Predatory fish find their prey with olfactory cues, by vibration cues using their lateral line and by visual appearance. Predatory fish, somewhat like hawks falcons and eagles, have sophisticated, highly-evolved eyes that help them find their meals.
Evolution is a warfare that works both ways however. Because vision is a fundamental and indispensable tool for recognizing prey among fresh water predators, prey species have evolved counter measures to fight against predatory visual systems. The primary tool prey species use is camouflage.
Fishermen are predators too. Bait and lure fishermen exploit olfactory and lateral line smell and vibration cues that may or may not be outside the scope of fly fishing. Our discussion here relates only to visual system warfares waged between aquatic predators and their prey.
Early 17th and 18th Century vaguely insect like flies with names like Cow Dung, Professor, Wickam's Fancy and McGinty followed a few oft-repeated structural patterns with creative but semi-random color themes layered on top. Wings were tied parallel to the shank in some cases or at right angles to the shank in others, lashed on over various yarns flosses tinsels and feather fibers. Color choices didn't bear much correlation to imitating real aquatic insects with most such flies. A century or so later fly tying in in both England and the USA gradually began to focus more sharply on imitatation with the works of Alfred Ronalds, George Marryat, Louis Rhead, Theadore Gordon and even with the Salmon Fly fancies of Western USA tiers like Jack Boehme and Norman Means.
USA fly tying took a dramatic turn in the 1950s with the game changing works of Vincent Marinaro and Earnie Schweibert--A Modern Dry Fly Code and Matching the Hatch. Almost over night and even to this early 21st Century day most tiers confidently assume the best flies visually mimic the minnows, crustaceans and insects they imitate--as seen by the human eye.
But what about the fish's eye? A lot has been written about the conical Snell's Window above the fish's eyes that grants visual access to the world above the water's surface, which otherwise appears as a mirror of the stream bottom below. Snell's Window merits further discussion in a later dry fly context. Right now I want to focus on wet flies: on the minnows, nymphs, larvae and crustaceans that represent the great bulk of a cold-water fish's diet.
In their predominantly sub-surface food world what trout do see--or do their best to see--are ephemeral prey-like outlines amidst a chaotic stream bed background, hidden from easy view by elaborate visual pattern camouflages produced by the long and never ending processes of evolution. The natural world is a dangerous place. Everything alive--save only the world's topmost predators--has to worry about getting eaten. No one is spared. Even eaters get eaten. Camouflage is nature's best and most effective armor. Camouflage is ubiquitous. Camouflage is the rule of the natural world day.
Hugh Cott's seminal 500 page 1940s classic "Adaptive Coloration in Animals" explains and catalogues the world of camouflage in exhaustive fine grained detail. From a big picture layman's perspective camouflage can be crudely digested into three main categories:
Terrestrial and aquatic animals both have a tendency to be darkly shaded underneath when viewed in any environment where light shines down from above. The dorsal (top) portions of top-lighted creatures tend to be brightly illuminated and light while the ventral (bottom) portions are are shaded and dark. Shading enhances and exaggerates the natural 3D appearance of depth and contouring. Shading makes animals stand out. Shading makes prey species easier for predators to see and to notice. Visible shading makes prey animals more likely to be eaten by foraging predators.
Evolution's response--among prey species that suffer from contour-enhanced shading--has been to gradually and methodically select slightly counter-shaded prey as survivors. Minnows that appear darker on their backs and lighter on their bellies when viewed in your hand appear flat-lighted, not shaded and relatively contour-less when suspended in the water column. Counter shading makes minnows harder for predators to see.
Minnows also tend to have shiny reflective scaling which mirrors their surroundings. Flat lighted counter shaded minnows suspended over a cobble stone background tend to look like cobblestones. Counter shaded minnows can be almost impossible to see.
Suspended minnows usually employ counter shading as their best adapted camouflage while bottom dwelling creatures like sculpins and bottom crawling nymphs, like the impressively-large Drunela Nymphs more often employ mottled bottom-matching colors instead of counter shading. Background mimicking among bottom-dwelling creatures dramatically increases their rates of survival. Background mimicking dramatically increases the chance foraging predators will go hungry.
Many terrestrial and aquatic animals exhibit high contrast and boldly displayed blotching or striping as a way to hide and obscure their real overall shape. High contrast boldly striped or blotched visual patterns on prey fishes are more common in coral reef habitats than in fresh water. Examples of disruptive fresh water coloring do exist. Many fresh water Perches exhibit bold vertical striping. Young salmonids often display rows of vertically oriented striping on their sides--sometime referred to as parr marks--that help at least somewhat in obscuring the vaguely cigar-shaped outline or silhouette of the minnow.
You might be thinking "Uh oh. Here we go again. He's trying to make fishing part of a graduate degree in complexity now!" But no. It isn't so. We fishermen have indeed been missing a key part of the overall aquatic landscape all these years. The inescapable implications of camouflage however, make it all a lot simpler in the long run. Bear with me. I'll get there soon enough.
In his 2003 book How Fish Work fisheries biologist Thomas Sholseth, in a chapater about the importance of light in understanding fish behavior, emphatically suggests outline recognition is a mechanism piscivorous predators use as a counter measure to the evolving and ever more sophisticated camouflage exhibited by their aquatic prey. That's a bit of a mouthful. Sholseth's chapter is a bit hard to follow but I do think that's the gist of it.
In other words prey species like Sculpins have evolved mottled stream bottom colorings because it works. Camouflage disguises their presence. Camouflage confuses the predator's prey recognition mechanisms. So in an evolutionary tit for tat response predators have evolved auxiliary outline recognition capacities--in order to break those camouflages--that may involve 'outline enhancement,' perhaps even utilizing polarized light. Perhaps even using polarized UV light. I
This back and forth evolutionary appearance warfare is not yet well understood. Deciphering exactly now outline recognition works is an active and lively area of research. Outline recognition using polarized UV light has been demonstrated in cuttlefish. Do vertebrate fishes use the same mechanisms? From a fisherman's perspective it doesn't really matter how outline recognition works. What does matter is that it happens. "Amodal outline recognition capacities" are almost universally assumed to be used by predators, both terrestrial and aquatic, as a means for breaking camouflage defenses.
Thomas Sholseth strongly infers high contrast flies and lures are the best way for fishermen to leverage predator outline recognition. This not only begins to make sense, it intuitively suggests rather obvious arguments explaining why unnaturally high contrast flies like the Prince Nymph the Zebra Midge and various high contrast minnows were heretofore so inexplicably effective. Contrast has moxey.
Is the corollary true? If Matching the Hatch implies meticulously Matching the Camouflage a strong case can be made Matching the Hatch is counterproductive, at least in the nymph-wetfly-streamer context. But what about dry flies? "What about a Pale Morning Dun hatch," you might ask. "If I use a big bushy Stimulator in the middle of a size #16 - #18 Ephemerella hatch I won't do well. So Matching the Hatch does matter," you might argue.
I agree. But dry flies are a subject I'd like to postpone until a later section. In the meantime we're talking here about anything and everything food-like that lives below the surface of the water.
If Sholseth and other fisheries biology professionals are on target--if contrast and outline recognition are important tools predators use to break through camouflage defenses--then high contrast absolutely is a better choice than Matching the Hatch. The best attractors, moreover, are not necessarily flies with the brightest flashiest colors. The best attractors are flies that create the most contrast with their backgrounds. That's why jet black leeches, black Woolly Buggers and black streamers of all kinds are so effective. The blackest blacks absorb all the available colors and appear as an outline against a chaotic stream-bed background no matter what. White or homogeneously chartreuse streamers do much the same. I often find Blacks and Whites to be more effective than multi-colored bright flashy streamers. Almost always in fact.
Bright bright colors often work well in the early season when water levels are high and the currents are a bit off color. But I also find the brightest flies are equally counterproductive late in the season when water levels are low and clear. Dull tan or brown nymphs seem to work better in low clear water conditions. High contrast blacks and whites do too. High contrast blacks and whites work well all year long. Early and late, in high off color waters and in late season low clear waters.
How many times has the Prince Nymph saved the day for you? Other than the Prince Nymph's outline it's coloration details don't look like anything real. It is a high contrast fly. So is the semi-magic Zebra Midge. So too are the wide tinsel bands on a traditional British style Gold Ribbed Hare's Ear.
Are Match the Hatch enthusiasts who substitute thin gold wire ribbing for the more traditional wide-banded tinsel ribbing on a British-style Gold Ribbed Hare's Ear damaging the effectiveness of their lures? Are they making their flies harder for fish to see? I think so. So does Thomas Sholseth.
Do terms like Matching the Hatch sometimes infer a little more than they should? What about Matching the Behavior?--which is both a looser term and another way of saying "Go with what works and don't worry so much about why."
Flies like Don Martinez' Black Nymph have fallen out of fashion because they're a round peg in the square holes created by Matching the Hatch ideology. But the Martinez Black Nymph is a powerful fly--a fly that fits right into the rounder holes of high contrast outline recognition. I need to tie some more Black Nymphs. Now. Pat Barnes liked that fly a lot. Who's to argue with that?
Camouflage pervades the aquatic environment. Everything alive below the surface of the water is fair game as food. Everybody eats everybody. Every creature does its best to stay safe and uneaten. Camouflage is natural selection's safety tool of choice. Camouflage is so universally effective it's not surprising predators have learned how to cheat the system. Breaking camouflage is how predators survive.
I Polarized light is ubiquitous in aquatic environments. Light gets instantly and thoroughly polarized by the mirror-like surface of the water. It makes sense to think aquatic creatures would evolve to make use of it. It makes no sense at all think they wouldn't.
I The idea that peacock herl has special fish catching powers has been around for a long time. Sholseth says peacock herl "reflects polarized light like crazy." He also mentions polarized UV light a few times, but does not sufficiently elaborate. All light in aquatic contexts--at least in the first few feet below the surface--is highly polarized, by its interaction with the water's surface.