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

70
60
Q 50
c
40
30
20
133 182 200 230 93 117
River Mile
Figure 6. Mean annual thermal units (ATU) available for Colorado pikeminnow growth at
four locations on the Colorado River and two on the Yampa River for comparison. Dashed
horizontal lines indicate sites on each river with similar thermal regimes. River mile (rm) 133
is near the Colorado/Utah state line; rm 182 is 3 miles downstream of the top of the 15-mile
reach; rm 200 is 7 mules upstream of Cameo; rm 230 is at Rulison. On the Yampa River, rm
93 is at Juniper Springs (the Yampa reach where densities of Colorado pikemmnow are
higghest); rm 117 is near Sheep Creek (the site of Nesler's [1995] most upstream Colorado
pikeminnow capture). Data are from Osmundson (1999a).
only 67% of that at the state line, and at
Rulison, only 52% (Fig. 6).
Although abundant forage may compen-
sate for slow growth caused by low tempera-
tures, there is a point at which temperatures
are so limited that additional food cannot
make up for the temperature shortfall and
growth slows. What defines the upstream
range of this species is likely a temperature
threshold, below which annual growth is
insufficient. Colorado pikeminnow can likely
gauge about where this minimum thermal
regime occurs and individual home ranges
are therefore established no further upstream
of this point.
In the Yampa River, fish have unre-
stricted upstream access. Nester (1995)
found adult Colorado pikeminnow in the
Yampa as far upstream as river mile (rm)
117. This location has an average ATU
value of about 41. On the Colorado River,
the location with the same mean ATU
occurs at approximately rm 199, or about
14 miles upstream of Palisade. Using the
Yampa River as a model, Osmundson
(1999a) suggested that Colorado pikemin-
now in the upper Colorado River should be
able to establish home ranges upstream
almost as far as De Beque, Colorado, once
passage is provided. Also, the thermal
regime in the reach where Colorado pike-
minnow were most abundant in the Yampa
River averaged 46.5 ATU (rm 93). The
analogue to this site in the Colorado River
10