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 2

based on local extinction or turnover. I found sufficient
movement of adults to mediate genetic connectivity. I felt that
between-generation mixing (I.e., the recruitment of juveniles
into a non-natal population) was even more likely than the
'straying' of adults. I did not assume that all sections of the
system had equal growth and density. I thought if some
stretches had negative growth that they would decline to a
lower density and then be "rescued" from extinction in a
source/sink manner through immigration. Overall, however,
the total system would have a positive growth rate (A > 0).

The genetic evidence suggested freedom from inbreeding and
mixing of alleles throughout the system. A thorough allozyme
survey will reveal spatial structure and could even, by the
method of Slatkin, establish migration rates.

Stochastic growth seemed slight. The population sizes were
too large to be affected by demographic stochasticity. And
environmental stochasticity at the time scale of the generation
seemed to be small. Further, the spatial complexity of the
system would provide between-population damping; that is, if
environmental stochasticity along one stretch pushed the
population down, immigration from adjacent stretches would
partially compensate for this. This is a kind of shifting
source/sink situation. The spatial complexity would also allow
a protection against most plausible catastrophes.

The demographic analysis of the data was weak, incomplete
and somewhat inconsequential to the PV A. The modeling was
weak because only a few hundred fish were recaptured after
marking and age was difficult to estimate. The first 5 to 7
years of life were modeled as a black box from which emerged.
after a delay, recruits into the adult population. For the adults.
a reasonable estimate of the death rate was obtained. This did
not show senescence. This demography, while imprecise, did
permit a fairly clear understanding of the life history of the
species, from which certain evolutionary arguments followed.

Density regulation. The natural assumption (the prior belief)
is that a fish population is density regulated, has a carrying
capacity, K, about which its density will be roughly stable. One
corollary of this is that sustainable harvest is possible. I found