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– Stream‐fish assemblage and environmental data for 13 sites in the upper Brazos River, Texas, USA during 1997 and 1998 were used to assess the relationship between environmental conditions, and seasonal and spatial variation in fish species abundance and distribution patterns. There was considerable spatial variation in environmental conditions among sites. Spatial variation in species diversity and species composition was related to variation in conductance (salinity), depth and current velocity among sites and streams. Species diversity increased downstream and species composition shifted from primarily cyprinodontids upstream to cyprinids downstream. Among all dominant species, spatial components of variation in fish abundance were greater than seasonal components, suggesting that assemblage structure is determined more by average or persistent differences in environmental conditions among sites than by seasonal variation in environmental conditions.

The fish assemblage of the Canadian River in Texas historically was dominated by Hybognathus placitus and Notropis girardi. These species represented > 90% of fishes collected fiom the Canadian River in 1954-1955. Construction of two reservoirs on the Canadian River in the 1960s (Ute Reservoir, New Mexico, 1962; Lake Meredith, Texas, 1965) altered hydrologic conditions in the river and affected the fish assemblage. Downstream fiom Ute Reservoir, mean annual discharge decreased by about 38% after impoundment, but H. placitus, N. girardi, and other mainstem species still dominate the assemblage. Downstream from Lake Meredith, mean annual discharge decreased by 76% and the historic mainstem fish assemblage has been almost completely replaced by species that formerly were restricted to tributary streams. The magnitude of postimpoundment changes in the fish assemblage of the Canadian River appears to be related to the degree that discharge has declined, especially during the spawning season.

We studied the relationship between stream discharge and reproductive success for five members of a prairie stream fish assemblage. Based on information obtained from the literature, we constructed seven models relating stream discharge to successful reproduction. To assess the utility of our models, we collected larval and juvenile Arkansas River shiners Notropis girardi, peppered chub Macrhybopsis tetranema, plains minnow Hybognathus placitus, flathead chub Platygobio gracilis, and red shiners Cyprinella lutrensis from the Canadian River, Texas, during 2000-2001. We removed otoliths and counted daily growth increments to determine ages. Models were parameterized by use of age and discharge data. Models were evaluated with the Akaike information criterion likelihood statistic to assess which model(s) best described reproductive success for each species. Of the seven models evaluated, the best overall model predicted that reproductive success was uniform across dates when discharge was present in the river; however, reproduction was predicted to be unsuccessful during periods when the river was confined to isolated pools. These results suggest that the presence of discharge in the river may be more important for successful reproduction of prairie stream cyprinids than the magnitude of discharge.

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