7025

• UCREFRP Catalog Number: 7025
• Author: Hawkins, J. A. and T. P. Nesler.
• Title: Nonnative Fishes in the Upper Colorado River Basin
• USFW Year: 1991.
• USFW - Doc Type: An Issue Paper.
• Copyright Material: YES

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Osmundson 1988; Lanigan and Tyus 1989; Marsh and Minckley 1989; Osmundson and
Kaeding 1989b). McAda and Wydoski (1980) attributed reproductive failure as the major
factor in the decline of razorback sucker. Lanigan and Tyus (1989) hypothesized that the
lack of recruitment in the Green River was due to predation on young razorback sucker by
introduced fishes. Kaeding and Osmundson (1988) suggested that Colorado squawfish were
vulnerable to increased early life mortality by introduced fishes because of their unusually
slow growth in the upper basin. If razorback sucker spawn in gravel pits connected to the
mainstream Colorado River, Osmundson and Kaeding (1989a; 1991) predicted young would
be subject to predation from common carp, green sunfish and largemouth bass. Osmundson
and Kaeding (1989a) also suggested that predatory introduced fishes "probably have
important negative effects on the endangered fishes in the 15-mile [24 Ian] reach and
elsewhere in the Colorado River", but which introduced fishes were a problem was unknown.
'Common carp, the most abundant nonnative fish in Dinosaur National Monument, was
suspected of having a negative impact on native fishes by egg predation (Karp and Tyus
1990b ).

Predation on endangered fishes has been documented in the upper and lower basins.
Coon (1965) found Colorado squawfish in two of 58 stomach samples of channel catfish
collected from the Dolores River in 19.63. Catfish sizes ranged 130-452 mm TL; prey sizes
were not reported. Taba et al. (1965) found Colorado squawfish and Gila spp. in the
stomachs of black bullhead from the Colorado River near Moab, Utah.

Other native species are also vulnerable to the effects of predation. Remains of
humpback chub have been found in stomachs of channel catfish collected from the Little
Colorado River (W. L Minckley, personal communication in Karp and Tyus 199Gb).
Crescent shaped wounds were found on some humpback chub from the Little Colorado
River, Arizona and were attributed to channel catfish; suggesting failed attempts by channel
catfish to prey on humpback chub (Kaeding and Zimmerman 1983). Similar bite-like marks
were observed on humpback chub collected in Dinosaur National Monument and were
attributed to channel catfish, the only piscivorous fish capable of such marks (Karp and Tyus
1990b).

Nonnative predator fishes were implicated in the decline of roundtail chub in the Gila
River Drainage, New Mexico (Best~en and Propst 1989). Deleterious effects were inferred
by past and current distributional patterns of roundtail chub and introduced fishes.
Roundtail chub, once common in the study area, were apparently displaced by smallmouth
bass that were common. Small mouth bass were restricted from further upstream distribution
in one tributary creek by barrier falls. Although rarely found below the falls, large roundtail
chub populations were abundant above the barrier falls where bass were absent. Floods in
1983 and 1984 severely reduced smallmouth bass and channel catfish populations, concurrent
with an increased abundance of native fishes in 1985.

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Several studies attempted to document predation by examining stomach contents of
predacious nonnative fishes. Most of these have been unable to provide evidence of

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