I haven't written anything instructional for a while, so I thought I might share my feelings on "picky" trout. This is a little complicated, and I don't want to write a thousand words, so let's see how concise I can be.
Trout are not picky. That suggests an intelligence they do not possess. When they are hungry, they are looking for food, and the way they do that is similar to how the FBI tries to find a match for a finger print found at a crime scene.
If you've ever had your prints taken, you know how carefully they roll your finger over the ink and then over the paper in order to obtain the perfect print. Prints at a crime scene are never perfect. At best, they're smeared or incomplete. So the computer doesn't look for a perfect match. It looks for points of similarity. If they're casting a wide net to identify suspects, law enforcement may set the standard at just a handful of points of comparison, which doesn't really prove anything in court, or they might set it for 10 or 20 points of comparison. That narrows the results, but it also provides more compelling evidence of a legitimate match.
Trout sometimes only look for a couple of points of comparison between your fly and what their brain recognizes as food (i.e. color and size). Sometimes they look for more similarities. There's an interesting reason why, and it's counter-intuitive. It's not that they're educated. It's because they are as dumb as a bag of doorknobs.
A trout's itty bitty brain can really only focus on one or two things at a time. In regards to feeding, that leads to their zoning in on a set of qualities in the food they're actively eating. If there's a hatch going on, and they're feeding on a cloud of Baetis nymphs, they're simply hyper-focused. They may be looking at the size, shape, color, location, depth, and/or behavior. There's really not much else they can look for. That said, during Baetis season, I catch a ton of fish on #14-16 pheasant tails. Not an exact match, but pretty close. It works, because I'm matching the basic points of comparison to the food they're actively feeding on. That's it. If something like a #8 stonefly isn't working in that scenario, it's not because they are refusing it. It's simply that everything else becomes part of the background static. They've keyed in on specific qualities and tuned out on everything else. During these times, they even forget to be nervous about predators, because they can't really focus on more than a couple of things at a time. So, good news, you can walk right up on them if you're careful about it.
This behavior carries over to circumstances other than hatches, by the way. If river A has a crap-ton of bugs in it, the trout will become accustomed to the average or typical bug. So if most of the bugs are smallish light colored mayflies of various species, they'll typically look for (1) about the right size, (2) about the right shape, (3) about the right color, and/or (4) about the right behavior, and something like a Hendrickson or Hare's Ear nymph will probably work fine most of the time. I know that seems picky, but it's actually lazy, and what makes it possible for them to be lazy is the fact that there's a "crap-ton" of bugs. They don't have to worry about starvation (in fact, they're almost never actually hungry), so they key in, sit still, and let the river feed them. However, if you move over to River B, you may find the bug population is weaker. This is one of those classic good news/bad news situations. With fewer bugs, river B will have fewer and smaller fish than river A. These fish also tend to be spookier. Since they don't spend as much effort focusing on food identification, they can focus more on safety. That's the bad news. The good news: those fish can't be nearly as lazy as the river A fish. With less food, they must feed more opportunistically, meaning they'll only look for one or two points of comparison on your fly. Sometimes, it just has to be drifting in the water and visible (i.e. glo bugs). Other times, as long as it has the basic shape of a bug and is within reach, you're in business. In rivers like this, the trout are often hungry, and they'll generally go for the largest potential bite of food in the current. They'll also move a greater distance to intercept that food. These also tend to be good rivers for fishing streamers, if the conditions are right.
Taking it to the next step, River C may have a terrible forage population, which means the trout there are required to feed aggressively and competitively in order to avoid starvation. And, again, it means you'll find fewer and smaller fish. If you've ever fished a creek where it seemed like those 7-inchers wouldn't leave your fly alone, bouncing your giant dry fly into the air, because their mouths are too small to eat it, you may have stumbled into this type of situation. EDIT: To the best of my knowledge, we don't have trout streams in Missouri that would qualify as a "River C" type of fishery. That's due to our karst topagraphy keeping our water on the alkaline side, which bolsters our bug populations. Trout may not be native here, but our rivers are apparently made for them.
To put this knowledge into practice, you just need to learn which type of river you're on: A, B, or C. Are there a million big fish all over the place (i.e. Taneycomo, White River, etc.)? Figure out their primary food sources and try to match them in a mostly general way. The San Juan River, for example, has massive numbers of massive fish, but if you're not prepared to fish #20 midges (or smaller), you're probably going to struggle. It's their primary food, and they can afford to be lazy. You don't have to match a specific bug exactly, but you'll do well to be in the general ballpark. Moderate quantities of fish in a variety of sizes? You'll probably have luck using more generic or attractor flies like pheasant tails, hare's ears, zug bugs, glo-bugs, and san juan worms, mostly in larger sizes. But since these fish tend to be spookier, longer casts, quiet wading, and proper technique are more important. Fishing a creek that seems to only hold below-averaged sized fish in smaller numbers? You'll probably have a great time, but you may have to cover some territory to find them. Just be sure to take your camera, in case you manage to catch that elusive 11-inch trophy.
FWIW, 1088 words. :-|
See you on the river!
Head Trout Honcho in Charge
Walt Fulps. RETIRED fly fishing guide and instructor, published author and columnist, and public speakr. My past career life was in the fields of Therapeutic Recreation and Adventure Therapy.