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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perturbations. Changes in size structure of the white sucker group would be an indirect
measurement of changes in predation rates due to pike reduction, and or other environmental
stresses. A decrease in sample size of large white and white suckers crosses was observed in
2001 compared to earlier years. Eventually geriatric fish expire and a certain amount of
recruitment is required to maintain numbers. This could have been happening in 2001, but
changes in size structure and species composition were apparent for the entire fish community at
Duffy. Given this, a reduction in population size oflarge white sucker more likely indicates a
change in carrying capacity than attrition of older individuals.

The size structure for carp was basically the same in 2001 at Sevens, Duffy and Lily Park
as it was in prior years. Minimum sizes of 46 cm at Sevens (Figure A31), 58 cm at Duffy
(Figure A 32) and 37 cm at Lily Park (Figure A33) also show predation is impacting small carp
throughout the Yampa River. The length frequency histograms for carp on the Colorado River
(Figures A 35 and A36) clearly show that 2001 was a very good year for YOY carp. YOY were
much more common in 2001 at Com Lake (Figure A35), the lower flow year compared to 1999
(Figure A54) the high flow year. This is an indication of more nursery habitat availability in
2001. Carp are already quite numerous in the Colorado River and it appears the carp
reproduction had a positive response to low flows in 2001. YOY carp and most of the small carp
(less than 30 cm) were taken from backwater habitats, but large carp occupy the main channel
shoreline with boulder or tree snag cover. Another factor that appears advantageous to carp in
the Colorado River, compared to the Yampa, is the magnitude of allochthonous input from
treatment pond outlets that provide increased feeding opportunities. Carp size structure in the
Dolores River was the same as other species, fewer larger carp and more of the smaller carp in
2001. Fewer large carp suggests reduced carrying capacity in 2001 in the Dolores River.

Size distribution for channel catfish for the Yampa River in 2001 at Sevens and Duffy
(Figures A37 and A38) was fairly similar to 1998, 1999 and 2000. Smaller mean lengths of
catfish were observed in 2001 at both Sevens (48.1 cm) and Duffy (46.4 cm) than in 2000, as
was also found for other species. Mean size was also less in 2001 (Figure A 39) at Lily Park
than in 2000 (Figure A 54) because of an increase in number offish between 20 and 27 cm.
Catfish smaller than 29 cm have not been found at either Sevens or Duffy during fours years of
sampling. At Lily Park, the smallest catfish in the sample was 19 cm. The smallest catfish on the
Colorado River in 2001 was 26 cm (Figures A41 and A42), except for one that was only 12 cm.
Catfish mean size in 2001 increased on the Colorado River at both sites from last year. Catfish
mean size in 2001 decreased on the Dolores River from last year as also happened with all other
species in the Dolores. Catfish mean size in 2000 was 28.7 cm compared to 25.8 cm in 2001
(Figure A 40). Except for an obvious shift in the 2001 histogram, about 3 cm toward the y-axis
the shape of the histogram was fairly similar between the two years.

Apparently catfish do not reproduce in the Yampa River near the Sevens and Duffy sites
or this part of the river lacks some important aspect of habitat for spawning; probably
temperature. Tim Modde of the USFWS routinely finds high numbers of small catfish (<30 cm)
in Dinosaur Canyon (per. comm.). It has been proposed by Recovery Program biologists that
large catfish migrate to Dinosaur Canyon for spawning and move upstream after they reach a
minimum size of near 30 cm. The catfish size data from this study has been consistent with this
concept. It was also observed in 2000, a year with very low flows, that there was a very high
number of catfish at Lily Park. This would happen in years with low flows that prevent
upstream movements over shallow rimes and also Cross Mountain Canyon may be a migration