9432 ‎(2)‎

• UCREFRP Catalog Number: 9432
• Author: Modde, T. and M. Fuller.
• Title: Feasibility of Channel Catfish Reduction in the Lower Yampa River.
• USFW Year: 2002.
• USFW - Doc Type: Vernal.
• Copyright Material: NO

of the three native fishes collected in the fish trap was combined on a weekly
and biweekly basis and plotted against the mean weekly and biweekly discharges,
respectively. It did not appear, then, that the higher flows were responsible
for the reduced number of Colorado pikeminnow and three, non-listed native fishes
that moved through the passageway in 1999 between 31 July and 9 October.
There was a trend apparent, however, from 1996 to 2000 during the post-
runoff months of July, August, September, and October: the numbers of these three
non-listed, native fish declined steadily (Table 3). This same trend is
supported by the "Ratio Method" employed and discussed earlier. The reason for
this is unclear. This reduced number of native fish over the five years during
this period, possibly, might be attributed to a slow, long-term attrition or
depletion of the numbers of native fish in downstream river reaches as fish moved
through the passageway. There was also a steady, diminishing trend for the total
aggregate number of 12 nonnative fishes collected in the fish trap during this
same period between 1997 and 1999. Other than 1996, the number of nonnative
fishes collected in the fish trap declined in 1998 and 1999 from 1997 (Table 3).
Runoff Discharge vs. Passageway Use
Some of the highest catches of native fishes occurred during the runoff
months of May and June and during April and early-July, immediately prior to and
following runoff, respectively, when discharge was usually highest in the
Gunnison River. Therefore, high discharges certainly did not affect the number
or ability of native fish to migrate through the passageway. High discharges
during runoff may have actually prompted and triggered large numbers of bluehead
and flannelmouth sucker to migrate at this time, and may have been associated
with these two species' spawning period.
Pikeminnow Demographics and Passageway Use
The reason that the number of Colorado pikeminnow collected in the fish
trap declined in 1999 (5) and 2000 (4) from the high of 1998 (23) and 1997 (18)
is perplexing. If seasonal variations in river flow volume or discharge did not
affect catches of listed and non-listed native fishes in the fish trap, then what
did? Pikeminnow population demographics that include net catch rate data, length
frequency, body condition, and population estimates collected during 1998-2000
spring and early-summer sampling were examined and interpreted to offer a
possible explanation for this decline.
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