CONSERVATION GENETICS OF PACIFIC SALMON .2. EFFECTIVE POPULATION-SIZE AND THE RATE OF LOSS OF GENETIC-VARIABILITY

A simulation model for genetic change in Pacific salmon (Oncorhynchus spp.) populations is extended to examine the loss of genetic variability. The study was designed to address three major questions: 1) What is the rate of loss of genetic variability from Pacific salmon populations as a consequence of finite population size? 2) How can existing population genetics theory be used to predict patterns of genetic change in Pacific salmon? and 3) What do the concepts "effective population size" and "generation length" mean with respect to Pacific salmon? Results indicate that effective population size per generation (Ne) is equivalent to the effective number of breeders per year (Nb) times the average age at spawning, or generation length (g), and that the use of Ne= gNb in standard equations for the variance of allele frequencies, loss of heterozygosity, and loss of alleles over time accurately predicts the rate of genetic change in the population as a whole. With Pacific salmon, however, genetic data are seldom gathered for a population as a whole; instead, data are typically taken from individual brood years, which represent only a portion of the individuals comprising a generation. The magnitude of genetic change among brood years is larger than predicted using standard equations. In addition, uniting of progeny from different brood years in each year's spawning population creates a Wahlund effect that slightly reduces the observed proportion of heterozygotes. Formulas are provided to adjust standard population genetics equations for the effects of measuring change among subsets of the population. It is shown that low frequency alleles are subject to rapid extinction in Pacific salmon populations with Nbless than about 100 per year, and the implications of this result for the conservation genetics of wild and cultured populations is discussed. © 1990 The American Genetic Association.