It is an "egg head" idea that is probably a lot more important to you and your fly fishing success than you know right now. Let's see if we can change that.
Optimal foraging theory (OFT) is a fairly simple idea that informs us about why fish are where they are, why the are eating what they do, and when the are most likely to be foraging. OFT states that organisms forage (feed) in a way that maximizes the difference between energy consumption and energy expenditures. It is quite simply an ecological cost-benefit analysis. The best place to feed is where an individual can realize the largest difference between intake of food and expenditure of energy.
Like any model, it is less than perfect but it is a good model because it is useful. It provides a mechanism to think about why individuals make the foraging decisions that they do. Individuals have a number of choices to make. Not all foods are created equal - some take more time and energy to capture and consume, some fight back, some have more calories than others - etc. And each location has different energy requirements which is why if you find fish in the riffles, they're almost certainly actively feeding. And fishes are always worried about predators - they are willing to move into shallow water but the rewards really need to outweigh the risks. Of course, if you are a little fish, the risk might be cannibalism so they are often pushed into the margins of the stream. These are the sorts of decisions that OFT help us understand and predict trout behavior.
Image from: https://outofmygord.com/2014/01/10/the-psychology-of-usefulness-how-our-brains-judge-what-is-useful/
We have all heard the idea that 10% of the anglers catch 90% of the fish. I would argue that this is explained by the fact that experienced anglers fish when, where, and how the fish will be eating. My single best piece of advice for catching more fish is to fish more often when more fish are feeding. Simple, isn't it? The old adage goes, fish during the part of the day that is most comfortable for you. It is at least partially true. During much of the season, it means that early and late are the times to be fishing. I say partially true as winter fishing can be pretty slow if you wait until too late in the day when the snow melt lowers stream temperatures and thus trout metabolism. During winter and early spring, particularly when there is snow on the ground, I like to be out from maybe 9 AM to 1 PM. Lastly, in the middle of summer - July and August, the middle of the day, particularly on slightly windy days, can be pretty fun for fishing terrestrials.
Probably the most overlooked places on streams are the riffles between the pools and the runs. If there are fish in them - and they are not always there - they will be hungry and likely looking up. Why? They are there because they are looking for food. They can not afford not to be looking for food as they are expending some energy to be there. Even then, they are tucked behind rocks, they are on the bottom, and they are tucked up along the edges - all places that reduce the current velocity. They may not be smart, but they are not dumb, or at least they are survivors and "understand" cost-benefit analysis.
These ideas are based upon the fact that trout are mostly drift feeding fishes. As such, they generally prefer to sit in a place where they can expend minimal energy but consume the most food by being out of the current but next to currents that are carrying food. They do no want to be on the conveyor belt but right next to it. If that location tucks them into a spot where they are safe from predators, all the better. On many streams, these are just off the foam lines where they can look for prey carried in the foam lines.
Foam lines near undercut banks, deep water, and slow currents - these are the prime lies. Trout have cover, food, and protection from current so they expend relatively little energy to get food.
Food Choices
One of the main ideas behind OFT is that individuals will select locations where they can maximize their food intake for the effort that it requires. This explains a lot of what we see on the stream. They are in the riffles or at the head of the pool that the riffle feeds when insects are hatching or actively drifting. They will move into these places when their eyesight gives them advantage over the prey they are searching for. And they will move into these places when they get a little desperate. They are quite often at the bottom of pools to avoid predators and to avoid having to expend energy batting the current. They might eat if food comes by - they are quite opportunistic - but they are generally not in the bottom of pools because there is a lot of food there. This is why you can often watch your nymph or streamer move by the trout without a reaction - the trout are mostly in these places to seek refuge from predators and current, not to forage.
From: Pizzul et al. (2009) - Energy Density of Brown Trout and it's Main Prey Items in an Alpine Stream of the Slizza Basin (Northwest Italy)
The table above illustrates how prey items of trout vary in how many calories they have per weight (the last column is joules per gram - 1 Joule = 0.24 Calories). There are some food sources that have 2-3X as many calories per gram. In other words, there are some prey items that a trout would have to eat a half or a third as many to get the same amount of energy. This is one way to conserve energy - eat higher calorie food. I don't think we have a very full understanding of how organisms pick the food items they do based on their energy density but we have all experienced some selectivity. Ever buy that mixed seed bird food and notice that all the black oil sunflowers get picked out to the exclusion of everything else?
In general, the best places to forage are some combination of prey density, the energy density (calories per weight) of the food, their digestibility, how easy / difficult they are to catch, and how much energy it takes to capture them. There are obviously a lot of variables and fish aren't exactly that smart but they're not dumb either. Evolution, OFT would tell you, has shaped their behavior to be efficient, if not "smart". There are times that prey density outweighs the other factors. During a heavy hatch of Baetis - even if they are not the highest in energy density - Baetis are what is for dinner because a fish can fill up on them rather quickly and with little energy expenditure.
OFT also informs us about the ontogenetic shift that occurs in trout as they get larger. It is a fancy way to say that as trout get larger, their diet shifts to more profitable foods. Until 14 inches or so, trout are mostly drift feeders. As they grow larger, they require more energy for "maintenance activities" (to stay alive) which means that they often shift to a more piscivorous diet (they eat more fishes). To put it in other terms, an 18 inch Brown Trout could sit and eat a whole bunch of blue-wing olives or catch a sculpin, a mouse, or a little Brown Trout to consume the same number of calories but probably spend less effort in doing so.
Habitat Choices
Where trout are will dictate the amount of food that is near them or passes by them, the amount of energy required to hold position, and their risk of predation. Riffles are great for food delivery but they require more energy to stay in place. Pools minimize energy expenditures but they do not deliver a ton of food expect where the riffle dumps into the pool or the pool tailout which concentrates food.
In many of our streams, the most difficult trout to catch are those in the tails of pools because there is a lack of security from predators in these places. Trout are there to feed but they understand they are vulnerable to predators here. You can not cast from behind them because the current of the riffle will catch your fly line. You can not get much closer because they are just a couple of feet in front of the riffle. And casting to trout in shallow, unbroken water from upstream without spooking them is really difficult. My best success has been get away from the stream and put most of my cast on the grass. It is incredibly rewarding to catch these difficult fish but it is usually a "one and done" opportunity. Catch one and you almost certainly spook the rest.
Habitat choices will change during the course of the day. In particular, once it starts getting darker, insect activity picks up, and trout move into riffles, runs, and the heads of pools. During the middle of the day, getting out of the sun is often at a premium. Look for undercut banks, shadows, deep water, and broken water - all places that keep trout out of the sun. Fish may not be actively feeding during the middle of the day but if you put a good size morsel in front of their faces, they may just reach out and grab it. I think this is what is happening during a lot of the time I am fishing terrestrials. The fly is large enough to make it worth their while much more so than dragging a midge or small mayfly past their noses.
Predator Avoidance
Trout have no shortage of predators so they are wary. There are birds (eagles, herons, kingfishers), otters, humans, and other predators that all pose a risk. And if you are a smaller fish, other fishes are significant predators that affect habitat choice. Brown Trout are particularly territorial and cannibalistic, pushing Brook Trout and small Brown Trout into the stream margins. The problem with stream margins is they are shallow which means predators can get them and they lack that nearby food conveyor belt.
They don't like shadows - the surest way to put fish off is to be silhouetted above them. Many years of experience has shown me that there is no need to ever be standing on the high bank. Your chances are much better if you can get to a place where you are out of their vision. Remember that with refraction in water, trout do not see along the horizon very well. There is about a 15 degree angle above the water they do not see. This generally means if you are behind them - in their blind spot - and not high above them, you are more or less invisible to them. Being on that high bank is asking for trout to see you and you are likely to look like a potential predator to trout.
Shallow water is particularly troublesome for a trout. There are two problems with shallow water, 1) it is where predators are more effective, and 2) shallow and fast requires a lot of energy to say in place and shallow and slow lacks food. Yes, there are times you find bigger trout in shallow water but much of the time, it is the domain of smaller trout. Not because they want to be there but it is where territoriality and predation risk pushes them. Larger trout will be in fast water but they will be wary and they will not spend more time there than food dictates. Even then, the larger trout will tend to be in the locations where there is the least current.
Food x Habitat x Predator Choices
We talk about "prime lies" as place where there is an abundance of food, trout are safe from predators, and they are in slow current so they do not have to expend much energy but they are close to currents that carry food. These are the undercut banks, places in front or behind boulders, where a riffle drops swiftly into a pool, and other places where there is an abundance of food nearby, a lack of current, and safety from predators. Relatively few of these places occur in most streams and they are generally occupied by the larger fishes.
Without getting to deep into ecological theory, fish want to live in a good "neighborhood". Much like us, They spend much of their time not actively feeding so a primary need is security and lack of energy expenditures. Much of the time, security and not expending energy are the priorities. Food doesn't need to be present all the time. However, when they are on the prowl, they want that source of food to be nearby. If you find a deep hole with a nice safe undercut bank and maybe some large woody debris, look for feeding lies near that safe location. Save that place in your mind until a nice hatch or early in the morning or late in the evening when bigger Brown Trout are more likely to be active. The other option is to throw something big and try to take advantage of the trout's opportunistic nature but you will stand a much better chance at a time when that fish is actively feeding or conditions (sun and shadows, water color and flow, etc.) are in your favor. Trout don't get large by being over opportunistic. They feed when, 1) energy density of their prey is high, 2) the conditions are in their favor (sunrise, sunset, and after dark), and 3) they want to...
How to be one of the Ten-Percenters
It is really rather simple, fish a lot and fish when and where the fish are most likely to be active. Yes, a good angler can catch fish all day long (most days) but without question, being in the right place at the right time will get you more fish. If you want to have REALLY good days, you need to be there at the right time. Part of that is simply fishing enough that you are more likely to intersect with those really good times. We can't all fish as much as we'd like but we can put put the odds in our favor by fishing when the fishing is most likely to be good. None of this is a substitute for fishing more often. You will not become one of the ten-percenters without putting in some effort to build those skills. Nothing makes will make you a better angler faster than fishing more often.
Learning the lessons about when to fish can be hard earned. In my younger days, I spent a lot of long days of fishing. Much of these days was spent catching a fish here and there interrupted by some fantastic fishing. Slowly the wheels start turning and I figured out that I could fish smarter and not harder.
How do I use optimal foraging theory on the stream? To be honest, it's not something I give a lot of conscious thought to while fishing or planning a trip. However, I am thinking about what time of the day, where I want to fish, and what bugs I expect to see. Whether we recognize it or not, OFT is an ecological theory we have all come to understanding in our own ways from our own individual experiences.