Dave and others -
While I'm certainly not an expert I have several observation about the willingness of coho to bite.

1) It is not a hatchery - wild issue - currently nearly 90% of the coho in the Snohomish are wild and as we all know they are among there worst biters in the Sound.

2) Just as clearly fishing pressure (harvest) makes a big difference. During the 1980s and early 1990s on those years that coho harvest was generally prohibited on the Skagit and Stillaguamish I found the coho were "pests" while cutthroat fishing. Yet within hours of an open season they became much more difficult.

3) It seems to me that coho return to the terminal areas and local rivers as more mature fish than decades ago. They have "shut down" their feeding making them much less agressive - this is often occurs well out in the Sound. While initially they are willing to "chase", follow or swipe at active lures or bait they seem to have lost interest in feeding. Have noticed that even out in the off-shore rips of the inner Sound that the fish often have to be tricked into taking. I can believe that this is a response to the intense harvest over the decades - those less agressive fish were more likely to escape the fisheries to spawn and those make more of these "timid" fish.

4) I feel that curiosity is a factor in tiggering a bite from heavily fished freshwater coho. In clear water the fish can see a typical lure at quite a distance and seem loss interest in it long before the path of the lure and fish cross. I think that is why often the best fishing is on a falling water with poor visibility - with only a foot or two of vis the fish takes the lure before it losses interest. That may explain why the more finesse lures (Dick Nites, small baits) work better than others in clear water. (an aside - I first saw Dick Nites on coho used 30 years ago on the Nooksack in quite turbid water).

It would also explain why odd ball approaches or a change up approach often produces while the tried and true doesn't.

I have also noticed that once several fish are taken from a pod they often shut down. Additional fish can sometimes be coax from the pod by changing to a different lure, color or retrieve. If there are not other anglers around resting the pod for time also often will allow another fish or two to be taken.

While the charge of a fresh coho to the fly is one of fishing most exciting momments the fishing over literally thousands of fish and being greeted by yawns and snubs is one of the most frustrating.

Tight lines
S malma