AbstractPacific salmon are semelparous but have overlapping year classes, which presents special challenges for the application of standard population genetics theory to these species. This article examines the relationship between the effective number of breeders per year (Nb) and single-generation and multigeneration effective population size (Ne) in salmon populations that fluctuate in size. A simple analytical model is developed that allows calculation of Ne on the basis of the number of spawners in individual years and their reproductive contribution (productivity) to the next generation. Application of the model to a 36-year time series of data for a threatened population of Snake River chinook salmon suggests that variation in population dynamic processes across years reduced the multigeneration Ne by ∼40–60%, and reductions may have been substantially greater within some generations. These reductions are comparable in magnitude to, and in addition to, reductions in Nb within a year due to unequal sex ratio and nonrandom variation in reproductive success. Computer simulations suggest that the effects of variable population dynamics on Ne observed in this dataset are not unexpected for species with a salmon life history, as random variation in productivity can lead to similar results.