Color Vision in Trout Eyes

Human Eyes:
In order to understand how colors are perceived by trout, we must first understand how color is seen by us humans. Color is a physiological effect which is a sensation that occurs as the brain responds to neural signals arriving from the retina of the eye. The retina has "rods" and "cones" that are two types of receptors. The rods register the presence of light in black and white (monochrome) while the cones register the colors. Note that there is no such thing as colored light, but only light of different wavelengths.

There are three sub-types of cone receptors in the retina of the human eye. Each interprets a different wavelength and sends its own signal to the brain: red, green, or blue. This is the same way the RGB leads feed color to a computer display. Combinations of these signals offer variations of the complete spectrum of colors. We see a rainbow as a graduation of colors because the wavelengths overlap. The ratio of these signal strengths determine the shade of color.

Each object has some chemical characteristics that absorb most wavelengths of visible light and reflect only a narrow band. This narrow band is perceived as color by the mechanism of the eye. Remember, we are not seeing a colored object, but an object that reflects light in different wavelengths.

The three types of cones in the human eye each contain a photo-sensitive pigment that has the capacity to absorb a range of light wavelengths. Each cone is characterized by the wavelength at which maximum or peak absorption occurs. The three are as follows:
erythrolabe; peak absorption at 565nm; red
chlorolabe; peak absorption at 535nm; green
cyanolabe; peak absorption at 440nm; blue
Light wavelength is measured in nanometers and the visible spectrum ranges from 700nm (red) to 400nm (blue-violet).

Trout Eyes:
Trout, on the other hand, have four receptors, and the four peaks are 600nm, 535nm, 440nm, 355nm.
The second and third conform to the green and blue cones in humans. The first is similar to the human red, but its sensitivity range includes longer wavelengths than humans. The fourth is outside the band of wavelengths visible to humans and is referred to as "ultra-violet". However, the fourth class of cones disappears by the time a trout is two years old.

It is thought the small fauna which feeds the immature trout, reflects the UV radiation and therefore the small fauna are more visible to the trout. It is also suggested that UV cones reappear annually in mature trout in time for spawning runs. It is also speculated that these UV cones are used to track polarized light as a means of navigating to the spawning locations.

It is interesting to note that the long wave (red) cone response of the trout is peaked at a point where the human’s response of the "red" receptor is diminishing. This means that where humans see a dark reddish color, the trout sees a much brighter color and in a lower visible light condition. Researchers tell us that the trout's ability to discern small differences in shade is highest in blue, second but much lower in red and lowest in green. Therefore shades of green will be less important than the contrast of the body or thorax.

Trout Environment:
Although trout have color vision similar to humans, there are major differences due to the available light in their environment. Their vision is limited by the quality of light which enters the underwater world. The advantage of their 4-cone system can be realized only if the full spectrum of sunlight from infra-red to ultraviolet is available to them.

In clear water, the short blue to ultraviolet wavelengths are dispersed causing the background appear blue. This is what occurs in the atmosphere causing the sky to appear blue and even bluer over water. Therefore when a trout sees the shiny scales of a fish, the image of the fish is blurred at short distances and invisible at longer distances.

Directional sunlight passing through water will tend toward red and becoming redder with increased distance just as it does in the atmosphere at sunrise and sunset. Hence, the old adage “Red at nite, sailors take delight, red at morning, sailors take warning”. However, water absorbs long light wavelengths; therefore, the energy of the longer wavelengths, corresponding to the red end of the spectrum, is absorbed and converted to heat. At longer range, the absorption of the long wavelengths and blurring of the image due to scattering become significant. For example, a red object seen through 12 feet of water has no wavelengths and will appear black. Note that the reflection of light diminishes very quickly as distance increases, so at 6 feet, there may be very little color perceived. Near the surface, reached by the full sun, at close range, it is reflected brighter red than seen by humans.

However if the object is white and capable of reflecting all incident wavelengths, it would remain visible at longer ranges. So what! The flash of mirror like reflection from a shiny surface such as tinsel or the scales of a fish will be seen over a much greater distance than body color of your fly.

It is clear that trout do indeed have the mechanism for full color vision and in a somewhat wider range as well. Red is brighter to the trout but the color diminishes quickly with distance while white will be visible over greater distances. Impurities in the water make color less important but white will be seen more readily. Water impurities, like minerals or staining, can selectively filter out various wavelengths of light. These impurities tend to remove the ultraviolet and blue wavelengths in a short distance and allow long wavelengths to penetrate the farthest but again not as far as clear water. To summarize, the color vision of the trout is limited to relatively clear, shallow water and at short distances.

What about fluorescent colors?
Fluorescence occurs where a surface has the property of absorbing ultraviolet radiation and converting its energy to be reflected as a longer wavelength within the visible range of the eye. This converted reflection is added to the reflection of normally visible light wavelengths, causing it to appear more intense than one would expect to be possible. Divers have noted that in tainted water fluorescent red, orange, and yellow are the most visible, and in clear water any fluorescent paint will do. At long distances or in deeper water, fluorescent yellow and green are more visible. Note that UV penetrates deeper than the visible blue wavelengths, so all fluorescent colors are visible to the UV limit, which is beyond the depth at which their natural color becomes invisible.

Effects of Low Light:
However, in tea stained water often found in trout streams, the opposite is true. The UV wavelengths are filtered out first, but the distance affecting the red wavelengths is not affected by the stained water. Therefore, fluorescence is useless in stained water a short distance below the surface. However, near the surface where it receives UV rays, the red and orange fluorescence will be visible at a greater distance than the shorter wavelength colors of blue and green.

An important feature of the trout's vision is that the rods and cones physically swap places at the start and end of daylight. In the evening the cones that need high light levels to operate and that provide the color response are withdrawn into the surface of the retina and the rods tend to rule. At dawn the reverse action occurs. This change is not instantaneous, but occurs over a period of time. Therefore, as night approaches, the color response in trout diminishes until at night a trout has no color response at all. Under these conditions, black and white is likely to be the most effective combination. Tinsel may have some value if the moonlight is significant.

Conclusions:
1. Trout do indeed have color vision, but it is limited to relatively clear, shallow, water and short distances, so at close range, the trout can see the full detail of color.
2. Trout can discern differences in shades with the highest in blue, then red and then green shades.
3. The color red appears brighter than it does to humans, but quickly becomes black at greater distance.
4. The ability to detect color is greatly impaired and completely eliminated within 12 feet.
5. Impurities in the water or stained water makes colors less significant, but under these conditions, white will remain the best.
6. In the low light conditions of dawn or dusk, trout can not distinguish color. Black, then, becomes the most visible.
7. In clear water, fluorescent colors are more visible with red, orange and yellow being the most visible. In deeper waters, fluorescent yellow and green stand out the most. However, in stained water fluorescent is useless.

Tips to create and select flies:
Trout will closely examine a slow moving fly like an emerger or nymph, and as we have seen, trout are very sensitive to colors. This is a strong feature in selecting fly dressing such as ribbing. At a greater depth of water, a fluorescent or shiny rib will have a significant effect. On the other hand, insects sometimes carry a bubble which would have high visual impact. Its visibility is not due to color but to a difference in optical density between water and gas. This difference can be imitated by transparent pearly white mylar ribbon.

A dry fly is seen by trout as a footprint in the surface tension and color is not as important as it is with a submerged fly like an emerger. In dry flies, translucent color is much more likely to be visible from below than an opaque mass of color. Therefore, a tightly wound body of a fly will be less effective than a loose winding of feathers or dubbed wool.