What Trout Eat

Trout eat a host of aquatic insects, terrestrial insects, other fish, crustaceans, leeches, worms, and other foods. The food items that are most important to trout and fly fishers are the aquatic insects that spend most of their life cycles underwater in rivers, streams, and still waters. They grow to maturity underwater and transform to flying, air-breathing adults that mate in the air above our favorite waters. 

This movement from the water to the air exposes the insects to predators such as trout and birds, and often causes a feeding frenzy. This event is called a "hatch" and is the situation all fly fishers search for.

During a hatch, when insects emerge en masse, trout become so focused on this one food item that they will often eat nothing else. This is called selective feeding.

"Matching the hatch—another common term you'll hear in fly-fishing circles—is the art of choosing the right fly and presenting it in the correct manner to fool selectively feeding trout. To do this you must be able to identify the insect, be familiar with its behavior, size, shape, and to a certain extent color, so it's important to have a working knowledge of the most important types of insects: mayflies (Ephemeroptera), caddisflies (Trichoptera), midges (Diptera), and stoneflies (Plecoptera).

Don't worry, you don't need to know the Latin names of each type of insect, but it may help later on when you learn to discern one type of mayfly from another and want to accurately describe the insect to fellow fly fishers. A good text on identification is Hatches II by Al Caucci and Bob Nastasi. A more up-to-date guide is troutnut.com, which would be better named as bugnut.com because the site is almost entirely devoted to current freshwater insect identification and information.

During your journey toward being an expert fly fisher, you should make a habit of picking up rocks from the riverbed and examining nearby bushes to identify the important insects in that stream. Some fly fishers eventually evolve into amateur entomologists and take and keep samples of the insects they find with the idea of tying flies that more accurately imitate them.

Mayflies

When fly fishers think of a hatch, they usually think of a mayfly hatch because mayflies create the most elegant fishing situations, are important foods in all trout waters, and have been studied and written about by fly fishers for hundreds of years.

Mayflies begin life as an egg, and hatch into an aquatic stage known as a nymph. Nymphs usually live about a year but may last two years or more, or just a few months, depending on the species. Some mayfly species have two broods per year, making them important in the spring and again in the fall when the next generation matures.

Mayfly nymphs range in size from 4 mm to 40 mm and most often have three tails (sometimes two).

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Some mayfly nymphs are burrowers, others have adapted to cling to rocks in fast water, so each nymph species has a different body shape and design. Most are dark on top (mottled brown, tan, or dark olive) with a lighter-colored underside.

There are sophisticated fly patterns designed to accurately imitate specific mayfly nymph species on specific waters, but in most instances, general-purpose patterns such as Hare's-ear Nymphs, Copper Johns, or Pheasant-tail nymphs in sizes 8 through 18 are fair imitations of nearly all important mayfly nymphs.

When a mayfly nymph rises toward the surface and splits its shuck, the insect that emerges is called a dun (technically a subimago or pre-adult). They have two large, upright wings, two or three tails, and most have two very small hind wings. The wings are opaque, and their bodies are often drab-colored.

Duns are the mayflies that ride the water's surface in an upright position while their wings dry before taking flight. It's a cliché, but fly fishers often compare mayfly duns to miniature sailboats.

When duns are on the water, you are in the hatch situation fly fishers live for, and it's time to fish with dry flies, which float on the surface of the water.

Hopefully you'll find trout rising to the surface to eat these mayflies, and if you can get the right fly into the right place, you'll watch the trout rise to the surface and close its mouth around your fly.

Your local fly shop has bins full of dry flies to imitate the dun stage of each local mayfly species, but for most trout you don't need an exact imitation. As long as you have a fly that is about the right size and shape, and you deliver it accurately, you will catch some fish.

A Parachute Adams is a good fly to imitate all mayfly duns. It has the right profile, the white wing post is easy for you to see on the water, and the gray body fools many trout. You'll need them in sizes 8 through 20 depending on the size of mayflies in your local waters. Other good mayfly dun imitations include René Harrop's Hair-wing Dun or Caucci and Nastasi's Compara-dun.

After they hatch, mayfly duns fly to streamside vegetation, where they molt or shed their skins and enter the adult or imago phase fly fishers call "spinners." The change from dun to spinner often results in a different body color, and spinner tails are longer than dun tails. The most noticeable difference is that the wings of mayfly duns are opaque or cloudy. Spinner wings are usually clear.

A short time after molting into spinners—usually within 24 hours—the mayflies fly back to the water and gather in large swarms over riffle areas, where they mate. This most often happens late in the evening or early in the morning.

The females lay eggs and then die in the egg-laying process. Males continue to fertilize eggs until they also fall spent to the water, with their outstretched wings flush with the water's surface.

Trout sometimes prefer spinners over duns because they have learned that spinners have no chance to escape—they are dead—and are easier meals. Also, duns hatch over a relatively long period of time, while spinners fall to the water en masse, creating an irresistible feeding opportunity.

Because spinners lie flush in the surface film, you may need a spinner pattern such as the Rusty Spinner (or other body color) with outstretched wings. One of the reasons we suggested a Parachute Adams for the dun stage is that it also works well for the spinner stage. The parachute hackle leaves a footprint on the water that is similar to the outstretched wings of a mayfly spinner, and the trout often ignore the upright parachute post. Flush-floating mayfly spinner imitations are tough to see, and tough to fish, so it's probably a good idea to give the Parachutes a try first, and move to more exacting patterns later if the fish refuse them.

Caddisflies

In most streams there are at least as many caddisflies as mayflies. Because caddisflies are more tolerant of water pollution, caddisflies are by far the most dominant stream-bottom insects in some streams.

Caddisflies begin life as an egg on the river bottom and have three additional life stages: larva, pupa, and adult. The eggs hatch into small worms (larvae) which is the longest-lasting part of the life cycle, and often exists for a year or more.

There are more than 1,000 different caddis species; but to begin you need only to recognize two general types: one where the larva builds a case from sticks or sand and pebbles on the stream bottom, and another free-ranging type with no case. Cased caddis are normally cemented to the rocks, but they become important when there is a dramatic flush of water that churns the river bottom. George Anderson's Peeking Caddis is the best and most famous cased caddis larva imitation.

Free-ranging caddisflies—sometimes called green rock worms—look like tiny inchworms (about 1/8" to 1/4" long) with black or dark brown heads. They frequently lose their grip on the bottom, and are more important to trout than the cased variety. Green Weenies, green Serendipities, or Green Rockworms (#12-18) are good flies to imitate these larvae.

When it matures, the larva makes an underwater cocoon where it changes into a pupa. The pupa is the transition stage. When it is ready, it emerges from the cocoon and rises to the surface. The pupa drifts in the surface film while its back splits open, the adult insect crawls out onto the surface, and eventually flies away. During this transition time, you'll often need a caddis pupa imitation such as Gary LaFontaine's Sparkle Pupa or Mike Mercer's Z-Wing Caddis (#10-16).

Adult caddisflies look like small moths when they fly through the air. At rest on a branch or the water's surface, they fold their wings back into a horizontal position that looks much like the pup tent you had when you were a kid. They are usually mottled tan, gray, or brown, but some have bright green colored bodies, and there are important Black Caddis species on some rivers.

When caddis hatch—and also when they lay eggs—they often skitter along the surface of the water. This causes the trout to slash at them and causes a splash, rather than the slow, steady feeding you see with mayfly duns and spinners. When you see trout making splashy rises in the middle of the river, you can guess caddis are hatching.

In riffle water, Al Troth's Elk-hair Caddis (#10-16) is a decades-old favorite, but in slower currents you'll probably do better with a lower-riding pattern such as Craig Mathews's X-Caddis or a CDC & Elk (#10-16).

Adult caddis live longer than mayflies—often many days or a week before they mate and die. When the females return to the water to lay their eggs, they most often either dip their abdomens into the water, using water tension to release the eggs, or else they dive into the water, swim to the bottom, and release their eggs directly onto rocks and debris.

These egg-laying females die in or on the water and create another excellent feeding opportunity for trout. Mike Lawson's Spent Partridge Caddis (#10-16) is a good fly in these situations.

Midges

Midges are from the family Chironomidae, sometimes called "true flies" because like common houseflies they have two wings shorter than the body, and they don't have tails. As the name implies, most midges are small, size 22 to 28 or smaller.

The midge life cycle has four distinct phases: egg, larva, pupa, and adult.

Adult midges lay their eggs over the water, and the larvae emerge and thrive in many water types, but especially in slow water with a silty, muddy bottom covered with debris and aquatic vegetation.

Fully developed midge larvae are small and can range from 1/2" (#10) down to 1/8" (#22) or smaller. The largest species are mostly found in lakes and other stillwaters. Most midges in flowing waters are #20 and smaller. They come in a variety of colors, but cream, brown, black, olive, and red are the most common.

Midges are especially important on spring creeks (where the water flow comes primarily from underground springs and not snowmelt or rain) and on tailwaters, where the water flow comes from a dam and reservoir. If your local trout water is a spring creek or tailwater, you should become familiar with midges, midge imitations, and how to fish them, as they will be a particularly important food item for your local trout.

Midge larvae look much like thin, helpless worms with distinct segmentation but no noticeable body taper. A head and tiny legs at each end are usually apparent only under magnification. The red color in some species comes from an internal store of oxygen-rich hemoglobin, a reserve that allows them to survive at least temporarily in polluted environments or in littoral areas that are occasionally dewatered.

Some larvae are free-swimming. Other species attach themselves to the bottom but periodically let go of their anchors and drift downstream to populate new feeding grounds. Wading anglers and water-level fluctuations constantly dislodge larvae, making them an important year-round food source for trout.

The best midge larva imitations are made of slightly stretchy synthetic materials wrapped on a curved hook. John Barr's Pure Midge Larva and the Jujubee Midge are good examples of these types of patterns.

Anything with a bead or with a pronounced thorax has the wrong profile. Take a seine or net with you and match your pattern to the specimens in the stream.

When they are fully developed, midge larvae transform into pupae. Some species build cocoons on the river bottom, others change inside a hardened skin of the last larval stage, and some are free-swimming as pupae. Unlike midge larvae, which are thin and stringlike, midge pupae are relatively short and squat with a segmented, tapered abdomen and a pronounced thorax.

Think long and slender for midge larvae, short and squat for midge pupae. A pupa's thick thorax contains the adult's developing wings. Pat Dorsey's Black Beauty, the Miracle Nymph, or Brassie (#18-24) are good midge pupa imitations for flowing waters.

When fully formed, midge pupae rise to the surface to hatch. Peak emergences change with the seasons but normally occur during midday in the spring, fall, and winter, and near dark on hot summer days.

When midges begin to emerge, their budding wings become prominent, and they sometimes carry gas bubbles that trigger trout to feed.

This phase of the transformation has been studied extensively by some expert anglers, who have developed deadly imitations. Rim Chung's RS2 pattern was created for finicky South Platte trout and has been widely adapted on the San Juan and elsewhere.

When a pupa reaches the surface, the adult escapes from its sheath, pushes through the meniscus, and rides the surface currents until its wings dry and it flies away to mate, die, and start the cycle over again.

Adult midges look something like a mosquito without the proboscis. They have six long legs; short, stubby clear or off-white wings that lie flat back along their bodies; and fluffy antennae.

When trout rise to adult midges on the surface, you can fish a tiny dry fly that imitates a single adult midge, and on some tough waters, this is the only dry that works. Luckily, some trout take bigger patterns, and if you are just getting started fishing midge dry flies, start with a small (#18-22) Parachute Adams or a Griffith's Gnat. Midges often clump up into clusters, and these flies are about the same size and shape.

Midge fishing is often considered the ultimate challenge in fly fishing for trout, because the flies are extremely small, and as a result the tippet end of the leader is light. If you are just getting started in fly fishing, you should avoid places that are well known for their midge hatches, and seek out rivers known to have good populations of larger mayflies, caddisflies, and stoneflies. Midges can make even experienced anglers want to pull their hair out.

Stoneflies

Caddisflies, mayflies, and midges include hundreds of different insect species. Stoneflies (Plecoptera) are a relatively small order of insects but because they are relatively large as individuals, and populate clean, cold, fast-flowing, and sometimes infertile streams, they are important to trout.

Stoneflies range in size from #4 to #18. Adults have four long, shiny wings, which lie flat over the back when at rest. The life cycle is egg, nymph, adult. As with mayflies, there is no larva or pupa stage.

Stonefly nymphs live in fast-flowing water and as a result have flat bodies with strong legs for clinging to rocks. They have two stubby tails, long antennae, and, unlike mayfly nymphs, they have no gills on the abdomen. When the nymphs mature, they crawl to shore and emerge. Therefore the "emergence" phase is not as important to fly fishers as it is with some aquatic insects. However, in the period when the nymphs migrate toward shore, they are often washed into the current, and trout gather in the shallows to feed on them. Check under streamside rocks to match your fly to the most prevalent nymphs.

Stonefly adults live for days or weeks in streamside vegetation and are poor fliers. They often fall or are blown into the water and trout feed on them ravenously. They mate on land and later, egg-laying females drag their abdomens on the water's surface to release their eggs.

Unlike mayfly, caddis, and midge hatches—where you are likely to observe trout feeding steadily on the surface—you don't often see a trout eat a stonefly. However, if you know there are stoneflies around, you can fish a big stonefly imitation with sometimes startling results.

Salmonflies are the largest and most well-known member of the stonefly family. For nymphs, use black Kaufmann's Stones, or Yuk Bugs (#4-8). For adults, use orange Stimulators or Sofa Pillows (#4-8).

Golden Stones are slightly smaller and lighter colored with yellow or tan highlights and mottled brown bodies. Use Mercer's Biot Epoxy Stones or 20-Inchers (#6-10) for the nymphs and yellow or orange Stimulators (#6-10) for the adults.

Other locally important stoneflies include Skwalas (#8-12), Little Black Stones (#12-14), and Yellow Sallies (#12-16). Inquire at your local shop about the best regional patterns.

Terrestrials

Trout, bass, and other fish eat mostly insects that are aquatic. However, aquatic insect hatches are seasonal—with most of the feeding opportunities concentrated in the spring. In the summer and early fall, trout rely heavily on other food sources such as terrestrial (land-based) insects that fall or fly into the water.

Grasshoppers may be the most important terrestrial insects, mostly because they are large food items and notoriously poor fliers. They love the long green grass along riverbanks, and are sometimes driven to the riverside in swarms when farmers cut their hay.

Hoppers come in all sizes from small (#12-16) in the early summer to large (#4-10) in the late summer. Observe the hoppers along your river and try to match the size and (less important) color as closely as possible. The Whitlock Hopper, Letort Hopper, and general attractors such as the Chernobyl Ant are all good during hopper season.

Trout love hoppers, but if your river is heavily fished, and the trout are caught and released frequently, they can become extremely cautious. In these circumstances, you will frequently see trout rise to the fly, give it a close inspection, and then disappear back into the depths.

Legs can be important on hopper patterns. In some circumstances the trout pluck at the side of the fly as if wanting to submerge it first, and then consume it. On slow-water Slough Creek in Yellowstone National Park, trout sometimes grab a grasshopper by a single leg, pull it under, then swallow the fly in a secondary motion.

Fish that exhibit these finicky feeding behaviors are difficult, if not impossible to catch on hoppers. The good news is, they've shown you their location, they've shown you they are hungry, and they've shown you they are familiar with land-based insects. Try following up with an ant or beetle imitation.

Beetles come in many sizes and shapes, from large emerald Japanese beetles to tiny black beetles. It's impossible to match all the color and size combination of this large insect family, but luckily simple black Foam Beetles (#8-14) are rarely refused.

Beetles float low in the water and with no high-profile wing to spot, even relatively large beetle imitations can be frustratingly difficult to see on the water. Purchase beetle imitations with a bright-colored indicator on top that sticks up so you can see it. A painted dot on top does little to help you see in low-light conditions.

Trout love ants and at times eat them in the middle of a heavy mayfly hatch. Unlike beetles and hoppers, which mostly fall in the water as individuals, ants tend to act as a group. Flying ants mate and migrate in swarms, and red, black, and cinnamon-colored ants often march one after another off a log or rock into the water. Use foam and fur ant patterns with two distinctive body lobes—trout seem to key on this body silhouette. A hackled ant (#14-18) sits on top of the surface—not down in it—and is easier to see than a large beetle, so you may not need a parachute post for visibility. Find what works for you. It's often effective to fish a grasshopper pattern as an indicator fly and fish a beetle or an ant as a trailer fly 12 to 18 inches off the bend of the hopper hook. Watch the hopper. If you see a splash near the hopper, or the hopper moves or jerks unexpectedly, set the hook.

Other seasonal or regionally important terrestrials include cicadas, crickets, inchworms, and spruce moths. Discover how important these insects are by taking time to look in streamside bushes. You can hear the buzz of cicadas as you walk through the forest, and if inchworms are lowering themselves onto your head and shoulders, they are certainly getting into the water.

Crustaceans

Shrimp, scuds, sow bugs, and crayfish are not insects like most other trout foods: They are crustaceans. Mysis shrimp are an introduced food source that thrive in some Western reservoirs. When these white "fairy shrimp" flush into the river below they can be important to trout.

These shrimp migrate from one lake depth to another through the seasons, and are only important downstream when their daily or seasonal migrations put them close to the dam outflow.

Some fly fishers mistakenly refer to scuds as shrimp but they are distinctly different. Scuds have a segmented exoskeleton and seven pairs of legs (plus antennae). They come in colors often matching the stream vegetation where they live: mottled rust, tan, olive, or other colors.

Scuds are not important on cold, clear mountain freestone streams but thrive in fertile lakes, spring creeks, and weed-bottomed tailwaters.

A relative of scuds are cress bugs, commonly called sow bugs by fly fishers. They come in similar sizes and colors.

Unlike scuds, sow bugs do not swim freely in the water column; they crawl like a pill bug along the stream bottom, avoiding light by staying under rocks and debris or moving and feeding only in the low-light hours. They thrive in cress beds, elodea, and other aquatic vegetation in spring creeks and tailwaters especially.

Crayfish—also called crawfish or crawdads—thrive in fertile warmer-water environments.

You won't find crayfish in glacier-fed streams, mountain headwaters, or Montana spring creeks, but you will find them in large lakes (also the Great Lakes), big tailwater streams, and other major Eastern, Midwestern, and Western watersheds.

Big trout in these environments look for molted crayfish or immature crayfish, as do smallmouth and largemouth bass, carp, and other large predators. In many places, a tan or rust-colored conehead Woolly Bugger (#4-8) or Slumpbuster makes a fine crayfish imitation.

Leeches

Leeches live in all water types and trout, bass, and other fish love them because they move slowly and offer a high calorie count.

Some large fish make a habit of feeding almost exclusively on leeches. On several occasions, I have caught large brown trout (22+ inches) on dark Woolly Buggers, and the fish threw up a handful of undigested leeches into my net or hand, proving that "matching the hatch" is not just a dry-fly concept.

A black Woolly Bugger is a fine leech imitation—especially in fast water on big rivers—but leeches come in all sizes and colors, so also carry rust and olive Woolly buggers (#4-10).

Other Fish

Almost every gamefish eats other fish as a regular part of its diet—the bigger the fish gets, the more piscivorous (habituated to feeding on other fish) it becomes.

In fresh water, trout and bass feed on minnows and small bottom-dwelling prey species such as bullheads and sculpins. Woolly Buggers and Zonkers work well to imitate many types of small minnows, but the Clouser Minnow may be the best general-purpose minnow imitation for fly fishers because it is easy to cast and quickly gets to the bottom where the fish are.

For bullheads and particularly sculpins, you need a fly pattern with a bulbous head so carry Muddler Minnows, Zoo Cougars, or Bow River Buggers.

Other less important aquatic foods trout eat include snails, craneflies, dragonflies, and damselflies—which are important in many stillwaters in spring/summer.