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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Osmundson and Smith (1995) reported that reduced flows and habitat modification
have allowed nonnative species to proliferate. Currently, shoreline and backwater habitats
required by larval pikeminnow are dominated by several introduced fish species (McAda et
aI., 1994) and if competitors are abundant, risk of starvation may be high in spite of
optimal flow and temperature regimes. Because Colorado pikeminnow larvae did not co-
evolve with predator species, avoidance of predation was not an advantageous behavior
for this life stage. This suggests that larval pikeminnow may be vulnerable to exploitation
by introduced predators. Bestgen (1997) believed predation to be a significant cause of
mortality on larval Colorado pikeminnow. The abundance oflarval competitors and
predators are also probably correlated with environmentaVhabitat factors. Therefore, even
in years with optimal flow and temperature conditions, Colorado pikeminnow larval
survival and the resulting year-class strength may be very poor due to abundant
competitors and predators. It may be that pikeminnow larval survival is highest in years
with sub-optimal environmental conditions if those conditions reduce numbers of their
competitors and predators.

This study was designed to collect passively drifting fish larvae in the water
column over a five-year study period. At the time of collection, Colorado pikeminnow
larval were typically in a size range of 8 to 11 mm which back-calculated to between six
and 15 days post hatch. This size and age is very early in larval development and therefore
these collections do not represent year-class strength. However, it is believed that these
collections represent levels of success during the spawning, incubation and hatching life
stages and may be a prerequisite for development of strong year-classes. If correlations
are not found between larval abundance in the drift and end of season young-of-year
abundance, biological interactions are suggested as the primary regulator of year-class
strength. If so, this further suggests that environment variables that limit population size
of introduced species may provide the most promising mechanism to improve numbers of
young-of-year endangered fish.

Data were extracted from similar studies in Utah (Trammel and Chart, 1999a and
1999b), to gain a better understanding of relationships between flow regime variables on
production of larval Colorado pikeminnow and their downstream transport, and
survivorship in nursery habitat in Utah. This data is needed by the Colorado River
Recovery Program to assist in formulating a Biological Opinion on operations of the
Aspinall reservoirs. Comparable studies have been done on the Green River to assess
affects of Flaming Gorge Reservoir releases on reproduction and recruitment of Colorado
pikeminnow (Bestgen et al. 1998). This study has the following objectives:

1) To determine the relative abundance of Colorado pikeminnow larvae
produced by wild adults in the upper main stem Colorado River within
Colorado.

2) To determine the effect offlow manipulation and habitat availability upon
growing season and size of Colorado pikeminnow young-of-the-year as it
relates to potential over-winter survival.