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<br />. <br /> <br />. <br /> <br />. <br /> <br />Lt. Colonel Dorothy F. Klasse <br /> <br />8 <br /> <br />nonnative species. which perhaps have been exacerbated by alterations in the <br />natural fluvial environment. Valdez (1990) reported that densities of three <br />nonnative cyprinids (red shiner. sand shiner. and fathead minnow) in the <br />Colorado River were much lower following high-flow years and increased three <br />to four times in a 2-year period during normal- and low-water years. <br />Platania (1990) noted that. during the 3 years of studies on the San Juan <br />River. spring flows and Colorado squawfish reproduction were highest in 1987. <br />He further noted catch rates for channel catfish were lowest in 1987. Recent <br />studies also found catch rates for young-of-year Colorado squawfish to be <br />highest in high water years. such as 1993 (8untjeret al. 1994. Lashmett <br />1994). <br /> <br />Biolooy <br /> <br />The life-history phases that appear to De most critical for the Colorado <br />squawfish include spawning. egg fertilization. and development of. larvae <br />through the first year of life. These phases of Colorado squawfish <br />development are tied closely to specifiC habitat requirements. Natural <br />spawning of Colorado squawfish is initiated on the descending limb of the <br />annual hydrograph as water temperatures approach 20' Celsius eC). Spawning. <br />both in the hatchery and in the field. generally occurs in a 2-month timeframe <br />between July 1 and September 1. although high flow water years may suppress <br />river temperatures and extend spawning in the natural system into September. <br />Conversely. during low flow years when the water warms earlier. spawning may <br />occur in late June. <br /> <br />A natural hydrograph with a large spring peak: a gradually <br />declining/descending limb into early summer: and low. stable flows through <br />summer. fall. and winter are thought to create the best habitat conditions for <br />endangered fishes while maintaining the integrity of the channel <br />Geomorphology. Tyus and Karp (1989) pointed out the importance of peak flows <br />(spring runoff) associated with reproductive activities of Colorado squawfish. <br />They further stated that alteration of this hydrological event may affect <br />initiation of Colorado squawfish migration and spawning. Additionally. <br />maintenance of low stable flows in summer and fall are necessary for growth <br />and survival of young Colorado squawfish. <br /> <br />Temperature also has an effect on egg development and hatching. In the <br />laboratory. egg morta1~ty was 100 percent in a controlled test at 13' C. At <br />16' to 1B' C. development of the egg is slightly retarded. but hatching success <br />and survival of larvae was higher. At 20' to 26' C. development and survival <br />through the larval stage was up to 59 percent (Hamman 1981). Juvenile <br />temperature preference tests showed that preferred temperatures ranged from <br /> <br />( <br />\ ' <br />