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

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FiGURE 5.-Response surface of probability of capture of
Colorado pikeminnow larvae by red shiner predators, as a
function of prey total length2 (IL 2) and predator-prey TI..
interaction.

were 0.44 mm/d in 1991 and 0.31 mm/d in 1992. Red
shiner size structure was that used for simulations
described earlier. Mean daily water temperatures in
simulations for corresponding periods were obtained
from U.S. Geological Survey records from nursery
habitat reaches of the middle (gauge 09261000) and
lower Green River (gauge 09315000). Simulations
concluded on dates that corresponded with timing of
autufffil field sample collections of Colorado pike-
minnow (McAda et al. 1994), usually late September or
early October.

Results
Laboratory Experiments
Probability of capture.-Predation experiments
conducted in aquaria demonstrated that red shiners
were moderately successful and persistent predators on
Colorado pikeminnow larvae (Figure 5). Mean number
of attacks by individual red shiners in a 20-min trial
was 21 (0-72); inspection of data showed no
relationship between number of attacks and red shiner
length. Capture success was size-dependent for both
predators and prey. Probability of capture ranged from
near zero for small shiners attacking large larvae, to
nearly 0.30 for large shiner attacking small larvae. Red
shiners captured Colorado pikeminnow as large as 23-
rom TL.
Attack rate experiments.----Dver all experimental
conditions and prey sizes in the 6-h mesocosm
experiments, red shiners captured an average of 7.9%
of the Colorado pikeminnow prey available. Attack
rate was size-dependent, larger larvae being attacked

more frequently than small ones. Solutions for equation
(3) revealed that a 15-rom larva was attacked about
twice as often as a lO-rom larva, and a 20-rom larva
was about 4 times as likely to get attacked as a lO-rom
larva Relatively large and positive coefficients for
turbidity and alternative prey in equation (3) demon-
strated that presence of each factor significantly
reduced attack and predation rate. Over all size-classes
and in the absence of alternative prey, red shiners
captured about 11.6% of pikeminnow prey in clear
water but only 2.9% in turbid conditions. In clear
water, red shiners captured about 11.6% of pike-
minnow prey in the absence of alternative prey, but
capture rates declined to 3.0% when alternative prey
were presenL Capture rate of pikeminnow prey when
both turbidity and alternative prey were present in
mesocosm trials was 1.0%.

Field Studies

Distributions of hatching dates.-Hatching dates of
larvae in the Green River in 1991 and 1992 were
episodic, showing one or more distinct modes, which
were often early in the reproductive season. The middle
Green River group represented fish from the Yampa
River spawning site that drifted downstream and
occupied backwaters from near Jensen, Utah, down-
stream to Desolation Canyon. The lower Green River
group represented fish from the Green River spawning
site that drifted to backwaters downstreana of Green
River, Utah. In the middle and lower Green River in
1991 and in the middle Green River in 1992, larvae
captured in drift nets began to hatch between 18 and 22
June (Figure 6), but in the lower Green River in 1992,
they hatched as early as 8 June. In nearly all years and
reaches, most hatching was completed by mid-July.
The vast majority of larvae captured in drift nets were
8-10 rom TL and 6--8 d old, although larger and older
larvae were occasionally captured later in the season.
Larvae arrived in backwater nursery areas about 8-10 d
after hatching, based on ages of larvae at upstream
capture locations and downstream transport rates
determined by stream flow rates.
Colorado pikeminnow were collected by seine
sampling in late July or early August to follow their
growth and survival in backwaters. Distributions of
hatching dates of Colorado pikeminnow surruner
juveniles revealed that most fish captured were from
either the first cohort of larvae hatched or the early
portion of the second. Larvae with later hatching dates
were not represented in the distributions, probably
because the fish were not susceptible to the sampling
gear used in backwaters or had not yet arrived.
Comparisons of distributions of hatching dates
showed that large cohorts of Colorado pikeminnow