Winners and Losers of River Morphological Change: Species- and Trait-Specific Fish Responses in Carpathian Rivers

Anthropogenic stressors increasingly threaten freshwater biodiversity, with fish communities particularly sensitive to habitat modification. This study evaluates how river morphological alterations influence fish assemblage structure in 114 mountain rivers of the Southern Carpathians, assessing whether such changes cause species loss or drive shifts toward disturbance-tolerant communities. Using a multi-scale analytical framework integrating non-metric multidimensional scaling, redundancy analysis, and variance partitioning, we quantified the contributions of spatial, catchment, and local habitat variables to community patterns. Spatial- and catchment-scale factors explained the largest variance in fish assemblages (12% in adults and 17% in small-bodied fish). However, morphological pressures proved significant in shaping community structure with clear ecological consequences. Weirs and embankments reduced abundances of rheophilic species (flow-dependent) by 27–38%, potamodromous by 23–42%, invertivorous by 26–49%, benthic by 40–46% and lithophilic taxa by 27–41%, indicating the loss of habitat specialists. In contrast, limnophilic taxa (preferring slow or still water) increased 25 times, phytophilic spawners by 17–41%, and tolerant species by 10%, reflecting biotic homogenization. By integrating a trait-based approach, this study highlights functional shifts that may be overlooked in species-level assessments. It underscores the need to couple local habitat restoration with catchment-scale management to conserve fish biodiversity and maintain natural ecological gradients in mountain river systems.