The chocolate bar
… and another nine fly-fishing theories
(part 1)

Framed in empirical, scientific reasoning, but not exempt from good doses of imagination and fantasy, the many theories developed in the fly fishing world awaken the curiosity of many an angler that seek answers to bewildering and bizarre questions.

I just had to write this article after making so many teasing references to various, often bizarre, theories about fly-fishing. It's about time I compile, classify and describe in some detail (to the extent of existing information and my experience) some of the more suggestive theories. Hopefully they will be useful for understanding our beloved hobby.
One other thing. I haven't given any of these theories the maximum reliability score, which I've set at 10. If it were 100% reliable, it would cease to be a theory. Besides, you wouldn't want me to stick my neck out too far, would you?

Wariness Scale theory

Scientific basis: high-average-low
My own experience: well verified-occasionally-hardly ever
General reliability rating (from 0 to 10): 9


Undoubtedly my favorite theory, the one I most often mention and the one that has given me a new understanding of many aspects of trout behavior. In a nutshell, it says: conditioned by many circumstances, the trout passes through states in which its feeling of security, state of confidence and degree of vulnerability vary constantly. These states enormously condition the trout's willingness to take your fly, independently of the pattern or the presentation.
This theory is developed in detail here: Wariness part 1

Chocolate bar theory

Scientific basis: high-average-low
My own experience: well verified-occasionally-hardly ever
General reliability rating (from 0 to 10): 4


This theory developed in the United States where they like to fish with big attractor patterns (Wulffs, Humpies, etc.). And eat chocolate bars. Imagine it's a hot, hungry summer afternoon and you're eating cherries from a luxuriant tree laden with the sweet, juicy gems. You're delighted with your find and, with a gnawing appetite, nobody and nothing can dissuade you from your gastronomic binge. But, suddenly, among the leafy branches of the tree, you come across a magnificent, luscious, yellow pear. Would you interrupt your cherry banquet for an instant to sink your teeth into this other larger, juicier fruit? The theory says that this is what often happens.
If a trout, very selectively feeding on the surface in the middle of a huge, single-species hatch, keeps rejecting your most exact imitations impeccably presented, this theory says the thing to do is to seduce the elusive lady with something radically different. You entice the trout out of its small insect fixation with something luscious (like a chocolate bar), big and juicy, something impossible to reject. You offer it something completely different in shape and size from the fly that is hatching but that especially surprises and incites the trout's already highly-stimulated, insatiable appetite in the midst of its blissful, rapturous feeding spree.
Well, OK. I've only given it a four.

Vulnerability traits theory

Scientific basis: high-average-low
My own experience: well verified-occasionally-hardly ever
General reliability rating (from 0 to 10): 7


This is an updated, more accurate version of this theory than what I wrote some months ago in the Spanish magazine Jara y Sedal. Its importance more than justifies including it here.

The trout is an expert in interpreting messages and attitudes of the creatures in its surroundings when they transmit some sensation of vulnerability. It's a clear defensive advantage when the message comes from one of its own species or from another fish. But the trout also knows how to use this skill when it comes from any other possible animal food, interpreting it as weakness and easy prey. Easy prey implies saving energy and, in the final analysis, this is the key to survival: knowing how to assess the calorie expenditure of catching that prey in relation to the calories that it provides.
Any insect or fish showing signs of weakness or vulnerability is interpreted by the trout as an opportunity for a positive calorie balance, which it certainly doesn't want to miss.
It behooves the fly fisherman to know the vulnerability traits of certain insects that the trout can interpret as such:

  • blurry, erratic silhouette of an insect trapped in the surface tension
  • one or two wings lying on the surface in duns and spinners
  • a missing or defective wing
  • shuck stuck to or slightly separated from the abdomen
  • erratic movements in insects below the surface or struggling in it
  • characteristic and imprecise silhouette of two insects joined in the process of mating
  • iridescent wings in spents (Marinaro effect)

According to this theory, adding some vulnerability trait when you tie your flies (or in their presentation) will sometimes get good results. So, cripple your flies.

Water cushion theory

Scientific basis: high-average-low
My own experience: well verified-occasionally-hardly ever
General reliability rating (from 0 to 10): 5


This theory developed from studies of trout vision carried out by the German biology professor, Dr. Muntz. He bases his findings on that small blind area trout have right in front of their snouts (due to the lateral position of their eyes).
We often try to catch duns floating down the stream to see which fly they are. It isn't easy to do with your hand. When you move your hand in the water towards the insect, you create a small cushion of water that moves in front of your hand towards the insect. This cushion catches the insect like a surfer who manages to catch and ride a wave.
This theory says that a lot of trout, usually the large ones with lots of experience, in slow waters (runs and pools), create this water cushion with their snout and delicately push the fly an inch or two before deciding whether to eat it or not. This way, they achieve two things:

  1. They observe the movement of the insect. They check that the way the fly floats is completely natural.
  2. They lengthen that instant in which they no longer see the fly after it enters the blind triangle. It is as if they needed more time to decide or as if they had a doubt at the last instant.

I don't know. Although I've occasionally observed this behavior, I may have been too generous with the reliability rating.