You’ll understand small-stream fly selection much better by thinking terrestrial insects, rather than the aquatic insects that most fly fishers study and discuss. Trout stomach content analysis done by scientists has shown that up to 90% of a trout’s diet in small streams is terrestrial insects, and that percentage seldom falls below 50%. Bigger rivers have large expanses of insect-producing riffles, and trout then position themselves in places where the current funnels this food to them. In small streams you don’t have these big insect factories, they are typically less fertile than larger rivers, and trout are forced to live in the few places with enough depth to protect them from predators. The banks of small streams are alive with ants, beetles, crickets, inchworms, spiders, grasshoppers, crickets, and leafhoppers. The air above a small stream hosts moths, bees, wasps, and many species of true flies like deer flies and houseflies. Mayflies, caddisflies, and stoneflies do live in small streams, but if a trout had to depend on these for subsistence it would starve.
Do small-stream trout key in on one species of beetle or ant and feed on it selectively? Probably not. In fact, small stream trout can’t afford to be selective, no matter what they’re feeding on. As long as you show them something that looks sorta like something they’ve eaten in the past, a meal that looks like food and not a threat, your fly will probably be eaten. It doesn’t hurt to pay attention to the insects you see along the stream, though. If you’re fishing a rocky, hemlock-lined stream a size 8 grasshopper fly might not be a good bet because those trout have never seen a grasshopper. Something that looks like a beetle or an ant or a deerfly would be a better choice because these are the insects you see in mountain forests.
Now before you start loading up your fly box with little ant and beetle imitations, let me suggest that you may already have the right flies in your box. Even though you now know that small stream trout eat a lot of terrestrials, many of the terrestrial imitations designed for spring creeks and other quiet waters are tough to fish in small streams. You have to be able to see your fly when fishing this kind of water, so something that floats high or offers a white wing will help you fire your fly right into a deep slot and keep an eye on it. An Elk Hair Caddis does a fine job of imitating a moth in size 12 and 14, and a leafhopper in smaller sizes. A Humpy or a Royal Wulff can suggest the bulky profile of a beetle or big carpenter ant. A Parachute Adams does a fine job of imitating a house fly or wasp. Anything that suggests the bulk and shape of a terrestrial insect works; just save your low-floating and delicate emerger patterns for bigger rivers.
You’ll notice so far I’ve talked only about dry flies. I find them to be most effective in small streams, probably because trout here are used to seeing their food falling in from above. However, small streams do host aquatic insect larvae, plus terrestrials get pulled under the surface, so nymphs are effective as well. Sometimes trout in a deep pool that won’t come to the surface for your dry (or don’t see it) can be fooled on a nymph. The best tactic is to tie a 10-inch piece of tippet to the bend of the hook on your dry fly and hang a Beadhead Hare’s Ear or Prince Nymph under your dry fly. The dry acts as your indicator and at least half of the fish you catch will be on the dry, but now you’re covering the subsurface feeders as well. Another approach is to tie a low-floating ant or beetle dry to this dropper. You won’t be able to see the smaller terrestrial, but you’ll know if a trout takes it when the high-floating dry darts upstream.
How about streamers? I don’t think you’ll find them as effective as dries and nymphs, but early in the season you can sometimes do well with a small (size 10 or 12) Woolly Bugger, Blacknose Dace, or a Mickey Finn. Minnows and crayfish do live in small streams but since most trout in small streams are not much bigger than the streamers you might fish on a big river, keep your streamers to a size that won’t threaten these little guys.
Because small stream trout are opportunists and will usually rise for anything that resembles food, larger (size 10-14) dries and nymphs work better here than they do on most large rivers, where trout can feed selectively on smaller but more abundant aquatic insects. To push these bigger, more wind-resistant flies, it’s more efficient to use a 4 or 5-weight line than a delicate 2- or 3-weight. Those lighter lines are fine for spring creeks and tailwaters, and are fun to use on small trout, but to really get the job done without working too hard I’d suggest a rod that will push a bigger fly but is still fun to use on small fish. Our Superfine Trout Bum Rods are the perfect tool — light and fun to fish, with an action that loads perfectly on short casts, yet makes a tiger out of a 9-inch rainbow. I suggest the 664-3 for the tiniest brushy streams, the 704-4 for mountain streams with plunge pools, and the 795-4 for wider meadow streams or mountain streams with wide flood plains where brush does not encroach on the river banks. And, yes, if you really want to go with the lightest wand for tiny trout, a 602-4 is lots of fun. Just scale down your flies to a size 16 and keep your casts short.