As someone who has avidly fished for salmon in the NW for the last 30 years (~75 days per year), I can at least give my opinion that this is mostly BS.

Pervasive, non-selective, commercial and tribal gill-netting that is optimized to remove the largest specimens from the gene-pool is a big part of where I point my finger. Throw in the dams and massive increases in non-native, invasive California sea lions in the lower Columbia and you have a mess in the making. Lots of factors but I believe most of the problems are in-river, not in-ocean.

Also disclaimer, my opinions mostly come from experience on the Columbia (The largest Chinook runs in the world), not the Sound or further north.