UNDERSTANDING SELECTIVITY
Fly fishers, especially those that fish for trout, are inordinately attuned to matching the hatch and selectivity. At certain times trout can be very choosy or selective about what they eat, and this singular trait has been elevated to a level approaching a religious experience. Some have suggested that this ability to be selective is a mark of intelligence, giving the trout an ability to apply logic and reasoning when they are feeding. In reality we can be very thankful that nothing is further from the truth.
The largest trout in the creek has a brain that is slightly larger than a peanut. The major part of a trout’s brain is devoted to visual acuity, and the cerebrum [cortex] that controls the thought processes and memory are relatively small. Trout are visual predators and that’s the part of brain that is most developed. The trout’s ability to logically process information and benefit from experience is extremely limited. A trout may spook if it detects a sudden movement, a unexpected shadow passing overhead, a disturbance in the water near its position like a carelessly cast fly, but these reactions are more a result of an inborn response [once called instinct] rather than the results of learned behavior. However intelligent or wise that old brown trout seems to be his IQ is not much higher than the mayfly that he just ate.
To illustrate this reality think of your favorite trout stream. Midges are the food of the hour and every trout in the creek is feeding ‘selectively’ on the emerging midges. Now these midges are small, and it takes a goodly number of them to fill the energy needs of anything but the smallest trout in the stream. In the midst of this blanket emergence of midges an occasional mayfly floats along on the surface. In comparison to the midges these mayflies are like Gulliver was to the Lilliputians and one of these behemoths would contain more calories than 1000 of those tiny midges. They flutter and hop around on the surface trying out their wings, but not a trout rises to intercept them. One by one they fly away while the trout continue to gorge themselves on the tiny midges.
Now imagine that you have been reduced to foraging for your food. You have only been able to find a few small things to eat, let’s say rabbits or other small animals. Although the rabbits and other small animals are abundant you have to spend most of your waking hours gathering enough of these small animals to maintain your life. One day while you are out foraging you happen upon a deer. You have the ability to kill the deer, but when a rabbit runs passed you kill the rabbit and let the deer run away. Ask yourself, was that an intelligent decision?
Selective feeding allows the trout to eliminate the process of trial and error. Once the trout establishes that those tiny midges are food it then becomes possible to focus on that one food source to the virtual exclusion of everything else that is floating passed in the drift. It’s similar to our illustration of the rabbits and the deer. If you know that rabbits are edible, but you are uncertain about the deer, when presented with a choice you choose the rabbit.
Now it should be understood that there is a certain ‘economy’ in selective feeding, unfortunately trout do not always seem to understand this principle. Most anglers that have spent any time attempting to catch trout feeding on specific flies have come across the situation where there is a blanket hatch of insects and not a single fish is feeding on them. It would seem to be a ‘no-brainer’ but the trout show no interest whatsoever, but they may be feeding on some other obscure bug that only appears randomly.
Trout determine what is edible by picking up lots of various objects that they find floating around in their environment. If it looks like it might be edible a trout will give it a try. Once it’s in their mouth, if it’s determined to be edible it will be swallowed. If, however, it is determined to be a non-food item it is quickly rejected. Even if the item is actually edible it will sometimes be rejected. Years ago I observed a number of fingerling trout in an aquarium. In a rather crude experiment the trout were feed some fish food that came in flake form. The flakes were shades of reds and greens, and the tiny trout would sample the flakes as they floated on the surface or drifted down through the water column. Often a piece of food would be picked up by a fish, spat out, picked up again, spat out, and sometimes it would be allowed to sink to the bottom without being eaten. All the flakes of fish food were the same; the only difference between the flakes was the color. I sprinkled tiny grains of sand into the water and instantly the tiny trout began to sample the tiny sand grains as they sank in the tank. Sometimes they ate the grains of sand just like they did the flakes of fish food!
If the angler is going to be successful when trout are feeding selectively it is important to understand what behaviors are at work that triggers this behavior. Trout feeding selectively are neither intelligent nor even especially cunning, but rather they are controlled by their own genetics. Certain triggers, such as abundance, size, shape, color, and actions of the prey species are all triggers that govern selective feeding. The key to angling success during selective feeding periods is to determine which of these triggers is the primary characteristic that is motivating the trout to feed selectively on that particular life form.
When faced with selective feeding I normally start by imitating the size of the insect that I believe the trout is eating. This is an arbitrary decision on my part, but my personal experience has convinced me that size is what triggers the initial response in a selectively feeding trout. If my imitation causes the trout to inspect but refuse my offering then I know that the size was correct but something else caused the trout to refuse my offering.
My next concern is appearance and color. If the trout moved to my fly but refused it then I attempt to more carefully match the overall color and appearance of the food that the trout is eating. This is where a well stocked fly box containing several different styles of the same pattern can be especially critical. A fly box containing standard patterns, parachute patterns, no-hackles, cripples, and shuck-trailers in a variety of colors, many only subtle variations of the overall color of the fly, may be the key to success or failure when trout are feeding on a very specific hatch.
If, after I have matched the size, color and appearance of the prevalent insect, and I still have not solved the puzzle I will often resort to an old angling trick by giving the trout something exactly opposite what they are eating. For some unknown reason; at least unknown to me, if you give a selectively feeding trout an ant or a beetle they will often take it despite the fact that you have not seen a single ant or beetle on the surface. Another technique that will sometimes work; and again I don’t know why, is rather than matching the size of the naturals use a fly one size larger.
However, there are times when I have given the trout everything in my box and they have snubbed their collective noses at my best efforts. Ah, humbled again by a creature with a brain the size of a grain of rice. Somehow there is a certain symmetry in that.