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<br />, <br />- , <br /> <br />were changed by more. stabilized seasonal discharges with rapid daily <br />fluctuations (Vanicek et ~. 1970). <br /> <br />- Concurrently, various non~natfve fishes -were intentionally or accidently <br /> <br />introduced. In 1976, the Colorado River Wildlife Council listed 20 species <br /> <br />( <br />(40%) as native to the system and 30 (60%) as introduced (Richardson 1976). <br />At the same time, Holden and Stalnaker (1975a) reported 10 native fishes <br />(34.5%) and 19 non-native (65.5%) in the upper basin. By 1982, the non- <br />native species had increased to 76% of the total of 55 fishes known in the <br />upper basin (Tyus et ~. 1982a). It is believed that competition and <br />predation by introduced fish are major factors that are adversely affecting <br /> <br />the endemic fishes. <br /> <br />Competition for Water <br /> <br />In the lower Colorado River basin, drought in the later 18005, followed by <br />prolonged flooding in the early 1900s, stimulated demands for controlling the <br />flows of the Colorado River (Pradkin 1984). Construction of Roosevelt Dam in <br />1913 on the Salt River and Hoover Dam in 1935, followed by other mainstem <br />dams, changed much of the free-flowing river to a lacustrine environment. <br />Streamflow and temperature regimes in the remaining river were greatly <br />altered. These events were closely followed by declines in the fish'es in the <br />lower bas in. The Colorado squawfi sh was ext i rpated, and bonyta il, humpback <br />chub, and razorback sucker were drastically reduced in numbers and <br />distribution. Only large, old adults of bony tail and razorback sucker were <br />still found in reservoirs of the lower basin by the 1980s (Minckley et sl., <br />Chapter 17). A viable population of humpback chub remained in the Little <br />