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2) the Upper Colorado River reach and the Gunnison River reach are similar with respect to this carrying capacity based on similar geohydrology and predominating native fish communities based on observed native/nonnative species relative abundance as described in Burdick (1995) and Anderson (1997). 3) Depletion and extirpation of adult populations above the barriers occurred as the result of some combination of downstream spawning movements by adults over. the barriers, and prevention of upstream dispersal movement and recruitment by late juvenile or adult fish, or return migration by spawning adults. 4) As top predator, Colorado pikeminnow biomass as a percent of the system should be, and would naturally be low (< 1 %). 5) the Colorado River reach from which biomass estimates in Anderson (1997) were made was at carrying capacity. Further, the following species interactions and recovery actions are anticipated: 1) Biomass and production of system occupied by common caIp, white sucker, and other nonnative fish species may be reduced by control efforts, expanding production potential within the native fish prey base for Colorado pikeminnow. 2) With successful establishment of this species, abundance of other native species may be reduced as a result of predation. A target biomass objective for Colorado pikeminnow of 1 % of the total fish community biomass in each of the river reaches designated was arbitrarily selected. The numbers of Colorado pikeminnow sucker required to equal this biomass was estimated to be 10 fish/mile using average weight (2500 gm) and length (650 mm) estimates for adult fish. Another consideration in selecting a target density and biomass of Colorado pikeminnow is the biomass of available prey fish species. From Anderson (1997), a prey fish biomass for the Palisade to Rifle reach of the Colorado River was estimated for the main channel based on electrofishing results for fish < 300mm in length, and for the backwater habitat based on July 1994 seine results. The average main channel biomass for prey fish species was estimated at 119 kg/mi. Main channel prey species were dominated by flannelmouth sucker, bluehead sucker, roundtail chub, and white sucker. For backwater habitat in this reach, a prey fish population estimate was extrapolated from the number of fish sampled, the backwater area sampled, and the estimated total backwater area based on 53 backwaters recorded and measured in the 52.6 mile reach. The biomass of this population was estimated by partitioning based on the relative abundance of each species, the fish size components for each species < or > 150 mm in length, and calculating the weight of each size component using length-weight relationships calculated in Anderson (1997) for each species. An average weight of 3 gm for a 64 mm 15