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Squawfish Population Viability Analysis --July 1993 Page 39 3.7 Stochastic Growth The Colorado squawfish population sizes are well above the threshold for demographic stochasticity. Thus, neither genetic stochasticity or demographic stochasticity threatens them. This leaves environmental stochasticity and catastrophes. Environmental stochasticity is commonly computed in terms of growth rate and variance in growth rate (Goodman, 1987). We have estimates for neither the growth rate or for its variance, for the same reason we do not have the proper data to do trend analysis. Nonetheless, I believe we can also discount this stochastic force. Observers do not seem to notice great local fluctuations in population size (and such would be inconsistent with the relatively high adult survival rates). Further, diffusional movement of adults into local populations would tend to damp these out. The idea of "storage effect" is that periodically an age cohort survives through to the adult stage where it is quite invulnerable to environmental influences. This is seen as a way to defeat environmental variation. To a limited extent, the Colorado should be able to "store" viability in old adults in a manner that would defeat high frequency (e.g., the period shorter than a decade) environmental variation. 3.8 Catastrophes A catastrophe in the context of the Colorado squawfish would be something that would, with a single act, eliminate all the squawfish, adults and juveniles, in, say, the entire reach of the Green River. A rotenone spill at Flaming Gorge did not do this. An oil spill in 1987 (Tyus, pers. com.) did not do this. An exotic disease could possibly have such an effect, but the high fecundity of the species might permit some genetically based adaptive response to such a disease. 4.1 Introduction to Space Based Population Analysis The Colorado squawfish is distributed over about 1000 miles of one- dimensional river habitat. Some recognition must be given to this feature of its ecology. 4.2 Fragmentation and Metapopulation Analysis