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

Adult channel catfish collected with block nets in 1988-1990 show concentrations at RMIs
175.3-175.2, 163.3, 148.5, 147-144, and 137.2-128.1 (Figure 7). Approximately 84% of the
adult channel catfish collected came from these backwater locations. The largest concentrations
ofadultswerefoundatRMIs 175.3,163.3,128.5 and 128.1. Over 50% of the total number of
adult catfish captured were collected at the latter two backwater sites. Channel catfish young were
rarely encountered, Only at RMI 135.5 in 1988 were notable numbers of young catfish collected.

Eighteen adult Colorado pikeminnow were collected in association with adult channel
catfish during block and shock sampling in 1989-1990. Pikeminnow were captured at nine
backwater sites from RMI 179.2-146.3, and were associated with catftsh collections ranging from
0-15 fish per backwater (Figure 7). Colorado pikeminnow were not collected in backwaters where
30 or more channel catfish were captured.

Length frequency distributions
Centrarchids collected from backwaters were mostly small fish, presumably young-of-the-year
and juveniles (Figure 8, Appendix Table A-I). . No age analyses were performed to associate ages
with these length frequency distributions. For largemouth bass, the bulk of the fish collected (94%)
ranged in length from 50-150 mm with a noticeable peak at 100-110 mm. All bass 180 mm and larger
were collected with block and shock sampling, and only 11 bass were collected in the 150-290 mm
size range. This length frequency distribution suggests small fish are more prevalent, but it is biased
by mismatched sampling effort between seine and block and shock sampling among years. From
Tables 1 and 4, only 16 small largemouth bass were collected by seines in 1989-1991 compared to
15 large bass collected by block net and electrofishing in the same years.

For green sunfish, a similar length frequency pattern was evident (Figure 8, App. Table A-I).
While potential for bias due to unequal sampling effort among gear types among years was possible,
Tables 1 and 4 indicate 168 green sunfish were collected by seines in 1989-1991 compared to 52 by
block net and electrofishing in the same years. Overall, most green sunfish were captured by seines
and 92% were 20-100 mm long. Green sunfish 20-40 mm long dominated the sizes of fish collected
(61 %). A few larger green sunfish ranged up to 180 mm long. Four bluegill ranged in length from
50-80 mm and 16 black crappie ranged in length from 40-190 mm.

For ictalurids, more larger adult fish were collected compared to the other gamefish species.
Seine-caught black bullheads were mostly 30-80 mm long, with most in the 40 mm size category
(Figure 9, App. Table A-I). Blocknet-caught bullheads were mostly 150-310 mm long with an even
frequency distribution. Forty-three juvenile channel catfish were collected using seines, and most
were 50-80 mm long (Figure 9, App. Table A-2). The length frequency distributions for larger
channel catfish captured using block-and-shock methods demonstrated similar peaks in the 400-450
mm size categories for both 1989 and 1990 (Figure 10, App. Table A-2). More large catfish
500-650 mm long were captured in April 1989 (n=13) compared to 1990 (n=O). A cohort of catfish
100-150 mm long was evident in 1989. It appears that this same cohort appeared in 1990 as 200-250
mm long fish.

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