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samples. Trammel and Chart (1999a) conducted a seine sampling efficiency study using block nets and repeated seine hauls and found channel catfish YOY and juveniles were consistently under-represented by seining; most young catfish were captured via entanglement in the blocking net. Larval drift, juvenile habitat use and sample gear bias may be affecting sampling results for young catfish. Catfish fry aggregate near cover in protected, slow flowing riffles in clear water or along the shallow edges of turbid flowing channels (see references in McMahon and Terrell 1982). Backwaters or shorelines with considerable cover are difficult to seine effectively, though these backwater types were not common in the study reach. The occurrence of channel catfish young as drifting larvae has also been documented in the Colorado River at Black Rocks (RMI 134) (Haynes et al. 1985), but it is not certain how downstream transport of young catfish affects size distribution and recruitment in the study area. Nesler (1987) found that larval drift characteristics of channel catfish and Colorado pikeminnow in the Yampa River were very Rlmilar with respect to seasonal dates and time of day, but quite different spatially across the river cross- section. Colorado pikeminnow larvae catch rates in drift nets were found to be statistically similar across the river cross-section between mainchannel and near-shore surface and bottom sampling zones, while most drifting channel catfish larvae were captured in the mainchannel bottom zone. It is notable that Anderson (1999) did not collect channel catfish larvae in near-shore surface drift nets in the Colorado River from RMI 185-154, and in the Gunnison River from RMI 171 downstream to the Colorado River confluence. Trammell and Chart (l999b) collected small numbers of channel catfish latvae at sites above Westwater Canyon near the Colorado-Utah border at RMI 132. McAda and Ryel (1999) collected only 48 channel catfish larvae or young-of-year in the Colorado River between the Gunnison and Green river confluences in 1983-1985 and 1988- 1994 from 1,645 samples and almost 354,000 specimens. Anderson and Stewart (2002, in draft) report that the relative abundance of channel catfish > 150 mm long in the Colorado River in the 15-mile reach from 1999-2001 was 5-6% at 400-500 fish/km (less than flanneImouth sucker, bluehead sucker and common carp); and the relative abundance of channel catfish < 150 mm was <0.1%. Spawning by channel catfish in the Colorado River is most likely affected by the temperature regime. Temperatures of 21-25C are considered necessary for successful reproduction by channel catfish, with temperatures above 25C considered optimal (Clemens and Sneed 1957, McMahon and Terrell 1982). Ramsey and Graham (1991) characterize channel catfish spawning as occurring from mid-May through July in temperate zone waters at temperatures of 21-28C. Gerhardt (1989) found spawning by channel catfish in the Powder River in Wyoming occurred over a 15-day Period when water temperatures reached 25C, and described spawning activity by this species as negligJ."le at temperatures < 25C. Water temperatures of 28- 30C are considered optimal for channel catfish fry (McMahon and Terrell 1982). Water temperatures in the Colorado River from Cameo to Stateline certainly allowed for successful reproduction from June through August in 1989-90, but only briefly reached the optimum in July 1989 at State1ine. Fry rearing temperatures never became optimal in the Colorado River in 1989- 1990. It is logical to assume that channel catfish adults were seeking a more favorable thermal 34 I I I I I I I I I I I I I I I I I I I