Fly fishing is more akin to hunting than a lot of other forms of fishing.
The fly fisherman “stalks” his prey by using their understanding of the characteristics and mannerisms of his quarry and its particular environment i.e. the natural course of the waterway and the debris it contains. Fisherman do this by “reading the water”.
Trout are much like any other animal surviving in nature – they try to find sufficient food to nourish themselves while at the same time hiding from predators, including human fishermen.
Fish often will find food in a streams “bubble line”. The bubble line is the visual sign that is made on the water surface as water flows over objects or it drops in depth. Trout will often hold in, or adjacent to, the bubble line devouring food as the water action throws it up.
Fishing the downstream end of the bubble line and methodically working subsequent casts upstream can often pay dividends.