9555

• UCREFRP Catalog Number: 9555
• Author: Bestgen, K. R., C. D. Walford, A. A. Hill and J. A. Hawkins.
• Title: Native Fish Response to Removal of Non-native Predator Fish in the Yampa River, Colorado.
• USFW Year: 2007.
• USFW - Doc Type: 140,
• Copyright Material: NO

low compared to historical samples and showed no consistent pattern between control and
treatment reaches. Only in 2003 was the percentage of native fish substantially larger in the
treatment reach than in the control reach (5.1 % vs 1.2 %), but that was in a year when nearly no
fish removal occurred in the treatment reach and when most native fishes in the treatment reach
occurred in isolated pools, a special habitat type. Native fish abundance in 2004 was highest in
the entire study period 2003-2006, and was consistent in control and treatment reaches at 9°/~ and
9.7%, respectively. This was also the year when smallmouth bass relative abundance was
relatively low in the study area, 2l .5% in the control reach and 17.1 % in the treatment reach.
Native fish abundance was lower in 2005-2006 than in 2003-2004 throughout the study
area. In 2005, native fish comprised 2.2% of the fish community in the control reach, and 0.3°/,
in the treatment reach. This pattern of reduced native fish abundance in 2005 and 2006
compared to 2002 and 2003, and reduced abundance in treatment reaches compared to controls,
is opposite the pattern one might expect if removal of predaceous smallmouth bass was having a
positive effect. Ironically, this occurred in the first year, 2005, that substantial removal of age-0
(with electric seine) and larger smallmouth bass both occurred. smallmouth bass abundance was
much higher in the control reach (55.3%} than in the treatment reach (29.2%), perhaps due to
removal efforts for those age-0 fish. Non-natives sand shiner and white sucker were more
abundant in treatment reaches than in the control reach in 2005, perhaps indicating a release
from predation due to the effect of removal smallmouth bass. Reductions in relative abundance
of age-0 native fish in the reach in 2005-2006 was consistent with reductions oflarger-bodied
native fish and increased abundance of smallmouth bass in boat electrofishing samples in the
same period (J. Hawkins, 2007 draft report).
In 2006, a pattern similar to that for 2005 was observed as native fish comprised 2.3% of
the fish community in the control reach, and 0.3% in the treatment reach. However, smallmouth
bass abundance was higher in the treatment reach (46.2 %) after substantial removals had
occurred than in the control reach (37.4%). Years 2005 and 2006 had the highest composite
abundance of smallmouth bass in the study area, regardless of treatment or control reach, which
was surprising given substantial efforts to remove age-0 and larger smallmouth bass in these
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