2013-03-06_REVISION - C1992081 (6)

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9 Colorado squawfish in the 10 -mile reach immediately downstream of the confluence of the Gunnison River (river miles 160 -170) warranted classification of this reach as a "Young -of -Year Nursery Area" by the Basin Biology Subcommittee (U.S. Fish and Wildlife Service, 1984). Nonsspawning Adult Occurrence Although the sampling effort was similar between the reaches upstream and downstream of the Gunnison River, over 60 percent of the adult -size Colorado squawfish collected during spring surveys by the Fishery Project (Miller et al. 1982; Kaeding and Osmundson 1987) came from the upstream 15 -mile reach. Kaeding and Osmundson (1987) reported that adult Colorado squawfish catch rates in the upstream 15 -mile reach were twice as high as those in the adjacent downstream river reach. Adults were most abundant in a 0.6 -mile segment (river miles 174.4 - 215.0) of the 15 -mile reach during high water, particularly in two gravel -pit ponds that were accessible during high flows. These fish may have moved into these ponds to feed and rest, or they may have been attracted to the warm, productive environments that the ponds provided (pond temperatures were as much as 10.5 °C warmer than the adjacent river). Some of the squawfish captured from one pond were well tuberculated by June 3, when nearby river temperatures were only 10 °C -13 °C (Kaeding, pers. comm.). It has been hypothesized by some investigators that thermal energy units above those provided in the mainstream are important to gonadal maturation. If this is true, then access to these sheltered off - channel pools may be very important to successful spawning in the upper reaches of the Colorado River. The 15 -mile reach has experienced major agricultural water depletions for many years. During late summer and early fall, this reach can be severely dewatered. Water depletions in the 15 -mile reach have been identified as a limiting factor for Colorado squawfish. Therefore, the Service has recommended a flow of 700 -1200 cfs during July, August, and September (Osmundson and Kaeding 1989). The objectives of these flows are to provide near maximum aggregate amount of run, pool, and riffle habitat for adult Colorado squawfish and to enhance the first -year growth of age -0 Colorado squawfish in the Grand Valley area by increasing water temperatures. Flow recommendations for the rest of the year call for high spring flows that are critical for shaping the river channel, determining substrate composition, and influencing the abundance of various species for the remainder of the year. Flow recommendations for winter call for flows between 1,000 and 2,000 cfs (no change from current flows). Razorback Sucker The razorback sucker, an endemic species unique to the Colorado River Basin, was historically abundant and widely distributed within warmwater reaches throughout the Colorado River Basin. Historically, razorback suckers were found in the main stem Colorado River and major tributaries in Arizona, California, Colorado, Nevada, New Mexico, Utah, Wyoming, and in Mexico (Ellis 1914; Minckley 1983). Bestgen (1990) reported that this species was once so numerous that it was commonly used as food by early settlers and, further, that commercially marketable quantities were caught in Arizona as recently