I find this whole idea pretty fascinating, in no small part because I think we can never truly understand what it is trout are thinking and how they sense the world. This is not unique. How can we begin to understand how a dog with a sense of smell 10,000 to 100,000 times better than ours senses its world? Or how birds with vision into the ultraviolet range see their world? And this is just the physical part of equation. Each taxa have brains that are different, having evolved to their senses, not ours. In the animal world, humans have much better than average eyesight but our sense of smell and taste - which are fairly strongly linked - are rather average to mediocre. Every species senses their world differently and processes those signals differently.
Dogs and birds at least live in "our world", whereas fishes in their waterworld, exist in such a different environment, one in which sound and vibrations travel at about four times the rate they do in air. Water is (nearly) incompressible and that means that unlike in our air-filled environment, every little movement in water is transmitted. As such, fishes have evolved a lateral line and other adaptations which allow them to sense their world by "feel", by the compression waves created by our wading, by the plop of a grasshopper on a stream, or its kicks to propel itself to safety. Fish see their world different then we do and the light waves they are seeing are slowed by their travel through water. And light above the surface of the water is reflected ("bounces off") and refracted ("bent") so that only part of the light above the water enters the water - much is reflected (that is why polarized glass are an angler's best friend). And what light does enter the water is refracted and attenuated, particularly if the turbidity is high.
I always think of it this way, there is only so much brain power to allocate to senses so species have evolved their senses to fit their environment. No species is great at all senses, each evolves to have a few well adapted senses and the rest are not as strong. Biologists often talk about the idea that there are no free lunches - that is to say, all things come with tradeoffs - like the timeshare pitch you had to sit through for that free lunch. Brains require a lot of "fuel" and the more processing brains have to do, the more fuel they require. Providing energy to a brain means that less energy is able to be put into growth or reproduction. Less energy allocated to growth means predators may be more likely to eat you or females will be less likely to choose you because you are not large enough. Less energy into reproduction means you make fewer eggs or sperm or are less likey to attract a mate. Everything has tradeoffs. Trout, like any other species had trade-offs that needed to be made so their brains were capable of quickly assessing the world around them.
For more on the topic of bioenergetics - how food (consumption) is processed and allocated - read the post, Fish Bioenergetics and You.
How Trout Sense our Flies
Despite our best efforts, I am not convinced that man will ever really understand how trout sense their world. That however will not keep us from trying.
A few of my favorite books about trout fishing are Vince Marinaro's A Modern Dry Fly Code and In the Ring of the Rise and Datus Proper's What the Trout Said. These authors tried to figure out what trout are thinking; what it is trout are looking for and how they are sensing potential food. And they are not the only ones. Gary LaFontaine was well known for his rather unique patterns and his writing and discussions of "triggers". And a host of other authors have written similar books, magazine articles, or blog posts about how trout sense their world or what it is we think they are looking for in a fly pattern. The truth is, at least I think, these endeavors are helpful but we will never truly understand what the trout are thinking until they start talking to us.
However, there are some interesting anecdotes about what flies trout seem to prefer and we can infer a bit about what it is trout are looking for. Lance Egan wrote about his The Iron Lotus fly pattern,
Because Baetis nymphs do not have anything red or gold on them, I've tried several versions of this fly with black or black nickel beads and without the red thread hot spot, thinking picky tailwater fish might prefer a more natural looking fly. For me, the more lifelike colorations have been completely out-fished by the original sporting red and gold.
This, of course, seems a bit counterintuitive as a more realistic imitation should be preferred over the flashier, less realistic flies. But like Lance Egan, I have seen this over and over again as well. Think of the Perdigon nymphs that many of us are fishing even if we have not gone "full Euro nymph". They look little like the real thing and have none of the legs and gills so many tyers try to imitate but Perdigons get down deep, quickly - even better than your run of the mill beadhead fly. It begs the question, what is it that trout are looking for in a fly?
As noted above, Vince Marinaro, Datus Proper, and others have attempted to answer the question, particularly for surface feeding fishes. Both Marinaro and Proper found that refraction of light by water makes the the wing of a mayfly very conspicuous. So as good fly designers do, they designed flies - Marinaro's Thorax Dun and Proper's Perfect Dun - to better imitate this. The way they hackled their flies also set the flies lower in the water, another key observation both of these fly angling scientists made.
My question is, "Is the tall wing really necessary?" After all, wouldn't a wing that was the same height as the natural be exaggerated by the refraction of light the same way as the natural and look more like the natural? Or is this one of those "triggers" we hear and read so much about but are really only guessing at? Is the exaggerated wing what make the fly effective like the exaggerated flashiness of Egan's Iron Lotus? Maybe what fish are really drawn to is that the fly sits lower in the water and provides a better "optical footprint" (yes, these are the sorts of words we invent to try to understand what fish are "thinking"). Personally, this best fits my observations. I find parachute flies and Compara/sparkle duns to be my most effective mayfly imitations.
The general idea about triggers is that fishes are looking for particular aspects of a natural that signifies food and they are looking for reasons to bite. And with this, there seems to be evidence from a few hundred years of successful fishing that fish are in fact looking for reasons to bite. As my friend and mentor Bob Blumreich would say, that giant steel hemorrhoid that your imitation is carrying does not seem to be much of a deterrent. And if you give it a bit of thought, if fish were capable of ignoring imitations where hook eyes and bends were visible, we would have to tie our flies much differently. Despite hundreds of years of angling with exposed hooks, fishes have not seemed to figure out that steel hemorrhoid signifies danger.
What is the trigger that a gold bead provides? Or is it a trigger at all? Maybe it just more effectively gets our flies down to where the fish are. How does a gold bead's effectiveness differ as water clarity or the amount of sun entering the water changes? How do fish react to flash in a streamer? I often see anglers state that flash can be a deterrent - a negative trigger - and that anglers should minimize flash in clear water. Then I see "gear guys" throwing huge bucktails with their spinning blades. Some popular inline spinners sport skirts tied of nothing but Flashabou.
It is all much less than clear why trout hit - or fail to hit - our flies. Maybe it is a bit masochistic but I still love to catch a live grasshopper or two, toss them into a stream, and watch them float downstream. I am still surprised to see how often that grasshopper travels for 50 yards or more and makes is back to shore. And typically, my first thought is, "what chance do I have if the kicking natural goes unscathed?" And often I go on to have a fine day of hopper fishing. Who knows?!
And thus far, we have not even addressed what is probably most fishes' best and most unique sense, their ability to "feel" their environment. We have all seen fish turn and smash a fly as it felt your fly hit the water and we have experienced fishes scurrying for cover at other times when our flies or line hit the water. Certainly the "gear guys" have a better understanding and appreciation for how sound and vibration attract fishes. Spinner and chatter blades, built in rattles, the "thump" of a blade bait, and even sonic attractor weights are all commonly used in different applications. And thinking about how different baits "wobble" and thus have a different "feel" in the water are things crankbait anglers think about. Among fly anglers, musky anglers are about the only group you see/hear this discussion occurring commonly. Rattles and rubber tails are common in musky flies as are heads that are meant to push water, which not only makes the long, flow hackles move but felt by predatory fishes.
What it Means for Fly Tying
To be completely honest, I am not entirely sure what it means other than you should experiment and have a little fun with it and see what works for you. There are some simple experiments - tie a few of the same nymph and change the bead color. See for yourself if conditions make a difference. Are there times a non-beadheaded fly outfishes a beadhead? Is there a such thing as too much flash in a streamer? Do fish react differently to internal versus external flash steamers? Make the parachute post or upright wings on your dry flies a little taller and see if it makes a difference. Try your hand at some lower floating dry flies and see if changes your success.
Fly fishing is often quite conservative and traditional. We often shun new, non-tradtional ideas like building spinner blades, rattles, and crankbait lips into flies. Why not give one - or Hell, all of them at once - a try? I promise no "fly fishing god" is going to strike you down. I know people that still won't use or minimize the use of synthetic materials - and to each their own. Explore the fishes' acoustic world and incorporate a little sound and vibration and see what happens. Experiment and have a little fun. Isn't that what it should be about?