Your theory could very well explain the schizo coho numbers.
Chum, on the other hand, had not yet arrived in the basin with the early rains. Moreover, they have a habit of moving upriver regardless of rain or low water. They seem to just GO because that's what their internal clock says... and it should have happened in the week straddling Halloween. I believe in all probability that the poor commercial chum catches accurately reflect a run-size shortfall.
"Let every angler who loves to fish think what it would mean to him to find the fish were gone." (Zane Grey)
"If you don't kill them, they will spawn." (Carcassman)