9591

• UCREFRP Catalog Number: 9591
• Author: Nesler, T. P.
• Title: Interactions Between Endangered Fishes and Introduced Gamefishs in the Colorado River, Colorado, 1986-1991.
• USFW Year: 2002.
• USFW - Doc Type: 91-29,
• Copyright Material: NO

(Figure 2). In 1989, flows in March were around 2,0004,000 cfs at Cameo and Stateline gauging
stations, and spring runoff was characterized by three peaks occurring from late April to early
June at 5,000-7,000 at Cameo and slightly less or more than 8,000 cfs at Stateline. In contrast
in 1990, river flows remained around 2,000 cfs for both gauges during March and April, then
climbed rapidly to a spring peak near 10,000 cfs in early June. For both years, flows decreased
to 6,000 cfs in the second half of June and decreased below 4,000-2,000 cfs through July. In
April, water temperatures reached 14-15 C at Cameo and Stateline in 1989 and 15-18 C in 1990.
In July, water temperatures ranged from 22-27 C at Cameo and Stateline in 1989, and from 21-25
C in 1990 (Figure 3). In 1989, 466 fish were captured compared to 444 in 1990, but 60% of the
fish in backwaters were captured in April in 1989 under the early, multi-spring peak scenario and
81.5 % were captured in July in 1990 following the single, later, large spring peak scenario. More
native fishes (all four species) and common carp were captured in backwaters in Apri11989, when
water temperatures were colder, comPared to April 1990; more channel catfish were captured in
July 1990 compared to total catfish captured in 1989.

Sampling in backwaters at night in July 1991 revealed that as many largemouth bass, green
sunfish, black crappie, and black bullhead were captured in three nights of sampling at four
backwaters (51 fish) as were captured in 40 backwaters sampled during daylight in the previous
two years (54 fish) (Table 4). During July 23-26, 1991, results of larval drift samples
demonstrated that Colorado pikeminnow latvae were present at both RMI 153.8 and 134.5. These
larval pikeminnow (16) were captured in association with larval and young-of-year stages of Gila
sp. (62), speckled dace (41), bluehead sucker (19), common carp (5), and channel catfish (1).
All larval pikeminnow collected were 8.2-9.9 mm in length, while specimens of Gila sp.,
bluehead sucker, speckled dace, channel catfish were in the 10-20 mm size group. One blue head
sucker collected was in the 21-30 mm size group. No larval fish were collected from drift nets
set at RMI 147.0. Colorado pikeminnow larvae comprised 11 % of the drift population, and were
among the smallest fish in the larval drift population on those particular dates.

Gamef'lSh species distribution and relative abundance by backwater location
Collections of largemouth bass and green sunfish from backwaters in 1987-1989 indicated
areas of concentration against a background of more or less continuous occurrence in these
habitats from RMI 185-128. Also, these concentrations appeared to vary in location from year
to year for these two species. Largemouth bass were concentrated in backwaters at RMI 175,
147.5 and 131.5 in 1987, and perhaps again at RMI 131.5 in 1988 (Figure 4). No locations of
concentration could be discerned in 1989. For green sunfish, concentrations occurred in
backwaters at RMI 182.5, 174, 159.5, 159, and 147.5 in 1987, 1988, 1990 (Figure 5). No
concentration sites were evident for green sunfish in 1989.

Black bullhead collections in 1988-1990 (seine and block net data) show concentrations of
young at RMIs 154.5, 147.5 and 131.5 in 1988 and RMI 175 in 1989 (Figure 6). No
concentrations of adult bullhead were evident at any backwater locations sampled in 1990.

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