What Do Trout Eat
Updated: May 2
Mayflies, Caddisflies, and Stoneflies comprise the vast majority of the diet of most trout in the cold water streams throughout the world. While the majority of these insects are available to trout in their nymph forms, it is their adult forms that take to the sky and captivate fly anglers and gluttonous trout during abundant hatches. In the world of stream biologists these three insect families comprise the EPT (Ephemeroptera are the Mayflies, Plecoptera are the Stoneflies, and Tricoptera are the Caddisflies) species that are indicators of good water quality. These three species aren't the only insects on the trout's menu as midges, dragonflies, damselflies, and a variety of terrestrial insects can also make a nice meal for hungry trout. In this article we will go over the variety of foods trout can find throughout the year and the most important of these species we see on our home waters of North Georgia as well as the major hatches in other streams across North America. The old expression used by bass fishermen "to understand the owl, you must study the mouse", couldn't hold more true for fly fisherman and any type of angler for that matter. Though many fly anglers don't enter the sport to study aquatic insects, any angler looking to improve their skills and understanding of trout will invest the time to learn the basics on these bugs.
Mayflies and Trout
Mayflies are likely the most recognizable trout food of all the insects that comprise the trout's diet. Their propensity to hatch in large numbers, sailboat shaped wings, long tails, and clumsy flying are easier to distinguish compared to the other insects on the stream. Their nymphs are identifiable by their three tail appendages and overall triangular body shape as seen in the photo below.
Mayflies lives consist of 3 stages: nymph, duns, and spinners. Most of this time is spent as a nymph (1 year), living beneath the rocks and wood scoured throughout the stream. The second stage occurs when the nymphs emerge and rise to the surface during a hatch in their dun forms. This form is the most vulnerable and is the time period many trout look to take advantage of the mayflies as they are not able to hold their own against the current in the stream and are incredibly weak flyers once they reach the surface. If they manage to survive the hatch and escape to the surface in their dun forms they will find cover where they will molt about a day later.
Once they have molted, they are now sexually mature adult spinners and ready to reproduce and return the eggs of future generations to the water. In less than a week as a spinner, the adults will expire and the spinners will fall back to the surface of the stream giving the local trout another opportunity at an easy meal. It is a common misconception by anglers that trout are feeding on these mature spinners when they are seen on or around the river. However, it is often the emerging nymphs and clumsy duns that are much more likely to appeal to hungry trout. Using emerger patterns or dry flies that ride low along the surface will be the most effective at these times. Klinkhammers, Sparkle Duns, or Comparadun patterns are great choices.
When searching for what insects are actively hatching or found in any given stream, look for spiderwebs to be the indicators on what might be around. You do not need to flip rocks, or be proficient at wrangling these flies as spiders are masters at catching these insects. After getting an idea of what is hatching, you need only to best replicate the insects that seem to be the most abundant. Often these insects have been around for some time and trout will be accustomed to seeing them.
Common Mayfly Species and Hatches
Blue Wing Olives or BWOs are often the first and last mayfly hatches of the year. Smaller than most of the mayfly species (typically in the 18-24 size range) these BWOs can be found in colder weather conditions from October through March. Here in North Georgia BWOs comprise the most prolific hatches in the area and lead to the first surface action of the year on our wild trout streams as early as January.
Sulphurs, Pale Morning Duns (PMDs), and other members of the Ephemerella family hatch in mid spring and into the early summer. Their lighter complexions often fall on the yellow to manila spectrum much like the Cahill. Flies in the size 14-20 make the best imitations. This group of insects will typically hatch during the lowlight periods of early morning or late evening. Here in North Georgia hatches ramp up from some time around mid April and into May for Sulphurs and a bid later in the Spring for the other Ephemerella members which are much less common in the area.
Many other mayfly species are found throughout the world. Isonychia, Drakes, Mahoganies, Baetis, Calibaetis, March Browns, Tricos, are all popular in the regions they inhabit but few garner the attention that the Hex receives. This is one the largest mayflies by size anywhere in the world. Hexes can be found throughout North America but is truly renown in the great lakes region where swarms can be spotted on Dopplar radar during their spring hatches. All fish species of the region will participate in the feeding frenzies that accompany these hatches each spring.
Caddisflies and Trout
Nothing gets the trout more excited than the large caddisfly hatches of spring. Referred to as sedges in Europe, the caddis has been a staple in fly fishing since its origins. As nymphs, caddis are the builders and filterers of the stream, constructing cases and webs from the various organic debris and small pebbles within the stream. Many individual caddis species are distinguished by their behavior as nymphs and whether or not they construct casings.
As seen in the picture, this casing is made from thick woody debris while others such as the Grannom Caddis use much thinner materials and the Case Caddis use small pebbles. Other caddis such as the Rhyacophila seen below are free swimming and are often found with the mayfly nymphs under rocks or in leaf debris deposits.
These free swimming caddis nymphs tend to look less like grubs then their case building relatives. Much like the mayflies, caddis nymphs are at their most vulnerable when they leave their casing to emerge to the surface when they hatch. Their hatching process takes a bit longer than mayflies but they are much better flyers once they take to the skies. Once in the air they can be identified by their monotonous pattern of flight. Caddis can be quick but often look like they have no idea where they are flying to as they zig and zag aimlessly above the surface. After cool mornings in mid to late spring, you can often shake the stream side shrubs and dislodge many caddis waiting for the sun to warm the area around the stream. In hand, they are recognizable from their tent shaped wings, two antennae and compact bodies.
Common Caddis Species and Hatches
Black Caddis are often the first caddis species to emerge each year. Here in North Georgia, we begin to see them as early as February, but very late winter and early spring is more typical. These caddis are often smaller but more numerous early in the season. They often accompany the hatches of BWOs, March Browns, and Winter Stoneflies. These Black Caddis don't quite fire the trout up like the caddis hatches later in the Spring, but are a great sign of the increasing activity level of the stream.
The Grannom Caddis make up many of the best Caddis hatches throughout North America each year. From Southern Appalachia to the Pacific Northwest, many guides and visiting fly anglers count the days to the biggest Grannom hatches of the year. Timing in these ranges does differ from river to river but the periods of high flows in mid to late spring are the most common times to see the hatches occur. Trout will binge on caddis during these times, and while they can feed with reckless abandon they can get selective for solely these caddis. Elk hair caddis and x-caddis patterns are excellent options in the size 14-18 range.
While not the hatches of Spring, the Fall hatch of October Caddis is not one to overlook. In a season when hatches can be few and far between, the October Caddis can be some relief. While bigger hatches exist throughout much of the U.S. , here in Georgia they are infrequent. As nymphs, they can be found in the healthiest streams. These nymphs are the closest thing I have found that a mop jig may imitate as they certainly look like a juicy meal for any trout. Maybe the fly purists will give the mop a try now.
Stoneflies for Trout
Stoneflies are typically the largest of the aquatic insects trout can regularly add to their diets. Unlike the mayflies and caddis which typically only spend one year in their nymph forms, stoneflies can live in the stream anywhere from one to three years in the stream. The stonefly nymphs are a favorite for trout, especially in the winter months when food availability is low and they can provide a large snack for the fish. As nymphs, stoneflies look similar to mayfly nymphs with three distinguishing factors. Stonefly nymphs will have 2 tail appendages, 2 antennae, and a more uniform shape from top to bottom than the more triangular mayfly nymphs. Adults can be identified by the double set of wings they possess and are typically visible to the eye upon flight. When stationary, these wings lay flat down the stonefly's back.
Unlike the caddis and mayflies, stoneflies do not have to emerge through the water column when hatching. Instead, they migrate to the shoreline boulders or vegetation and pull themselves out of the water where they split their carapace and hatch into adults. This migration tends to occur under the cover of night and hatching occurs as air temperatures rise in the morning hours. It is when the stoneflies return to the water to lay their eggs can the trout take advantage of them as a food source once again. Here in Georgia, anglers can find several stonefly species. However, few adults likely make it onto the menu as hatches are typically in very low numbers.
Common Stonefly Species and Hatches
One of the only insects Georgia anglers can find in the coldest days of winter along with midges, the small black Winter Stonefly is a beacon of hope for warmer days ahead. I have personally never seen a winter stone turn a trout's head here in North Georgia but it certainly wouldn't be out of the realm of possibility. Typical sizes are small but the size 16-22 range is the most common with the larger bugs emerging during warmer conditions.
The Golden Stonefly and Yellow Sally are the only stoneflies here in North Georgia that really get any attention from the trout. Mid Spring will find them in their most abundant numbers. Here in Georgia sizes 14-18 are more typical but these bugs tend to be much bigger in neighboring states and out west. These stonefly hatches will often accompany the sulphur hatches here in Georgia. Yellow can certainly be the go to color for late April and early May.