As we knew on Rainbow Trout Facts, trout in streams and rivers have remarkably catholic diets, feeding on a vast array of aquatic and terrestrial insects and other invertebrates as well as occasionally on small fish.
In the following articles we consider the rainbow trout diet, characteristics and life-cycles of the main orders of aquatic insects upon which trout and grayling fish live in running water, a couple of other basic aquatic food forms and, relatively briefly, the main terrestrial creatures they take advantage of when the opportunity presents itself.
Essentially, the aquatic insects that form the staple diets of trout in streams and rivers can be divided into four main groups:
A basic understanding of the distribution, appearance and life-cycle of each of these groups is important to the angler. Doing so can help him to identify what the fish are feeding. Later he can select an appropriate artificial from his fly box. The descriptions that follow cover the main insect families. Detailed descriptions of the individual species are also covered in the following posts.
Rainbow trout hunt by reading subtle cues in the current. In fast riffles they hug the head of a seam, darting from an eddy to snatch drifting nymphs and larvae.
In slower pools they cruise along banks or hover over submerged structure, feeding more methodically. During spring runoff and high flows, focus on faster water where trout can hold with their nose into the current.
In calm late summer pools they tend to suspend near rocks or drop-offs where insects emerge. Understanding these positional preferences helps anglers predict where feeding activity will be most concentrated.
Young trout, including fry and juveniles gorging on tiny mayfly and chironomid nymphs, rely on dense riffle runs where these insects drift constantly. As they grow, nymphs in the size 12 to 18 range, especially stoneflies and caddis larvae, become their primary fare.
In the transitional emerger stage when insects are rising to hatch, trout key on the shimmering silhouettes breaking the surface. Dry flies imitating emerging mayflies, sedges, and caddis are deadly during these periods.
Adult trout, especially during peak hatches, take fully winged insects including mayflies, caddis, and stoneflies. They even consume terrestrial bugs blown into the river.
They also gladly eat larger scuds or aquatic worms in slower runs. This varied diet reflects the opportunistic nature of rainbow trout feeding strategies.
Water temperature is critical to understanding rainbow trout feeding behavior. Between about 45 degrees Fahrenheit and 65 degrees Fahrenheit, trout metabolism peaks, so they feed aggressively.
Expect constant movement and big rises, particularly during spring and fall. As temperatures climb beyond 70 degrees Fahrenheit, feeding slows and trout move deeper or into cooler feeder creeks.
They forage less frequently and prefer early-morning or evening bites. Below 40 degrees Fahrenheit they still feed but do so very sluggishly, sticking to steady drifts of nutrient-rich nymphs near the bottom.
These temperature-driven patterns dictate when and where anglers should focus their efforts. Monitoring water temperature with a thermometer can dramatically improve your success rate on any given day.
The best times of day reflect both temperature and insect activity. Dawn and dusk remain top choices year-round because insects hatch then and cooler water energizes the trout.
In spring and fall, mid-morning through mid-afternoon can also be productive if cloudy or windy, keeping water temperatures stable. In hot summers, concentrate on pre-dawn and after sunset.
During cold snaps, midday when the sun warms shallow riffles can trigger short bursts of feeding. Learning to read these daily patterns gives anglers a significant advantage.
To identify active trout, watch for surface clues including periodic pops, midges, or the wide, positive rings of a rise. A steady train of ripples trailing upstream, the wake of a cruising trout, signals focused feeding.
Look for darting slashes along seams. Those trout are moving laterally to intercept drifting bugs.
On calm days, a shimmering feeding lane just below the surface often betrays several trout sipping emergers. Pay attention to insect activity: visible mayflies or caddis midday mean the trout are likely rising in predictable locations.
Pair these observations with matching presentations. Use slow nymphs in fast water, emergers when you see trout sipping, and dries during full hatches to stay ahead of the rainbow’s appetite.
Mayflies are thin-bodied, delicate creatures with upright wings and long tails. Anglers watch several suborders including Baetids which are small and quick, Pale Morning Duns with their pale yellow-green bodies, and the larger Hendricksons and Adams.
Their life cycle includes nymph, emerger, dun, and spinner stages. Trout can key on any of these depending on hatch timing.
Nymphs crawl in riffles, then rise to the surface to emerge. Duns sit upright while spinners fall back to the water.
Mimic the stage trout are keyed on by using beadhead nymphs in the feeding lane for early activity. Use a soft-hackle emerger in shallow riffles, and a classic dry fly such as parachute or elk hair patterns for a calm dun surface.
Match size, typically ranging from 12 to 18, and tint. Pale Morning Duns should be pale green, while Adams patterns work best in gray.
Caddis have thicker bodies and tent-winged profiles, lacking the split tails of mayflies. Larvae build cases of gravel and feed along the bottom.
Adults are fluttery, mothlike insects that often ride the current or get blown into pockets. Trout favor them because they are more robust and plentiful later in the season when caddis hatches pop in runs and slower seams.
Nymphs require patterns with a darker thorax or wing case and beadhead for subtle bounce. Adults are imitated with elk hair caddis or post-wing patterns.
Learn the local hatch calendar. Summer evenings and early fall see the biggest caddis activity, and strong emergers melt into the surface film.
Trailing a caddis emerger behind a larger dry can be deadly. This tandem approach covers multiple feeding zones simultaneously.
These are larger and heavily built insects, with two tails and stiff wings. They clamber over rocks and emerge in spring or fall, especially in faster, oxygenated currents.
Trout key into stonefly nymphs in tailouts and runs. Adults shake off rocks or fall into the current.
Match these with robust nymphs such as Pat’s Rubber Legs in size 8 to 12, fished on a short leader with a bit of weight to get deep quickly. When adults are active, look for them in riffles and on rocks near seams.
Use terrestrial-style patterns like Stimulators or stonefly duns. Target the tailouts of riffles on sunny afternoons when trout face upstream and refuse other offerings.
Tiny midges dominate winter and tailwater feeding. These midges, also known as Chironomids, hatch all year.
Trout feed on emergers and pupae in the film even during cold water. Use micro nymphs in size 18 to 24 with pale bodies and red or olive thoraxes, often with a dropper setup to cover different depths.
In tailwaters, fish are often keyed to midges drifting near the bottom. Index lines slightly upstream and let the nymph drift through.
Non-midge small flies such as small baetids can be imitated with comparaduns or small sparkle-wing wets. Sometimes throwing a very small dry in Size 20 Adams pays when the surface is calm.
Grasshoppers, beetles, and ants are trout candy in summer. When grasshopper falls start, swing a foam hopper or popper along banks and let it ride near the surface.
Beetles and ants frequently drop into slower pocket water. Use small foam ants and sticky beetles.
Trout are opportunistic and will smash these offerings. Use a longer rod tip to keep drag low and allow the fly to hang in the surface film.
Remember that terrestrials often land near shore. Focus on edges, under overhanging brush, and overhanging rocks.
Observe insect size, color, and behavior. It is critical to match both profile and silhouette.
Use emergers or soft-hackle patterns when trout are sipping just below the surface. Switch to a true dry when duns are on top.
Adjust your leader length for the stage. Use short leaders for heavier stonefly nymphs, longer leaders for small midges.
When terrestrials dominate, fish a slack line and allow the fly to rest in the seam. Trout often take with a slow, lazy drag.
This focused approach keeps your offerings subtle, precise, and aligned with what trout see in every season. By understanding the complete rainbow trout diet and matching your presentation to current feeding behavior, you will dramatically increase your catch rates throughout the year.
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