If you look at the history of sockeye returns in the Fraser River, you can see that there have been certain environmental effects that have caused considerable reduction. For example, the slide in Hell's Gate in the early 1900s had a devastating impact on the overall returns of sockeye, and so you will see cyclical differences between the populations.
As you have noted, the sockeye come back every four years. Every one of those years in between is a different population, and there will be different strengths, so you're going to see differences between one year and the next because you're looking at different populations of sockeye.