The Bighorn River is one of Montana's most prolific tailwater fisheries. The river below Bighorn Reservoir produces cold and stable flows that allow aquatic insects and trout to flourish. Hatches on the Bighorn can be proflific and anglers need to understand what insects are prevelant when they visit to improve the odds for success.
Midges
The Bighorn River is often described as one gigantic spring creek. And in most spring creeks, midges account for a massive portion of a trout’s diet. Because the Bighorn River is a tailwater fishery—its flows originating from releases from Yellowtail Dam near Fort Smith—an understanding of midges is crucial. Midges range in size from 16 to 26, with most midges on the Bighorn River size 18 to 22. A trout on the Bighorn River may consume several thousand midges in a feeding span versus a few hundred mayflies. Midges can hatch year-round and trout feed on midges subsurface throughout the year. Any subsurface tandem nymph rig on the Bighorn River should always include a pattern that imitates a midge, such as a Zebra Midge, Ray Charles, or Soft Hackle Sowbug.
Blue Winged Olives (BWOs)
Even though midges are active year-round and the most numerous hatch on the Bighorn River, BWOs are considered the first major hatch of the season. Beginning in early March and lasting through May and again in mid-September and lasting well into October, these mayflies can hatch on any day during these two seasonal windows. Overcast days may see the strongest emergence. The abundant long, slow runs on the Bighorn River create ideal habitat for trout to gorge themselves on a heavy emergence of BWOs. Look for rising trout in softer water feeding lies—the tail outs of long flats, the inside of current seams, and slow waters on the inside of riffle corners. Sunny days can experience very strong hatches as well, but the ideal BWO day on the Bighorn River is overcast, with scattered showers, and air temps in the low 50s. Spring BWOs range from size 14 to 16 and fall BWOs range in sizes 16 to 22.
Caddis
Caddis species make-up a large portion of a trout’s diet on the Bighorn River. However, two species of caddis—tan and black—are the primary caddis hatches. Unlike many other Montana streams, the Bighorn River doesn’t experience a Mother’s Day caddis hatch. Instead, it’s as if the river withholds its caddis magic until summer. In July both black and tan caddis hatch in very strong numbers. Small in size but big in distribution, these size 16 to 22 insects emerge a few weeks after PMDs. Because these caddis species are distributed throughout the river, wading and floating anglers can enjoy fishing these active insects. Tan caddis tend to hatch throughout the day while black caddis are most prolific just before dark.
Pale Morning Duns
If midges are the engine that feeds Bighorn River trout, Pale Morning Dun (PMDs) mayflies are the gas that makes it move. These summer-time mayflies hatch in abundance on the Bighorn River. PMDs begin to hatch in mid-June and last all through July. Primarily found in the river’s riffles and longer runs, PMDs emerge when water temperatures hit 58 degrees F. Hatching mostly from riffles and swifter water, making the skill of getting a perfect drift with a dry fly a little easier—making PMDs the most user friendly hatch on the Bighorn River. PMDs hatch on sunny or overcast days on the Bighorn River and they can hatch in either condition. Nymphs are available to trout year-round, but during a hatch, the hatching insects require time to allow for their wings to dry before taking flight, therefore easily available to a sipping trout. Most PMDs are size 14 to 18, with sizes 16 and 18 being the most common.
Yellow Sally stoneflies
While small in scope compared to midges, caddis, and BWOs; and, not a major source of surface feeding action for Bighorn River trout, this small stonefly species is a staple for Bighorn River trout. Yellow Sally nymphs range in size from 8 to 16, with size 10 and 12 being the most common. Because adult Yellow Sallies have very sporadic flying habits, trout rarely key on them, making fishing dry flies difficult, but the nymphs are consumed on a daily basis. Most Yellow Sally stoneflies on the Bighorn River occupy habitat near bank-side structure.
Trico mayflies
If a trico-loving trout were to create an ideal river for eating hatching tricos, the Bighorn River would be it. Because the Bighorn River is defined by shallow riffle corners and long flats, Bighorn River trout feed on hatching tricos with conviction. Tricos are small in size, emerging in the early morning hours, and seen only for a few weeks of the angling season, trico mayflies have a dedicated following of anglers on the Bighorn River. Tricos offer something for anglers desiring small fly, technical dry fly fishing. Tricos usually hatch at sunrise in mid- or late July and throughout August. An emergence rarely lasts longer than a few hours. Adult tricos swarm in mating swirls above the water, after mating they fall to the water and die, creating “spent” mayflies, also called “spinners”by many anglers. Most adult tricos are size 18 to 22.
Terrestrials: Grasshoppers, ants, crickets, and beetles
The Bighorn River is flanked by large fields of hay. As these grasses grow, so do the populations of terrestrials that inhabit the grasses. During late-summer when the grasses are at their tallest and when ranchers harvest the hay and grasses, thousands of terrestrials can land on the river. Trout look to land-based insects for food. Grasshoppers, ants, crickets, and beetles dominate a trout’s diet by late July. Having been blown into the river these land-dwelling insects are a common occurrence on the Bighorn River. Once on the water they become easy prey for hungry trout. Most grasshoppers are sizes 8 to 12 and most ants and beetles are size 14 to 18. Colorations vary but natural tones like gray and tan tend to fish better than bright colors.
October caddis
It is rare to see an October caddis flying in the air. However, they do hatch and a committed angler can bring a few trout to the surface by dedicating to fishing one of these size 8 to 10 dry flies. October caddis can hatch as early as late September and will typically peak in mid-October. Despite the Bighorn River having an abundant population of October caddis, an emergence of October caddis is limited to a few flying insects in a given section of the river.