Not a lot new in this discussion, W. E. Ricker in 1981 published a paper where he pointed out chinook had been decreasing in size since 1920. The reasons were all of the ones discussed in this thread. All salmon species and populations have been changing size and age, increasing and decreasing over time. I think of it as evolution, adapting to a changing environment. Animals with a short life cycle do it faster. We humans in some populations are slowly getting larger and older but it is not in all populations.