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12 . Executive Summary to occupy relatively higher velocities and utilize local supplies of food. Adults in Cataract Canyon were frequently observed moving to large eddies during runoff or when drift volume was high to feed on entrained materials (Valdez 1990). Adults in postdam Grand Canyon may be largely restricted to recirculating eddies as areas of sufficient food supplies with low velocity. The character of these large recirculating eddies may be changed from predam conditions as a result of nearly 90% sediment retention in Lake Powell, and subsequent scouring of sediments from the river channel downstream of the dam. Because adult humpback chub in Grand Canyon may prefer eddies, the volume of sand in the channel. and thus in eddies, may be important for shaping fish habitat and hydraulic characteristics. These relationships and resulting effects to the fish are not well understood. Possibly, some expansion zones contain less sand today than predam and may be more effective at entraining drifting material. FOOD HABITS Adult humpback chub in the Colorado River in Grand Canyon ate primarily simuliids, introduced freshwater amphipods (Gammarus lacustris), chironomids, other aquatic invertebrates (i.e., primarily beetles) and terrestrial insects (i.e., primarily ants). The algae, Cladophora glomerata. also made up a significant portion of gut volume, but it was not determined if this item was consumed incidentally or selected by the fish and of nutritional benefit as observed for rainbow trout (Liebfried 1988). The major food items (excluding Cladophora) of the LCRI aggregation were amphipods (45% by volume) and simuliids (40%), and the major items of the Middle Granite Gorge aggregation were simuliids (49%) and terrestrial invertebrates (30%, primarily ants and beetles) (Fig. 12). Johnson's electivity indices indicate that simuliids, chironomids, and amphipods were conswned in approximately the same proportions as available in the drift in both areas, while terrestrial invertebrates were lower in numbers in drift samples than in guts. This disparity is attributed to the irregular occurrence of terrestrial invertebrates washed into the mainstem by floods; these events may not be represented by periodic drift sampling, Although Cladophora composed the greatest volume Final Report in drift, with increasingly greater amounts in a downstream direction, this algae was absent from guts of Middle Granite Gorge fish, but composed 24% of gut volume of LCRI fish. Seasonal diet of adult humpback chub was distinct. The diet in spring was primarily amphipods (51%), simuliid larvae (24%), and terrestrial invertebrates (23%); summer diet was simuliids (47%), amphipods (30%), terrestrial invertebrates (14%), andchironomids (7010); and fall diet was amphipods (44%) and simuliids (44%). The relative abundance of amphipods and chironomids in gut contents was approximately the same as respective seasonal abundances in drift. but electivity indices indicated selection for terrestrials and other aquatic invertebrates in all seasons. Terrestrial invertebrates were difficult to monitor in drift Little Colorado River Aggregation n= 128 OIh Aqua!. Terrestrial er IC Invertebrates Invertebrates 940/< 0.3% ' 0 SE=O,1 ~ SE=2.7 Gammarus 44,8% SE=5.1 Simuliids 40,3% SE=4.6 Chironomids 5.3% SE=2.1 Middle Granite Gorge Aggregation n=24 T errestria/ Invertebrates 29.6% SE=7,3 Simuliids 49.1% SE=7.5 Other Aquatic Invertebrates 6.3% SE=1.5 Gammarus 10.4% Chironomids SE=5.1 4.6% SE=1.6 Fig. 12. Volumetric composition of invertebrates (excluding Cladophora) found in stomach contents of adult humpback chub from the Little Colorado River aggregation and the Middle Granite Gorge aggregation during 1992-93.