This is just a personal observation and I'm not a scientist just a life long naturalist, I believe that another thing (as Toolman mentioned) that the the Trans Alta diversion did with the huge silt deposit was have a terrible effect in not only choking out spawning areas but also affecting insect cycles. Areas that where clear pebble bottom or compact where choked with inches of silt. I really did notice a huge difference in quantity of nymphs and their resulting hatches after that point. There where dramatically fewer Stones, BTO's and a cross section of others in traditional areas where I would normally be able to see them flipping stones and studying bug seigning. The Net Builders disappeared as did many of the larger stones. With lessening of a food source and then the hit of sapro on the Trout it was a perfect storm. As far as Whitefish go I look at them like the canary in the coal mine. They are indicators of a rivers quality as they rely on fairly pristine conditions to survive.