Trout Vision & Their Perception of Color

Trout Vision & Their Perception of Color

By Sam R. Johnson... 

Trout have eyes for the same reasons as we humans do – to see what’s going on around them and ultimately do what’s needed to survive! For the most part, their eyes have the same basic components as ours, yet some of those components function differently. The details can be complicated, but the basics are relatively simple, and if you understand the basics, you’ll be able to imagine what trout are seeing in their watery world. This will ultimately enable you to sneak up and catch more of them. To understand how trout see and perceive color, we’ll examine some of the generally accepted tribal knowledge compiled over the past few decades, and then draw some conclusions. There’s some academia and science in all of this, but also a healthy dose of personal observations from me and others.

Basic Physiology of a Trout’s Eye

Let’s first consider how a trout’s eyes function to collect and process light into color  Although understanding the trout’s “cone of vision” in the water column and on the shore is also important, let’s just focus for now on the mechanics of their vision. That alone can enlighten us as to what bug colors might be best in a given situation, how to fish them, as well as what colors we should wear, or NOT wear, while fishing.

The biology of a trout’s eye is similar to ours. They too have an iris/pupil, lens, and retina – but that’s about where the similarities end. Where human iris/pupils dilate or constrict like a camera shutter to regulate the amount of light reaching the retina, the trout’s iris is fixed, and it’s their retina that adjusts to receive the incoming light. Like ours, a trout’s iris/pupil is basically round to aid peripherical vision. However, it’s different in that it’s also “notched” in the front to provide enhanced frontal vision. It’s believed this notch feature helps them see what’s coming straight toward them in the water column. A human eye focuses the incoming image of an object by morphing the shape of its lens, whereas a trout’s eye focuses by telescoping its lens (forward / backwards) to direct the image onto the retina.

The perception of color by humans and trout is a physiological sensation that occurs as the brain responds to signals arriving from the eye’s retina. One of the more significant differences between human and trout eyes is how the trout’s retina adapts to capture varying light conditions in its environment. To understand how it works we must first understand the basic light capturing components of their retinas.

Both trout and human retinas are covered with specialized cells called “cones” and “rods.” Rods capture black and white only, and work very well in low intensity light. Cone cells contain a photo-sensitive pigment that senses color by absorbing a specific range of the incoming light’s wavelength, and then sending a signal to the brain where combinations of these signals create the perception of a complete color spectrum. Cone cells work best in bright light. Note that bright / low light sensitivity will be an important factor as we discuss the significance of water depth and available sunlight on the trout’s vision.

Humans have three types of cones - each one is specifically receptive to either red, blue, or green wavelengths. However, trout have FOUR types of cone cell receptors – three that capture red, blue, or green, as well as an extra one that captures the UV spectrum. We humans can’t see into the UV Spectrum. Each of the four cone types in the trout’s eye is receptive to a specific color wavelength, and the peak absorption of each color wavelength can be measured in nanometers (nm). The “peak” color sensitivity for each of the trout’s four cones are:

  • 600nm – Reds (Visible to humans and trout – reds are much more vivid to trout)
  • 535nm - Greens (Visible to humans and trout – least vivid of all colors for trout)
  • 440nm – Blues (Visible to humans and trout – most vivid of all colors to trout)
  • 355nm – Ultraviolet (Visible to trout only – vivid, but thought to diminish as trout mature)

An important feature of a trout's eye structure involves how the rods and cones can swap places. In the evening, cones need high light levels to provide a color response. So, as it gets darker, they withdraw below the retina’s surface. The rods, with their low light sensitivity, then come forward and provide black / white sensitivity. At dawn, the reverse occurs, the rods withdraw, and the cones resurface to absorb the brighter light to provide color perception. Just as humans getting accustomed to bright / dark environments, this change takes place gradually.

Water’s Effect on Light and Color

The advantage of the trout’s four cone retina is maximized only when the full spectrum of sunlight is hitting the water - infra-red, visual, and ultraviolet. However, this is rarely the case, so their vision is impacted by the quantity of sunlight entering the water, viewing distance through the water, depth of water, as well as the water’s turbidity (clarity).

At best, water is a poor conductor of light and that affects the way trout see color and the sharpness of an object. Water absorbs light at different rates depending on depth and the light’s wavelengths. The combination of these two factors causes light’s intensity to decrease and colors to change. Long wavelength visible light such as reds and oranges are absorbed quickly as they penetrate water, while short wavelength blues, greens and violets are absorbed more slowly. The effect is simple, as visible light penetrates a volume of water, the warm colors (red) will slowly fade to black as the water gets deeper. The cooler colors (blue) go deeper and fade to dark slowly. For example, a bright red object seen through 12 feet of water depth will have most of its wavelengths already absorbed by the water and will appear almost black. On the other hand, a bright blue object seen through the same 12 feet will retain much of its wavelengths and appear a dark blue.

Additionally, the ability of an object in water to reflect visible light is reduced as the depth of the water increases. Therefore, at 12 feet there will be very little color reflected for red, but there will at least be some reflection of the blue. Near the surface, where the sunlight is brightest, both red and blue will be visible. Consider how this has a direct bearing on what colors trout see and how they might perceive bugs at varying depths or distances!

Ultraviolet (UV) light is another story! Trout can see it because they have that special fourth cone to collect it – we humans can’t. This enhanced UV visibility is an important difference to understand and use when selecting and using flies. Ultraviolet light causes some materials like tinsel, foil, flash, etc., to trap UV light and shift it to a longer wavelength thus creating intense color saturation. The fluorescence effect occurs when a surface absorbs ultraviolet radiation and reflects that energy as longer wavelength light within the visible range for humans and trout. This “converted reflection” is added to the normal reflection of visible light wavelengths, causing the object’s color to appear more intense than normal colors appear. Since UV radiation travels greater distances underwater, fluorescent colors can provide greater visibility in deep pools and runs both day and night.

Color and sharpness of an object are also affected by the chemical composition of the water, as well as foreign matter. Stained / tea colored water is characteristic of many mountain streams, and it absorbs UV light quickly. In these conditions warmer colors become more important. And while trout may see less color overall, their visual sharpness may actually improve. Consequently, water impurities, can also selectively filter out various wavelengths of light and make color less pronounced.

Trout’s Perception of Color – the Basics

As a general rule, as trout move from shallow to deeper water, their environment gradually gets darker, and colors become duller. At that point, when viewing an object such as a fly, the biology of the trout’s eye will affect its perception of the fly’s color based on how the water reflects the available light’s wavelengths at that given depth or distance. Trout also see objects under water with greater clarity and color than the same object on the surface. It’s also worth noting that objects on the surface are mostly backlit from the trout’s perspective and appear as dark silhouettes. This can make it difficult for a trout to see the color of a dry fly.  So, it’s the actual impression of that dry fly on the surface film of the water that seems to trigger a trout’s response. These impressions create attention by bending and focusing light and causing bright spots. These dimples caused by the weight of the fly in the surface film are a powerful incentive for the trout to strike an object whose profile plays on their instinct that the object is probably food.

Although trout have color vision, that vision is limited to short distances through water that’s relatively clear and shallow. The color trout have the most trouble seeing is green – the color they see best is blue. Red tends to be brighter to a trout than to humans. But because of red’s long wavelengths, that intensity diminishes quickly with depth or distance. The long wave “red” cone response of trout peaks at a point where human’s response to red diminishes. Consequently, where humans see an object as dark red, trout see that same object as a much brighter red.

Trout don’t have to be in deep water for color perception to be affected by the light absorption. This is also true for trout holding in shallow water viewing an object “horizontally” through the water. A red streamer running at a depth of two feet, where there’s plenty of light, will appear almost black to the trout viewing it from fifteen feet away. But as the trout closes on the streamer, the red will become vivid. However, the same is not true at a “depth” of fifteen feet. At that depth, the fly would appear almost black even at close range because of the water’s effect on the wavelengths of the light being reflected.

A trout’s rod cells are very sensitive in low light and give the trout excellent night vision – a time where everything is in shades of black / white for them. As light intensity lowers, the trout’s retina adapts. Cone cells recede and the highly sensitive rod cells become dominant, firing up the trout’s “night vision.”  So, as night approaches, the trout’s color perception is reduced, and their world becomes black / white. Under these conditions, as well as any low light conditions in trout water, bait with black / white features will likely catch fish.

Most of what we have talked about here is common sense. Under bright light conditions trout see color very well, and in dim conditions they don’t. However, actually defining bright and dim conditions is a bit more subjective. Keep in mind that a bright sunny day to us on the bank, won’t necessarily be to that trout holding at the bottom of a deep run.

10 Take-Aways…

Based on what we just discussed, here are 10 points regarding the importance of water depth, distance, and color when trout fishing:

  1. Most importantly, always try to “match the hatch” for the foods currently available to trout
  2. Muted, earth tone colors that blend into the landscape make it difficult for trout to detect your approach – the styles and colors you wear make a big difference!
  3. Trout have color vision, but it is limited to bright light, in relatively clear, shallow water and short distances
  4. The trout’s ability to detect color is greatly impaired by water depth and / or distance, and almost eliminated when they are viewing an object through 10 – 12 feet of water
  5. The trout’s highest color sensitivity is blue, then red – least is green. Red appears brighter to trout than to humans, but quickly becomes black with distance
  6. Fluorescent materials are great trout attractors in clear, deep water in low light conditions - but not in stained water where UV light is scattered
  7. In low light (deep water or time of day), trout respond better to contrast, like a black / white fly, than to one of blended colors
  8. Darker colors like black, blue, violet, and fluorescent colors are good attracters in low light conditions
  9. Impurities or stained water makes color less significant
  10. In deeper / dark water, fluorescent colors stand out well. However, in stained water fluorescent is useless. In clear water, fluorescent red, orange and yellow are most visible
Sam R. Johnson is a Partner in Wild Bearings, LLC, and the author of “Fly Fishing the Blue Ridge Parkway – NC Section.”  His next book about fly-fishing the VA Section of the Parkway is due to be released sometime in 2023. He is a lifelong freshwater fly fisherman, freelance contributor to several fly-fishing magazines, maker of bamboo fly rods, member of Mojo Sportswear Co.’s Pro-Staff Team., and a general outdoors do-gooder.
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