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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increased in 2001 to 32% (Table AI). At the Sevens station, white sucker and white crosses
were stable between all years at 13% in 1998, 15% in 1999, 17% in 2000 and 16% in 2001.
Small mouth bass at Sevens comprised only 1% in 1998, 2.5% in 1999, and 0.5% in 2000, but
increased to 5% in 2001 (Table AI).

A major shift in species composition was observed at the Lily Park station for fish over
15 cm between 2000 and 2001. In 2001, flannel mouth sucker was 68% compared to 48% in
2000 (Table A3). Sample size was similar for flannel mouth sucker between the two years and
the increase in the flannelmouth percentage was strictly due to much fewer catfish caught in the
2001 sample. In 2000, catfish comprised 40% of the fish sample with a sample size of 1,631, but
in 2001 catfish composition was 18% and sample size dropped to 528 (Table A3). The very low
base flows in the Yampa River in 2000 (Figure 2) appear to be a main factor explaining this
change in catfish species composition at Lily Park. It was suggested in Anderson and Stewart
(2001) that catfish were probably atypically overabundant in the Lily Park in 2000 and this was
attributed to a suspension of channel catfish migration due to very low flow conditions that
appeared to interfere with upstream movements. The 2001 data is consistent with this
explanation and catfish composition in the neighborhood of 18% is probably a more accurate
long-term description of this site.

Bluehead sucker, roundtail chub and Colorado Pikeminnow composition were similar
between years (2001 versus 2000) at Lily park: (9% versus 7%), (0.02% versus 0.03%) and
(0.07% versus 0.03%), respectively (Table A3).

The only species at Lily Park to show a strong shift in species composition (independent
of catfish sample size) was small mouth bass. In 2000, small mouth bass comprised only 0.8% of
the fish over 15 cm. In 2001, small mouth bass were 5% (Table A3).

Fish less than 15 em

Species composition was less variable between stations for fish less than 15 cm in 2001
than it was for> 15 cm. For the first time during the 4-year study period, smallmouth bass
YOY was the dominant fish for this size group at all three Yampa stations. Smallmouth bass
were 80% at Lily Park, 58% at Sevens and 98% at Duffy (Table 4). These rates were the highest
of the sampling period and strongly indicate the increase in YOY small mouth bass is a function
of increased habitat availability in 2001 for this species compared to earlier years. At Duffy,
species composition ofYOY (n or sample size) small mouth bass collected by electrofishing in
1998, 1999,2000 and 2001 was 45% (673), 42% (703), 84% (741) and 98% (3,698),
respectively (Table A2). The very large increase in YOY bass sample size at Duffy in 2001
more dramatically demonstrates the increased habitat availability for this fish in 2001 relative to
earlier years. At Sevens, small mouth bass composition (n) in 1998, 1999,2000 and 2001 was
0.3% (1),26% (3), 14% (64) and 58% (82), respectively (Table A2), and indicating a widespread
experience for improved YOY bass habitat in 2001 compared to 1998 and 1999.

The less than 15-cm fish composition for native species was very rare at both Duffy and
Sevens compared to earlier years. At Duffy, speckled dace were only 0.2% and mottled sculpin
were at 0.8% in 2001 (Table 4). The species composition (n) at Duffy in 1998, 1999,2000 and
2001 for speckled dace were 13% (196),8% (143), 1.2% (11) and 0.2% (8), respectively and was

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