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I I I I I I I I I I I I I I I I I I I cohorts into a notable adult population in the river or in backwaters. Meals and Miranda (1991) observed that age-O bluegill and crappie migrate from littoral habitats to limnetic zones a few days after hatching, while largemouth bass stay in the littoral zone. These traits may help explain the presence of largemouth bass and the comparative scarcity of bluegill and black crappie in the "littoral habitat" present in Colorado River backwaters. According to Meals and Miranda (1991), bluegill leave the shallows at < 11 mm and return at sizes between 11-25 mm. Bluegill progeny deposited or spawned in Colorado River backwaters would be unlikely to survive and return to a backwater habitat after leaving. Black crappie at 45- 60 mm leave littoral habitats for deep water. There is no surrogate for deepwater lentic habitat in the Colorado River. The lotic habitat and fluctuating backwater habitat of the Colorado River likely promote high mortality for bluegill, black crappie and their progeny. Largemouth bass young would tend to stay put in backwater habitats and persist as long as favorable conditions existed. Sample results targeting larger-bodied fish indicate that channel catfish and common cazp were the most abundant adult fish in Colorado River backwaters, at least during April and July. Native adult fish appear to use the backwaters in the spring only. Mainchannel fish sampling data, however, demonstrates a different picture of relative abundance from backwater sampling. Using dipnet, seine and electrofishing in the Colorado River from RMI 149-170 in 1983, Wick et al. (1985) captured roundtail chub, speckled dace, flannelmouth sucker, bluehead sucker comprising 74% of the total sample, whereas common carp and channel catfish were much less common at 5 and 0.4% of the catch. Anderson and Stewart (2000, 2001) estimated channel catfish at 195-660 fishlkm and common carp at 300-590 ftsh/km in the 15Mi1e-Reach of the Colorado River in 1999-2000. The relative abundance of select fish species in the Anderson and Stewart samples were channel catfish (4-14%), common catp (11-16%), flannelmouth sucker (31- 38 %), bluehead sucker (23-40 %) and roundtail chub at 3-7 %. Relative abundance of backwater species indicate nearly complete dominance by nonnative cyprinids and channel catfish, whereas relative abundance of mainchannel adult fish populations suggests native species are more abundant. Similar contrasts between backwater and mainchannel fish communities were found by Anderson (1997) in the Colorado River above the study site and by Burdick (1995) in the Gunnison River above Red1ands diversion dam near the Colorado River confluence. Osmundson (1999) reported bluehead and flannelmouth sucker as the most abundant fish species collected in the Colorado River from Palisade, Colorado to the confluence with the Green River in Utah. Anderson and Stewart (2002, in draft) report flannelmouth and bluehead sucker > 150 nun long as the most abundant in the Colorado River in the 15-mile reach. Despite the apparent concentration of adult channel catfish in the Colorado River backwaters based on blocknet sampling, and the densities of adult channel catfish reported by Anderson and Stewart (2000, 2001), young-of-year (YOY) of this species were relatively scarce in fall backwater seine samples. It is possible that this life stage of catfish may be using riffle-run habitat during daylight hours, and/or may become more active in backwater habitats during crepuscular periods. This diel behavior would account for their relative scarcity in daytime seine 33