I agree with Spokey. Selective harvest has been removing the largest genetics for generations before non Indians got here.
Salmon have continued to get smaller, just at a way faster rate in recent times. Their is no maximum size limits in Washington, so sport and commercial harvest has also has contributed.
If anything, Pink Salmon are a example of this. Because of there fixed life cycle, they have shrunk less. Our harvest model attacks the multi year class fish most.
The average Alaskan chum this year is less than 8 pounds.