The Stillwater River is a cold and swift mountain freestone river. Hatches on the Stillwater are more sporadic than on other fisheries in the region, but they are still important. The Stillwater can produce excellent dry fly fishing in the spring or in the summer months after runoff. During these windows when trout are willing to rise to the surface it is important to understand which aquatic insects are emerging (or which terrestrials are prevelant).
Blue Winged Olives (BWOs)
On the Stillwater River, these small mayflies are the first, and last, significant hatches of the angling season. They first emerge in late March and last through May and hatch again in mid-September lasting well into October. Overcast days see the strongest emergences, especially on the river downstream of Nye to the confluence with the Yellowstone River. Sunny days can experience very strong hatches as well, but the ideal BWO day on the Stillwater 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 size from 16 to 22.
Western March Browns
March Brown hatches on the Stillwater River can occur in late April and early May, but compared to hatches of BWOs or caddis, March Brown hatches are minimal. These mayflies can hatch on the entire length of the river, but conditions must be ideal—overcast, water temperature around 48 degrees F, and very light winds. If these conditions on the Stillwater River occur, a large emergence of March Browns can happen, making these large mayflies easy pickings for hungry trout. March Browns on the Stillwater River range in size from 12 to 16, with most being size 14.
Caddis
Caddis are a significant source of food for Stillwater River trout. These insects can hatch from late April through early September. The strongest hatches occur in early May—the Mother’s Day caddis hatch—then again after runoff in mid-June, then can last well into August. The Stillwater River is home to dozens of caddis species and knowing them all is not necessary for catching more fish. In summer—mid-June through August—caddis on the Stillwater River can hatch at any time during the day. Caddis nymphs on the Stillwater River are active year-round and run from size 10 to 22. Evening caddis hatches on the Stillwater River can be very strong, allowing anglers a pleasant hour or so of twilight fishing.
Salmonflies
Although not as pronounced as on the Yellowstone River, salmonflies also hatch on the Stillwater River. On the Stillwater River salmonflies hatch as soon as the river begins to drop after snowmelt runoff. Hatches of salmonflies last for a few days at the most. Water temperatures in the 55 to 58 degrees are ideal for large emergences of salmonflies on the Stillwater River. Because populations of salmonflies are small in comparison to some other nearby rivers, it is rare to see an abundance of salmonfly adults flying in the air, but because Stillwater River trout are opportunistic, trout will rise to a salmonfly dry fly even if only a few flying insects are present. Salmonfly nymphs crawl to bankside structure and remove their outer shell—also known as their shuck. When this occurs, an adult insect complete with legs and wings, emerges.
Golden stoneflies
On the Stillwater River Golden stoneflies provide plenty food for trout. These insects are size 8 to 12 and usually emerge slightly later than salmonflies, although on the Stillwater River they can emerge concurrently with salmonflies. In most years they hatch in late-June and last into mid-July. A Golden stonefly hatch occurs in the same sequence as a salmonfly hatch, however golden stoneflies are smaller. On the Stillwater River, trout will eat Golden stonefly dry fly patterns for several weeks after the main hatch has occurred. The fast, clear water of the Stillwater River forces trout to be opportunistic feeders and a well-presented size 8 to 12 dry fly is often gobbled-up by a hungry trout.
Yellow Sally stoneflies
Often mistaken for caddis or Pale Morning Dun mayflies, the nymphs of these small stoneflies are a crucial part of a Stillwater River trout’s diet. Yellow Sally stonefly nymphs become very active in mid-June, with adult insects hatching from mid-June well into July. These stoneflies may hatch concurrently with salmonflies and Golden stoneflies but are much smaller. Yellow Sally nymphs range from size 8 to 16, with size 10 and 12 being the most common. Because adult Yellow Sallies have very sporadic flying habits, trout do not actively feed on Yellow Sally adults, but the nymphs are consumed regularly on the Stillwater River.
Pale Morning Duns
Pale Morning Dun (PMDs) begin to hatch in late-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. These summer-time mayflies hatch in abundance out of the Stillwater River’s riffles, shelfs, and runs. PMDs hatch on sunny or overcast days on the Stillwater River. Nymphs are available to trout year-round, but during a hatch and on the water’s surface, adults require a few moments for their wings to dry before taking flight, therefore being easily available to a rising trout. Most PMDs on the Stillwater River are size 14 and 16, with size 16 being the most common.
Spruce Moths
For most of its run, the Stillwater River is flanked by evergreen trees. Spruce moths make their home in these evergreen forests, so they are not considered an aquatic insect, but when they emerge many spruce moths can end up floating on the river. These terrestrial based insects emerge as early as mid-July or as late as mid-August. Fluttering near pine trees and bankside bushes, Stillwater River trout can be very opportunistic during a strong spruce moth emergence. Most spruce moths are size 14 or 16.
Trico mayflies
While not a significant hatch on the Stillwater River, tricos offer small-fly, technical dry fly fishing. Most of the Stillwater River is fast-flowing, which makes seeing a tiny dry fly challenging. Tricos usually hatch at sunrise in late July and can last throughout August. Hatched 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.
Terrestrials: Grasshoppers, ants, crickets, and beetles
The Stillwater River’s crown jewel for anglers is the emergence of terrestrials. The river flows through abundant hay fields and grasslands. Because of this, grasshoppers, ants, crickets, and beetles dominate a Stillwater River trout’s diet from mid-July through early September. Having been blown into the river these land-dwelling insects are a common occurrence on the Stillwater 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. Tan, grey, pink, and yellow colored grasshopper patterns are all successful for trout on the Stillwater River.
October caddis
October caddis do not provide a significant source of food for trout on the Stillwater River, however they are a large aquatic insect that hatches in the fall. Compared to other hatches, an emergence of October caddis is limited to a few flying insects. But an opportunistic trout may feast on one of these large caddis species if it happens float by and they fish leaves its holding lie to gulp a large morsel of food. Most October caddis are size 8 to 10.