9406

• UCREFRP Catalog Number: 9406
• Author: Osmundson, D. B.
• Title: Importance of the '15-Mile Reach' to Colorado River Populations of Endangered Colorado Pikeminnow and Razorback Sucker.
• USFW Year: 2000.
• USFW - Doc Type: Grand Junction.
• Copyright Material: NO

Electrofishing surveys between the
Price Stubb Dam and Rifle, Colorado dem-
onstrate that an abundance of native suckers
and chubs live in this reach and could serve
as forage for adult Colorado pikeminnow
(Wydoski 1994, Anderson 1997, Osmund-
son 1999a). However, because of progres-
sively cooler water temperatures, the mar-
gin of the historic range for these warm-
water species occurs somewhere within
these upstream reaches. Upstream of Pali-
sade, a transition zone begins between a
downstream warm-water fish community,
historically dominated by chubs, native
suckers, and Colorado pikeminnow, and an
upstream cold-water fish community, domi-
nated by salmonid species such as trout and
mountain whitefish. How the endangered
fish utilize this reach once access is again
provided remains to be seen. In the interim,
predicting habitat suitability for these fish
requires an evaluation of the thermal re-
quirements of the species and the thermal
regime of the reach.
Relationship Between Temperature and
Fish Distribution
Positioning of adults, both locally and
along the length of the river, is likely driven
by growth maximization (Hughes 1998),
and growth is largely dependent on the
interaction of food availability and water
temperature (Weatherley 1972). Achieving
maximum growth potential enhances the
ability of the individual to survive and re-
produce successfully. In many fish species,
large females produce more and larger eggs,
thereby enhancing larval survival (Montele-
one and Houde 1990; Brandt and Kirsch
1993). Although temperatures in the
Palisade-to-Rifle reach are only a few de-
grees centigrade (C) lower than in the
Grand Valley on any given day, this differ-
ence can result in a significant reduction in
thermal units needed for fish growth over
the course of a year.
Controlled laboratory experiments have
shown that growth rate of yearling Colo-
rado pikeminnow is maximized at 25°C. At
20°C, growth rate is 54% of the maximum;
at 15'C, growth is only 18% (Black and
Bulkley 1985a); at 13 ° C, Colorado pike-
minnow cease to grow (Kaeding and Os-
mundson 1988). Other lab studies demon-
strated that 25°C is the preferred tempera-
ture of yearling and subadult pikeminnow
(Black and Bulkley 1985b) and preferred
temperature generally corresponds to the
optimum temperature for many physiologi-
cal processes including growth (e.g. Mag-
nuson et al. 1979). To reveal the effect of
low temperature on habitat suitability,
annual, accumulated thermal units available
for growth can be calculated for various
reaches of river. For comparative purposes,
Kaeding and Osmundson (1988) developed
an index whereby mean daily temperatures
are converted to values relative to the maxi-
mum potential (1.0) for growth at the opti-
mum temperature (25 ° Q. These thermal
units can then be summed to provide an
annual value for a specific location.
Using multi-year data from a series of
thermographs located at various sites along
the river (see Fig. 1), Osmundson (1999a)
converted daily temperatures to thermal
units for Colorado pikeminnow growth.
Average annual thermal units (ATU) at
Palisade were 79% of the average at the
Colorado/Utah state line (about 48 miles
downstream). At Cameo, the average was
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