7835

• UCREFRP Catalog Number: 7835
• Author: Osmundson, D. B., et al.
• Title: Studies Of Colorado Squawfish In The Upper Colorado River, Final Reports.
• USFW Year: 1997.
• USFW - Doc Type: Recovery Implementation Program, Project No. 14,
• Copyright Material: NO

Monte Carlo simulation based on the mean and variance of the growth increments for each
50-mm size class was also used to estimate mean length at each age. Twenty simulations
were conducted using different random number sequences. The mean and variance of 7-year-
old fish, as determined using scales, was used to generate the initial size distribution. For a
simulated population with 10,000 fish of each age, lengths for each age were calculated based
upon the simulated distribution of sizes in the previous age class. From these simulations, the
range of ages expected for a given length were also calculated.
To aid in constructing realistic growth simulations, statistical tests were conducted to
determine if growth of fish in one year was related to growth in the following year, and
whether differences occurred in mean growth among years. Using fish captured in multiple
years, contingency table analysis was conducted to test the relationship in growth between
years for individual fish. This analysis determined whether above (or below) average growth
was related to above (or below) average growth in subsequent years. We used analysis of
covariance (initial length as covariate) to test for differences in growth rates among years for
the period 1991 through 1995.
Survival
Estimates
Survival rates were estimated for Colorado squawfish Z 550 mm TL. We assumed survival
rates of juveniles and subadults were likely different from adult rates but had no way of
assessing these rates. These estimates were limited to adults from the upper reach only (Fig.
1) because longitudinal variation in size distribution coupled with unequal sampling effort
between lower and upper reaches would have biased whole-river length distributions.
Estimates of adult rates were limited to fish z 550 mm TL because Seethaler (1978) reported
immature fish as large as 503 mm TL. Estimates were made using a modified Chapman-
Robson (Seber 1982) approach where survival is based on declining numbers of individuals by
age in the population. Because captured fish could not be reliably aged, estimates of survival
were made by comparing length distributions from captured fish with theoretical length
distributions. The method used makes the following assumptions, which are consistent with
Ricker (1973):
1) survival rate is uniform with age over the range of ages examined;
2) survival rate is uniform over time, and does not vary among years;
3) recruitment to the first size examined is equal among years; and
4) sample is uniformly drawn from all ages/lengths considered, i.e., no effect of gear
selectivity.
Theoretical length distributions (termed "stable length distributions") were calculated from
age distributions assuming constant survival rates and constant recruitment into the youngest
age class (termed "stable age distributions"), and age-length distributions generated in this
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