In the first installment of this series on trout mentality, I described to you how to think like a fish by describing the underwater world and the many dangers a trout faces on a daily basis throughout his life. Therefore, in this article, I will instead describe what both the underwater and surface worlds look like from a trout’s perspective and explain why precision casting is so very important to a fly fisherman.
So, to begin with, imagine that you are a trout holding in your favorite prime lie under the surface of the water. Now, imagine that when you look up, what you see is a huge mirror image of the streambed. Thus, what you see above you is exactly what you see below you except for the distortions caused by the current. However, directly above you is a large, round, hole in the mirror through which you can see the surface world. But, because water and air have different densities and, because your eyes are specifically designed to enable you to see clearly underwater, your image of the surface is more akin to a vague, blurry, picture taken by someone who is unable to hold the camera steady than the crystal clear image of the subsurface world you are accustomed to. Now, imagine that the size of the hole in the mirror above you is dependent on how deep you are in the water such that the size of the hole grows larger as you descend deeper into your watery world and shrinks as you rise to the surface and that the relative size of this hole is approximately 2 1/4 times the depth at which you are suspended. Consequently, as long as you are comfortably lying deep in your favorite prime lie, you have approximately 330 degrees of horizontal vision but, you only have 97 degrees of vertical vision. Therefore, as you ascend to the surface to snatch a helpless May Fly dun, your “cone of vision” causes the hole in the mirror to shrink until you have a mere pin hole to see through as you reach the surface and, since you depend on your vision to keep you from becoming a meal for some avian predator, approaching the surface is not something that you do without trepidation. Of course, the reason for this limited “cone of vision” is a law of physics called Snell’s Law which states that any light wave that strikes the surface of the water at an angle of less than 41.5 degrees is reflected and, any light wave that strikes the surface at an angle of greater than 41.5 degrees is absorbed by the water.
However, even though you only have 97 degrees of clear vertical vision, you also have another 31.5 degrees of peripheral vision on either side of your cone of clear vision and thus, you actually have a total of 160 degrees through which to observe the surface world. Therefore, any unwary fly fisherman approaching your lie will be exposed to your sight unless he remains below your 10 degree horizon of peripheral vision and, since any movement above the surface of the water usually represents a threat to you, your first instinct upon seeing a fly fisherman approach your lie is to run for cover and to stay there until the danger has passed.
Thus, now that you understand how a trout sees his world, it’s time to explain why precision casting is so vital to successfully catching them. In fact, you may have heard someone say at some point that the trout were not looking up today in explanation of why they have ignored the angler’s dry flies. However, as I stated previously, trout have 330 degrees of horizontal vision and thus, below the surface of the water, they can not only see what is in front of them clearly, they can also see what is behind them with the exception of a 30 degree blind spot directly behind them. But, they can only see the surface world through the hole in the mirror and, as I stated previously, the size of that mirror is approximately 2 1/4 times the depth at which they are holding. Therefore, if a trout is holding at a depth of two feet, then the size of his hole in the mirror is only 4 1/2 feet in diameter and thus, in a swiftly flowing stream, any insect that passes through that hole tends to do so very quickly. Consequently, a trout only has a very short window in which to notice the insect, identify it as something that is edible, and then make up his mind whether or not it is worth the effort and risk of rising to the surface to seize it. Consequently, many experts believe that trout gain up to 90 percent of their food from the subsurface drift and thus, if a trout is distracted by something under the surface when your fly floats over his lie, then he may very well not even notice it. Plus, as I stated previously, the more shallow a trout’s lie is, then the smaller his cone of vision is and thus, if your fly does not drift directly over the center of his cone of vision but instead drifts over it near the edge, then the trout has even less time to notice your fly and make up his mind whether or not it is something that he wants to eat. Therefore, in order to maximize your chances of your fly being noticed by the trout, it must be visible to the trout for the maximum amount of time by drifting directly over the center of his cone of vision. Consequently, it is often imperative that you be able to land your dry fly very precisely on the surface of the water because, having it drift even a little bit to either side of the trout’s cone of vision may very well cause to be ignored or not seen at all by the trout.
Therefore as I stated in the first article in this series on trout mentality, as a fly fisherman, it is a wise idea to adopt the mentality and techniques of a hunter instead of that of the average fisherman by taking full advantage of any available cover such as streamside foliage, large boulders or logs laying in the water that may conceal your approach, as well as simply crouching down close to the surface of the water so that you remain below the trout’s 10 degree horizon of peripheral vision as you approach their lie. In addition, it is absolutely vital that you be able to land your fly precisely where it needs to be in order for the trout to notice it and have the maximum amount of time to decide whether or not he wants to rise to the surface to seize it.