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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Table 9. Median drift date at Lorna and size and date of Colorado pikeminnow
YOY sampled in backwaters in Moab (Trammel, 1999a).

SUMMER FALL
YEAR Median Sample Range Mean Date Mean Date
Date drift Size MM MM Sampled MM Sampled
1992 14-Jul 378 11-33 21 5-Aug 36.6 28-Sep
1993 29-Jul 47 13-32 17.8 13-Aug 35 5-0ct
1994 12-Jul 88 1 0-36 22.2 25-Jul 50.7 1 O-Oct
1995 19-Aug 1 12 12 19-Aug 31.6 29-Sep
1996 21-Jul 1101 9-35 16.4 2-Aug 43.2 6-0ct

DISCUSSION

Colorado Pikeminnow Distribution. Gunnison River

Numbers of Colorado pikeminnow larvae collected in the Gunnison were low, but
document the existence of a spawning population. Two larvae were collected at the lower
Gunnison station in 1992, the first year of the study. None were found either above or
below the Redlands diversion dam in 1993 and 1994. One larva was sampled above and
two below Redlands dam in 1995, and three larvae were collected from both stations in
1996. Although every low numbers, Colorado pikeminnow larval densities were similar
both above and below the diversion dam suggesting similar larval population sizes at both
locations. Colorado pikeminnow must be spawning upstream of the dam, but if they are
also spawning between the dam and the lower sampling station, the drifting larvae
population should be higher below the dam. Since the sample did not iI).dicate this, the
larvae sampled at the lower Gunnison station could have originated from the same source
as those collected above the dam.

The Redlands dam was constructed in 1918, and completely blocked upstream
migration of adult pikeminnow until construction of a fish ladder in 1997 (Burdick 1997).
Colorado pikeminnow larvae were found at Bridgeport (RM 29.3) in 1994, 1995 and
1996 which is about 26 miles upstream of the Redlands diversion dam. Larval production,
survival, and retention were apparently adequate over the last 87 years to maintain a small
resident population. Burdick (1995) speculated that movement of six of seven radio-
tagged adult pikeminnow appeared consistent with those expected spawning migration
movements. Those fish congregated in a location about four miles upstream of the
Bridgeport drift sampling station. Obviously if all larvae drifted below Redlands the adult
pikeminnow population of the Gunnison River would have been lost to attrition.
Abundance of Colorado pikeminnow collected immediately above the dam (n =;; 5) were
the same as at Bridgeport (n =;; 5), indicating that perhaps a high proportion of this rivers
larval population is vulnerable to drifting out of the reach. However, the slowed currents
and deeper habitats created by Redlands diversion dam may discourage all larvae from
drifting over the dam. Also, suspected spawning sites [28 to 32 miles, Burdick (1995)]
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