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GRAND CANYON NATIVE FISH MANAGEMENT 15 survival to sexual maturity (Kaeding and Osmundson 1988), reduced condition, lipid stores, and size that result in elevated overwinter mortality for young-of-year fishes (Thompson et aI. 199I), Iowered egg production by adults (McAcIa and Wydoski 1983), and reduced survivaI of developing embryos (Marsh 1985). " , As a result of post-clam hydrology, temperature, and sediment changes, major trophic shifts have occurred in the mainstem Colorado River. Standing crops of attached aIgae in the relatively stabIe, clear, armored, and nutrient-rich tailwater increased relative to pre-dam conditions (McConnell and SigIer 1959, Zimmerman and Ward I984). Sediment inputs from the Paria River, 25 Ian downstream from Glen Canyon Dam, abruptly reduce standing biomass of the periphyton community (Angradi et aI. 1992, Usher et aI. 1987, Blinn et aI. 1992). .':.,;.. ,-' .~; :0:: High.. densities and biomass of invertebrate populations contrast with low species diversity in the Glen Canyon Dam tailwater (Leibfried and Blinn 1986). Thermal constancy and low absolute water temperatures disrupt cues necessary for the compIetion of life cycles of many macroinvertebrates (Ward 1976, Hauer and Stanford 1982). The transitory character of nearshore habitats has rendered them largely unsuitabIe for coIonization by desiccant-intoIerant algae and invertebrates (Usher and Blinn 1990, Angradi et aI. 1992). ~i ( I';. . ~. C _~-.J-:.. Disturbances to pre-dam habitats and fluviaI processes have been detrimental to most native. and non-native fishes. Native forms, however, fared the worst; three species were extirpated and others are threatened. with extirpation. As forcefully argued by Minckley (1991), introductions of non-native fishes to the Colorado River Basilt is the most. critical factor . affecting the Iong-term status of native form.s. The canyon now supports populations of at least 10 non-native fishes (Maddux et aI. 1987, VaIdez et aI. 1992) and several non-native macroinvertebrate taxa (Blinn and CoIe 1991), the latter including the fish parasites Lenuzea cyprinocea (Carothers et aI. 1981) and Bothriocephalus achielognathi (Angradi et aI, 1992, Clarkson 1992, Clarkson and Robinson 1993). The sometimes subtle ways that non-native species may displace and eliminate native forms through competition and preclation were discussed by Minckley (1991) Ruppert et aI, (1993), and DougIaset aI. (1994). . ;..,. ~,- '':~ > ! :-'.( ?'~' ~ .:?:5!1iL.":~. . ;i,' .;~ ~~' ;;J ~~ :~~~ :.-. LegQl History-Prior to construction of Glen Canyon Dam, evaluation. of its Potential impacts on fishery resources was limited to expectations for sport fisheries development in Lake Powell and in the downstream tailwater (U .S. Bureau of Sport Fisheries and Wildlife 1958). Native -suckers and the CoIorado squawfish were mentioned only in passing, even though the latter species and razorback sucker already were decIining rapidIy in the lower CoIorado River basin (Minckley 1973). The three chubs (bonytail, humpback, and roundtail) were not cOnsidered,' perhaps in. part because of the co,nfusion surrounding their taxonomic differentiation (Holden and Stalnaker 19~. . ";""