7742

• UCREFRP Catalog Number: 7742
• Author: Gilpin, M.
• Title: A Population Viability Analysis of the Colorado Squawfish in the Upper Colorado River Basin
• USFW Year: 1993.
• USFW - Doc Type: A Report to the United States Fish and Wildlife Service, Denver, Colorado.
• Copyright Material: NO

Squawfish Population Viability Analysis --July 1993

Page 24

It is possible that recent disruptions to movement may have melded two
previously isolated populations. One scenario is that a dam prevented the
down-stream juveniles from returning to their natal spawning location.
Thwarted, they then joined the population of a different spawning site.
This story could explain the Ammerman and Morizot result: intermediate
gene frequencies at two loci, fixation elsewhere. The two isolated
populations would have to have had almost 0 heterozygosity and would
have had to fixed for different alleles at two different loci. Then, and this
is problematical, they would have had to merge in a 50-50 fashion,
otherwise there would be a preponderance of alleles from a single one of
the two populations and the total heterozygosity would remain low. While
not impossible, I judge this 50-50 mixing as highly unlikely. Nonetheless,
genetic sampling should be performed throughout the range of the
Colorado squawfish.

2.10 Current Ne: Inbreeding Today

Adult Colorado squawfish population numbers are in the range 103 to 104
(Tyus 1991). In 1980, Franklin (1980) and also Soule (l980) suggested a
species population size of 500 as a lower viability limit for long-term
genetic health. The basis for this conclusion has been questioned (Lande
1987), but it is clear that heterozygosity that is maintained by the balance
between drift and mutation will decline as the population size declines. The
input of genetic variability through mutation is one part in 106, while, at a
population size of 500, the output due to drift is one part in 103. Of
course, these are neutral genes of no immediate benefit to the population.
However, long-term evolution depends on genetic variability for traits that,
while currently unimportant, might have future significance.

Ne is the genetic effecti ve size of a population. The census size is normally
larger. Such a larger population acts genetically like a smaller population
for a variety of reasons. The two sexes may contribute differently. Some
families may produce many more offspring than other families. The total
population size may fluctuate dramatically over the course of generations.
The population may have a spatial structure where different local groups
have very different long term contributions to the system.

The correction to Ne for unequal sex ratios is carried out with the equation