My two labs pretty much stay out of stuff. I do think it has to do with the training though. I watch my dogs pretty close and if they are in the same spot for more than a few seconds I pay close attention. As soon as their head goes down (I assume they're about to eat whatever it is they were smelling) I give a firm yell of "NO!" It works.

As for the dead salmon thing in fall when walking the rivers. I watch them real close. They get obsessed with the rotten fish so I circle around and wait for that second when they are about to do the lay down and roll in it thing. As soon as they start I give the "NO" shout and smack 'em on the ass at the same time. The first dog learned to stay away from them after the initial encounter with my hand and my current lab learned it after a good swat on the 2nd try. This was when they were under 6 months old and I've never had to do it again. Now, if they are in the same spot for more than a few seconds, all I have to do is say no and they move on.

It takes some watching but is easy to do after the cast while the fly is swinging thru the run. Fish will give me a tug and then I pay attention to that.