Fragmentation by Dams and Habitat Quality Affect Body Condition of a Small Characid in a Neotropical System

ABSTRACT The structure of stream networks naturally fragments freshwater habitats, but this complexity is increasingly intensified by dams and reservoirs. How anthropogenic fragmentation interacts with natural isolation gradients to affect fish populations remains poorly understood. Here, we tested how natural (position within the river network, i.e., closeness centrality, and elevation) and anthropogenic (cumulative reservoir volume) isolation gradients influenced body condition of Psalidodon aff. fasciatus across stream reaches distributed in two sub‐basins upstream of Lake Paranoá, Upper Paraná River basin, Brazil. We estimated body condition as residual mass from the length–weight relationship using generalized linear mixed‐effects models, testing effects of isolation predictors and local environmental variables simultaneously. Populations exhibited positive allometric growth, contrasting with typical reports for the species. Contrary to our hypothesis, body mass was significantly higher at sites with greater cumulative reservoir volume downstream. Individuals from the right sub‐basin exhibited greater body mass than those from the left sub‐basin. Natural isolation predictors (closeness centrality and elevation) had no significant effect. Body mass was significantly positively related to water temperature and negatively related to substrate diversity. Our results suggest that body mass varied in response to environmental conditions and the presence of dams, which may benefit this generalist species by expanding foraging habitats and resource availability. These findings highlight that the impacts of fragmentation are species‐specific: while dams can severely affect sensitive species, opportunistic omnivores may exploit newly formed reservoir habitats, potentially reshaping community reassembly in increasingly dammed Neotropical systems.