This is a very good question.
It is probably safe to assume that any river near populated areas is experiencing lower returns of fish. But long time anglers like Chris can paint a more accurate picture.
Some rivers that have hatcheries might be keeping the numbers up, but the wild fish ("untouched" and "unmanaged" by people) are definitely on the decline.
Marine bioligists are trying to understand what is going on, how much of the decline is caused by people and how much by natural factors.
Some say lot of it is due to over fishing and declining productivity of the oceans. Others believe that warming is causing some of it.
All the damms and growing population isn't helping either.
I don't really know as I haven't fished for salmon for a long time and have nothing to compare to, but I have certainly caught a lot of jacks in recent years.
I don't keep any adult fish at all.