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Squawfish Population Viability Analysis --July 1993 Page 4 1.1 Introduction to Demography A Population Viability Analysis (PV A) attempts to estimate the viability of a population, where viability may be quantified in te~s of mean (or median) time to extinction. Both deterministic and stochastic factors are considered in such an analysis. Spatial structure is often important, and it may be modeled with a metapopulation approach or source/sink approach. However, a sensible PV A must be based on a sound understanding of the biology of the species under investigation. And even though much detail may ultimately be left out of final models, this must be done in a defensible manner, based on considerations of relative importance. The Colorado squawfish has a very complex life history. There is a long prereproductive period. Juveniles disperse long distances down stream and remain there, in sahnon-like fashion, for several years, ultimately to return upstream (possibly to their natal home) to commence reproduction. Adults continue to grow throughout their life, certainly becoming more fecund. One must make some sense of squawfish life history before adding the complications of genetics, space and environmental variation. What one would like is a Leslie Matrix for Colorado squawfish, a matrix of the form shown below, where the age specific "reproduction rates" are given in the first row of the matrix and the survival rates are given on the sub diagonal. In the case illustrated below there are constant survival rates for the first six years as juveniles, denoted j, and then different, certainly higher, survi val rates for adults, denoted s: 0 0 0 0 0 0 ffi7 ffig ffig J 0 0 0 0 0 0 0 0 0 j 0 0 0 0 0 0 0 0 0 j 0 0 0 0 0 0 0 0 0 J 0 0 0 0 0 0 0 0 0 j 0 0 0 o. 0 0 0 0 0 J 0 0 0 0 0 0 0 0 0 s 0 0 0 0 0 0 0 0 0 s 0 0 0 0 0 0 0 0 0 s