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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

in 1987 at 9 % relative abundance.

The catch rate pattern for gamefish species collected with seines per year show .the peak
in catch rate occurred in 1988 (Table 2, Figure 1.D). Gamefish catch rates were less than
1.0/loom2 in 13 of the 23 estimates, reflecting the small numbers of gamefish collected (Table
2). Catch rates for the four common gamefish species combined ranged from 3.31fish/l00m2 in
1986 to a high of 10.42 fish/100m2 in 1988 (Figure 1.D). The highest catch rates noted per
species were for largemouth bass in 1988 at 0.98 fish/100m2, green sunfish in 1990 at 4.94
fish/l00m2, black bullhead in 1988 at 4.19 fish/100m2, and channel catfish in 1990 at 1.82
fish/100m2 (Table 2). Three of 10 correlations were significant between yearly catch rates for
gamefish and Colorado pikeminnow and backwater habitat available or seine sample area (Table
3). For area seined, black bullhead catch rates were significantly and positively correlated. For
backwater area, green sunfish and channel catfish catch rate was positively and significantly
correlated. Catch rates for the gamefish species were positively correlated with both seine sample
area and backwater area available; catch rates for Colorado pikeminnow were negatively
correlated with seine sample area and backwater area available.

No correlation coefficients between specie's yearly catch rates in backwaters were
significant (Table 3). Colorado pikeminnow catch rates were negatively correlated with catch
rates for black bullhead, channel catfish and green sunfish. The correlation coefficient between
largemouth bass and Colorado pikeminnow was positive but nearly neutral (r=0.06). Channel
catfish catch rates were positively correlated with green sunfish and black bullhead.

Blocknet-electrof'lShing samples-abundance
A total of 910 fish were collected from 40 backwater habitats in four sampling trips
(Table 4). Backwaters available for large-bodied, adult fish sampling in 1989 and 1990 were
more limited than the number available for seining (1989: 9 vs 11; 1990 11 vs 14). Nonnative
fish made up 85 % of the total catch. Channel catfish made up 47 % of the total and common carp
32 % . Most of these channel catfish (85 %) were collected in the July samples for both years.
Numbers of common carp collected were nearly equal (136 vs 151) between April and July
samples. Native fish species captured included Colorado pikeminnow, roundtail chub,
flannelmouth sucker, and bluehead sucker; these species comprised 15% of the total catch in
1989-1990. Native fish were more commonly caught in April sampling (132 fish) than in July
sampling (5 fish). Nonnative gamefish species other than channel catfish made up only 5.9% of
the total catch in 1989-1990 and included largemouth bass, green sunfish, and black bullhead.
No bluegill or black crappie were captured. An observation made during electrofishing of
backwaters was that more green sunfish, and particularly larger sunfish, were collected in the upper
ends of backwaters than in the main body of the backwater. . Higher water clarity was noted in the
upper ends of backwaters and this may have attracted green sunfish.

Flow regimes are influential in the creation, persistence and physical characteristics of
backwaters, which in turn may influence fish use of these habitats according to species. Flow
conditions under which this fish sampling occurred were quite different between 1989 and 1990

9