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

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The length-weight (l-w) relationship for channel catfish demonstrated robust weight gain
(regression line slope >3.00) with length and little difference between the 1989 and 1990 curves
(Figure 11). Channel catfish in the Colorado River appear to put weight on in greater proportion to
increase in length starting at about 350 nun. Compared to Yampa River channel catfish (Nesler
1995), the l-w relationship for channel catfish in the Colorado River suggested lower weight gain per
incremental increase in length than for Yampa River channel catfish (Nesler 1995). The greatest
slope for Colorado River catfish (3.497) was less than the lowest l-w slope estimated for the Yampa
River population (3.588) during 1987-1991.

Gamefish Diet and Predation Potential
Largemouth bass, green sunfish, and black crappie had a similar body size to gape area
relationship based on the limited numbers and range of fish sizes collected in the 1991 nighttime
sampling effort (Figure 12). Assuming the cross-sectional body area ofa potential prey fish would
be equal to or less than the gape area of the fish predator, the largest largemouth bass and black
crappie examined could potentially ingest fish prey with a body area up to 700 nun2, or about 30 mrn
in diameter. If the modal size oflargemouth bass captured was approximately 110 nun in length, fish
prey with a body diameter of about 300 nun2 or about 20 mrn in diameter are vulnerable to ingestion
Based on observation ofISMP seine samples, most of the fish species captured via seine sampling
are smaller in body depth and width than 30 nun, and therefore likely vulnerable to predation by these
centrarchid species in the size ranges of centrarchids sampled. The 1990 ISMP seine data
demonstrates over 80% of 14,482 red shiner, sand shiner and fathead minnow were ~30 nun in
length, and would be potential prey (Figure 13). Since juveniles offlannelmouth sucker, bluehead
sucker, roundtail chub, Colorado pikeminnow and speckled dace appear more fusiform in shape as
opposed to deep-bodied, they would probably have to reach or exceed 75-100 nun in length to reduce
their vulnerability as prey to the median size of largemouth bass juveniles present in backwaters of
the Colorado River. This reduction in vulnerability would be due to length of prey making capture
by slightly larger bass more difficult rather than prey body depth or width exceeding bass gape size.

Stomach samples were taken from 39 largemouth bass, 119 green sunfish, 159 channel
catfish, and III black bullhead during 1987-1990. Among these four species, largemouth bass was
clearly the most piscivorous. Incidence of fish prey in the bass diet was much higher than in the diets
of the other three species. Fish prey occurred in almost 57% of the largemouth bass examined
(Figure 14). While unknown species and remains were most frequently observed, identified fish prey
included (in order of abundance) unidentified cyprinids, red shiner, fathead minnow, Gila sp.,
unidentified catostomids, sand shiner, and flannelmouth sucker. Other prey items were observed in
almost 300,10 of the bass stomachs, and included mostly aquatic insect adults, larvae and nymphs. Only
12% of bass stomachs were empty. The number offish prey items ingested per bass ranged from 1-3
and averaged 1.2.

For green sunfish, aquatic insect larvae dominated the majority of ingested prey items
identified, and non-fish prey items occurred in 86% ofall green sunfish examined (Figure 14). Fish.
and fish remains were found in only 7.4% of the green sunfish examined, and red shiner and fathead
minnow were the most frequently occurring fish prey identified. Only 7% of the green sunfish
stomachs were empty.

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