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Light Bottoms, White Flies and Their Sky Visibility

Your experiments and findings on tenkara fly-patterns and fly-tying.

Light Bottoms, White Flies and Their Sky Visibility

Postby Karl Klavon » Tue Dec 25, 2018 7:52 am

Merry Christmas, 2018. The following was written in answer to a question posed by Jason Klass, on his use of white flies when fishing streams with light colored bottoms, and said flies visibility to the fish when seen against the sky.

Hi Jason. Its raining here and we can’t do much outside, so I have been thinking about your white fly technique used on streams with light colored bottom structure. And I have no doubt that the aquatic insects living in such streams would take on a light body coloration to better blend in with their surroundings to protect them from predators. But I do not think the light coloration would necessarily make emerging insects or flies tied for them any more visible to the fish against the sky, and here’s why: Counter-shading.

Almost all fish use counter-shading in their body coloration schemes to protect them from predators, with white on their undersides so their bellies blend in well with the sky above, they are silvery on their sides, regardless of color shading and camouflaging pattern markings, to reflect the color of the water and other backgrounds around them predators will see them against, and they are colored darker on their backs to blend in well with the darker water and bottom colors seen by predators looking down on them from above, so a white fly would not necessarily be more visible to predatory fish. Against the sky, a black fly would provide the greatest contrast – day or night.

Black and white are not true colors. Black is the absence of light, meaning black objects do not reflect any color of light shined on them. White light is made up of all the colors of the spectrum, so white will reflect any color of light shined on it to some degree. Gray is a combination of black and white, so it will reflect gray toned light of many slightly different colors of a reduced intensity from what white will reflect.

Fluorescent colors are a double-edged sword in that they need to be used cautiously. Fish have no eyelids or diaphragms to reduce the light coming into their eyes, so the only way they can regulate the light entering their eyes is to find shaded areas, hide under rocks or to put a deeper distance of water between their eyes and the sun. If more than 30% of a fly’s surface area is made up of a FL-color, it will usually over load the cones in the fish’s eyes and they will back away from it, so you do not generally want a whole FL-colored fly. And even Hot Spots have to be used with caution. For flies cast upstream and drifted down to the fish, the Hot Spot should be located near the front of the fly. For fishing in lakes, where the fish usually overtake a fly from behind, the Hot Spot should be located at the back of or behind the fly. Some anglers have tried to cover all their bases by putting Hot Spots on both ends of flies, but that only serves to confuse and discourage the fish from taking a fly so tied.

Like us, fish eyes have both Rod and Cone vision cells. Young trout also have UV-Light Sensitive Cone Cells as well. But as the fish mature those cells are eventually lost and their function is taken over by the regular rod and cone vision cells of the adult fish. Rod cells are highly light sensitive but can only see black, white, and shades of gray, with very little detail being transmitted. The cone cells are for color vision and provide a much more detailed vision that only bright sunlight makes possible. But compared to human vision trout vision resolving power is about 14 times weaker than our vision is. Trout are nearsighted and they can see small objects and minute motion unbelievably well, but the overall vision quality of the picture their brain sees of a fly pattern is pretty crude compared to the picture of that same fly in our brains, which goes a long way in explaining why our crude representations of the food forms trout eat work so well.
Karl Klavon
Posts: 664
Joined: Wed Mar 02, 2011 10:01 am

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