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Baetis – The Time is Now

Apr 6, 2014 | Mike Lawson

If you're tired of slinging nymphs, now is the time to hook a large trout on a dry fly

With the weather of February and March it doesn’t seem to be April already. Thankfully there were more wet days than dry ones over the past few months. If it weren’t for that, we would be in dire straits for water right now. Since late winter the snow pack in the Henry’s Fork Basin has risen to well above normal and the reservoirs are sure to fill. This will render great dividends later in the summer. It also assures that high water flows will come as soon as the snow melts in the higher elevations. When that will happen is anybody’s guess. Normally the heaviest runoff in the lower Henry’s Fork occurs between the 2nd week of May and the second week of June.

Baetis are the first mayflies of the season to bring large trout to the surface

No matter when the runoff transpires, one of the best dry fly fishing periods occurs from early April until the runoff roils the water later in May. Three different aquatic insects are largely responsible for this grand dry fly opportunity. The first of these is a small two-tailed mayfly of the Baetis genera. Pay close attention to future blog pages for information about the other two aquatics – the Grannom (caddisfly) and the Western March Brown.

Baetis Dun photo: Jim Schollmeyer

Commonly called Blue Winged Olives, Baetis include a number of important species throughout North America. Most of them are multi-brood which means they reach maturity in a matter of months rather than a year. Baetis are small olive bodied mayflies with medium to light gray wings. Baetis tricaudatus, the specie common to the Henry’s Fork in April, May, and June is reasonably large for these genera of mayflies. Nymphs and adults are best matched with hook sizes of 16 or 18. The early season brood is usually a hook size larger than the late season brood which emerges in September, October and early November.

Baetis Nymphs are slender and streamlined for swimming

Understanding the biology of early season Baetis is essential if you expect to successfully seize on this opportunity. The nymphs are streamlined swimmers. Their preferred habitat is medium to slow water with rooted aquatic vegetation. At the commencement of emergence the nymphs are buoyed to the surface by internally generated gases within their bodies. The emergers frequently encounter difficulties in penetrating the surface film and float just under this barrier for considerable distance before breaking free. Emergers continue to struggle to break free from the nymphal shuck as they drift in the film.

Baetis often struggle in the surface film during emergence. Photo: Ralph Cutter

These Baetis are well adapted to survive the harshest weather conditions frequently encountered on our rivers of Eastern Idaho and Western Montana. Duns that survive the emergence process hasten to riparian areas where they can find shelter, rest and molt into the spinner phase. After that, they get right to the business at hand of finding a mate. Mating, copulation and egg laying usually occurs at the water’s edge rather than spinner flights in the air. In most cases egg-laden females simply crawl into the water to deposit eggs sometimes followed by horny males. For this reason typical spent-wing spinner imitations are not as effective as wet sunken spinner patterns.

This is an exceptional photo by Ralph Cutter of a Baetis Spinner Underwater

I’ve seen a number of intricate Baetis nymph imitations but it is hard to find a more effective pattern than a #16 Pheasant Tail Nymph. The Skinny Nelson is another good option along with the Tungsten Bruised Baetis designed by our own Devan Ence. Don’t forget that the nymphs are streamlined swimmers and patterns to imitate them should be slender. Emergers are also essential. I like floating nymphs like the Half Back Emerger and Barr’s Flashback Emerger. Thorax, Hackle Stacker and Nohackle are great patterns to imitate the duns.

Chris Lawson battles a strong trout on the Henry's Fork below the Chester Dam.

Baetis are common throughout the Henry’s Fork, South Fork and Madison. The best hatches occur on the slower water. The soft water on the Upper Henry’s Fork above Mack’s Inn and at Last Chance provides good Baetis activity until early June. Further downstream the river becomes more subject to spring runoff.  Durinig the runoff period the water remains fishable above the confluence of Fall River. Below Fall River you need to get after it while you can. When the weather heats up Fall River turns the water dark and unfishable. Hopefully we have another month before that occurs. I like to focus on the backwater areas above the Chester Dam and the Fun Farm.

Understand the biology of Baetis can pay great dividends

Just because these mayflies are small doesn’t mean larger trout won’t target them. Baetis constitute the first concentrated food opportunity of the year. I like a #4 or #5 weight rod with a floating line tapered with a 12’ leader to 5X. You might need to go to 6X but you’ll have a much better chance to put the brakes on a large, hard-running fish if you can get away with a larger tippet.

Next up: The Grannom or Mother’s Day Caddis and the Western March Brown. Stay Tunned!