9413

• UCREFRP Catalog Number: 9413
• Author: Osmundson, D. B.
• Title: Flow Regimes for Restoration and Maintenance of Sufficient Habitat to Recover Endangered Razorback Sucker and Colorado Pikeminnow in the Upper Colorado River.
• USFW Year: 2001.
• USFW - Doc Type: Grand Junction.
• Copyright Material: NO

by low stable base flows. The cause-effect explanation offered by McAda and Ryel (1999)
is as follows: high, bed-mobilizing flows are needed to create and maintain ideal substrate
conditions for egg deposition and incubation and the effect of this sorting and cleaning of
the gravel-cobble substrate is carried over into the following year when moderately high
flows are then adequate to remove any additional fine sediment deposited between runoff
events; moderate flows also allow for earlier spawning, a longer first-year growing season,
and more backwater habitats for the small fish to settle in. In conclusion, for high YOY
numbers, two conditions need to be met: high larval production in summer followed by high
larval survival and retention in the nursery area until fall. This combination occurs when a
particular set of hydrological conditions are provided.
Diet, growth, body condition and carrying capacity
Once Colorado pikeminnow are over a year old they switch from a diet consisting of
invertebrates to one consisting almost entirely of fish (Vanicek and Kramer 1969, Muth and
Snyder 1995). Backwaters are good nursery habitats because they are warm, lack current,
and contain relatively abundant supplies of zooplankton, macroinvertebrates, and small-
bodied fish (Graboswski and lEebert 1989). Even after Colorado pikeminnow move to
main-channel habitats, small-bodied fish, primarily non-native minnows, provide a
substantial part of their diet until pikeminnow reach a length of at least 550 mm. Larger
individuals are thought to require larger forage items (Osmundson et al. 1998), as is the
case with other warm-water piscivores such as northern pike Esox lucius and muskellunge
E. masquinongy (Scott and Crossman 1973, Gillen et al. 1981, Diana 1987). Information
from limited stomach content data suggests that relatively large, soft-rayed, fusiform-shaped
fish are eaten, including various species of sucker. Consumed suckers were as long as 47%
of the Colorado pikeminnow's length (summarized by Osmundson et al. 1998).
Theoretically, positioning of Colorado pikeminnow within the river, at both macro-
and micro- scales, is driven primarily by growth maximization, and growth is largely
dependent on the interaction between temperature and food availability (Weatherley 1972).
Achieving maximum growth enhances the ability of the individual to survive and reproduce.
When food availability is low, growth slows and body condition declines. To the extent
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