2004-06-29_PERMIT FILE - C1981038 (3)

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s~ had reached 20° C and durine a time when runoff flows were dropping off sharply. A second agsregation was noted at river mile 17.3, 12 days after the initial observation. Driftine trammel nets throush an area occupied by nvo fish equipped with transmitters yielded an additional male Colorado pikeminnow in spawning condition. During this same time period, an adult female was captured near river mile 175 that weiehed nearly 1 pound more than when previously captured a month eazlier, suggesting the development of spawning (gravid) condition. Larval Occurrence Fishery Project studies included the routine sampling of the larval-fish community both within and downstream of the 15-mile reach. During 5 years of investigation, 701arva1 pikeminnow were collected with fine-mesh hand nets from the two Colorado River reaches in the Grand Valley immediately upstream and downstream of its confluence with the Gunnison River. Although the sampling effort was similar in the two river reaches, 96 percent of the larval captures occurred downstream of the Gunnison River confluence (river miles 162-164). Only two (3 percent) of the larvae were collected from the upstream reach. These observations may indicate that most fish were spawned in the downstream reach or that the larvae were deposited in the upstream reach and drifted downstream to the azea where most of the captures were recorded. Postlarval Young-of--Year Occurrence No postlarval young-of-yeaz Colorado pikeminnow greater than 25 mm total length were collected from above the Gunnison River confluence in a total of 57 samples collected in the fall of 1982-1986. However, a total of 62 Colorado pikeminnow were collected in an 18-mile reach below the confluence of the Gunnison River (54 samples). The 1982-1984 catch rate of young-of--year Colorado pikeminnow 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--Rear Nursery Area" by the Basin Biology Subcommittee (U.S. Fish and Wildlife Service 1984). Nonspawning,Adult Occurrence Osmundson and Kaeding (1989) reported that adult Colorado pikeminnow catch rates in the upstream 15-mile reach were twice as high as those in the adjacent downstream river reach. During 1986-1989 adults were most abundant in a 1.3-mile segment (river miles 174.4-175.7) 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 pikeminnow 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 aze important to gonadal