Big Horn River Blue-Winged Olives
Blue-Winged Olives, frequently referred to as Baetis or BWOs, are the first significant mayfly hatch of the season, and
their arrival is highly anticipated by anglers and the trout each year. Unlike other mayflies, Baetis produce several broods throughout the year. Spring Baetis hatch predominantly in March, April, and May. I have seen the spring hatch last continuously into August, typically during years when water temperatures are colder than normal. The spring Baetis emerge in size 16 to 18 and hatch in abundance. The body color of the Blue-Winged Olives is directly correlated with the amount of sunlight present when the hatch occurs. This is why adults that hatch in March will be darker than adults that hatch in May or June. I have seen Baetis present in a wide variety of colors including olive brown, olive gray, and even a light yellowish olive. I tie all my favorite BWO patterns in each color.
The most significant factor affecting the BWO emergence each spring is water temperature. I have seen Blue-Winged Olives hatch on the lower river down near Two Leggins Access a good two weeks before seeing any on the upper river. The magic temperature for this hatch seems to be right around 43 degrees, which is typically reached in the afternoon, during the warmest part of the day. For this reason carrying a thermometer in your vest or pack is a good idea While we all enjoy fishing to rising trout on the surface, the nymphs and emerging nymphs offer tremendous opportunity to the angler. Rainbow and brown trout thrive and quickly fatten up each spring by gorging on Baetis nymphs dislodged from the river bottom and ascending to the surface.
Nymphs dead-drifted near the bottom are always effective. Once the nymph’s abdomen and wing case become almost black in color, the nymphs are ready to emerge. Once the water temperature warms each day these dark nymphs will begin to emerge to the surface. Classified as swimmers, Baetis nymphs emerge rapidly to the surface and are taken readily by trout. The Wonder Nymph, created by longtime Bighorn River guide Brad Downey, is effective at imitating these emerging nymphs. A great way to induce a strike when Baetis nymphs are emerging is by stopping your flies mid-drift and allowing them to rise to the surface. On countless occasions I have had clients begin to strip their line in to recast and a fish takes their fly. Another technique that works well is right before your drift is about to drag out below you, give your flies a few slow but deliberate strips. This will cause viscous strikes by feeding trout. Once the emerging mayflies have reached the surface film, or are drifting on the surface as duns, trout switch to adults on the surface. BWO duns can range in size from 16 to 22 throughout the year, with duns being the largest at the start of the yearly emergence. While a drag-free presentation and accurate drift are always required, fish are not nearly as selective when it comes to fishing this hatch early in the spring. Large pods of surface feeders allow the angler to approach relatively close. The trout seem to let their guards down during the climax of the Blue-Winged Olive emergence. I know a hatch of BWOs is at its best when the entire surface of the river is blanketed in insects and the geese and swallows are feeding as ferociously as the trout!
While Bighorn trout will take high-riding adult imitations regularly, stealthier patterns that sit lower in the surface film are often more effective. When mayflies hatch, they break out of their nymphal shucks in the surface film and then need to dry their wings before they can leave the surface of the water. At this time the insect is helpless and in its most vulnerable state to the trout. Insects that are half in and half out of their nymphal shuck are represented by your typical emerger pattern. A BWO dun that is trapped in a partially opened shuck, or that has not been able to dry its wings, either because it is too cold or too wet to do so and cannot leave the surface of the water, is called a cripple. The colder and wetter the conditions, the longer it takes for a mayfly to fully emerge and take flight. Bighorn trout have been conditioned to take advantage of this stage of the emergence and the trout can feed on this stage exclusively at any given time. Surface emergers and crippled emerger patterns should be a must in every angler’s fly box for the Blue-Winged Olive hatch. While the fall Blue-Winged Olives carry the same characteristics as the spring Baetis hatch, they are not found in as great abundance as in the spring. You should be prepared to see fall Baetis throughout October and November in size 18 and 20. It is the Pseudocloeon in the fall that are most prevalent and they should have your greatest focus when looking for surface action in the fall.
Pseudocloeon in the past were classified as part of the Baetis genus. Recently these pale olive size 20 to 24 mayflies have
been given their own genus: Pseudocloeon. For the avid dry-fly angler, the Pseudo can create some terrific technical dry-fly fishing, displaying similar characteristics to that of Blue-Winged Olives. This hatch occurs on overcast afternoons in September and October. Pseudos can emerge in amazing abundance, to the point where they will blanket the surface and you can scoop them up by the handfuls where they collect along the river’s edge. An exacting presentation and accurate cast, with just the right imitation, are a must when the fish are feeding on them. For anglers not willing to tempt their fate with a dry fly, Bighorn trout readily take these tiny nymphs as they emerge off the river bottom also. A Skinny Bill, tiny Pheasant Tail, or Pseudo Wonder Nymph are great nymph imitations.
