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

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YOUNG COLORADO PIKEMINNOW RECRUITMENT

within or among years, an understanding of recruitment
variation at multiple temporal and spatial scales can
help identify factors that regulate fish communities and
focus conservation efforts for rare species (Hjort 1914;
Methot 1983; Rice et al. 1987; Gaines et al. 1985;
Connell 1985; Doherty 1991; Garvey et al. 2002).
The Colorado pikeminnow Ptychocheilus lucius is an
endangered species (under the U.S. Endangered
Species Act) endemic to the Colorado River basin in
the American Southwest Distribution and abundance
of this large, piscivorous cyprinid have been reduced by
human-induced changes to the physical and biological
environment Physical changes are largely due to
construction and operation of danIs, which restrict
movement of Colorado pikeminnow; reduce seasonal
discharge variability, water temperature, and sediment
load; and increase daily hydrograph variation (Vanicek
and Kramer 1969; Holden 1979; Tyus and Karp 1989;
Tyus 1991; Stanford et al. 1996). Biological changes
are mostly related to establishment of over 60 nonnative
fishes, which may compete with or prey upon native
fishes (Carlson and Muth 1989; Minckley and Deacon
1991). As a result, distribution and abundance of
natural populations of Colorado pikeminnow have
declined and are currently restricted to a few individuals
in the San Juan River and more substantial populations
in the upper Colorado River basin upstream of Lake
Powell (platania et al.. 1991; Tyus 1991).
In upper Colorado River basin streams, the Green
River population of Colorado pikeminnow is thought
to be the largest, but recruitment of age-O fish in
autumn varies annually (Tyus 1991; Tyus and Haines
1991; Osmundson and Burnham 1998; Bestgen et al.
2005). Recruitment variation may be due, in part, to
intra-annual and interannual variation in abundance of
nonnative cyprinids in backwaters, which may compete
with or prey upon co-occuring early life stages of
Colorado pikeminnow throughout their first summer
and autumn of life (Tyus and Haines 1991; Tyus and
Saunders 2000). Nonnative fishes include cyprinids
(<80 mm total length [TL]) such as red shiners
Cyprinella lutrensis, fathead minnow Pimephales
promelas, and sand shiners Notropis stramineus, which
compose 90-99% of the fish community in backwaters
(Haines and Tyus 1990; J. M. Bundy and K. R.
Bestgen, unpublished report evaluating the interagency
standardized monitoring program sampling technique
in backwaters of the Colorado River). Although these
small minnows have generally been overlooked as
piscivores, predation by red shiners on cypriniform
larvae has been documented (Ruppert et al. 1993;
Brandenburg and Gido 1999). Red shiners have been
widely introduced in many streams in the western U.
S., and given their high densities, even modest rates of

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predation may be an important source of mortality for
early life stages of native fishes, including Colorado
pikeminnow. Understanding sources of Colorado pike-
minnow mortality during the first growing season
merits particular attention because strong year-classes
of juveniles (30-70-mm TL) are thought responsible
for subsequent strong year-classes of adults in the
Colorado and Green rivers (Osmundson and Burnham
1998; Bestgen et al. 2005).
The purpose of this investigation was to enhance
understanding of factors that control the recruitment
dyuamics of young Colorado pikeminnow in their first
summer of life. We first conducted small-scale exper-
iments to evaluate effectiveness of nonnative red shiners
as a predator on Colorado pikeminnow larvae. Red
shiners were used because they are abundant, preda-
ceous on cypriniform larvae (Ruppert et al. 1993), and
their abundance can be affected by mechanical removal
or high stream flows (Haines and Tyus 1990; Tyus and
Saunders 2000). Second, we examined large-scale
patterns of growth and recruitment of early life stages
of wild Colorado pikeminnow in the Green River by
analysis of daily increment patterns in otoliths (pannella
1971; Campana and Neilson 1985; Bestgen and Bundy
1998; Bundy and Bestgen 2(01). Intra-annual patterns
of growth and survival of cohorts of larvae were then
related to time of hatching and environmental events
that may influence recruitment (Methot 1983; Crecco
and Savoy 1985; Post and Prankevicius 1987; Rice et al.
1987; Limburg 1996; Mion et al. 1998; Garvey et al.
2002). Third, we developed an individual-based simu-
lation model (lBM) that integrated results of experi-
ments, field studies, and environmental variables and
used it to explore potential mechanisms for growth and
recruitment patterns observed in wild populations of
age-O Colorado pikeminnow through their first summer.
We also used mM simulations to predict recruitment
patterns of Colorado pikeminnow under hypothetical
management actions (e.g., predator reductions, water
temperature changes), and compared simulation predic-
tions to patterns observed for wild populations to test
model generality. Synthesis of these investigations
allowed us to evaluate relative effects of various
physical and biological factors on growth and survival
of early life stages of Colorado pikeminnow.

Colorado Pikeminnow Natural History
The Colorado pikeminnow has a complex life
history depicted in a conceptual model that links
developmental stages with some important biotic and
abiotic controlling factors (Figure 1; Bestgen et al.
1997). Large-bodied adult Colorado pikeminnow
(>450 mm TL) in the Green River basin of Colorado
and Utah migrate to two spawning areas in spring