9410

• UCREFRP Catalog Number: 9410
• Author: Wydoski, R. S. and E. J. Wick.
• Title: Ecological Value of Floodplain Habitats to Razorback Suckers in the Upper Colorado River Basin.
• USFW Year: 1998.
• USFW - Doc Type: Denver.
• Copyright Material: NO

I
I
I
I
I
I'
t
I r
I I B.
I l
I t
! t
'r
"
,I
I
I
J
I

bottom so that such ponds drain as high streamflows subside to provide
ephemeral floodplain habitats rather than permanent ones.

Four floodplain terraces and four floodplain depressions were reconnected
with the middle Green River as part of the Recovery Program Habitat
Enhancement/Restoration Subprogram. Nonnative minnows dominated the
catches of fyke/trammel net catches from these sites in 1997. Fathead
minnows composed between 14.9 and 76.2% of these catches and red shiners
accounted for 3.1 to 30.8% in the four terrace sites (G. Birchell, 1998,
personal communication). In one site, sand shiners dominated at 48.1% of
the catch but this species made up only a small portion (1.1 to 9.1%) of
the catches at the three other terrace sites. In the four floodplain
depressions that were reconnected with the middle Green River, fathead
minnows composed between 28.5 to 70.2% of fyke/trammel net catches in 1997
(G. Birchell, 1998, personal communication). Red shiners were not very
abundant in the reconnected depressions with catches that ranged between 0
and 9.5%. Green sunfish composed over one-fourth (25.2 and 26.5%) of the
catches at two depression sites but formed a relatively small portion of
the catches (2.6% and 9.8%) at the other two sites. Black bullheads
(Ictalurus melas) also made up from one-fourth to nearly one-third (24.3%
and 31.9%) at the same two sites where green sunfish were numerous but
were absent from one of the other reconnected depressions and made up 9.4%
of the catch at the remaining site. Carp composed nearly one-third of the
catch in one reconnected depression (29.3%) but were nearly absent at the
other three sites (0, 1.1, and 1.4% of the catches).

Competition. Although competition for food among larval and juvenile
fishes in the Upper Colorado River Basin has not been well documented
(Hawkins and Nesler 1991), the main reason is because competition among
freshwater fish species is often difficult to document due to the lack of
specialization in food habits by freshwater fish, resulting in much
overlap in their food habits (Larkin 1956). However, the extremely low
densities of zooplankton in backwaters during the spring runoff where
larval razorback suckers are now found and the high percentage of
nonnative fishes provides evidence that competition may also be an
important factor in the Upper Basin. Competition would probably be
reduced if productive floodplain habitats, with higher densities of
alternate food organisms occur, are reconnected with the main channels.

Beyers et al. (1994) documented in the laboratory that competition between
larval Colorado squawfish and larval fathead minnow occurred where growth
of both species was reduced through competition. Beyers et al. reported
that the "negative competitive effects were quantitatively greater and
more frequent for Colorado squawfish than for fathead minnows". Adult
nonnative minnows and juvenile razorback suckers feed on the same food
organisms so that nonnative minnows are not only predators on larval
razorback suckers but may also be important competitors with juvenile
razorbacks.

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
(Connell 1983; Crombie 1947; Elton 1946; Hardin 1960; Larkin 1956; Li and
Moyle 1993; Moyle et al. 1986; Schoener 1982; Tilman 1982; Underwood
1986). Species that are more efficient at capturing and converting food
into biomass will persist for a longer time as food resources become
scarce (Schoener 1982; Tilman 1982). Such competitive interactions can
change the structure of a fish community (Werner 1984). Density-dependent
processes are often reflected in reduced growth rather than direct
mortality in juvenile and adult fish (Schoenherr 1977) .

Dietary overlap was reported between nonnative and native fishes in the
Upper Basin (Jacobi and Jacobi 1982; Grabowski and Hiebert 1989; Muth and

25