Here is a wonderful explanation of sonar returns and fish arches. Maybe the "big ones" aren't biting because they're not there. All this time I thought I was just a lousy fisherman. :smiled
My understanding is that Salmon have a verys small air bladder, so don't really sho well on souners anyway.
Side note: When I sold marine electronics for a living, I would often get calls for people looking for GPS / Fishfinder combo units. What's so unusual you ask? They beleived that the satillites would show where the fish were...