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

19% (278), 27% (467), 5% (44) and 0.8% (28) for mottled sculpin, respectively (Table A2).
Both dace and sculpin were common in 1998 and 1999, the years with normal base flow, but rare
in 2000 and 2001, years with low flows and dewatered riffle habitats. In 2001 a deliberate effort
was made to collect all dace observed up to the fIrst 40 for preserving as museum specimens.
Because extra attention was made to collected dace, it is believed that the 8 dace and 28 sculpin
counted collected in 2001 at Duffy (4 passes of7.2 km each) is an accurate indication that these
species were indeed very rare in this section of the Yampa River.

Dace were also very rare upstream of the Duffy site. Electrofishing surveys were made
from RM 124 to 105 in July 2001 for the purpose of obtaining DNA samples for native species
and hybrids. During this electrofishing survey only 3 speckled dace were collected from the
riffles that were shocked. Several riffles appeared to be suitable habitat for speckled dace, but
instead were occupied by large numbers ofYOY small mouth bass.

Low densities or abundance of dace and sculpin was also observed at Sevens. Speckled
dace species composition (n) in 1998, 1999,2000 and 2001 was 38% (123), 13%(10),2% (11)
and 2% (3), respectively and for mottled sculpin it was of 5% (16), 0%, 0%, and 0%,
respectively (Table A2).

Sand shiner was the most common small fIsh sampled or observed by electrofishing at
Sevens in all years except 2001 and were at 34% in 2001 (Table 4). At Duffy, sand shiners were
rare in 2001 at only 0.2%. White sucker YOY were common at Duffy in 1998 (10%) and 1999
(18%) and fairly common in 2000 (6%), but were very rare in 2001 (0.8%) (Table A2). At
Sevens, white sucker YOY were 6%, 18%,26% and 6% in 1998, 1999,2000 and 2001,
respectively (Table A2). Intuitively low flow regimes should provide abundant habitat
availability for both sand shiner and white sucker YOY, since shiners and YOY fish occupy
shallow, slow habitats. Their numbers should not be adversely impacted by low flows but low
flows may allow improved survival of bass. Juvenile size (100 -160 mm) small mouth bass
would be an efficient predator on all YOY size (20 - 90 mm) fish and a large increase in juvenile
bass would likely account for reduced numbers of sand shiners and white sucker YOY at these
stations.

Species Composition - Seining collections (fish<15 cm)

Results of seining in 1999 were given in Anderson and Stewart (2000) in Tables A2.3.
A2A and A2.5. Large shifts in species composition were observed for Duffy with the seine
samples for roundtail chub, speckled dace, white sucker, and small mouth bass (Table 5).
Number of speckled dace collected in 1999 was 538 or 24% ofthe total, but in 2001 seining
collected zero speckled dace. The number of white sucker collected in seines in 1999 was 497
for 22% of the total catch, but in 2001 the 11 white sucker caught were only 1% of the total.
Smallmouth bass numbers were 35 for 1.5% in 1999 but increased to 540 bass for 67% of the
total catch in 2001 (Table 5). The seining data shows independently of the electrofIshing data
that YOY small mouth bass numbers greatly increased in 2001. Habitats (riffles and shoreline
with cover) that contained dace and sucker in 1999 were either empty or filled with YOY bass.
It was also observed from length frequency histograms that 2001 had a larger population of
juvenile bass compared to earlier years. It appears that juvenile bass are very efficient predators
on fish under 80 cm and that juvenile bass survival was higher in 2001 compared to earlier years.

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