir, Brazil. Sampling in April 2023, we collected 22 native and 11 non-native species, increasing the reservoir's richness to 65 species, comprising 51 native and 14 non-native species. Both the richness and abundance of native species were higher in the upper reservoir. These species predominated throughout the entire longitudinal gradient except in the most lacustrine portion (50 ​km from the dam), where non-native species prevailed. Physical habitat condition had little influence in the tested models. Except for total solids, water quality variables were correlated with distance from the dam. Total solids were associated with increased richness and abundance of both native and non-native fish species. We recommend protecting riverine zones in large reservoirs, because of the greater prevalence of native species in them. • The longitudinal gradient explained most of the variation in richness and abundance. • Higher native species richness and abundance in the riverine region. • Non-native species displayed a more homogeneous distribution throughout the reservoir. • Most water properties were correlated with the distance from the dam. • Physical habitat complexity increased the chances of capturing at least one fish.