7226

• UCREFRP Catalog Number: 7226
• Author: Kaeding, L. R. and D. B. Osmundson.
• Title: Slow Growth of Colorado Squawfish in the Upper Basin and Its Effect on Recovery.
• USFW Year: 1986.
• USFW - Doc Type: \
• Copyright Material: YES

populations of slow and fast-growing fish. We assumed fish in the
"slow" population grew at the rate of upper basin Colorado
squawfish (Seethaler 1978), whereas those in the "fast" grew like
squawfish in the Osmundson (1986) pond (Figure 2). Because
E maturity in upper basin Colorado squawfish begins at about 428 mm
TL (6 years of age) (Seethaler 1978) but the pond-raised fish
probably would reach this size by their third year (Figure 2), we
assumed maturity in both simulated populations occurred at 410 mm
TL, 6 years of age for "slow" fish and 3 for "fast." Simulations
were made with 80, 90, 95 or 99X annual mortality in the smallest
length class, but with 209: annual mortality in subsequent length
;g classes. Such rapid, early-life mortality and a reduced, constant
rate for later ages is typical of many freshwater fishes (cf.
) Weatherley 1972).
~ The simulations show fewer fish reach maturity as early-life
mortality increases, and that growth rate has a pronounced effect
on survival (Table 1). Markedly more "fast" fish reach maturity
than do "slow" under each rate of early-life mortality and this
3 disparity increases with additional early-life mortality. The
effect of increased early-life mortality therefore is much greater
on the "slow" population than on the "fast."
The combined effect of low survival to maturity and late age
~¢ of first reproduction is reduced potential for population growth
!~ (cf. Cole 1954). We simulated growth potential of our "slow" and
"fast" populations using survival to maturity when early-life
mortality was 95 or 999; (data from Table 1). Computation of vital
statistics for these populations is shown in Appendix Tables 1 and
2. Net reproductive rate (R„ the number of mature female
offspring produced in the lifetime of a female parent) is much
greater for "fast" than for "slow" females in both simulations.
Mean length of generation (G, the mean period between the birth of
the parent and that of offspring) is 6.5 years for "fast" fish and
8 for "slow." Finite rate of increase (f) is the multiplication
factor by which the adult female population wi17 annually grow if
that particular value of Ro is maintained. The simulation shows
growth potential of the "fast" population is markedly greater than
that of the "slow," especially when early-life mortality is 9992
(Figure 4). Weatherley (1972, Chapter 6) gives a similar example
of reproductive potential of "slow" vs. "fast" populations, but
also considers important differences in size-specific fecundity
between populations. Consideration of such differences would make
,;~ tt~i d-ispari ty between growth potential of our "slow" and "fast"
populations even greater.
In addition to precluding rapid growth, temperature regimes of
upper basin rivers have another important effect on Colorado
squawfish. Water temperature is a cue for spawning of temperate-
zone fishes. Colorado squawfish begin spawning when temperatures
reach 20-22 C (Hamman 1981, Tyus and McAda 1984, Haynes et al.
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