There are plenty of fish in both fresh water and salt water habitats willing to bite a fly fisherman's line. In fresh water, fishermen may land large and smallmouth bass, landlocked and Atlantic salmon, perch, Arctic char, trout and walleyes, to name a few. In sea water, fly fisherman may gun for barracuda, striped bass, bluefish, mackerel and sailfish, among others.
Trout is a common target for fly fishermen, and it's found in the stereotypical fly fisherman's flowing streams. But to catch trout, you need to know where it's likely hang out and what it's likely to eat. For example, according to Hughes, trout generally prefer waters between 45 and 65 degrees Fahrenheit. They become worn down by the current, so they usually hang around in nooks away from swift-moving waters. They have a pretty good sense of smell, which is why your waders will come in handy -- they'll block your body's scent from seeping into the water. And as for flies, you should choose one that's soft. Trout won't like the taste of a fly, no matter what. But they also won't like the feel of a hard fly and will be quick to spit it out. So you should use one that's soft. Mayfly, caddisfly and stonefly are trout favorites.
The best way to land a fish is to research it. Does it have a good sense of hearing? If so, keep quiet while you wade in the stream. How is its eyesight? Should you crouch while you cast your fly or cast it from further away in order to stay outside its range of sight? Knowing these types of things about the fish will help you to cast your line in the proper place with the proper fly.