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There are many things you could use as bait but if you are using a fly rod then you need to use flies.
Flies are not actual living flies because using live bait in some states or countrys can be heavily ILLIEAGL!!!!
Flies are tied on the front of the line and can be launching far distances due to the wrist motion in the cast.
There are many kind of flies out there in the Fly Fishing world but the three main flies are the Nymfs, Dry flies, and streamers.
The Nymfs are the flies that will sink all the way to the bottom of the body of water you are fishing in,
Dry flies are different because they float on the water so when the fish are feeding on top of the water they will come up and eat the fly
and it is very easy to see whe the fish bites due to the fishes head being out of the water.
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There are still so many other flies in this world and yet that list wouldn't put a dent in the Types