9404 - Laserfiche WebLink

Home Browse Search

also Minckley, 1983). They were also eliminated from the Green River above the mouth of the Yampa River when Flaming Gorge Dam became operational in 1962 (Vanicek et al., 1970; Fig. 1--2a). Glen Canyon Dam.---The operation of Glen Canyon Dam precipitated major changes in the Marble/Grand Canyon ecosystem of the Colorado River (Marzolf, 1991:33). Some occurred immediately upon closure of the dam in 1963 (e.g., decreased water temperatures; reduced sediment loads; diminished salinity; alteration of flow regimes). Others developed over a much longer time frame (e.g., geomorphic adjustment of channel; secondary succession of terrestrial vegetation; modification of aquatic species- composition) (Committee, 1991). All have severely impacted the natural ecosystem; some are irreversible. Indigenous fishes inhabiting Glen, Marble and Grand canyons were impacted following closure of Glen Canyon Dam (Holden and Stalnaker, 1975; Suttkus and Clemmer, 1977; Minckley, 1991). Many (including G. cypha: Holden and Stalnaker, 1975; Anonymous, 1980) persisted in Lake Powell, but were unable to reproduce (Holden, 1973:4). Downstream from the dam, the fish community shifted from predominantly warm-water native and introduced fishes to one dominated by either cold-water fishes [i.e., rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta)] or those with broad temperature tolerances. Within GCNP, five of eight indigenous fishes still persist in low to moderate numbers. These are usually restricted to warmer habitats such as tributaries and backwaters. While terrestrial species in GCNP adapted to the post-dam Colorado River ecosystem (Carothers and Brown 1991:147; Johnson, 1991), indigenous fishes generally did not (Kaeding and Zimmerman, 1983:592). The Little Colorado River as habitat.---Temperataure and flow conditions in the LCR are similar to those of the pre-dam Colorado mainstem, and thus suit habitat requirements of indigenous fishes shaped over evolutionary time. Kaeding and Zimmerman (1983) argued that G. cypha persisted within the Canyon, whereas other endemics were eliminated, because a portion of its population spawned within the LCR. They also argued that, given post-dam temperature disparities between LCR and mainstem, significant reproductive success for G. cypha must occur within t`-?e LCR. Thus, selection should be strong for development of a spawning migration (Kaeding and Zimmerman, 1983). Critical though these observations are to the ecology and conservation of G. cypha, they have yet to be substantiated. While data presented herein do not address movements of G. cypha from the mainstem into the LCR, they do suggest that staging occurs at the confluence. Our data do demonstrate that adult G. cypha actively move up the LCR in spring (primarily to reproduce), and often remain within the LCR for long periods, possibly the entire year. These observations are based both on monthly population estimates by reach, and on seasonal recaptures of tagged G. cypha. Before 8