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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INTRODUCTION

Construction and operation of the Aspinall Unit has had a major effect on flow
regimes and frequency of high spring-runoff flows of the Gunnison and Colorado rivers.
Maximum annual discharge (highest mean daily flow) has decreased by 47% in the
Gunnison River and 37% in the Colorado River from 1914 to 1989 (McAda and Kaeding,
1991). Mean montWy flows for May and June, the primary runoff period, have declined
43 and 47%, respectively, in the Gunnison River, and 31 and 37%, respectively, in the
Colorado River. The Aspinall Unit is responsible for 75 to 85% of the flow changes
observed between 1965 and 1989 in the Colorado River during May and June (McAda
and Kaeding, 1991).

Endangered fish species in the Colorado River Basin, i.e. Colorado pikeminnow
(Ptychocheilus lucius), razorback sucker (Xyrauchen texanus), humpback chub (Gila
cypha), and bonytail chub (Gila elegans), as well as other sympatric native fishes, evolved
with and adapted to the natural cycle of flow patterns of these river systems (Modde and
Smith, 1995; Tyus, 1989; Osmundson et aI., 1995). Flow alterations during the spawning
periods may be especially disruptive to reproductive processes of sensitive fish species.
Colorado pikeminnow spawn on the descending limb of the hydrograph when flows are
dropping and water temperatures are rising (Osmundson et aI., 1995). Also Colorado
pikeminnow larvae,have been found to passively drift in the current (Haynes et aI., 1984;
Nesler et aI., 1988; Nesler, 1986), presumably to nursery habitats which can be
considerable distances downstream of spawning sites (Tyus and McAda, 1984; Tyus,
1986; McAda and Kaeding, 1989; Osmundson and Kaeding, 1991). If environmental
variables have become less favorable for reproduction because of flow alternations,
endangered fish species may have reduced potential for recruitment and long term
population stability compared to natural conditions.

Year-class strength strongly influences subsequent adult population sizes and is
typically delimited at a very early life stage (Mills and Mann, 1985). Fluctuations in year-
class strength are mostly attributable to environmental factors that regulate survival of
eggs and larvae. Colorado pikeminnow begin spawning in a range from 160 to 220C
(Bestgen, 1998), which is usually from mid June to mid July in the Colorado River. Egg
survivability can be directly impacted during this time by flow and temperature conditions
and the amount of silt in spawning sites (Mills and Mann, 1985; Bestgen and Williams,
1994). Flow conditions determine depths and velocities over spawning sites, which
agitate and oxygenate eggs (Naesje et aI., 1995).

Significant causes of mortality on larval fish are starvation and predation (pepin,
1998). Emergence of larvae from nests must be synchronous with an adequate supply of
forage items and larval fish habitats for recruitment to the next life stage. Significant
alteration of flow and temperature conditions during the larval life stage have high
potential to negatively impact year-class strength by disrupting the timing between larval
emergence and forage and habitat availability. Colorado pikeminnow larvae are dependent
on runoffflow for dispersal arid transport to nursery habitats (Tyus, 1986).