8272

• UCREFRP Catalog Number: 8272
• Author: Wydoski, R. S. and E. J. Wick.
• Title: Flooding and Aquatic Ecosytems.
• USFW Year: 2000.
• USFW - Doc Type: 238-268, Chapter 9
• Copyright Material: YES

256

Flooding and Aquatic Ecosystems

Table 9.7. Mean number of zooplankton
(cladocerans and copepods) from various habitats
along the Green River, upper Colorado River basin'

Habitat

Range of mean number of organisms
per li ter

Main channel t
Backwaters
Floodplain habitats

0-1.3
0-13.1
4.2-81.5

'Mean numbers of zooplankton were obtained from the fol-
lowing studies: Grabowski and Hiebert (1989); Cooper and
Severn (1994a, 1994b, 1994c, 1994d) Mabey and Schiozawa
(1993). Number of samples taken, sampling gear used, sea-
sons (late spring and summer), and years of sampling dif-
fered so ranges were used to illustrate relative productivity
of zooplankton.
tMost (Four) investigations were made on the Green River.
However, the data include one investigation on the Colorado
River and one investigation on the Gunnison River, a major
tributary of the Colorado River.

season despite the presence of large numbers of nonnative fishes (631,760
in 1995 and 548,970 in 1996). Over half ofthese nonnative fish consisted of
common carp (Cyprinus carpio), fathead minnow (Pimephales promelas),
red shiner (Cyprinella lutrensis), and green sunfish (Lepomis cyanellus).
Peak zooplankton densities in the wetland produced 43 organisms per liter
of water in 1995 and 54 organisms per liter in 1996 (Modde, 1997). This
density was within the range required for good survival of razorback sucker
larvae (Figure 9.3). The presence of emergent aquatic vegetative cover in
this wetland likely increased survival of the YOY razorback suckers by
lowering predation by green sunfish, adult red shiners and adult fathead
minnows and/or preventing competition by juvenile carp, fathead min-
nows, and red shiners. Larval razorback suckers could feed on zooplank-
ton with less risk of predation by nonnative fishes in vegetative cover than
in open-water habitats. Survival of larval and juvenile razorback suckers
in isolated backwaters along Lake Mohave. in the lower Colorado River
basin was also attributed to dense vegetation (Minckley et al., 1991).

Predation and Competition by Nonnative Fishes
on Endangered Fishes

Nonnative fish species compose 76%(42 of 55 species) of the fish com-
munity in the Upper Basin (Tyus et al., 1982). In addition, the abundance
of nonnative fishes in backwater habitats constitute most of the fish com-
munity (96.7 to 99.6%) in the Upper Basin (McAda et al., 1994a, 1994b,
1995).
Predation on larval fish is a significant factor that decreases their sur-
vival (Leggett, 1986). Minckley et al. (1991) considered predation by non-
native fishes as the single most important factor in recruitment failure of
razorback suckers in Lake Mohave of the lower Colorado River basin. Com-
petition by two species occurs when food is limited, the food is shared,
and one of the two species is adversely affected by sharing food (Moyle