9697

• UCREFRP Catalog Number: 9697
• Author: Bestgen, K.R., D.W. Beyers, J.A. Rice and G.B. Haines.
• Title: Factors affecting recruitment of young Colorado pikeminnow
• USFW Year: 2006.
• USFW - Doc Type: synthesis of predation experiments, field studies, and individual-based modeling.
• Copyright Material: YES

YOUNG COLORADO PIKEMINNOW RECRUITMENT

minnow food abundance. Growth reductions may have
extended the time that Colorado pikeminnow were
susceptible to predators and that flow-predation
interaction may be a cause for low recruitment of fish
hatched during that period.
Physical factors such as river flow are suspected of
having a strong influence on the recruitment patterns of
fishes in other systems as well (Crecco and Savoy
1985; Tyus and Haines 1991; Mion et al. 1998). For
example, poor recruitment of American shad produced
early in the year in the Connecticut and Hudson rivers
was associated with high discharge, cold water
temperature, and slow growth of larvae (Crecco and
Savoy 1985; Limburg 1996), whereas higher survival
of later hatched cohorts was correlated with lower
flows, warmer water, and higher food abundance.
Similarly, river-spawning walleyes Sander vitreus also
showed high intra-anmual recruitment variation driven
mainly by negative effects of higher discharge and
water turbidity (Mion et al. 1998). Very high and
extended summer flows also negatively affected
abundance of age-O Colorado pikeminnow in backwa-
ters of the Green River in 1983 and 1984 compared
with other years in the period 1979-1988 (Tyus and
Haines 1991).

Management Applications
Management strategies to reduce fish predator
abundance in. Green River backwaters may enhance
recruitment of early life stages of rare native fishes,
including Colorado pikeminnow and endangered
razorback suckers Xyrauchen texanus (Bestgen 1990;
Tyus and Saunders 2000). Bundy and Bestgen
(unpublished report) were able to temporarily reduce
abundance of nonnative cyprinids by 90% or more with
depletion sampling in backwaters of the Colorado
River. Based on results presented here, similar
reductions of red shiner and other potential predator
fishes should have a positive recruitment effect on
native fishes such as Colorado pikeminnow, if
removals are timed with endangered fish reproduction.
This may have occurred in the lower Green River,
Utah, where intensive removal of abundant red shiners
and other nonnative cyprinids in a short river reach was
associated with persistence of relatively large early life
stages of razorback suckers (Trammell et al. 2004).
Less intensive removals over longer reaches in other
years did not produce similar results. Because high
discharge events also reduce abundance of nonnative
fishes in backwaters (Haines and Tyus 1990; Stanford
1994; Gido et al. 1997), further exploration of the link
between hydrology and nonnative fish abundance
dynamics is warranted.
Simulations portrayed the complexities of recruit-

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ment dynamics of juvenile Colorado pikeminnow
observed in the field and could be used to generate
predictions of the relative effects of various manage-
ment schemes on the recruitment and growth of
Colorado pikeminnow. For example, Muth et al.
(2000) recently proposed more natural flow patterns
and water temperatures for Flaming Gorge Dam
releases; this may enhance survival and growth of
endangered fishes in the Green River system (Ward
and Stanford 1995; Poff et al. 1997). The mM offers
one means to evaluate the relative benefits of flow and
temperature modifications from Flanaing Gorge Dam
and effects of different anmual hydrologic and temper-
ature regimes on Colorado pikeminnow growth and
recruitment Establishing reliable links between sur-
vival and baseline growth rate of young Colorado
pikeminnow relative to base flow level or flow
fluctuations would make mM predictions particularly
useful. Effects of changes in temperature regimes could
be simulated directly and would be most important in
reaches close to the dana. Simulations could also
predict size-structure of autumn juveniles, if seasonal
production patterns of Colorado pikeminnow larvae
were available. Managers could then better understand
potential effects of late autumn and winter flow
regimes from Flanaing Gorge Dam that may influence
growth or overwinter survival of Colorado pike-
minnow, particularly if the autumn juveniles were
small (Thompson et al. 1991; Haines et al. 1998). This
may be particularly important in years when flows are
relatively high and cold because Colorado pikeminnow
reproduction is usually delayed under those circum-
stances (Nesler et al. 1988; Bestgen et al. 1998).
The abundance of juvenile Colorado pikeminnow
has declined in the Green River since the mid-1990s
(Muth et al. 2(00), and that decline has been implicated
as a main reason for the decline in abundance of adults
since 2000 (Bestgen et al. 2005). As a result, additional
research was initiated to understand factors associated
with the recent decline of early life stages of Colorado
pikeminnow, including the potential role of red shiners
and other predators in backwaters. Increased survival
of Colorado pikeminnow larvae, particularly those that
hatch early in the reproductive season and grow to a
relatively large size by autumn, should be investigated
because those are the ones most likely to overwinter
(Haines et al. 1998; Muth et al. 20(0).
Investigations should consider habitat availability
and quality, as well as the effects of nonnative fish
predators, to minimize the effects of interacting biotic
and abiotic factors that limit recruitment success of
early life stages of endangered Colorado pikeminnow
in the Green River Basin.