9375

• UCREFRP Catalog Number: 9375
• Author: Kimball, J. F.
• Title: Flow Effects on Humpback Chub ‎(Gila cypha)‎ in Westwater Canyon.
• USFW Year: 1999.
• USFW - Doc Type: Salt Lake City.
• Copyright Material: NO

(n=2; 20mm TL) collected on that date were estimated to have been spawned on 25 May when flows at
the Stateline gage were 35,700 cfs and main channel temperatures were 14°C. This early pulse of
spawning continued through the peak flow of 44,000 cfs, recorded 28 May 1993, and sporadically on to
06 June as flows dropped to 27,000 cfs. Based on the flows and temperatures at the time of this early
spawning in 1993 and the trends established throughout this study, it seems unlikely that the 24 YOY
collected on 25 June were spawned in the main channel. Perhaps these fish were produced in a tributary
stream or intermittent wash, many of which were running in the immediate area.
Growth of Age 0 chubs
Hamman (1982) reports newly hatched larvae (6.3 to 7.5 mm) growing to 20 mm during their
first month. He also reports larvae size increasing 5-7 fold during 56 days of culture at 12.8-25.5°C.
Growth rates of Age 0 chubs collected in this study were calculated for the months of July and August __
and found to be positively correlated with the degree days > 20°C for each respective month. A lowest
daily growth rate recorded during the present study, O.15m/day, occurred during August 1995 while
flows averaged 8365 cfs and temperatures were warming to an average 22°C. The greatest growth rate
(0.83 mm/day) occurred in August 1994 while flows in Westwater Canyon averaged 2742 cfs and main
channel temperature averaged 25.9°C and were in the process of cooling.
A length frequency analysis of two collections of YOY chubs from Desolation Canyon of the
Green River in 1993 indicated a growth rate there of 0.36 mm/day (Chart and Lentsch 1998). Day and
Crosby (1997) reported excellent, long term conditions for YOY chub growth in Desolation Canyon, in
1994. In that study, a strong year class of YOY chubs had grown to 60.8 mm (avg. TL) by rnid-
September 1994.
Year Class Strength and Flow
YOY chub catch rates were significantly greater in and above Westwater Canyon than
immediately downstream of the canyon. Sampling for adult chubs in the lower portion of Westwater
Canyon in 1992 revealed low densities of roundtail chubs and no humpback chubs. Unlike Colorado
pikeminnow and razorback sucker, there is no indication that humpback chub larvae drift long distances.
Upon emergence from gravels, humpback chub larvae swim in an upstream direction and adhere to near
bottom surfaces (Marsh 1985). We assume roundtail chub larvae behave similarly. We relegated much
of our flow analysis to the above Canyon sub-reach (roundtails present) and Westwater Canyon sub-
reach (both roundtail and humpback chub present). Our feeling is that the YOY chubs were collected in
the vicinity of where they were spawned.
Densities of YOY chubs (as measured by # of fish / m2 of seined habitat) varied greatly
throughout the study. The greatest density of YOY chubs (2.28 / mz) was observed in the above canyon
sub-reach on 02 August 93. In Westwater Canyon, the greatest catch rate (1.42 YOY chubs / m2) was
recorded on 15 August 96. Annual average catch rates were extremely high above the canyon in 1993
(0.679) and 1996 (0.607). Catch rates within Westwater Canyon were also very high in 1996 (0.673), but
not in 1993 (0.086). During early investigations of the Westwater fish community, Valdez et al. (1982}
report combined YOY Gila spp. catch rates of 0.127, which is within the range of catch rates reported in
the present study (see Table 4). Young of year sampling in Desolation Canyon, 1992 - 1996, where chub
populations densities are lower than Westwater, revealed a project high density of 0.09 YOY chub / m2
in 1993 (Chart and Lentsch 1997). Day and Crosby (1997) recorded their highest YOY chub catch rate,
21.38 / 100 m', in the summer of 1994.
YOY densities in Westwater Canyon rival those reported recently for the humpback chub
population in the Grand Canyon. In their study of the ecology of Grand Canyon backwaters, Arizona
Game and Fish Department (1996) reports densities as high as 0.24 YOY humpback / mZ in the vicinity
of the Little Colorado River.
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