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The Eddy Line: Blue Winged Olives



Discover the first mayfly of the spring

By Patrick Straub EBS Fishing Columnist

March is a month of transition on our local waters. With warmer temperatures, a longer duration of daily sunshine, and lower elevation snowpack beginning to melt, it’s time to shift out of the winter fishing mode. Don’t fret just yet night owls – there’s still no need to get to the river early and you can feel good about returning in time for happy hour, as fish do most of their early springtime feeding between 10 a.m. and 5 p.m.

The most welcome sign of spring is the arrival of Blue Winged Olives. Midges are nice and they can ease the winter doldrums, but in the next few weeks both trout and anglers relish in the season’s first legitimate hatch. Here’s some advice to help you navigate this hatch.

BWOs are in most trout streams. Because of the insect’s small size, many streams will have multiple generations over the course of the year. Unlike salmonflies, golden stones, and most species of caddis, Blue Winged Olives often have three to four cycles of nymphs and adults. This means more bugs, which means happier trout and anglers.

Get your naming right. Or not.
If you want to impress anglers during a high society cocktail party, then study your Latin classification tables. They are most commonly called Baetis. But over time, anglers have gravitated to simply calling them “Blue Winged Olives” or “little olive mayflies” due to their olive bodies and bluish wings. The most abundant species locally is Baetis tricaudatus. Several other species exist, but be ready for some obtuse looks if you walk into your local fly shop and say, “The fish ceased rising to genera B. tricaudatus and commenced their post meridiem caloric intake of B. brunneicolor.”

Timing is everything. BWO nymphs cling and crawl along rocks and underwater structures during most of their existence. When water temperatures are in the low 40s F, the nymphs become more active. During the next several weeks, you’ll find the warmest water temperatures between noon and 3 p.m. The rise in water temp creates internal gases in a nymph’s body, which force the nymph to drift or swim to the surface. As the nymph hits the surface, these tiny bugs often struggle to break through the water’s surface tension. The colder and wetter the weather is, the more these hatching insects struggle and the more crippled or stillborn bugs occur.

Fish the correct habitat. Most BWOs will be found in slower sections of freestone streams. On the Gallatin this will be the back end of longer pools and runs. On spring creeks and tailwaters, many insects can also be found on aquatic growth. Keep in mind the nymphs will be moving through the water column before you see insects on the surface, so position yourself accordingly.

Emergers are very important. For many anglers, this may be the biggest adjustment you make this spring. When trout are feeding on mayflies, the emerging stage – gas bubble formation to fully winged insect on the surface – is the stage when they are most susceptible to hungry trout. During this long transition, these little bugs must cast off the nymphal shuck, penetrate the surface film, and then become airborne. This may take only a few seconds on our clocks but to a mayfly it must feel like hours.

Adults are the prize. Mayfly adults floating on the water’s surface epitomize dry fly fishing for trout and allow us to endure three long winter months of fishing double-nymph rigs. Mayfly adults, also called “duns,” float along the surface with their wings upright like the mast of a miniature sailboat. Trout rise slowly and deliberately to adult BWOs, as if they’ve been preparing all winter.

As the seasons move from winter to spring, so goes the fishing. The beauty of March is the answer to the most often asked question in Big Sky, “To ski or to fish?” It’s easy, do both.

Pat Straub is the author of six books, including “The Frugal Fly Fisher,” “Montana On The Fly,” and “Everything You Always Wanted to Know About Fly Fishing.” He and his wife own Gallatin River Guides in Big Sky, he is co-director of the Montana Fishing Guide School, and co-owns a guide service on the Missouri River.

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