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

Black bullhead diet was similar to that of green sunfish, with aquatic insect larvae dominating
the non-fish prey items, which were found in 55% of all bullhead stomachs examined (Figure 15).
Unidentifiable fish and fish remains were the only type offish prey observed, occurring in 14.4% of
bullhead stomachs. Almost 31 % of black bullhead stomachs were empty.

The diet of channel catfish was also similar to the diets of black bullhead and green sunfish.
Non-fish prey items dominated stomach contents of catfish at 63% frequency of occurrence (Figure
15). Plant matter and aquatic insect nymphs and larvae were the most abundant non-fish food items
observed. Fish prey occurred in 11.3% of catfish examined and over half of these fish prey were
categorized as remains or unknown. Fathead minnow, red shiner and Gila sp. were the most
frequently occurring prey species in order of relative abundance.

DISCUSSION

Objective 1: to determine trends in abundance, distribution, reproductive success, and size
structure of non-native, non-saImonid gamef"lSh populations in the Colorado
River.

Results indicate that young-juvenile nonnative centrarchid gamefish were ubiquitous in
Colorado River backwaters, but represented a minor numerical component of the fish community,
During the study period, the relative abundance of all nonnative gamefish species actually
declined in backwaters, as did the relative abundance of native fish species. Results from ISMP
data subsequent to this study period (1993,1995, 1996, 1997) demonstrate nonnative gamefish
comprised only 0.6, 0.4, 2,0, and 11.3 percent of the samples for those four years, respectively
(McAda et al. 1994, 1996, 1997, 1998). The relative abundance of gamefish species overall and
for individual species in this investigation were similar to the ISMP results for these later years.
Green sunfish were most abundant for two out of the four years, and largemouth bass or black
bullhead were the most abundant in the other two years.

From a potential impact perspective, the relative abundance of fathead minnow, red shiner
and sand shiner suggests these species should be of much greater significance, both as competitors
and predators in comparison to nonnative gamefish species. ISMP results for years outside this
study period confrrm that the relative abundance of all species in this investigation is characteristic
of the fish composition in Colorado River backwaters. ISMP results from 1993 and 1995-1997
demonstrate nonnative fish species comprised 87-96% of the fish sampled; and that red shiner,
fathead minnow and sand shiner made up 87-99 % of that nonnative fish population (McAda et al.
1994,1996, 1997, 1998). Bestgen et al. (1997) concluded "predation by red shiners may be a
major source of mortality for larval Colorado squawfish" based on model results. Conclusions from
Beyers et aI. (1994) for fathead minnow suggest competition between these species and Colorado
pikeminnow may be significant. McAda and Ryel (1999) showed catch rates of native fish species
were negatively correlated with the catch rates of red shiner, fathead minnow and sand shiner, and
positively correlated with Colorado pikeminnow. Catch rates among the three nonnative cyprinids
species were positively correlated. The authors attributed these results to the positive and negative

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