Rainbow Trout

The rainbow trout is Montana's most widespread and popular gamefish, celebrated for its explosive surface strikes, aerial acrobatics, and willingness to take a well-presented dry fly. Native to Pacific Coast drainages from Alaska to Mexico, rainbows were first introduced to Montana's waters in the late 1800s and have since established thriving populations in nearly every major river system in the state. On legendary waters like the Madison, Missouri, and Bighorn, wild rainbow trout grow fat on prolific insect hatches and provide some of the finest dry-fly fishing in North America.

Rainbow trout in Montana display remarkable variation depending on their home water. Fish in nutrient-rich tailwaters like the Missouri River below Holter Dam can exceed 20 inches with regularity, their deep bodies and broad tails reflecting the abundant food supply. Freestone river rainbows, such as those in the upper Madison and Gallatin, tend to be leaner and more athletic, shaped by faster currents and colder water temperatures. In many Montana rivers, rainbows spawn in the spring from March through June, and anglers should be mindful of spawning redds during this period to protect future generations of fish.

Rainbows are arguably the most versatile trout species for fly anglers. They respond well to dry flies during hatches, aggressively chase streamers, and feed heavily on nymphs subsurface throughout the year. Their fighting ability is legendary: a hooked rainbow will often launch itself completely out of the water multiple times during a battle. Montana's catch-and-release regulations on many blue-ribbon streams have produced exceptional rainbow trout fisheries with high densities of large, healthy fish that continue to draw anglers from around the world.

The rainbow trout's most distinctive feature is the broad pink to reddish lateral stripe that runs along each side from gill plate to tail, which gives the species its common name. This stripe can range from a faint blush on silvery tailwater fish to a vivid magenta on wild freestone rainbows, particularly during spawning season. The intensity of the stripe often increases with age and is typically more pronounced in males.

The body of a rainbow trout is covered with small, irregular black spots that extend across the back, dorsal fin, adipose fin, and most notably across the entire tail fin. This heavily spotted tail is one of the quickest ways to distinguish a rainbow from a brown trout or cutthroat, both of which tend to have fewer or no spots on the tail. The overall body color ranges from steel blue or olive-green on the back to silvery white on the belly.

To distinguish rainbows from the closely related Yellowstone cutthroat trout, which also appears in many Montana waters, look for the absence of the orange-red slash marks under the jaw that define cutthroats. Hybrid rainbow-cutthroat fish (known as 'cutbows') are common where the two species overlap and can display characteristics of both, including a faint lateral stripe combined with faint slash marks. Rainbow trout also have 8-12 rays in the anal fin and lack the small teeth on the back of the tongue (basibranchial teeth) that cutthroat trout possess.

Rainbow trout in Montana are opportunistic feeders with a diet that shifts with the seasons and available food sources. Aquatic insects form the backbone of their diet for most of the year, including mayflies (Baetis, Pale Morning Duns, Green Drakes), caddisflies (particularly Hydropsyche and Brachycentrus species), stoneflies (salmonflies and golden stones), and midges. During major hatch events like the famous salmonfly hatch on the Madison River, rainbows will feed almost exclusively on the surface, gorging on adult insects.

Between hatches, rainbows spend the majority of their feeding time subsurface, consuming nymphs, larvae, and emergers drifting in the current. Scuds, sowbugs, and aquatic worms are important food items in tailwater environments like the Missouri and Bighorn rivers, where these crustaceans thrive in the nutrient-rich water below dams. San Juan Worms and scud patterns are staple flies on these waters for good reason.

As rainbow trout grow larger, they increasingly supplement their insect diet with small fish, crayfish, and leeches. Rainbows over 18 inches become more predatory and will readily attack sculpin, juvenile whitefish, and other small fish. This is why streamer fishing with Woolly Buggers, Sculpzillas, and articulated patterns can be so effective for targeting trophy-class rainbows, especially during low-light conditions or when water is off-color from runoff.

Rainbow trout in Montana thrive in cold, well-oxygenated water with temperatures ideally between 55 and 65 degrees Fahrenheit. They prefer clean gravel substrates for spawning and are most commonly found in riffle-run-pool sequences typical of Montana's freestone rivers. In the upper Madison, Gallatin, and Yellowstone rivers, rainbows hold in moderate to fast currents, using rocks, ledges, and current seams to conserve energy while positioning themselves to intercept drifting food.

In tailwater fisheries like the Missouri River below Holter Dam and the Bighorn River below Yellowtail Dam, rainbow trout benefit from consistent water temperatures and flows that support year-round insect production. These regulated environments produce some of Montana's largest rainbows, with fish commonly exceeding 18 inches. Tailwater rainbows often hold in surprisingly shallow riffles and along weed beds, where they can feed efficiently on the abundant scuds, sowbugs, and midges that thrive in these alkaline waters.

Rainbow trout are more tolerant of faster current than brown trout but less tolerant of warm water temperatures. When summer temperatures rise above 68 degrees Fahrenheit, rainbows become stressed and seek thermal refuges near cold-water tributaries and springs. Montana's hoot-owl restrictions, which close fishing during the hottest part of the day on certain rivers, are specifically designed to protect rainbow trout and other cold-water species during these critical periods.

Dry-fly fishing for rainbow trout is the quintessential Montana fly-fishing experience. During major hatches (salmonflies and golden stones in June, Pale Morning Duns from June through August, caddis from May through September, and Baetis in spring and fall), rainbows will feed aggressively on the surface. Match the hatch with appropriate patterns in sizes 8 through 20, and focus on seams, foam lines, and the heads and tails of riffles where trout stack up to intercept drifting insects. Presentation is paramount: a drag-free drift with a long, fine leader (9 to 12 feet tapered to 4X or 5X) will consistently outperform a heavy-handed approach.

Nymph fishing accounts for the majority of rainbow trout caught in Montana, since trout feed subsurface roughly 80 percent of the time. A two-nymph rig under a strike indicator is the standard approach, with a larger attractor pattern like a Pat's Rubber Legs or Girdle Bug on top and a smaller dropper like a Pheasant Tail, Lightning Bug, or Rainbow Warrior trailing 18 to 24 inches below. In tailwaters, scale down to size 16-20 scud, sowbug, and midge patterns fished near the bottom. Adjust split shot and indicator depth frequently to keep your flies in the strike zone.

Streamer fishing is the go-to method for targeting the largest rainbows in any given river. Fish articulated patterns like Circus Peanuts, Dungeons, and Sex Dungeons on sink-tip lines, casting tight to banks and stripping with erratic, aggressive retrieves. Low-light periods (early morning, late evening, and overcast days) produce the best streamer action. Spring and fall are prime streamer seasons on Montana rivers, when larger fish are most actively feeding on baitfish. Do not overlook the classic Woolly Bugger in black or olive, which remains one of the most effective all-around rainbow trout flies ever tied.

Stable and widespread in Montana. Wild, self-sustaining rainbow trout populations exist in most major river systems across the state. However, hybridization with non-native hatchery strains and with native cutthroat trout is an ongoing management concern. Montana Fish, Wildlife & Parks has discontinued stocking rainbows in many waters to protect native cutthroat genetic integrity.