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Catching the Hatch

A pre-summer primer for chasing trout on the Upper Henry's Fork

Published in the April 2014 Issue Published online: Apr 25, 2014 East Idaho Outdoors Steve Smede
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If you've ever driven through Island Park on a bright early-summer afternoon, I'm betting you've seen at least a few grasshopper-sized critters going splat! on your windshield. If you're new to the area and don't have a penchant for entomology, the sight of these orange-bellied monstrosities could send you fishtailing off the highway.

The insect in question is none other than the venerable Salmonfly. Some locals call it the Trout Fly; others call it a Giant Stonefly. Affluent high-brow tourists from Back East call it Pteronarcys californica (and yes, you can actually hear the italics as they enunciate the words).

Beginning with the early-June arrival of these bugs from Jurrassic Park, you can also find fly-casters of all stripes scattered along the Henrys Fork of the Snake River, each hoping to find and match the hatch. After the salmonflies taper off, the river's more placid stretches in Harriman State Park reawaken with the arrival of the Green Drake mayfly in late June and a long summer's roster of flying, crawling and hopping fish foods that lasts well into autumn.

During this hectic season, parking lots from Ashton to West Yellowstone provide a rich catalog of license plates from every corner of the country. When you pare it down, however, there are really only two types of angler on the upper Henry's Fork: waders and drifters.



The money tree in my back yard does not provide an endless supply of fruit, but if it did, I'd be quick to pluck some of it for a nice drift boat. Casting off the bow of such a vessel is a pure delight, even on those rare occasions when the fishing slows to a crawl.

Short of buying a boat for yourself, you can always book a guided trip. Resident anglers tend to brush off this idea, but even if you're a skilled caster and comfortable in your "local knowledge," you might be surprised how much you can learn from a professional guide. If nothing else, you'll get a chance to throw the latest and greatest fly patterns at the biggest fish in the river.

Wading, while more of a chore, has its advantages as well. You can stake out a patch of water and fish it thoroughly, and you can experience an insect hatch as it unfolds around you. Whether you're catching trout or not, the process can be educational. While the salmon fly hatch is a dicey proposition without a drift boat, the parade of mayfly hatches will provide great opportunities for wading all summer long.

Trouble is, even if you know what the fish are eating, you may not know what that meal looks like to the fish. Volumes have been written on the subtleties of matching the hatch. What's more important, most of the time, is catching the hatch -- even if that means your fly selection is just a rough approximation of the actual items on the menu.



For the angling-inclined, fly shops are like toy stores and candy parlors. Most of us can get an emotional high from just walking in the door. This is especially true if you're an aficionado of fly patterns but find yourself too busy, lazy or easily distracted to tie your own. (There's also the fact that East Idaho is home to some of the most prolific and highly skilled fly-tyers in the world.)

High-end fly shops like Idaho Falls-based Jimmy's All-Seasons Angler offer patterns that range from basic approximations to extremely lifelike, but as the tyers themselves will tell you, sometimes the general approximations work best.

"Consider what a fish is really looking at from down below," notes Northwest veteran guide (and self-described Henry's Fork junkie) Josh Larkham. "It's a circus down there -- all kinds of debris, currents, refraction. [The fish] aren't counting belly segments or the number of tails on your pattern." Presentation is key, he says, but so is having a robust pattern that has the key features to get the trout's attention.

For a gargantuan salmonfly, that means something that is orange-ish brown, bushy and buoyant as possible. Modern salmonfly patterns accomplish this in a variety of ways. Some of the newest versions employ highly buoyant foam indicators, or even foam bodies in lieu of more absorbent poly yarns. Traditional patterns take more work to keep afloat, but it's hard to beat a well placed Sofa Pillow with a bright wing of elk hair. With Sofa Pillows and even more modest-sized caddis patterns, the natural materials have an added bonus -- they get worn and torn, which further enhances their "bugginess" -- especially in rougher waters.

Mayflies and their signature sailboat wings require a little more definition when replicated. On the upper river above Coffeepot Rapids, for example, you'll find an amazing hatch of Brown Drakes. They're half-pints by salmonfly standards, but are easily one of the largest mayflies in the West. An artificial version can vary somewhat on the color or even the size, but the huge wings must be visually obvious from below the surface.

You could say that the same idea holds true for the famous hatch of Green Drakes at Harriman, but on this stretch of river (known to many as The Railroad Ranch), the equation is far more complicated. Insect numbers are prolific, to say the least. On top of that, the trout here see a lot of fake bugs. The water is clear and shallow, and the flat currents are fast and complex. If you want to fool your quarry, a more exacting fly pattern may be called for. To boot, the fish are so selective that they will often pass on adult mayflies floating directly above their noses. Instead, they may select only the drakes that have not fully broke free from their nymphal shucks. Easy pickings. The question is whether you have a pattern to match that precarious stage of the insect's life.



It's been preached many times before by fly-fishing experts from Yellowstone to New Zealand, but it's certainly worth repeating: Most of a trout's diet is fulfilled under the surface. On the Henry's Fork, trout gorge on salmon fly nymphs for weeks leading up to the hatch. Fishing nymph imitations at this time can produce 20-fish days. You may find yourself catching a 10-inch rainbow yearling on one cast and a 5-pound bruiser on the next.

If you encounter the short window when trout turn from the nymphs underwater to feasting on adults at the surface, consider yourself blessed. Catching the salmonfly hatch can be maddeningly difficult, even when the bugs are swarming. But if you're ahead of the hatch and the adults are not yet appearing, chances are good that your quarry is enjoying a buffet line down below.

Fat black or brown rubber-legged patterns can work miracles when the trout are focusing on the stonefly nymphs. Proper weighting is a must to get the fly down deep. Resulting snags are a fact of life, but so are the hookups. In the waters of Box Canyon, don't hesitate to throw on the biggest strike indicator you can find. The fish won't care if the dinner bell is ringing.

Regardless of conditions and insect activity, keep in mind that the upper river supports a vibrant population of trophy rainbow trout, and that fact doesn't disappear when catch rates taper down. The hefty food supply makes great fishing possible, but it also produces highly selective feeding habits. The upside is that when you hook into a Henry's Fork trout, one thing should be clear to you when that reel starts screeching: That trout just didn't get hooked; it got fooled. Even as you release the fish to fight another day, you can bask in the success of your efforts long after the season is over.


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