Flies and Fly Tying...What fish eat...
Without a fly at the end of your line, your chances of hooking a fish are somewhat diminished...pretty obvious. It is also true that without the right fly your chances may be about the same...nothing. So, the obvious question is "What is the right fly?" The answer to that question is simple..."It depends!". It depends on the species you are chasing, it depends on the type of water you are fishing, it depends on what kind of "fish food" is available to the fish in that particular piece of water and probably a few other things.
Trout eat primarily insects...more specifically aquatic insects and terrestrials. Generally, this means mayflies, caddisflies and stoneflies...often in that order. There are certainly other aquatic insects such as damselflies, dragonflies, crane flies, mosquitos and a whole group of what are termed "midges". As if this is not complicated enough, it gets worse. Aquatic insects have several life stages and trout may well be "keyed into" one of these life stages and almost ignore the others. For example, mayflies have three life stages (nymph, subimago [emerger/dun] and imago or spinner). If you want to get picky, eggs are a 4th. A mayfly nymph will live 1-3 years underwater and the two adult stages combined live only a couple of days.
Trout will also eat other small fish (a streamer pattern), leeches, crayfish, fresh water shrimp and scuds. Some larger trout (especially brown trout) will eat mice frogs, crayfish, etc.
Bass are a very predatory fish. They primarily eat other fish and this includes other bass including their young. Insects are also important to bass as insects are a primary food source for the young bass...generally not for larger bass. Other common bass food includes crayfish, frogs and even baby birds/ducks. Unlike trout that tend to be more "selective", bass are opportunistic feeders and will often consider anything that moves as food. A fishing guide in Florida once told me "Bass will eat just about anything that doesn't eat them first and can fit in their mouth". I recently read a good article on the Bassmaster web site. The title of the article is "What bass eat". Much of the information was provided by a fisheries biologist that primarily studies bass. See more of tying flies for bass here...
Steelhead and Salmon:
Both salmon and steelhead are only really available to the fly fisher once they leave the ocean and enter the rivers on their annual spawning runs. These fish are there to spawn and, as a result, tend not to feed as you might imagine. In both cases, these fish have prepared for the long journey by feeding heavily before leaving the saltwater. Steelhead will feed opportunistically along the way often on salmon eggs. Salmon feed very little while in the rivers. Many flies for salmon are "attractor" flies and their purpose is to irritate the fish into striking. Flies to imitate salmon eggs are also common. Salmon are known to eat salmon eggs. It is thought that they eat the eggs of other salmon out of an instinct to preserve their own species. The use of multiple bright colors is a common theme for steelhead and salmon flies.
Pike and Muskies:
Pike and muskies (muskellunge) are both carnivorous fish and are voracious feeders. Their diet consists primarily of fish but they also eat other things like frogs and smaller waterfowl. Both are ambush predators that lurk in the vegetation, making no movements, just waiting for a creature to get close enough to strike it.
Virtually all saltwater species that are of interest to the fly fisher primarily eat other fish. Some species like permit and redfish eat crustaceans (shrimp, mollusks and small crabs). Flies (streamers) that imitate a wide variety of "baitfish" are certainly most common. If fishing for permit, crab imitations are very popular.