7996

• UCREFRP Catalog Number: 7996
• Author: Wydoski, R. S. and E. D. Wick.
• Title: Ecological Value of Flooded Bottomland Habitats to Endangered Fishes in the Upper Colorado River Basin.
• USFW Year: 1996.
• USFW - Doc Type: \
• Copyright Material: NO

predation at that time by minnows since other food organisms are scarce to the
abundant nonnative minnows.

Potential Role of Comoetition on Survival of Fish Larvae. Although competition
for food among larval and juvenile fishes in the Upper Colorado River Basin has
not been documented (Hawkins and Nesler 1991), documentation of competition is
difficult because proof is often lacking that food resources are limiting fish
numbers.

Competition 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 et al. 1986;
Li and Moyle 1993). Low numbers of zooplankton occur in the main channel and
backwaters of rivers (i.e., food is limited for early life stages of fish) in the
Upper Colorado River Basin (Tables 1 and 2; Cooper and Severn 1994a, b, c, d;
Grabowski and Hiebert 1989; Mabey and Schiozawa 1993). Dietary overlap was
reported between nonnative and native fishes (i.e., food is shared) in the Upper
Colorado River (Jacobi and Jacobi 1981; Grabowski and Hiebert 1989; Muth and
Snyder 1995). Nonnative fishes expanded rapidly in the Upper Basin between 1986
and 1994 as indicated from the Interagency Standardized Monitoring Program where
they constitute 96.7 to 99.6% of the tot a 1 numbers of fi sh collected from
backwaters by seining (McAda et al. 1994a, 1994b, 1995). Native and nonnative
fish share the same backwater habitats. Although direct competition among
endangered fi shes by nonnat i ve fi shes has not been documented, 1 i mited food
resources, sharing of the food resources, and use of the same habitats with low
water velocity supports the belief that competition may have contributed to the
decline of some native Colorado River fishes. Red shiners and fathead minnows
were found to constitute 90.4% of 149,489 fish collected by seines between 1986
and 1994 from primary backwaters of the Colorado and Green rivers during the
Interagency Standardized Monitoring Program (McAda et al. 1994a, 1994b, 1995).
These minnows feed on the same foods items that are used by juvenile endangered
fishes (Grabowski and Hiebert 1989; Jacobi and Jacobi 1982) and, more than
likely, compete with the endangered fishes -- particularly during the early
spring when food organisms are scarce in the high and turbid upper basin rivers.

Sand shiners (Notroois stramineus) are also abundant in the backwaters of the
Colorado and Green rivers (McAda et al. 1994a, 1994b, 1995) but this species is
not been reported to feed on fish larvae. Sand shiners are generalists that feed
on a variety of aquatic or terrestrial insects and crustaceans (Muth and Snyder
1995). Although sand shiners may not be predators upon the endangered Colorado
River fishes, they could be potential competitors with them.

XI. LOSS OF CRITICAL LOW WATER VELOCITY HABITATS
SUCH AS
FLOODED BOTTOM LANDS IN THE UPPER COLORADO RIVER BASIN

Flooded bottomland habitats occurred in low gradient river reaches with
unrestricted meanders that occur in broad valleys. In the Upper Colorado River
Basin, these reaches have been channelized by levee development to contain the
river near population centers and agricultural areas (Bestgen 1990; Fradkin 1983;
Quartarone 1993). It is hypothesized that the loss of flooded bottomlands is one
factor has adversely affected the food base required by larval endangered fishes.
The lack of recruitment of the endangered Colorado River fishes is also linked

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