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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

Colorado pikeminnow data do not indicate interactive or strong density-dependent
relationships for over-winter mortality. It appears that flow conditions influence standing
stocks since there are fewer fish in backwaters in the years with high spring runoff flows
and reduced temperatures. This occurred in spite of the fact that higher runoff years had
higher Colorado pikeminnow larval populations estimated in the drift. Regardless of
runoff flows, YOY Colorado pikeminnow must co-habit with a much larger NNC
population with which it did not co-evolve. The NNC fall backwater estimates ranged
from 288 to 7205 times higher than pikeminnow estimates. Perhaps because pikeminnow
densities were very low, the data could not identify relationships between NNC density
and Colorado pikeminnow YOY densities. The largest numbers of both NNC and
Colorado pikeminnow YOY occurred in the same year, 1996 (intermediate flow year)
suggesting an elevated productivity that year which was beneficial to both native and
nonnative groups.

Colorado Pikeminnow Larval Drift and YOY Backwater Density (ISMP)

Prior to the larval drift net study, Colorado pikeminnow YOY had been collected
in small numbers in backwater habitats in Reach 2 (1986 to 1992) (McAda et al. 1998).
Efforts to collect young-of-year Colorado pikeminnow by seining backwaters in Colorado
were unsuccessful in late summer and fall 1992 when 96 seine. samples collected a total
catch of71,440 fish (Snyder 1993). No Colorado pikeminnow fry were collected in
Colorado in spite ofthe fact than larvae were fairly common in the drift sample that year.
Ninety-six percent of the total catch was comprised of just three nonnative species, .
fathead minnow, red shiner and sand shiner (Snyder 1993). Based on these results, fall
seining of backwaters was terminated from the project and ISMP sampling was used to
indicate trends in pikeminnow abundance between years in backwater habitats.

No Colorado pikeminnow larvae were sampled in backwater habitats during ISMP
sampling effort in Reach 2 between 1993 and 1997 (McAda et al1998). Since
pikeminnow were sampled in this reach prior to 1992, this could suggests a recent change
in larval production by spawning adults, or lack of suitable nursery habitat, or low larval
survivorship in backwater habitats upstream ofRM 139. No cause was determined for
this sudden absence of young of year Colorado pikeminnow in this river reach, but it is a
cause for concern.

Colorado Pikeminnow Larval Drift (LOMA) and Backwater Density (Utah)

Bestgen (1998) reported a lack of concurrence of transport abundance estimates
and juvenile recruitment in the fall. There was a negative correlation between larval
Colorado pikeminnow drift density estimates at Loma, Colorado and the average densities
for YOY Colorado pikeminnow collected during summer and fall in Utah backwaters.
The relationship was very weak with a r2 of 0.20. Again the data suggests that year-class
strength determined in Moab was not related to reproductive success as measured by
number of larvae in the drift at Lorna. The reason the relationship was negative was due
to the fact that the best year for larval abundance was the worst year for backwater
abundance (1995) and the worst year for larval abundance was the best year for backwater

25