what about the bubbles coming out of water you notice almost every time out fishing, would that be trout too or perhaps just a buildup of oxygen underneath ?
Two options, bubbles caused by water running over rocks/rapids and floating off downstream, or, on slower bits of river, escaping gas from silt build up on the bottom.
It is fair to say that on a still water containing fish such as Tench, Bream and Carp, there is a distinct possibility that bubbles are caused by the fish rooting about in silt. Eels will also cause the same, but normally as a sudden large, long patch of small bubbles as they roll in the silt, kicking up food items.
I have yet to see a river trout rooting about in the bottom, but I guess it is possible.