Perhaps we are hijacking this thread a bit Lew, but yeah, I remember catching lots of whitefish "grayling" back in the 60's, when I started fishing the Vedder. Interesting post, as today I fished the upper river with an old friend and soon to retire FO who also fished the Vedder in the 60's and 70's ... we both could not come up with a plausible explanation for the demise of this once so numerous species.
Studies done have linked commercial cures used on eggs to juvenile mortalities...
I wonder if it's possible that commercial cures used on eggs have the same effects on white fish (super sensitive species). I could see them eating up any eggs that fall off a hook (caught quite a few on my roe flies over the past couple years, so I know they go for chunk roe)!
I could also concur with Lew, that siltation caused decline in food species.
I'd say the Vedder overall though, is getting healthier by the year. I'm catching far more large resident rainbows and bull trout down low the past 2-3 years than ever before, I never use to be able to count on 3-4 bullies day to save my days steelheading like I do expect now! In the past 3 years I've also started catching whitefish somewhat commonly again, which I had never seen before up until then.