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Seasonal differences in pooled fish abundance (lowest in spring, intermediate in summer, and relatively high in autumn) were what we expected. Spring abundance was lowest presumably because of high overwinter mortality or few fish occupying relatively cold backwaters, summer abundance increased because of successful reproduction, and autumn abundance was perhaps highest because most fish were susceptible to the capture gear used. Several abundant non-native species are very small at hatching and are difficult to capture in summer because they are not susceptible to the mesh size of the seine, but grow sufficiently by autumn so that capture efficiency is high. Highest fish abundance in 2003 seine samples was likely due to presence of a few very large samples of sand shiner, red shiner, and reside shiner in some reaches or seasons. Highest species richness in seine samples in summer and autumn is likely due to increased presence and abundance of age-0 life stages of rare species that survived to that point in the year. Highest species richness in 2003 was due to presence of a few rare species that were absent in other years and to the large number of fish captured, which increased our chances of detecting rare species. Reach and seasonal abundance patterns of each fish species depended, in large part, upon individual thermal tolerances and timing of spawning, and differed among native and non-native fishes. Relatively high abundance of most native taxa in summer (bluehead sucker, flannelmouth sucker, and speckled dace) was likely because most were late-spring or early- summer spawning species. Thus, age-0 fish were not present during spring seine sampling, but were relatively abundant and large enough in summer to be captured in seines. Mortality resulted in lower autumn abundance. Additional reasons for lower autumn abundance could be that fish in some taxa grew sufficiently to move from backwaters to main channel habitat or were fast enough to avoid capture by seines. Overwinter mortality and habitat shifts likely resulted in additional reductions in native fish abundance in the subsequent spring. Seasonal abundance patterns were similar in the Green River in the mostly regulated reach upstream of the Yampa River and in the partially regulated reach downstream of the Yampa River. Highest abundance of native taxa in Lodore and Whirlpool canyons is likely a 43