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The Llanos is an extensive area of savannas and floodplains in central and western Venezuela that encompasses a gradual elevation gradient from the río Orinoco to the foothills of the Andean piedmont. The río Portuguesa is one of the major rivers in this region that until recently had escaped major anthropogenic impacts and still maintains substantial seasonal fish migrations. However, little work has been conducted on fish ecology in this river. The present study analyzes museum collections sampled at 28 locations along the longitudinal gradient of the río Portuguesa to assess similarity of species composition from the foothills of the Andean piedmont to the lowland llanos floodplain. The standardized samples used in this analysis contained greater than 133 species representing 6 orders and 27 families, dominated by characiforms (61 species) and siluriforms (52 species). Detrended correspondence analysis (DCA) ordination of samples revealed a continual pattern of compositional change, and species are added at a faster rate than they are lost as one moves from the foothills of the piedmont to the low llanos. Based on DCA, samples from three elevational segments were found to significantly differ in fish species composition. Assemblages in the upper reaches contained unique species of loricariid catfishes, small pimelodid and trichomycterid catfishes, and small characiforms not observed at lower elevations. The lowland reach contained species of cichlids, large catfishes and characids not collected from the other two regions. Samples from the middle region revealed transitional species composition. Longitudinal species turnover probably reflects differences in environmental characteristics such as a water velocity, substrate composition and disturbance regime. Findings from this broad-scale analysis contribute to a baseline for future studies of fish ecology in this region

Compositional change in fish assemblages along the Andean piedmont - Llanos floodplain gradient of the río Portuguesa, Venezuela