Sunday Classic / What Does The Trout See?

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Try to see it my way Photo by Louis Cahill

Try to see it my way Photo by Louis Cahill

Did you ever wonder why fish eat bright pink worm patterns, or Thingamabobbers for that matter?

Ever watch a trout refuse your dry fly and wonder what he saw that he didn’t like? A trout’s eye serves the same purpose as ours but it functions in a very different way. The subject of how trout see the world is a complicated one but the basics are well worth your time. Understanding how the fish eye works can help you imagine the watery world they see, and it may give you some insight that will help you catch them. The following are some simple principals to keep in mind.

Water as a visual medium

Water is a poor conductor of light at its best. It affects the way fish see color as well as their visual acuity. Water absorbs light at different rates depending on its wavelength or color. Long wavelength light, colors like red and orange, are absorbed quickly while short wavelengths like blue and violet are absorbed more slowly. This means that as light passes through more and more water, warm colors fade to black while cooler colors fade more slowly. Overall, as a fish moves into deeper water his environment becomes darker, at which point the biology of the fish’s eye affects his perception of color as well.

It is not necessary however for a fish to be in deep water for its vision to be affected by the absorption of light. The rules hold true for a fish in shallow water, viewing an object at a distance. A red streamer, for example, that is running at a depth of one foot, where there is plenty of red light, will appear black to a fish viewing it from fifteen feet away. As the fish closes on the fly, however, the red will become vivid. The same would not be true at a depth of fifteen feet. At that depth the fly would remain black to the fish, even at close range.

Ultraviolet light, which we do not see but trout do, is scattered in water. Colors like white and reflective materials like flash are visible to fish at long distances but may appear blurred by this effect. These flies will get a fish’s attention from a distance and become sharper as the fish draws near.

Color perception and visual acuity are both affected by the chemical composition of the water as well as what foreign matter is present. Tea stained water, which is present in many mountain streams, absorbs UV light quickly, changing the rules dramatically. In these conditions warmer colors become more important and while fish may see less color overall their visual acuity will improve. When water is dirty, light is scattered by foreign particles and the fish’s environment becomes darker with little visual acuity.

The biology of a trout’s eye

The biology of a trout’s eye is similar to ours in some ways and very different in others. Their eye has an iris, a lens and a retina with both cone cells and rod cells, much like our eyes, but each functions in a different manner. The human iris or pupil dilates and constricts to regulate the amount of light reaching the retina. The trout’s iris is fixed and the retina itself adapts to changing light levels. Human eyes focus by changing the shape of the lens. A trout’s eye focuses by moving the lens closer to, or further from, the retina. The trout’s vision is very sharp in its focus but its depth of field, to use a photographic term, is limited.

The function of the trout’s eye which is of most interest to anglers is the adaptation of the retina to changing light conditions. To understand how it works we must first understand the cone and rod cells themselves. Cone cells see color and require bright light. The trout has four different types of cone cells. Humans only have three. Each type of cone cell is sensitive to a different wavelength of light. The trout’s extra cone cells see the UV spectrum and in some species dwindle with age. The trout’s eye is also more sensitive to the red spectrum than the human’s. The color it has the least ability to discern is green and the color it sees best is blue. Rod cells are very sensitive in low light and give the trout excellent night vision. These cells do not see color. To the rod cell the world is black and white.

During times of bright light the trout’s retina is dominated by the cone cells giving it very precise color vision. Still, the fish’s ability to discern color and its visual acuity are governed by the physics of its watery environment. As the light becomes lower the retina adapts. The cone cells recede and the sensitive rod cells are exposed, engaging the trout’s night vision and turning the world slowly to black and white. This is a physical change and takes time. The trout, like almost all fish, experiences a brief period of diminished vision as conditions change.

What does this mean to the angler

The idea is simple. Under bright conditions trout see color very well and in dim conditions they do not. It becomes a bit trickier when we start to define bright and dim conditions. Obviously a sunny day is bright but not to a trout sitting at the bottom of a deep pool. At a depth of ten feet much of the light has been absorbed and all of the red spectrum is gone. A trout sitting in shallow water on a sunny day will discern color very well at close range but not when looking at a submerged object ten feet away.

When fishing deep water or stained water that bright pink worm will appear much duller to the trout. Fluorescent materials, which trap UV light and shift it to a longer wavelength creating intense color saturation, are great triggers for fish but not in stained water where the UV light is quickly scattered. In low light fish will respond more quickly to the contrast of a fly tied in black and white than to carefully blended colors. For this reason I tie black and white stonefly nymphs which are highly effective in dirty water. When you are fishing Yellow Sallies on a bright day a red egg sack will improve a fly’s productivity.

Another important thing to remember is the fish’s point of view. A fish sees a submerged object with greater clarity and color than an object on the surface. The refractive index of its cornea is almost exactly that of water, meaning a fish sees much more clearly through water than air. In addition, objects on the surface are backlight from the fish’s perspective, appearing mostly as silhouette. This makes it hard for a fish to see the color of a dry fly.

Color that is not opaque, like the translucent body of a mayfly, is much easier the fish to see. This is why dry flies with loose dubbing, translucent wing material or flash underwings work so well. It is also one of the reasons that flies which sit in the film, like parachute patterns, are effective.

What trout respond to primarily in surface patterns is the impression of the fly on the surface film. The dimples on the surface caused by the weight of the fly resting on the water are a powerful trigger and their profile tells the trout that the object is likely to be food. These impressions focus the light, causing bright spots. The translucent color of a Thingamabobber combined with the way it focuses light often makes it irritable to hungry fish.

Flies and indicators are not the only tackle that create these impressions. It is often the bright dimples from curly tippet that give it away to fish. This is why I fish dry flies on fluorocarbon tippet. It sinks and vanishes. (There will no doubt be comments on this. I will write a separate article on the subject)

The important thing is to understand how the fish sees in a variety of conditions and what the triggers are that make him eat. Armed with this information you can make smart choices both on the stream and at the vise. Don’t be afraid to try new things. Trust your gut and see what works. Just try to see it his way.


Come fish with us in the Bahamas!

Louis Cahill
Gink & Gasoline
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4 thoughts on “Sunday Classic / What Does The Trout See?

  1. I love a healthy dose of biology on a Sunday morning.
    Mix in reflection and refraction and your head starts to spin.
    A video titled Discovery by Ozzie Ozefovich was posted here a few months ago. If you have not seen the Underwater World of Trout
    series, Trout Vision and Refraction is a must.

    By the way Louis, How’s your vision coming along?

    • Thanks Rick, It’s still a process. I’m over a month past my second lens replacement and have had 2 of probably 3 laser procedures. I’m doing pretty well. Not 100% yet. It’s been a little complicated but I’m confident that we are closing in on it.

  2. Thanks for the article. Any information on how fish see is good information to have both at the vise and on the water. Also have to agree with fluro on dries.

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