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 7

1.6 Speculations on the Evolutionary Basis of Squawfish Life History

The biggest minnow sees evolutionary tradeoffs differently from lesser
minnows. The Colorado Squawfish has been king of the creek for most of
its evolutionary history. Importantly~ at some minimal size (let's guess 400
mm) it largely escaped predation, for juveniles possibly the strongest of the
mortality factors. As an adult, it entered a regime of extremely low
natural mortality (let's guess about 10% per year). The figure below
(Figure 1.1) is a guess at the curve of reproductive value with age for pre-
human disturbance squawfish.

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Age

Figure 1.1

The reproductive value of squawfish is drawn as increasing with age, this
in distinction from the typical curve for a mammal, which usually declines
after an early peak. There are several justifications fer this view. First,
the size of squawfish increases with age~ thus reproductive output increases
with age. Second~ mammals often senesce~ while cold blooded creatures
typically do not. That is~ the death probabilities for squawfish do not
increase with age; and~ if size is any protection~ they may actually decrease.
With increasing size with age~ and with low to nil predation on it~ the
squawfish has an evolutionary incentive to sacrifice immediate high growth
and high reproduction for higher survival, as she will be able to produce