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<br />Diet overlap values were considered biologically important only for comparisons with certain <br /> <br /> <br />size-interval, river-reach groups of five fishes. <br /> <br /> <br />Because interspecific demand for resources might not exceed supply, Bowen (1983) <br /> <br />noted that even extensive diet overlap is not conclusive evidence for competition. <br /> <br />Accordingly, McAda and Tyus (1984), who also used Schoener's index to examine diet <br /> <br />overlap between YOY Colorado squawfish and nonnative fishes in the Green River, <br /> <br /> <br />suggested that the high diet overlaps they observed between Colorado squawfish 22-40 <br /> <br />mm TL and channel catfish 19-55 mm TL (overlap value = 0.60) and especially red shiner <br /> <br /> <br />15-69 mm TL (overlap values 0.70-0.80) might reflect shared use of abundant resources, <br /> <br /> <br />primarily immature dipterans, rather than competition. The same may be true for the higher <br /> <br />diet overlaps we observed. Ward et al. (1986) reported that chironomids, the principal food <br /> <br /> <br />category resulting in high diet overlap, were among the more common benthic invertebrates <br /> <br />in the Colorado River System. <br /> <br /> <br />We observed that overlap values were generally higher and, for most fishes, diet <br /> <br /> <br />diversity was greater in the lower than upper reach, perhaps because food resources were <br /> <br />more abundant and diverse in backwaters of the lower reach. Based on observations <br /> <br />during summer and fall 1979-1988, Haines and Tyus (1990) found that backwaters in the <br /> <br /> <br />upper and lower reaches were similar in mean surface area, but that those in the lower <br /> <br />reach were shallower and warmer, conditions that may favor higher productivity. Also, <br /> <br /> <br />within the upper reach, Grabowski and Hiebert (1989) noted that during summer and fall <br /> <br />1987-1988 concentrations of backwater nutrients, particulate organic matter, phytoplankton, <br /> <br /> <br />zooplankton, and benthic macroinvertebrates (particularly chironomid larvae) increased <br /> <br />progressively downstream. They suggested that this trend was due to attenuation of flow <br /> <br />releases from Flaming Gorge Reservoir at downstream sites which reduced the degree of <br /> <br />13 <br />