9299

• UCREFRP Catalog Number: 9299
• Author: Anderson, R. and J. Mumma.
• Title: Aspinall Studies
• USFW Year: 1999.
• USFW - Doc Type: Annual assessment of Colorado Pikeminnow Larval Production in the Gunnison and Colorado Rivers, Colorado 1992-1996.
• Copyright Material: NO

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abundance (I 996). The 1995 results could be attributed to increased spawning efforts
and larval production stimulated by high summer flows that year. However, Colorado
pikeminnow larval size was small and survival poor in 1995 because of a short growing
season and reduced habitat availability. Thus, even though many larvae were produced,
few survived.

However the 1996 data does not lend itself to flow related explanations. The
lowest total larvae production including Colorado pikeminnow, other native and non-
native species at Loma was in 1996. This indicates either reduced spawning (adult
numbers) or larval survival for all species. In contrast, 1996 was the best year for both
yay Colorado pikeminnow and NNC (all species) in Utah backwaters. The fact that
both Colorado pikeminnow yay and NNC densities were highest in 1996 suggests
increased productivity and reduced competition compared to other years. Also the
improved 1996 productivity was in the intermediate flow year, conditions not expected to
be more productive for NNC populations than low flow years. The fact that there was
decreased upstream productivity (Lorna) simultaneously with increased downstream
productivity (Moab) suggests a possible relationship between the two. An unusual
situation occurred about that time, the mud slide event at Glenwood Canyon in the
, September 1995. This is another example of a non-flow related variable that could
potentially have unknown impacts on fish reproductive success. For example,
mobilization of mud early in the spring 1996 runoff could have resulted in abnormally
heavy siltation during the spring in the upper river concomitant with displacement of
invertebrates and/or nutrients downstream.

With only five years of data collection it is not possible to tell if the exceptional
Colorado pikeminnow yay density found during backwater samples in 1996 is due to
aberrant environmental conditions or if those results would be repeated given similar
runoff flows. However even if the 1996 data is ignored, data from 1992 to 1995 strongly
indicate that intermediate flow years should provide optimal conditions for survival and
recruitment of yay Colorado pikeminnow.

Non-Native Cyprinid Backwater Density (UTAH and ISMP)

Examination ofNNC (fathead minnow, red shiner and sand shiner) abundance has
relevance to this study in two arenas. First NNC spend their entire life cycle in backwater
habitats. If we assume a positive ecological relationship between NNC abundance and
habitat and forage availability, so their abundance in a given year is an indicator of
resource abundance. Secondly, since Colorado pikeminnow utilize the same habitats as
NNC competition occurs when mutual resources (habitat and forage) are limiting.
Therefore trends in NNC density are useful for explaining trends in Colorado pikeminnow
yay densities from backwater habitats.

The NNC density was higher in the low runoff years probably due to extended and
improved conditions for growth, reproduction and survival. If average NNC density
estimates are a good indicator of annual productivity potential, then in general it appears
that low flow years had better potential than high flow years. However 1996, the