Contact the Yakima WDFW office and ask what information they have on the river's whitefish population. Whitefish require cooler water temperatures than trout. If the Yakima has had some warmer than average summers recently, that could be a factor that is depressing the population. Over-harvest is a possibility, but it would be news to me to learn that very many people are keeping them. Whitefish congregate to mass spawn in November and December, so if you're fishing for them where they aren't, that could give you the impression that their numbers are less than they really are.