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5 <br />were primarily confined to the cold water area between Hoover Dam and <br />Eldorado Canyon (Jonez and Sumner 1954). <br />Hoover Dam was frequently operated from the upper intake gates <br />(1045 ft. elevation ) until 1954. Discharge temperatures were lower than <br />historic river temperatures, but seasonal temperature cycles were still <br />maintained in the river (Baker and Paulson 1980). Since 1954, Hoover Dam <br />has been operated continuously from the lower intake gates (880 ft. <br />elevation), and discharge temperatures remain 12-13°C year-round. <br />Aquatic insects have disappeared from the river since the Jonez and <br />Sumner (1954) investigations (Paulson et al. 1980a). This apparently <br />resulted from the loss of breeding habitats (Jonez and Sumner 1954), but <br />was probably also affected by the lack of seasonal temperature cycles in <br />the river. These often are required to trigger reproduction of many <br />aquatic insects (Paulson et al. 1980a). Oligochaetes, midges, and <br />amphipods now comprise the dominant invertebrate fauna in Black Canyon <br />(Bryant 1977; Paulson et al. 1980a). <br />The physical and biological features of the Colorado River between <br />Hoover Dam and Davis Dam have been changed considerably since the <br />1930's. The changes in habitat, trophic interrelationships, and species <br />interactions could directly or indirectly influence the abundance of <br />bonytail chubs and razorback suckers in Lake Mohave. In this study, we <br />attempt to evaluate how changes in temperature regimes, habitat, and <br />predation could be affecting reproductive success of the native species. <br />2.0 DESCRIPTION OF THE STUDY AREA <br />Lake Mohave was formed in 1951 by the construction of Davis Dam. <br />The reservoir extends 108 km south from Hoover Dam to Davis Dam. The