Hey Paul it's just starting.
Here's the trick, look for the slower water so you can see what is going on (dead slow with a dry line is even better). After spotting your fish move around so you can come up from behind them, making double shure you don't send any ripples in their direction. After you are in position (where you can cast comfortably), fire you fly well ahead of them so that it will be at their depth when it drifts into their zone. Now for that little thing that makes all the difference; once the fly is within their zone start a fast retrieve. I have found that many times Coho take no notice of a dead drifted fly, but will turn and attack one that looks like it is trying to escape. This works well with minnow patterns, leach patterns, and very well with egg sucking leach patterns. From what I have observed minnows and lampreys will prey on eggs in the reds, so it stands to reason that a coho would want to kill one of these preditors before it can prey on the coho's eggs.