9433

• UCREFRP Catalog Number: 9433
• Author: Anderson, R.
• Title: Riverine Fish Flow Investigations.
• USFW Year: 2002.
• USFW - Doc Type: Federal Aid Project F-288-R5,
• Copyright Material: NO

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Table 7. Species composition for fish Over and Under 15 em at the Big Gypsum site on the
Dolores River, July 2001.

Big Gypsum Big Gypsum Big Gypsum Big Gypsum
2001 2000 2001 2000
Species > 15 em > 15 em <15 em <15 em
Flannelmouth Sucker 57.5% 16.0% 9.9% 5.2%
Bluehead Sucker 5.8% 2.2% 14.2% 0.0%
Roundtail Chub 24.5% 54.9% 16.5% 48.0%
Channel Catfish 8.3% 15.8% 0.4% 1.4%
Carp 1.7% 3.4% 0.0% 0.2%
Green Sunfish 1.4% 2.0% 1.5% 5.7%
Pumpkinseed 0.0% 0.5%
Brown trout 0.6%
Black Bullhead 0.6% 5.2% 0.5% 0.2%
Speckled Dace 0.0% 17.5% 33.8%
Mottled Sculpin
Red Shiner 0.0% 36.3% 5.2%
Sand Shiner 0.0% 2.3% 0.2%
Fathead minnow 0.0% 0.5% 0.2%
Native species 87.9% 73.1% 58.1% 87.0%
Sample size 636 501 2159 577

The considerably large shift in composition between years could indicate an
environmental change between years. Since the percent of native fish increased, it appears the
shift could be a positive adjustment to changes in habitat conditions or predation rates. In spite
of the fact the native species composition (88%) and diversity for fish over 15 cm increased in
2001, and was highest of any sites sampled, the native population on the Dolores appears to be
highly stressed and unnatural. This is better indicated by trends in size structure and density.

Size Strueture for the Yampa. Colorado and Dolores Rivers

Length frequency histograms for each station sampled in 1998 and 1999 are available in
the progress report Anderson and Stewart (2000) and histograms for the 2000 sample are given
in Anderson and Stewart (2001). Refer to these progress reports to make comparison between
this year's data (Appendix Figures Al to A53), and earlier years. Some histograms from 2000 at
Sevens were incorrect (bluehead, flannel mouth and catfish) or not included (Lily Park,
small mouth bass) and are included here at the end of the Appendix.

At Sevens, the length frequency histograms for bluehead sucker in 2001 (Figure AI) had
no fish under 19 cm, which was also found in 2000 (Figure A50). Both mean length and sample
size were smallest in 2001 at Sevens (Table 8) and its 30.0 cm mean length was the smallest
mean size for any site on the Yampa or Colorado Rivers. Bluehead sucker under 34 cm were
rare at Duffy in 2001 (Figure A2), as was observed in earlier years and its mean length of38 cm
was the highest for any site on the Yampa or Colorado River (Table 8). Also, Duffy was the
only site where mean lengths were not significantly different between years (Table 8). At Lily

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