Genetic Monitoring Reveals Genetic Stability within and among Threatened Chinook Salmon Populations in the Salmon River, Idaho

Identifying and understanding temporal genetic changes within fish populations is important for the management of these populations, especially those of conservation concern. Such changes are often the result of genetic drift, which can be exacerbated when the size of a population decreases. Using molecular-genetics techniques, we monitored nine populations of Chinook salmon Oncorhynchus tshawytscha in the Salmon River, Idaho, to determine how the genetic characteristics within and among these populations have changed over time. We found no evidence of change in the level of heterozygosity or allelic richness over three to four generations in eight of the populations. This is probably due to the fact that the populations all maintained a sufficiently large effective size, even though a few of the populations did show a decline in effective size. Also, the genetic structure among the populations did not change appreciably over time. Populations that had been supplemented with hatchery-reared fish showed genetic similarity to the within-basin hatchery source population, presumably because of the extensive use of native fish for hatchery brood stocks and minimal out-of-basin stock transfers. The lack of a detectable decline in these populations' levels of genetic diversity is encouraging, given the species' threatened status under the U.S. Endangered Species Act.