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

abundance in the control reach also declined rapidly, perhaps due to mortality, movement out of
relatively shallow and colder, low-velocity areas that we sampled, or due to increased turbidity.
However, control reach bass abundance was still more than two times that of the treatment reach.
Treatment reach smallmouth bass abundance in mid-October was similar to that in late
September.
In 2006, smallmouth bass abundance was estimated at 1,347 fish > 150 mm TL; 642 of
those fish were removed from the treatment reach of the study area with boat electrofishing over
six removal passes, a removal of about 48% of the population (Hawkins 2006). An additional
7,909 small-bodied smallmouth bass (most age-0) were removed by electric seining efforts from
]ate-July to early September. Again, we do not know what proportion of smallmouth bass were
removed because there was not accompanying abundance estimate. Age-0 smallmouth bass
abundance in samples in the treatment reach started at a low level in late-July based on electric
seine sampling results (Fig. 7B). Bass abundance increased rapidly by mid-August, and was
about two times as high in the control reach as in the treatment reach. By late September,
smallmouth bass abundance in control and treatment reaches declined, but smallmouth bass
abundance in the control reach was still more than twice that of the treatment reach in October.
Over the 2003-2006 period, smallmouth bass removals via boat electrofishing were
sometimes large, but the treatment effect for these larger bass was not sustained because bass
abundance was high in the beginning of each spring removal period (> about 1,300 fish > 150
mm TL, Hawkins, 2005, 2006, 2007). In 2005 and 2006, removal of age-0 smallmouth bass in
the treatment reach was also substantial, and levels in the control reach were often twice as high
as that in the treatment reach. Declines in abundance did not occur until relatively late in
September, and there was also a large seasonal component in each reach, as age-0 bass
abundance apparently declined in both control and treatment reaches. Consistent declines in
each reach suggest mortality of age-0 bass, reduced capture probability, or both.
Number of fish captured in control and treatment reaches varied among years but was
consistent between the reaches within years (Table 5), reflecting a similar number of habitat
areas sampled in each reach (Table 2). The percentage of native fish in samples was consistently
13