Pacific Salmon: Ecology and Management of Western Alaska’s Populations

Genetic Health and Variables Influencing the Effective Number of Breeders in Western Alaska Chinook Salmon

Jeffrey B. Olsen, Steve J. Miller, Ken Harper, and John K. Wenburg


Abstract.—In this study, we used genetic and demographic data to estimate and evaluate an indicator of genetic health, the effective number of breeders per year (Nb), in Chinook salmon Oncorhynchus tshawytscha populations from western Alaska. Many of these populations show male-biased (70%–85%) sex ratios. Four such populations were examined: two from the Gisasa and Tozitna Rivers in the Yukon River drainage and two from the Tuluksak and Kwethluk Rivers in the Kuskokwim River drainage. Our objectives were to: 1) evaluate the genetic health of each population, and 2) infer the influence of annual fluctuations in census size, male-biased sex ratio, and variance in family size on Nb. Four genetic estimates of Nb were computed for each population to account for possible bias when using low frequency alleles. The lowest Nb estimates ranged from 225 (Tozitna River) to 4,859 fish (Kwethluk River) and were not indicative of a high risk of long-term loss of genetic diversity. Demographic and genetic estimates of the ratio Nb/N, where N is census size, suggested the observed sex ratio bias is unlikely to adversely impact genetic diversity at current population sizes. Variation in family size within each population likely had the largest affect on Nb over the time period examined. However, the time period was relatively short (3–14 years) and the estimates of Nb, could decline if populations show large future fluctuations, including significant decreases, in census size.