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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intermediate flow year, had double the NNC density in the fall surveys than the two low
flow years. The fact that the best year for NNC productivity occurred in the intermediate
flow year, suggests NNC productivity may be not on a continuum in regard to flow.
Increased NNC densities in 1996 could be the result of improved environmental conditions
that were independent of flow.

Density estimates for NNC also varied seasonally with highest densities in the fall.
Runoff conditions seem to explain the low spring densities in 1993 and 1995. The high
spring NNC density in 1995 after the low year of 1994 indicates there can be a large range
in variability between years. Since data was not collected in the spring of 1997, it is not
known how over-winter mortality would have affected year-class strength of the abundant
NNC population identified in the 1996 fall.

Fall NNC densities in backwaters in Reach 2 were higher in the two low flow years
than in the two high flow years, but the intermediate flow year (1996) had the lowest
density of the period. Trends in NNC abundance were not consistent between Reach 2
(upstream) and Reach 1 (downstream) during the five year study period. If 1996 NNC
densities had been intermediate trends in abundance would have been higWy consistent
between reaches and with correlated with flow conditions. But flows do not explain why
NNC abundance, and resources that determine their abundance, were least in the upstream
reach (2) and greatest in the downstream reach (2) during the same year.

SUMMARY AND CONCLUSIONS

· The fact that larval Colorado pikeminnow were collected in the lower portion of the
15-Mile reach and in the Gunnison River above and below Redlands diversion dam
documents that reproduction is occurring in those reaches.

· Colorado pikeminnow larvae appeared earlier in the sample in the low flow years
when water temperatures warmer earlier suggesting that water temperature was
strongly associated with the initiation of spawning.

· Transport abundance of Colorado pikeminnow larvae was strongly and positively
correlated with discharge. The high flow years had the most Colorado pikeminnow
larval densities at Loma, and Moab (1995) and Westwater (1993).

· The year (1995) that produced the most native and pikeminnow larvae at Loma had
the largest runoff flows, the coldest summer water temperatures and the least number
of nonnative larvae.

· The lowest flow year (1994) had the lowest Colorado pikeminnow larval drift density
in Westwater and Moab. The lowest drift density in Loma was in 1996 and
intermediate flow year.

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