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

years compared to 2003 and 2004. Sand shiner was more abundant in the treatment reach than
the control reach in 2006, and white sucker abundance was about equal. High abundance of
black bullhead that occurred mainly in the control reach was unusual and most were captured in
a few samples.
Continued low native fish abundance, and inconsistent response of native fishes and
selected non-native taxa to smallmouth bass removal, suggested that removals conducted at the
present levels are not having the expected effect. Mechanisms may include predation rates that
are too high to allow recruitment, changes in habitat quality, or levels of reproduction by adult
fishes that are too low to permit sufficient recruitment to occur. It appears that smallmouth bass
abundance levels in the main channel of the Yampa River remain too high, even in the relatively
low abundance year of 2004 (about 19 % relative abundance), to allow recruitment of native
fishes at levels that were observed as recently as 1999.
Yampa River fishes, isolated pool and main stem river comparisons.-Support for the
thesis that smallmouth bass may be negatively impacting abundance of native fishes in low-
velocity habitats comes from comparison of the few places where native fishes do exist in the
study area in relatively high numbers, isolated pools, to places where native fishes do not
survive, the main channel in both the control and treatment reaches. A total of 19 isolated pools
were sampled in the study area in the four years of study, 10 in the control reach and 9 in the
treatment reach (Table 2).
In general, the proportion of samples from isolated pool samples were low, with the
exception of 2004, when isolated pooled were relatively common and fewer main channel
samples were obtained. Native fishes were more common in isolated pools than in the main
channel (Table 6). For example, 13 of 19 (68 %) isolated pools that held fish supported native
fishes, while only 50 of 230 main channel samples (22 %) supported native fishes. From 2003-
2006, native fish relative abundance in isolated pools was 14%, compared to < 1 % in the main
channel. Conversely, mean relative abundance of smallmouth bass in isolated pools was < 5 %,
but in the main channel approached 50 %.
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