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 Larval Catch Between Years and Between Stations. Colorado

Initiation of spawning appeared to be strongly associated with warming water
temperatures. Date of first spawning at Loma was earlier in the low flow years of 1992
and 1994 and was estimated to be June 12 and June 17, respectively. The date offirst
spawning in the intermediate flow year (1996) was June 27, and it was July 3 and July 21
in the two high flow years. Water temperatures warmed faster and reached 180C on June 9
and June 10 in 1992 and 1994, respectively which was three and seven day prior to
estimated first spawning in those years. Water temperatures reached 18C after the
estimated first spawn date by three, seven and eight days in the other three years (1996,
1993 and 1995, respectively). The strong relationship between warming temperature and
date offirst spawning and suggests temperature strongly influences initiation of spawning
for Colorado pikeminnow. Contrary to these results, Bestgen (1998) reported that there
was no clear relationship between water temperature and initiation of reproduction by
Colorado pikeminnow on the Yampa River. On the Yampa River the dates for first
spawning occurred between June 13 and July 1 (1992 and 1996) suggesting a temporal
factor cues reproduction (Bestgen 1998).

The accumulated degree-days on the date of first spawning was lower in the
warmer years (1,456 and 1518 in 1992 and 1994, respectively) and was highest in the
1995 (1,799) the year with the slowest warming. Degree-days to first spawning found in
the Colorado River is near the range reported by Bestgen et al. (1998) for the Yampa
River.

In spite of seemingly similar flow and temperature conditions, Colorado
pikeminnow and other native larvae drift estimates were not similar for 1992 and 1994.
The second highest seasonal pikeminnow larval drift estimate (106,00/hr) was recorded in
1992 and was 4.2 times the 1994 estimate, which was the poorest (25,000/hr) of the study
period. The seasonal drift estimate for native larvae in 1992 was six times that of 1994,
and it was 1.8 times higher for nonnative fish larvae. The larger number of larvae
collected in 1992 could have been due to more ova produced by larger numbers of adults
spawning (Colorado pikeminnow and other native species) that year. Another possible
explanation is there was better survival of eggs and larvae in 1992 compared to 1994. A
general observation by the field crew leader was that algae quickly clogged drift nets in
1992 and was much more ofa problem in 1992 than in 1994. Higher alga production in
1992 could have had a positive influence on larval survivorship, by presumably increasing
forage availability, which in turn reduces competition for limited resources. The end result
would have a higher larval carrying capacity in 1992 given similar flow conditions.
Speculation about higher alga production in 1992 is somewhat supported by the fact that
runoff conditions prior to 1992 were mild (reduced scouring of channel), whereas 1994
followed a high runoff year (increased scouring of channel).

The highest Colorado pikeminnow and larvae density and drift estimate
(268,000/hour) at Lorna occurred in 1995. All stations sampled had their largest density
for native species and their lowest densities for non-native species in 1995. This year was

21