These different types of trout foods may not only be specific to a stream, but seasonal as well. Trout are limited to what is presented to them much like many animals in the wild. Typically spring and summer offer a great abundance of food choices. Winter may only provide limited food supplies. Trout adapt to the cold water by naturally reducing their metabolisms.
Familiarity with different food sources is one of the fundamentals of successful fly fishing. Let's have an overview of these trout foods.
Aquatic Insects - mayflies (Ephemeroptera), caddisflies (Trichoptera), midges (Diptera), and stoneflies (Plecoptera)
For many, fly fishing is centered around the life cycle of aquatic insects as much as it is the trout themselves. Many anglers unwittingly become pretty good entomologists in pursuit of fly fishing. These insects are a significant part of any trout's diet throughout the year. Most aquatic insects live almost 98% of their lives in the water. Trout will feed on these bugs during all times of the insect's life cycle. Most notably trout will key in on active or passing nymphs in the water. For a brief period at the end of these insects' life, they hatch from the water to mate, lay eggs and die.
For many fly fishing anglers, mayflies are the belle of the ball and can be found hatching in significant numbers from April thru July. They are found during all times of the year, but just more sporadically. Under the correct conditions, a few streams even have small occasional hatches of blue-winged olives (BWO) in the dead of winter.
Midges, stoneflies, and caddisflies are very common in streams and have similar life cycles. Specific behavior with all these insects can vary greatly beyond the living, molting, emerging, mating and dying cycle. Certain types of caddis live under rocks with little wooden stick homes protecting them, while some mayflies burrow deep in the muddy ends of pools rarely being seen until they emerge. There is a lot of diversity and behavior between these insects that should be understood.
Fish - small trout, minnows and sculpins
A wide variety of small fish can be considered part of a trout's diet. There are many types of smaller fish including young trout, darters, minnows and sculpins that are trout favorites. Habitat and water conditions influence which type of small fish patterns are the most successful.
Terrestrials- ants, beetles, grasshoppers and caterpillars
These are all those bugs that don't live in the water but can be found by late spring thru the fall landing in the water as trout food. About any insect that can fall off the banks or out of a tree can find itself in trouble with actively feeding trout. I have seen trout gorge themselves on caterpillars falling out of trees in June but also quietly picking off ants by the edge of a stream in September. Out west grasshoppers are all the action during late July and August.
Crustaceans (Crustacea)- crayfish, freshwater shrimp and scuds
While crayfish are very common, scuds and shrimp are more often found in nutrient-rich streams with abundant plant life in limestone-fed waters. Scuds and shrimp need this type of habitat to survive. In limestone streams, trout can be seen nosing into the weed beds feeding on these scuds. Crayfish can thrive pretty well in streams with just rocks and a modest bottom structure.
Mammals - mice and other small rodents.
Trout can be pretty aggressive predators. On some streams, larger trout can key in on a mouse swimming across a stream that they can easily prey on. Anglers will typically try this approach in the evening since rodents are generally nocturnal creatures.
Trout and other fish deposit eggs during their spawning seasons. Trout will commonly follow up behind these spawning fish and take advantage of this opportunity to get an easy meal. Suckerfish spawn in late winter and very early spring. Rainbow trout spawn in the spring, with brook and brown trout spawning in the fall.
Beginners can follow along and learn more in the Beginners Forum.
FlyFisherman - What trout eat
Other Suggested Books
Handbook Of Hatches: Introductory Guide to the Foods Trout Eat & the Most Effective Flies to Match Them by Dave Hughes
Trout and Their Food: A Compact Guide for Fly Fishers by Dave Whitlock