Sounds plausible to me, but, there must be some chinook who hang around the Columbia all year and are not coming from northern waters. I only say this because there are chinook available just off the mouth during the early chinook only season and throughout the summer before the upriver brights and tules arrive to ascend the Columbia and make up the Buoy 10 fishery. I don't think those chinook are there yet. But, there certainly are some chinook around now.I believe the coho do turn south but mostly head west where oxygen concentrations for forage is good and temperatures are favorable. I also think the deeper the water the less the "bouncing around" they get and ocean currents stir up "food fish". My unproven theory has a reasoning I believe is logical... When Coho are starting to inundate the "home spawning grounds" it is very typical that we get them south and west of the CR. Schools used to be a regular thing but recently the schools are few and far between but can be found in krill and anchovie rich areas. They're not that different from Chinook as far as forage/fattening up is concerned but remember, Chinooks come from the cold water in the north.They get into shallow areas where forage like crab larvae, krill and small bait fish meets their demands v Coho coming from the west south west and finding forage near rich, structured, off shore pockets of forage. When the ocean is roughed up and the sandy seafloor gets stirred up, they move to deeper water just as the forage would for security as well as predation protection.
That's just my theory, I have no "science" to prove it and "my" reasoning hasn't been able to rebut it so far. HOWEVER, the evidence of where we go to get Coho V Chinook has proven itself year after year. Of course the two can be found in both areas but OVERWHELMINGLY speaking (as far as the CR area is concerned).......... Coho are South, higher in the deep water column and Chinooks are found North and deep in what ever water they are in, even in shallow water.