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intolerant of tUlbidity, which may interfere with reproduction and growth. Instream cover of 40- 60% of pool area is optimal. Optimal current velocities are ~ 6 em/see and velocities exceeding 20 cm/ see are unsuitable. Optimal spawning substrate for largemouth bass is gravel; silt bottoms are unsuitable. Current velocities> 10 cm/sec are avoided by spawning bass and stable water levels are considered best due to characteristic shallow nest depths. Bass fry cannot tolerate current velocities exceeding 0.27 cm/see and optimal temperatures for growth are 27-3OC. These temperatures were not reached in the Colorado River mainchannel between Cameo and Stateline in 1989 or 1990. Instream vegetative cover is considered important for fry survival. Spawning by largemouth bass is negatively affected by the predominance of silt substrate, fluctuating water levels, and rapid water temperature changes (Carlander 1977). Based on these habitat suitability criteria, the presence of flowing, cold, turbid water with rising spring water levels and storm-induced summer flow spikes in the Colorado River would likely not favor adult largemouth bass reproduction, and could disrupt all centrarchid species attempting to spawn in main-channel habitats, including backwaters. A change in side-channel backwaters to flowing channels and fluctuating water levels would make many backwaters unstable and unsuitable spawning habitats. Backwaters, as low-velocity habitats may provide higher water temperatures for centrarchids and other fish species. McAda et al. (1994) found a significant difference in water temperatures between the mainchannel and backwaters containing Colorado pikeminnow young-of-year in the Colorado and Green rivers in only 3 of 22 comparisons from 1986-1992, but suggested Colorado pikeminnow used backwaters during the warmest part of the day and left during nighttime cooling. Trammell and Chart (1999c) reported monthly mean water temperatures in the Green River mainchannel for May through October were higher than in backwaters in all but one occasion. They found the average backwater temperature was significantly lower than the mainchannel temperature and backwater maximum temperature was signifteantly higher than mainchannel maximum. These authors found degree day accumulation in backwaters based on maximum daily temperature was higher the mainchannel, but degree day accumulation in backwaters was lower than the mainchannel based on average daily temperature. Results of Trammell and Chart (1999c) suggest the thermal advantage of backwaters resides largely in being able to achieve higher daily maximum temperatures than the mainchannel, but this advantage may be mitigated by daily temperature fluctuations resulting in average backwater temperatures lower than the mainchannel. To exploit the thermal advantages of backwaters and mainchannel habitat, fish species must be adapted to move freely between and exploit both. Offchannel, standing water habitats that are infrequently affected by Colorado River flows and can maintain stable water levels and more optimal water temperatures suitable for centrarchids would be the most conducive to centrarchid reproductive success. This suggests that off -channel ponds and reservoir act as centrarchid sources to riverine habitats. The incidental presence of bluegill and black crappie collected in backwaters as well as the catch rate and length frequency data for largemouth bass support this hypothesis. This contn"bution may occur during high spring flows when floodplain ponds are breached by high water, or by simple escapement in outlet streams. While largemouth bass samples collected in backwaters are comprised of many young- of-year or juvenile fish, the length frequency distributions do not demonstrate recruitment of these 32 I I I I I I I I I I I I I I I I I I I