7227

• UCREFRP Catalog Number: 7227
• Author: Kaeding, L. R. and D. B. Osmundson
• Title: Interaction of Slow Growth and Increased Early-Life Mortality
• USFW Year: 1988
• USFW - Doc Type: An Hypothesis on the Decline of Colorado Squawfish in the Upstream Regions of Its Historic Range
• Copyright Material: YES

292
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J F M A M J J A S O N D
Month
Fig. 3. Comparison of temperature regimes (average mean-monthly temperatures) of the historic lower and middle Colorado River and
of the present upper Colorado and Green rivers. Horizontal lines are temperature thresholds for growth (13° C) and the onset of
spawning (2()° C) of Colorado squawfish. Numbers in parentheses are the relative availability of temperatures suitable for Colorado
squawfish growth provided by each temperature regime (see text for explanation). See Figure 1 for sites oftemperature-data collection.
Age-0 growth analysis
'There was a highly significant, positive relation
(p<0.01, r = 0.95) between the mean total length
of age-0 Colorado squawfish captured in fall from
the Green and Colorado rivers and the relative
suitability of the temperature regime for the year of
capture (Fig. 4). The coefficient of determination
(r2 = 0.91) indicated 91% of the variation in fish
length was explained by variation in the suitability
index.
Age-0 Colorado squawfish are most often cap-
tured from river backwaters (Holden & Stalnaker
1975). Although the temperature regimes of both
backwater and main-channel habitats are largely
dependent upon ambient air temperature and solar
radiation and therefore are closely correlated with
one another, backwaters generally have larger diel
temperature variation than does the main channel
(Robert Green, personal communication). Colora-
do squawfish might make Biel movements between
backwater and main-channel habitats to maximize
the use of temperatures near their physiological
optimum (e.g. Magnuson et al. 1979). The impor-
tance of such behavioral thermal regulation to the
relation shown in Figure 4, which isbased onmain-
channel temperatures, is unknown.
Population simulation
As one would expect, our simulations showed that
fewer fish reach maturity as early-life mortality
increases; more important, however, they showed
that growth rate can have a pronounced effect on
survival (Table 1). Markedly more fast-growing
fish than slow-growing ones reach maturity at each
rate of early-life mortality, and this disparity in-
creases asearly-life mortality increases. The poten-
tialeffect of increased early-life mortality is there-
fore much greater in populations of slow-growing
fish than in those of fast-growing fish.
The combined effect of low survival to maturity
and advanced age at first maturity is reduced po-
tential for population growth (Cole 1954), as pre-
dicted by our simulations (Fig. 5). Moreover, we
illustrated in Figure 5 that growth potential of the
populations of fast-growing fish is markedly grea-
ter than that of the slow-growing ones, especially
when early-life mortality is 99%. Computation of
vital statistics for our simulated populations is ex-
emplified in the Appendix.
Although our simulations are useful for showing
the importance of growth rate of individual fish as it
can affect potential for population growth, they
also provide examples in which the growth of the