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 PIKEMlNNOW RECRUITMENT

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TABLE I.-Recruitment patterns, mean growth (11..; mm), and sampling date for Colorado pikeminnow juveniles captured in
summer and autumn from backwaters of the middle and lower Green River in 1991 and 1992. The G-test was used to compare
the distributions of hatching dates of larvae captured in drift nets with those of juveniles in autumn captured with seines (Figure
6) to evaluate whether mortality varied with hatching dates. The relative sUlVival index is the observed proportion of autumn
juveniles in a cohort divided by the proportion expected from larval abundance; indices of 1.0 indicate that summer larvae
occurred in the same proportion as autumn juveniles in that cohort, whereas indices greater or less than 1.0 indicate relative
sUlVival greater or less than that expected. The predominant cohort was the one that contributed most to recruitment in that reach
and year. The predictions for recruitment and growth from individual-based-model (IBM) simulations are based on hatching date
distributions for wild larvae.

Mean TL (range) Statistic
Relative survival Predominant
Measured and predicted values Summer Autumn G-value df P indices of cohorts" cohort
Middle Gr=I River, 1991
Measurement 17.3 (14.4-20.8) 35.9 (17.6-53.4) 160.7 2 <0.0001 0.31, 3.06, 0.93 2nd of 3
(1-2 Aug) (20 Sep)
ffiM prediction 31.0 (12.8-54.0) 0.88, U14, 1.12 3rd on
Lower Gr=I River, 1991
Measurement 21.2 (12.9-27.0) 39.2 (22.0-65.0) 107.8 2 <0.0001 0.33, 0.66, 5.44 3rd of 3
(31 Jul) (20 Sep)
ffiM prediction 38.1 (15.5-62.3) 0.78, 1.03, 1.22 3rd on
Middle Gr=I River, 1992
Measurement 20.7 (14.8-27.0) 43.2 (21.5-66.9) 313.2 2 <0.0001 0.37, 1.68, 6.73 3rd of 3
(29-31 Jul) (25 Sep)
ffiM prediction 41.4 (16.9-62.3) 0.8, 1.26, 1.4 3rd of 3
Lower Gr=I River, 1992
Measurement 14.7 (9.0--29.9) 33.7 (20.0--59.0) 138.8 3 <0.0001 I, 0.38, 0.18, 25 4th of 4
(28-29 Jul) (19-20 Sep)
ffiM prediction 33.7 (11.4-60.9) 0.73,0.93, U19, 1.11 4th of 4
"There were Ihree cohorts for all locations and years except the lower Gr=I River in 1992, for which there were four.

larvae do not necessarily produce large cohorts of
autumn juveniles. In all cases, distributions of hatching
dates for larvae and autumn juveniles were significant-
ly different from each other (all P-values < 0.0001,
Table 1), in large part because larvae in the first cohort
were much reduced or nearly absen.t from distributions
for autumn juveniles. This was true even when early
hatched cohorts of larvae were abundant and remained
evident as summer juveniles. Second and third cohorts
of autumn juveniles in the lower Green River, 1992,
also had low relative survival indices. The bulk of
autumn juveniles in each reach and year were typically
larvae from the relatively late-hatching third or fourth
cohorts, which had relative survival indices greater
than 5. An exception was the middle Green River in
1991, when larvae from the intermediate second cohort
had the highest relative survival. The very high relative
survival index for cohort 4 in the lower Green River,
1992, was the result of poor survival of early hatched
larvae and relatively high survival of the few larvae
that hatched late. However, recruitment was very poor
that year because neither the larvae from the two main
modes detected by drift-net sampling nor the large
early peak of summer juveniles were present in
autumn.

Growth rates.-Within given cohorts, the mean
growth rates of Colorado pikemirmow summer juve-
niles were lower than mean growth rates of autumn
juveniles (Figure 7). This suggested that Colorado
pikemirmow that survived to autumn represented the
fastest growing subset of summer juveniles. For
example, in the middle Green River in 1991, mean
growth rates of autumn juveniles were greater in the
first (18%) and second (12%) cohorts than were mean
growth rates of summer juveniles in the same cohorts.
Mean growth rates of autumn juveniles in the first to
third cohorts from the lower Green River, 1992, were
56% higher than mean growth rates of summer
juveniles, which suggested lower survival of fish with
low growth rates. Mean growth of autumn juveniles in
the third or fourth cohorts was the lowest of any cohort
Growth rate comparisons between summer and autumn
juveniles for the third or fourth cohorts were not
possible because summer juveniles were not available.
Thunderstorm-induced high-flow events in summer
were associated with the slow growth of some
Colorado pikeminnow in the lower Green River in
both 1991 and 1992. Slow growth rates in 1991
occurred at a time coincident with a discharge increase
from 76.4 m3 to 88.3 m3/s in the lower Green River,
which began on 30 July and continued until about 12