Identifying Aquatic Insects and Selecting the Correct Fly
When you are on a stream for the first time, the most important thing for a fly fishing angler to do is to identify the types of aquatic insects and the stage of the hatch for those insects. From this information, the angler can intelligently select the correct fly pattern to duplicate the species and the life cycle of insects the trout are currently eating. You may select a nymph, dun, wet dun, spinner, cripple etc. depending on your findings.
The 
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Here is a report from C.S.Gibbs who is the man behind the Fishwater Maps that are so helpful in locating trout. He necessarily needs to do a good deal of fly fishing in order to create those magnificant maps.