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Squawfish Population Viability Analysis --July 1993 Page 19 Apparently, only one genetical survey has been performed on Colorado squawfish populations, namely the .1989 work of Ammerman and Morizot. These authors, sampling mainly in the southern reach of the Green River found roughly 5% heterozygosity. Somewhat unusually, this overall heterozygosity was due almost entirely to two polymorphic loci. 2.3 The Level of Heterozygosity The level of heterozygosity, although by itself not meaningful, is usually the first genetic trait assayed for a population. Expected heterozygosity levels for individuals, denoted H, are commonly measured at the enzyme or protein level, usually using electrophoresis. They tell the fraction of the gene loci for an individual that have two distinguishable alleles. Values for H are normally in the range 0% to 20%. The reason that H values per se are not meaningful is that there are so many different arrangements of ecological and genetic forces that can maintain any given levels of H th~t it is impossible to know anything about a population simply from a knowledge of H. For example, H may be the result of balancing selection, or of a balance between mutation and drift, or of population mingling of migrants from local populations with different selection pressures or evolutionary history. One might have guessed that the H level for the Colorado squawfish should be low. First, the quite comparable Sacramento squawfish (see its distribution in Figure 2.1) has heterozygosity of 1 %. Second, a different line of reasoning is provided by Mitton and Lewis (1989), who have studied the relationship of heterozygosity levels and life history traits for fishes. Among other things, they find that low H is correlated with late first reproduction, and also with K-selected species. From the previous section on Colorado squawfish demography, the Colorado squawfish seems to be K-selected, with low adult mortality rates. Third, Nevo (1978) shows that H levels are low for some anadromous fishes that return to spawn at their natal site. Since some believe that the Colorado squawfish also home to their natal spawing site, this would fragment their population and thereby contribute to a low H.