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Colonel John N Reese DRAFT 6 squawfish, that evolved under highly fluctuating flow conditions, were better able to survive and successfully recruit under those conditions than the introduced species. The decline.of endemic Colorado River fishes seems to be partially related to competition or .other behavioral interactions with nonnative species, which perhaps have been exacerbated by alterations in the natural fluvial environment. Valdez (1990) reported that densities of three nonnative cyprinids (red shiner, sand shiner, and fathead minnow) in the Colorado River were much lower following high-flow years and increased three to four times in a two-year period during normal and low water years. Platania (1990) noted that, during the three years of studies on the San Juan River, spring flows and Colorado squawfish reproduction were highest in 1987. He further noted catch rates for channel catfish. were lowest in 1987. I'Ippendix A, Figure 20, compares the 1987 flow of record to historical conditions for dry, average, and wet years. Recent studies also found catch rates for young-of-year Colorado squawfish to be highest in the high water years, such as 1993 (Buntjer et. al. 1994, Lashmett 1994). Critical Habitat Critical habitat has been designated within the 100-year floodplain of the Colorado squawfish's historical range in the following section of the San Juan River Basin (59 F.R. 13374). New Mexico. San Juan County; and Utah. San Juan County. The San Juan River from the State Route 371 Bridge in T. 29 N., R. 13 W., section 17 to Neskahai Canyon up to the full pool elevation in the San Juan arm of lake Powell in T. 41 S., R. 11 E., section 26. BioloQY The life-history phases that appear to be most critical for the Colorado squawfish include spawning, egg fertilization, and development of larvae through the first year of life. These phases of Colorado squawfish development are tied closely to specific habitat requirements. Natural spawning of Colorado squawfish is initiated on the descending limb of the annual hydrograph as water temperatures approach 20' Celsius (C). Spawning, both in the hatchery and in the field, generally occurs in a 2-month timeframe between July 1 and September 1, although high flow water years may suppress river temperatures and extend spawning in the natural system into September. Conversely, during low flow years when the water warms earlier, spawning may occur in late June. A natural hydrograph with a large spring peak; gradually decreasing flows into early summer; and low, stable flows through summer, fall, and winter are thought to create the best habitat conditions for endangered fishes wrriie maintaining the integrity of the channel geomorphology. Tyus and Karp (1989) pOinted out the importance of peak flows.(spring runoff) associated with reproductive activities of Colorado squawfish. They further stated that alteration of this hydrological event may affect initiation of Colorado squawfish migration and spawning. Additionally, maintenance of low stable flows in summer and fall are necessary for growth and survival of young Colorado squawfish. ". ". ...."..,'