WSP12249

• File Number: 8240.200.10.B
• Description: UCRBRIP - Riverine Fish Flow Investigations
• Basin: Colorado Mainstem
• Date: 6/1/2004
• Author: CDOW
• Title: Riverine Fish Flow Investigations 2004
• Water Supply Pro - Doc Type: Publication

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White sucker and their hybrids were 36% of the catch in 2003 compared to about
15% of the catch in the four previous years, Small mouth bass were 13% in 2003 compared
to about 3% in prior years, In the baseline years (wet cycle) both channel catfish and
nonhem pike were more common than in the later three years (dry cycle). Black crappie
composition was highest in 2003, but still only 2%, Carp were consistent in all five years at
near 4% of the catch,

The Sevens' total density estimates (all fish combined) were very similar in 1998
(I 79.'1Iha) and 1999 (179,11km) (Table 7), Total biomass at Sevens was also very similar
(162.2 kglha) in 1998 and (16S.4 kglha) 1999 (Table 8). The density and biomass estimates
for 1998 and 1999 represented the baseline carrying capacity at this site, The total biomass
estimate in 2000 (98.4 kglha) and 200 I (87,S kglha) were consistent to each other, but less
than the baseline years (1998 and 1999), The difference in density estimates from 1998 -
2001 were significant. They were alpha = O,OS both between years and simultaneously, The
reduced biomass in 2000 and 2001 likely indicated reduced carrying capacity since species
composition was consistent. In 2003 the biomass estimate was higher than 2000 and 2001
(Table 8), but less than 1998 and 1999, An increase in total biomass in 2003 resulted from
an increase in white sucker. carp and small mouth bass (nonnative species),

In 2003 density and biomass estimates for all native species was the lowest of the
study period, In the first four years at Sevens, tlannelmouth sucker was the most common
fish (larger> IS cm) in the catch, but ranked third in 2003. Flannelmouth sucker biomass
was similar in 1998 & 1999 (l',aseline years), but lowered in each successive year.
Flannelmouth sucker biomass in 2003 was about SO% of the baseline years (Table 8). The
species with the greatest change in biomass was bluehead sucker. In 1998, 1999 and 2000
bluehead sucker biomass was about 15 to 17 kglha, The bluehead biomass estimate was S
kglha in 2001 and only 0.6 kglha in 2003. Sample size was small for chub in all years and
recapture rates were not high enough to produce tight confidence intervals. Given roughly
equal sampling effon between years, however, it appeared that the biomass of round tail chub
also declined during the study period, but not as much as other native fish.

In contrast, the non native fish, -- white sucker, smallmouth bass and carp -- increased
in densities in 2003 compared to the baseline years (1998 and 1999), White sucker density
was nearly tripled in 2003 to the prior years, but its biomass was only slightly higher (Table
8) because small fish were more abundant. Yearling-sized white sucker were common in
2003 (Figure A 1-18). but not in prior years. Both density and biomass of small mouth bass
was much higher in 2003 compared to the prior years and the increase was roughly ten fold
for both categories. The higher number of Age-l carp accounted for the higher carp density
in 2003 (Table 7),

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