8159

• UCREFRP Catalog Number: 8159
• Author: Trammell, M. and T. Chart.
• Title: Aspinall Studies
• USFW Year: 1999.
• USFW - Doc Type: Annual Assessment of Colorado Pikeminnow Larval Production in the Colorado River, Utah 1992-1996 \
• Copyright Material: NO

o Non-native cyprinid drift density was not correlated to density in backwaters in the
fall, as measured by fall ISMP sampling and nursery habitat sampling.
o The summer sampling revealed that in high water years, densities ofnon-native
cyprinids were reduced over the runoff period, indicating both mortality and a lack of
available habitat. In low water years, densities increased over the runoff period and
the average total length of the fish decreased, indicating reproduction occurring prior
to the summer sampling. Densities peaked in the fall, after a period of reproduction.
Non-native cyprinid densities were reduced overwinter from fall to spring (pre-runoff).
o Overwinter survival ofnon-native cyprinids was higher in the winters following the
low flow years of 1992 and 1994.
o Non-native cyprinid drift densities in backwaters in the fall were negatively correlated
with high peak flows for 1992-1995. The intermediate flow year of 1996 was
anomalous, with the highest densities ofnon-native cyprinids of the five years seen in
the Moab reach.
o The high productivity in 1996 could be a result of improved environmental conditions
which aze independent of flow, and beneficial to both non-native cyprinids and
Colorado pikeminnow.
• Comparison of relative abundance of larval Colorado pikeminnow between sites in
Colorado and Utah
o On three drift stations on the Colorado River within Colorado from the confluence of
the Colorado and Gunnison rivers to Loma, annual Colorado pikeminnow larval drift
densities increased downstream.
o At the next downstream station, Westwater, Utah, annual drift densities had decreased,
then increased again at the Moab, Utah station.
o Seasonal drift densities at Loma and Westwater (48 km downstream) were not
correlated, and were only slightly more correlated with Moab, 160 km downstream.
o The assumption that Loma and Westwater drift densities influence downstream
abundance is not supported by this lack of correlation.
o The estimated onset of spawning occurred earlier at Westwater than at Loma in 4 of 5
years, and earliest at Moab in 3 of 5 years.
o Drift densities at Moab were higher than Westwater all years, and higher than Loma 4
out of 5 years, indicating additional reproduction occurring between the sites as well as
some input from upstream drift. The effects are inseparable.
o Few larvae over 10-12 mm are collected in the drift. Larvae as small as 11 mm were
collected in backwaters in the summer.
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