Baitfish Flies for Saltwater Fishing
Saltwater fish, especially in estuaries, favor minnows for bait. You may use a baitfish fly to mimic not only the coloring of the minnows, but also the reflection of the sun off their scales and the movement in the water to attract fish that hunt for food near the coastline or within estuaries. This is one of the types of flies that mimic another fish, not an insect. Use them for bluefish and stripers, but also for redfish.
If you are an angler who keeps a close eye on the hatching of the insects that are local to your fishing hole, you would use these types of flies to mimic the most current prey of your target fish. This requires a good understanding of changing seasons and weather conditions---as they impact the presence of certain insects in or near the water---but pays off with consistently having just the right lure for the fish you are after. Commonly used flies of this type are the nymph or streamer and also terrestrial fly. Target fish are trout, bass and carp.
Use wet flies when you are seeking to attract fish that feed just below the water's surface or just a bit deeper. Wet flies sink, depending on the weights you attach, slightly below the water's surface or all the way to the bottom of the lake or stream. These types of flies include streamers and you would use them most commonly for targeting bass.
Use dry flies to target fish that feed from the water's surface. These flies are smaller than wet flies, and they imitate an insect that might have fallen into the water but is too light to sink to the bottom. These are excellent flies for attracting trout in lakes as well as also slow, calm rivers. When shopping for these flies, you will sometimes find them under the moniker "terrestrial flies."
Target Species Flies
If you prefer to specifically target a species of fish rather than broadening the appeal of your lures, you may opt for target flies. You see them used most commonly by salmon anglers, who do not want to second-guess insect hatching conditions. Keep in mind that these specialized flies rarely appeal to other species of fish, and unless you are certain that your target species is plentiful at the fishing hole, it is unwise to use them.