On theory I have heard relates to the fishing pressure in the North Pacifci Gyre. From what I understand Chinook and other anadromous species typically make one

On theory I have heard relates to the fishing pressure in the North Pacifci Gyre. From what I understand Chinook and other anadromous species typically make one revolution per year (it's not uncommon for US born fish to be caught off Japan and Kamchatka). With an increase in fishing pressure fish just don't live as long any more without being caught. So the one, two, and three salt fish (two salt being a fish that has spent 2 years in the salt water), are more likely to pass on there genes. Being that how long a fish stays in salt water is largely genetic it would be easy to remove those genes from the gene pool in a relatively short time. I'm sure other factors like dams without fish passage on the Upper Columbia and Elwah (two rivers known for their large salmon) also played a factor. Additionally fish hatcheries largely select for fish that return at a younger age as they will return in larger numbers, and with such a huge amount of hatchery fish that would further drive down the average size. As would any spawning between wild and hatchery fish (although the reproductive fitness of hatchery fish is particularly bad).