Hydrological variation and fish assemblage structure in the middle Wabash River

Two years of fish assemblage data from 28 sites in the Wabash River Indiana, were examined for relationships with environmental variables using a multivariate approach, correspondence analysis. Upstream sites had lower mean daily discharge and lower coefficient of variation of daily discharge when compared to downstream sites. Although the fish assemblage changed along this 230-km river distance gradient, patterns were in contrast to the patterns in streams with unaltered flow regimes. We compared functional variables of fish species (species traits that describe habitat, trophic, morphological, and tolerance characteristics) by examining the proportion of their occurrences along the hydrological variability gradient (upstream downstream). The general pattern showed assemblages from hydrologically stable (upstream) sites had higher proportions of generalist species that tend to occur in small to medium streams, prefer fast current velocities, generally occur over rocky, gravel, and sand substrates, and have low silt tolerance. In addition, there was a pattern relating the hydrological/longitudinal regime and the overall morphology of species: species with higher caudal peduncle/caudal fin ratios and more fusiform body shapes occurred in higher proportion in upstream sites.