Fly fishing streamers is often over looked by fly anglers, especially trout anglers. Streamers is a category of flies fly fisherman have given to loosely mean food other than insects. Generally, being smaller fish, it could also be crayfish, leeches, and crustaceans. Bass and salt water anglers, are more familiar with the theory that big fish eat littler fish. However, as our sport is dominated by trout anglers we think insect first and many times that is as far as our thought process goes. However, big fish eat little fish, always have and always will. Fish are indiscriminate feeders and will eat what is presented to them. They don't have preferences, they take what they are given. And the nice thing about little fish is they are always present.
In most rivers and lakes there will be almost always be fish fry of one kind or another present. With the predominant season for fish fry being the spring, they easily will be present until late fall, and often year around. But even if fry are not around, minnows are present year around, sculpins, silversides, alewives, smelt, the list is endless. And even if you say there are no minnows available where you fish, well, trout are cannibalistic, if your fishing waters have big trout, then it also has little trout. In short, the 'hatch' is always on when you are fishing streamers.
Besides year around availability, another advantage of fishing streamers, is your average catch is going to be larger. Fish are masters of energy conservation and expenditure. They will not pursue food if it costs more energy to catch then they will receive from its consumption. That is why in the winter they will not chase food, you literally have to hit them in the nose with it. This fact, helps us to understand the feeding habits of large trout. A large trout holed down deep, not particularly interested in eating, will consume a minnow, if given the opportunity, no matter what. They might not rise up to devour size 22 Tricos, unless they are in a feeding mode, but given a minnow that they only have to lunge for, is an opportunity they won't turn down very often. That is a lot of energy consumed without a lot of energy exerted.
Techniques for fishing streamers vary from nymphs somewhat. A little action imparted by your rod tip, can help convince your targeted prey that the minnow is injured, thereby increasing strike potential. If casting into a river, the current will help you greatly in imparting 'swimming' action onto your streamer. Raisng and then lowering your rod tip, can also aid in the action. Vary the tempo until you find something that works. If fishing in lakes and ponds or from a boat on a river, you will need to retrieve. Again vary the tempo, using short bursts followed by a count or two of idleness. Allow your streamer to fall, often times the strikes happen when your fly is falling.
Favorite trout streamers include Woolly Buggers, both beadhead and non-beadhead. Mickey Finns, Jannsen's Minnows, and of course the veritable Muddler Minnow and its cast of imitators. Bass fisherman have also used the same patterns for years and years. Saltwater patterns all virtually fall under the streamer category with perennial favorites being the Lefty's Deceivers, Clouser Minnows, and Crazy Charlies. As well as various crab and shrimp patterns.
Trout fisherman might have to bulk up their gear a little, depending on what they are used to fishing with. 6 wt rods are usually preferable, with enough stoutness to cast the bigger fly, and fight those sure to come bigger fish.