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12 I I I I I I I I I I I I I I I I I I I downstream of the three major irrigation canals of the Grand Valley, and habitat structure. Fish surveys were difficult to complete at several sites due to increased agricultural return flows. Native fish exceeding 170 mm total length were floy tagged. Overall, 1,895 fish were collected. Thirty-two percent of these fish consisted of the four native fish species collected; the bluehead sucker (Catostomus discobolus), flannelmouth sucker, roundtail chub (Gila robusta), and speckled dace. Flannelmouth and bluehead suckers were collected at all seven sites sampled, while roundtail chub and speckled dace were each collected at five sites. Size distributions of the native fishes collected suggested that the bluehead and flannelmouth suckers, along with the roundtail chub may be reproducing and maintaining self-sustaining populations within several of these intermittent, irrigation-influenced tributaries. Eight nonnative, nonsalmonid fish species constituted 68% of the total fish captured. The fathead minnow (Pimephales promelas), red shiner (Cyprinus lutrensis), and green sunfish (Lepomis cyanellus) accounted for 94% of the total nonnative fish species collected. Species composition fluctuated from site to site, and may be influenced by habitat structure and flow. Water chemistry results indicated that during the irrigation season, these tributaries were highly conductive alkaline systems, with elevated total phosphorus (maximum of620 ugIL) and suspended solids concentrations. Instream metal concentrations did not exceed state water quality standards for aquatic life. Dissolved selenium concentrations ranged from 3.1 to 13.1 ugIL. The CDOW will increase monitoring efforts in 2002, assessing variability of fish community structure, habitat, and water chemistry in relation to changes across irrigation seasons. Such studies may help determine what role, ifany, these irrigation-influenced waters play in the life cycles of native and nonnative fishes of the Grand Valley. Trammell, M.t, and R. A. Valder ISWCA Inc., Environmental Consultants; 2R, A. Valdez and Associates Non-native Fish Control in Backwater Habitats in the Colorado River, 1999-2001. Predation and competition between small, non-native cyprinid species and young endangered fishes has been demonstrated in laboratory experiments (Ruppert et al. 1995; Muth and Beyers, ~uWished data), and is perceived to be influential in limiting survival and recruitment ofthe endangered razorback sucker {Xyrauchen texanus) and Colorado pikeminnow (Ptychocheilus lucius) in the wild. The objectives ofthis study were to significantly reduce the abundance of small, non-native cyprinid and centrarchid fish species present in backwaters in the Colorado River, significantly increase the survival and abundance of native and endangered fish species using backwater habitats, and evaluate backwater seining as an effective field method for controlling the abundance of small, non-native cyprinid and centrarchid fish species and for inducing a positive biological response within the native fish communities. Response to depletion was evaluated using Interagency Standardized Monitoring Program (ISMP) catch data Depletion sampling was conducted on the Colorado River in the 15- and 18-mile reaches in June-July, 1999, March-April, 2000, and June 2001. Four to five depletion passes were made in each year. In 1999,8,863 non-native fish were removed from 65 backwaters. In 2000, 7,054 non-native fish were removed from 58 backwaters. In 2001, 180,379 non-native fish were removed from 82 backwaters. Catch and catch rate of all fish initially declined after the first pass in all reaches and years, but increased by the fourth pass in most cases. The effects of depletion by seining were thus temporary at best. Comparison ofISMP fall catch rates prior to, and after, depletion efforts were