Editor: Mark

Part 1: What Trout Eat—Know Your Bugs

This is the first part of a three-part series. Understanding what trout eat, where they find this food, and how to imitate it will mean your fishing trips become more about catching that searching. With your new-found ability to catch more fish, you are in that enviable guard of fishermen who have more fun.
• Part 1 What Trout Eat: Know Your Bugs
• Part 2 What Trout Eat: Know Your Water
• Part 3 What Trout Eat: Know Your Technique
We all know that big fish eat little fish. We also know that many trout species are particular aggressive or predatory. I’m setting that aside for this series. We’ll cover that important subject on another day.
The primary insect types for trout fishermen to know are mayflies (Ephemeroptera), stoneflies (Plecoptera), and caddisflies (Trichoptera).The adult stage of these insects is displayed here (not to scale):

What trout eat
What trout eat

Mayfly Adult Stonefly Adult Caddisfly Adult

A trout’s primary food source are aquatic insects that spend most of their life cycles underwater—anywhere from mayfly species that hatch in a few months to stonefly species that require three full years. When these insects are ready for their adult and final stage, they leave the water to mate and lay eggs before dying. Moving from the water to the air exposes these insects to trout, often causing a feeding frenzy. In fly-fishing terms, this event is called a “hatch.”
During a hatch trout can become so focused on this one food item that they will often eat nothing else, which is called selective feeding. “Matching the hatch is another common fly-fishing term meaning choose the right fly and present it in the correct manner to fool selectively feeding trout. Now you understand the importance of entomology (insect study). Greater success awaits the fisherman who is familiar with insect behavior, size, shape, and to a certain extent color.

what trout eat stoneflies
Mayflies are one of the most important aquatic insect groups to Trout. The life cycle begins when the egg is deposited into the water. The nymph hatches and grows on the bottom of the stream for several months to a year until it swims to the surface as an Emerger. At this point, it will struggle to break the through the surface film as an adult or dun. The dun sits upon the surface of the water letting its wings dry before flying off. Mayfly hatches can be prolific, presenting great opportunities for trout (and for fishermen!).
Once hatched, the typically herbivore nymphs begin foraging. The nymphs grow and develop through a series of molts, a trait common to all aquatic insects and invertebrates. Some mayfly species may molt in excess of 20 times during their nymph cycle. Water temperature has a profound effect on growth and emergence timing and the experienced fisherman understands when the conditions are good for a hatch.
The adult dun changes into the sexually active spinner anywhere from one hour to 3 days after shedding its outer covering. Spinners have clear wings and form mating swarms in the air. When a female comes into the swarm she is seized by a male and mating takes place. After mating, the male usually falls spent to the water or ground and the female begins depositing her eggs on the water’s surface or sometimes underneath the water. Then she falls spent, creating a spinner’s fall. Knowing this cycle means you can make presentations that will be in concert with what is taking place naturally.

stone flies
Stoneflies can be among the largest insects in a creek or stream. Most stonefly nymphs, despite their fierce appearance, eat plant matter. However, many species are predatory, feeding on other aquatic insects and larvae. Stoneflies typically prefer rocky, stony or gravel substrata, with more species frequenting cooler, swifter water. Studies have shown that different species prefer different living conditions, for example under rocks or in plant decay.
Stoneflies are so large that they represent a lot of protein to a trout trying to pack on weight. A stonefly nymph or adult, for example might provide the equivalent nutrition of 100 lesser insects. And stoneflies, unlike other insects, crawl across the river bed and to up the banks, where they sit stationary until they are able to break out of their exoskeleton and emerge as a winged adult.
Anyone who has fished a stonefly hatch will tell you that it can be among the best of life’s fishing experiences. As these adults drop to the water to lay eggs, and as they fall into the water, the trout activity can become furious. To throw the right imitation into that boiling cauldron can hook you into lunkers that typically don’t feed with such abandon. It is prime time fishing!

caddis flies trout

Caddis flies are relatively large aquatic insects that provide hungry trout with an excellent source of nutritional protein. In many streams caddis represent up to 50% of trout food source. Caddis, unlike mayflies and stoneflies, undergo complete metamorphosis, experiencing larva, pupa, and adult stages within a typical life cycle, rather than just a single nymph stage.
During the larval stage, caddis look like tiny, segmented worms. These wormy creatures are classified as either cased caddis larvae or free-living caddis larvae. Cased caddis spend the duration of the larval stage protected by a self-constructed case. Cases are cleverly and resourcefully fashioned of vegetation, gravel, and other tiny bits of debris and held together by a sticky silk secreted by the bug specifically for this purpose.
When the caddis “hatches” it moves rapidly from its position on the bottom and swims to the surface with the help of a little gas bubble. And, unlike the mayfly, who must sit on the surface floating downstream while easy prey to a waiting fish, the caddisfly bursts through the surface and flies away. The caddis is commonly the dominant aquatic insect in many trout streams. You would do well to fish more caddis patterns.

The Application
Notice the bugs streamside. Pay attention to the adult species that are in the air and dipping on the water. See what is on the rocks and the bushes beside the water. Mayfly and Caddisfly hatches can happen over an extended period. Stonefly hatches rarely last more than a week at any given location. You can get general hatch information at your local Division of Natural or Wildlife Resources. Your best recourse is to learn the insects that live in the waters you fish; know their approximate seasons and cycles. Notice what is happening on the water. Is there a hatch coming off? If so, what is the bug and its approximate size and color. You have that very thing in your flybox, right?!!
You will notice a decidedly different behavior in fish feeding on mayflies and caddis flies. Fish will sit and slurp or such the mayflies from the surface, almost casually. The bigger the fish, the less effort expended. It’s fun at dark o’clock, to fish to the big guy you can hear slurping mayflies but can no longer see. In contrast, during a caddisfly hatch you will notice the fish to be more active, darting and swirling in the water (which also occurs when stoneflies are crawling from the water to dry land). You will sometimes see violent explosions as the fish attacks an egg-laying stonefly adult, or as the fish tries to get the swimming caddis emerger before it can become airborne.
Since the majority of caddis are consumed sub-surface it only makes sense that a fly fisher employ a nymphing technique, for which I recommend the sparkle emerger, a deadly pattern. The other way to catch a fish on a caddisfly is with the adult pattern fished on top of the water, which will rarely disappoint.
Now, for those of you wanting to complicate your casting but increase your catching, tie the bead-head emerger or pupae form of the fly to the bend of the hook of the elk hair caddis dry (though not a great idea in swirling wind). Sink the sparkle pupae down with enough line and weight to get it into the feeding lane, and prepare yourself for more hookups.
There you have it: a quick and cursory stroll through the basic insect groups. Dig in and enjoy the rewards of better understanding the entomology that fuels the feeding activity of all stream and river fishing.

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