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~. f!.~ I " [,','1 GRAND CANYON NATIVE FlSH MANAGEMENT 3 ~: 'i , physical characteristics .in the downstream river, further disrupting the seasonality of environmental conditions by reducing the annual range and mean of water temperatures. , These changes modified the temporal pattern of environmental conditions in the post-dam river, altering linkages between the native fishes and their habitats. . Perennial low water temperatures from deep reservoir reIeases limited or eliminated successfuI reproduction and decreased growth rates of native fishes in the CoIoradoRiver. The physical barrier presented by Glen Canyon Dam restricted their seasonal movements and migrations. Food sources and feeding relationships in the mainstem were altered following near cessation of particuIate organic matter transported from the Upper Colorado River Basin. In combination, these changes altered relationships between the mainstem and its tributaries, both of which are important in the life histories of the native fishes. , ,<~ ~.~ Populations of most non-native fishes .present in the mainstem prior to 1963 persisted following closure of the dam, although distributions and abundances of some species were altered. . The non-native assemblage was increased by massive new sportfish stocking programs initiated in Lake Powell and in the taiIwater below the dam to exploit the recreational opportunities in newly created systems both upstream and downstream. These actions were taken with little regard for their impacts on native fishes. f~; ~-, ." ~.Ai ~:~ '~" The endangered CoIorado squawfish (Ptychocheilus lucius) and bonytail chub (Gila elegans), and the roundtaiI chub (G; robusla) were eliminated from Grand Cariyon within a decade of thecIosure of GIen Canyon Dam. The endangered humpback chub (G. cypha) reproduces only in a single tributary, and its mainstem distribution has contracted. Another endangered species, razorbac~ sucker (Xyrauchen lexanus), is extremeIy rare in Grand Canyon (although pOsSibIy never abundant), with no current evidence of reproduction:-Successful mainstem reproduction by the remaining, more common native fishes, also was reduced or eliminated. .-: ;,.'~,. . RECOMMENDATIONS \ -~ -;' >'\." hJ,~ ~~ :~~ ~~ .... ,:;" tJ~. ~~;.:--,....'::r~:'-;" r. .::-,~ "_; .,:" 1" _)':<:':.' -., ..~' ".'-~ -. ~ ,.~ . I Recorirmendefl' HydioliJgy in the Absenceo! Temperature Modification-We contend that an approximation of the hydroIogical pattern under which the native CoIorado River fishes evoIved (Figure 1)' is one of the: major prerequisites for successfuI maintenance and enhancement of these populations in Grand Canyon. This contention is a tenet of the flood puISi concept (Junk et al; 1989), which was developed to expIain. adaptations of aquatic and tCirestrial faunas in Iarge floodplain rivers. We argue that these adaptations transcend. the geOinoIphic. differences between classical floodplain rivers and the canyon-bound Colorado River. In the U.S., reestablishment of !laturaI hydrological patterns has been advocated for \ .~.