It can be difficult to comprehend how small a fly really could be. The photo shows a few of the flies out of my midge box (click on the photo for an enlarged image.) They are really really small. With my eyes, I need a magnifier to tie them and reading glasses and a hemostat are almost required to tie them on a tippet to fish them. I forgot the reading glasses the other day on the South Platte and managed to tie them on without them, but it wasn't easy.
In the tailwater fisheries of the west, tiny flies are essentially required. These are the streams that flow out of bottom release dams and therefore offer the fish a year round consistent water temperature. In the summer it's nice and cool and in the winter it is relatively warm. The bottom release temperatures may actually be too cold to be optimal for trout, say as cold as 39-40°F. But some dams, like the one at Flaming Gorge in Utah on the Green River are designed to mix water from various levels to provide optimal temperatures for trout growth, around 50°F. And indeed, there are more trout per mile in the Green below Flaming Gorge than any other river on the continent. These dams simulate the great spring creeks like the Henry's Fork of the Snake and Silver Creek both in Idaho. Other great tailwaters beside the Green include the Bighorn in Montana below the Yellowtail dam, the San Juan in New Mexico below the Aztec dam and the North Platte below Grey Reef. There are more. I think of the Big Horn, the Green and the San Juan as the big three.
Because these waters provide such uniform temperatures, they grow huge trout. On the other hand, the aquatic bug life is pretty much restricted to smaller bugs; Chironomids (the midges), and a couple of species of smaller mayflies;
Baetis (Blue Winged Olives) and
Tricorythodes (Tricos).
Since these tailwater rivers have so many fish, they're fished hard and generally have mostly catch and release regulations in force. On the North Platte, you can keep one fish over 18" (I think) and similar rules apply on other rivers. That means that in a lifetime, most of these fish are caught many times, and that makes for so-called educated fish. They stick to eating the natural foods that are in the stream and maybe once can be tempted to take a bit of some larger buggy thing that they've never seen before. When they do, they learn quickly not to. They key into the natural foods available, and that is pretty much all they will take. They'd eat a real worm, but regulations in these places usually restrict the choice to artificial flies or flies and artificial lures only. Some places even make a restriction that the hooks must be barbless.
Anyway, there are lots of big fish, and they're feeding constantly and exclusively on the tiny aquatic insects; on the nymphal (mayfly) or larval (chironomid) forms underwater and on the winged adults when there is a hatch and they come to the surface.
I was on the San Juan a few years ago and when I started fishing, around 9 AM or so, there was a older guy fishing in the best spot on Texas Hole. This is a deep hole just by the parking area with an astounding number of really really large trout; mostly rainbows. Texas hole fishing is done combat style, shoulder to shoulder, and altough it's less than an acre in size, all the drift boats stop to fish it for a while too. Anyway, this older guy was in perhaps the best spot to fish the hole; in the braided water that flows in on the south side. I fished upriver most of the day and around 4 PM I was wandering back to the parking lot and saw the same older man wading out of the good spot. He'd been there all day. I decided that, well, since the best spot was opening up, why not fish it for a bit. As he walked out, I asked him how he'd done. He was disgusted. Amazingly, with huge trout continually feeding literally right at his feet, he hadn't caught a single fish all day! People all around him were regularly catching them, and he hadn't hooked one. He was about to explode. I asked him what he was using. "Oh, my best fly." He showed me a pattern called a
halfback which is a brown fly about an inch and an half long. A great stonefly imitation, but useless on the San Juan. He added hopefully, "It always works on the White River back in Arkansas." I showed him some of the tiny midge patterns, and gave him a couple to use the next day. "But how the hell do you tie 'em on?" He asked. I just shrugged and told him that if he wanted to catch a fish, he'd have to use what they were eating. I stepped into his spot and hooked up right away with a 20" rainbow and caught a half a dozen more before I decided it was time to quit and go get some dinner.
My favorite reference for tying tiny flies is
Ed Engle's book
Tying Small Flies. Even on these small flies different tiers exhibit different styles. I like Ed's style.
Sometimes it's only the teeny tiniest of flies that will enable you to catch a big trout.