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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studies that contained information on feeding habits of introduced fishes. Many studies have
examined dietary overlap but none have concurrently identified whether food resources were
in limited supply.

Jacobi and Jacobi (1981) examined stomach contents of five introduced fishes (YOY,
juvenile, and adult) and six native fishes (YOY and juvenile) to identify dietary overlap and
possible competitive interactions. Introduced species included channel catfish, fathead
minnow, sand shiner, red shiner, and an unidentified killifish Fundulus spp. Native fishes
included Colorado squawfish, roundtail chub, possible humpback chub, tlannelmouth sucker,
bluehead sucker, and Colorado speckled dace. Food item preference was determined for
each species and whether preference was significantly different between native and other
species. There was dietary overlap for all species except adult killifish, YOY bluehead
sucker, and adult fathead minnow. Only native species consumed fish. YOY squawfish
apparently chose fish (YOY red shiner) over invertebrates.

Grabowski and Hiebert (1989) identified dietary overlap between Colorado squawfish
and many introduced fishes sampled from backwaters on the Green River. Overlap
occurred between Colorado squawfish and sand shiner, common carp, channel catfish, green
sunfish, and black bullhead. They suggested that if chironomids, the major food item,
became limited then competition could occur.

McAda and Tyus (1984) found high dietary overlap for Colorado squawfish (22-40
mm TL), small channel catfish (15-53 nun TL), and red shiner. Overlap decreased as
squawfish became larger. There was no significant overlap between squawfish and fathead
minnow. Although dietary overlap supported the hypothesis that competition was occurring,
they cautioned that food availability was not measured and it was unknown if resources were
limited. They warned that dietary overlap may have reflected common use of an abundant
food rather than competition. Tyus and Nikirk (1990) suggested negative interactions
between channel catfish and native fishes were likely due to the abundance and omnivory
of channel catfish in the Green and Yampa rivers.

Karp and Tyus (1990b) identified a potential for competitive interaction between
humpback and roundtail chub with channel catfish. They based their theory on the high
number of channel catfish in habitats used by chub and overlap of food consumed Tyus
and Karp (1989) believed dietary overlap identified by Jacobi and Jacobi (1981) and McAda
and Tyus (1984) indicated the potential for 'significant interspecific interaction between
channel catfish and Colorado squawfish.

Forage supp1emi!ntation. - The abundance of nonnative prey species in the upper basin may
provide native fishes with an unconventional source of food. Whether or not this, food
source is now actually necessary or beneficial for the native species is difficult to determine.
Colorado squawfish is the only endangered fish species that is piscivorous. Karp and Tyus
(l990a) suggested that growth of juvenile Colorado squawfish may be enhanced by an
abundance of nonnative prey fishes. YOY Colorado squawfish feed on red shiner (Jacobi