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Final Report samples because their abundance varied with floods or behavioral patterns of local invertebrate populations. The proportion of terrestrial insects in the diet of adults was higher in Middle Granite Gorge fish than fish near the LCR, suggesting a decreased abundance of aquatic invertebrates and greater dependence on terrestrial taxa in downstream reaches. The numbers of organisms in drift above the LCR (248/100 m3), from the LCR to Hance Rapid (190/100 m3), and from Hance Rapid to Diamond Creek (96/1 00 m3) showed decreased drift food availability from eastern to western Grand Canyon and was consistent with the pattern of benthic invertebrates (Blinn et al. 1994). Average volume of drifting Cladophora was significantly higher during rising flows (i.e., upramp) than during steady flows, while densities of drifting organisms were significantly greater during falling flows (i.e., downramp 380/100 m3) than during rising flows (177/100 m3), indicating that Cladophora was dislodged with rising flows, but invertebrates drifted primarily during descending flows, Cold releases from Glen Canyon Dam have disrupted life cycles of aquatic invertebrates and greatly affected fish food diversity and availability. Humpback chub are opportunist insectivores, and adults have apparently adjusted to substantial changes in the invertebrate fauna; e.g., the amphipod Gammarus lacustris is a _ 250 non-native, introduced crustacean that E composed nearly 50% of the diet of ..s .c2oo adults. Although the effect of reduced t>> abundance.. and variety of ~ 150 macroinvertebrates on longitudinal ii abundance of humpback chub has not ~ 100 been fully evaluated, reduced densities of aquatic invertebrates in drift and the lower benthic productivity found by Blinn et aI. (1994) suggest a food shortage in downstream reaches. This shortage is probably caused by low shoreline production from persistent turbidity reducing photosynthesis and year-around cold temperatures that limit invertebrate diversity. 350 Executive Summary . 13 AGE AND GROWTH Average lengths of subadult humpback chub at the first three scale annuli formations were 96, 144, and 186 nun TL. A transition check on the scale also showed that the mean length of young fish moving from the warm LCR to the cold mainstem was 74 nun TL (range, 52-132 nun TL), indicating that most of the first year of growth for mainstem subadults occurred in the LCR. Average monthly growth rate of age 0 chubs in the LCR was 10.30 mm, which was similar to laboratory results of 10.63 nun at 200C (Lupher and Clarkson 1994). Average monthly growth rates for age I and II fish were 4.00 nun and 3.50 mm, which was higher than laboratory results of 2.30 nun at 100C; the difference was attributed to wild fish finding slightly warmed shorelines and backwaters. Average monthly growth rate from PIT-tag recapture information was 2.25, 2.79, 2.50, 1.16, 0.79,0.91, and 0,96 nun for fish in 50-nun length groups of 150-200 nun TL, 200-250 nun TL, 250- 300 nun TL, 300-350 nun TL, 350-400 mm TL, 400-450 mm TL, and 450-500 mm TL, respectively (Fig. 13), These adult growth rates were 300 A TL= 143.92' log.(Age+1) + 1.0938 8, TL= 114.43' log.(Age+1) + 14.921 ......-... 456 Age in Years Fig. 13. Logarithmic growth curve for humpback chub in the mainstem Colorado River in Grand Canyon (A). Hatching length of 7 mm from Muth (1990); length at 1-3 years from scale back- calculations; lengths at 50 mm increments for 4+ years from PIT tag recaptures. Growth curve for humpback chub in the LCR (B) from Minckley (1992) 50 o o (A) 2 3 7 10 8 9