Fly Selection and presentation
Another very important thing in fly fishing is the selection of flies. Also called matching the hatch, selection is very important on the river ability. If there is a hatch of caddis pupae to adult spinners, many times it is all the trout will eat. Presentation is also important. If you were to see the caddis pupae shedding their larval skin into adult spinner form, you will not catch much using a cased caddis larva.
To select a fly, you must first know what the trout are eating. There are two types of tactics you can use. The first would be using a locator fly, like a woolly bugger to find and catch a small brown or rainbow. Then you can see a little cooking pump and pump the stomach of the trout and see what it has been eating and match the size, color and shape as closely as you can.
The other type is careful observation. You can watch trout to see the types of rises, which tells you where the bugs are on the water. For example, f it leaps out of the water, you know whatever they are eating, it is in the air. Then you can look under rocks for larvae and in the air for flying bugs.
Also, for presentation, you must know what part of the water they are feeding in, and it can also help to know where the fish are, so you can let the current take the fly naturally with the water current.
The picture of a pumped stomach (scroll down a bit)