Vision is an important sensory system for most species of fish. Fish eyes are similar to terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rod cells and cone cells (for scotopic and photopic vision), and most species have colour vision. Some fish can see ultraviolet and some can see polarized light. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. The ancestors of modern hagfish, thought to be the protovertebrate were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators, and where it is advantageous to have a convex eye-spot, which gathers more light than a flat or concave one. Unlike humans, fish normally adjust focus by moving the lens closer to or further from the retina.
Fish and other aquatic animals live in a different light environment than terrestrial species. Water absorbs light so that with increasing depth the amount of light available decreases quickly. The optic properties of water also lead to different wavelengths of light being absorbed to different degrees, for example light of long wavelengths (e.g. red, orange) is absorbed quite quickly compared to light of short wavelengths (blue, violet), though ultraviolet light (even shorter wavelength than blue) is absorbed quite quickly as well. Besides these universal qualities of water, different bodies of water may absorb light of different wavelengths because of salts and other chemicals in the water.
Colour can play a major influence on trout and salmon flies without us really understanding why, just think for example we go out on a bright sunny day and wear sunglasses; whereby the trout don’t have eye lids and require to go deeper or into a shadow area to escape from the sun.
Trout 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.
Although trout have color vision similar to humans, there is some 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. 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
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 the body color of your fly.
Fluorescence occurs where a surface has the property of absorbing ultraviolet radiation and converting its energy to be reflected as a lower 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.
However, in dark 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.