Light and darkness, brightness and obscurity, or if a more general expression is preferred, light and its absence, are necessary to the production of color… Color itself is a degree of darkness.
~Johann Wolfgang von Goethe
Penetration of Light through Water
The ocean is divided into three layers, as far as light is concerned: the euphotic zone, the disphotic zone, and the aphotic zone. The euphotic, or sunlight, zone is the uppermost layer that receives most of the sunlight. All photosynthesis that occurs in the ocean takes place in this zone, and most of the truly abundant animal life resides here. In clear equatorial waters, the euphotic can extend to a depth of 655 feet. (Near the equator, the sun’s rays strike the ocean almost perpendicular to the ocean’s surface, meaning that more energy penetrates the surface of the water at the equator than at the poles, where sunlight hits the ocean at an angle.) The amount of light that penetrates also depends on surface conditions, the weather, and the time of day – choppy waters reflect more light than calm waters, sunlight at dawn/dusk is reflected more than light from the noonday sun, etc. Turbid, muddy waters may have a significantly shallower euphotic zone due to attenuation. When light hits a substance, it can do one of three things: it can be scattered, it can pass through, or it can be absorbed. Attenuation is the result of two of these processes: scattering and absorption. The scattering of light is caused by particles – or other small objects suspended in the water – and is somewhat similar to the effect of smoke or fog in the atmosphere. Coastal waters generally have more suspended material due to river input, sediment stirred up from the bottom, and higher plankton counts. Because of this, light usually penetrates to a lesser depth. In relatively clear offshore water, light penetrates to a greater depth.
Below the euphotic zone, down to about 3,280 feet, is the disphotic, or twilight, zone. Some animals survive here, but no plants. Although the amount of light is measurable at this range of depths, it is not sufficient for photosynthesis to take place. The layer of ocean where darkness reigns is called the aphotic, or midnight, zone. Over 90 percent of Earth’s ocean area is accounted for in this layer. Fun Fact: in calm weather, a diver can look upward to see the entire hemisphere of the sky compressed into a circle – a phenomenon called Snell’s window, caused by the bending of light as it enters water.
To study light in the sea, scientists use a wide range of instruments. The simplest method involves the use of a Secchi disk, a white plate about 12 inches in diameter. It is fastened horizontally to a rope marked in meters, then lowered into the water. The depth at which the disk is lost to sight is noted on the rope markings, providing a rough estimate of the extent of light penetration. There are several more sophisticated measures in use, such as nephelometers, optical backscatter and light scattering meters, transmissometers, a-c meters, etc. They measure scattered light and attenuation. Radiometers numerically describe the shape of the light field in the ocean. They can be lowered from ships, placed on submersible vehicles, carried by scuba divers, or even attached to Earth-orbiting satellites. The ones on satellites measure light reflected from the surface layer of the ocean and allow scientists to measure changing color over wide swaths of ocean. However, satellites view only the upper few meters of the water, and underwater instruments sample only specific places and times in the ocean. To visualize the entire ocean, computer models were developed in conjunction with available observations to simulate how light behaves and propagates through the ocean.
Water absorbs different wavelengths of light to different degrees. The longest wavelengths, with the lowest energy, are absorbed first. As light wavelength decreases from red to blue light, the ability of light to penetrate water increases. The colors disappear underwater in the same order as they appear in the color spectrum. Blue light penetrates best, followed by green, yellow, orange, and finally, red light. All objects that are not transparent/translucent either absorb or reflect nearly all of the light that strikes them. White objects appear white because they reflect all colors of light in the visible spectrum. Black objects appear black because they absorb all colors of light. A redfish appears red when swimming at the surface because, when struck by white light, its scales reflect the red wavelengths and absorb all the others. However, the deeper the fish goes, the less red it will appear because there is less and less red light to reflect. Even at a depth of just five feet, the fish can be noticeably grayer. Past the point at which red light can penetrate (50 feet, give or take, depending on water conditions), that redfish becomes a black fish because there is no red light to reflect, and the fish absorbs all the other color wavelengths. Blue wavelengths penetrate best, giving the deep ocean waters and some tropical waters their characteristic coloration.
This loss or alteration of visible colors occurs not only in the vertical plane, but also in the horizontal and diagonal. Vertical depth has roughly the same impact on color perception as horizontal or diagonal separation between object and observer. So the redfish that appears black 50 feet down would also appear black when viewed from the side at 50 feet away, even if it’s swimming right up near the surface. Fun Fact: your brain can compensate for the loss of color underwater. This is why sometimes you think you can see reds and oranges in deeper water – but if you take an ambient light shot with your camera, those colors aren't there!
Fish eyes are largely similar to those of other vertebrates. Light enters the eye at the cornea and passes through the pupil to reach the lens. Most fish species seem to have a fixed pupil size, but elasmobranches (sharks, skates, and rays) have a muscular iris which allows pupil dilation. Pupil shape varies between circular or slit-like. Lenses are normally spherical but can be slightly elliptical in some species. Compared to terrestrial vertebrates, fish lenses are generally more dense and spherical. In the aquatic environment there is not a major difference in the refractive index of the cornea and the surrounding water so the lens has to do the majority of the refraction. (Refraction is the bending of a lightwave when it enters a medium where its speed is different – air to water, water to fish eye, etc.) Visual focus in birds and mammals is normally adjusted by lens shape, but fish eyes adjust focus by moving the lens closer to or further from the retina.
Fish retinas generally have both rod cells and cone cells. Rod cells control scotopic vision, or vision for low-light conditions. Cone cells control photopic vision, for well-lit conditions and allow for the possibility of color vision. The ratio of rods to cones depends on ecology. Most fish can see color, but not all can distinguish the full color spectrum. Inshore fish have good color vision, whereas offshore pelagic fish have limited color vision and detect only a few if any colors other than black and white – not surprising from an evolutionary point of view, because nearshore waters are lit with many colors; offshore waters, on the other hand, are mainly blue or green. Physical studies of the eyes show that the majority of fish can at least obtain a clearly focused image, detect motion, and have good contrast-detection ability, even if color perception is limited.
The light that humans see is just a small part of the total electromagnetic radiation that is received from the sun. We call what we see the visible spectrum. Many fish, however, can see colors that we do not, including ultraviolet (UV), which advantageously extends their range of vision. Lots of animals contain compounds in their tissues that protect them against UV radiation by scattering, reflecting, or absorbing UV light. To a fish with UV vision, this makes those animals appear dark and silhouetted against an otherwise bright background of UV light. On the flip side, fish that use UV coloration as an alarm signal are a step ahead if their predators lack UV vision. The leading theory regarding the evolution of UV vision is due to its strong role in mate selection. However, it may also be related to foraging and other communication behaviors (such as the alarm signal, used by the two-stripe damselfish). UV vision is sometimes used during only part of the life cycle of a fish. For example, juvenile brown trout live in shallow water where they use ultraviolet vision to detect zooplankton more easily. As they get older and move to deeper waters where there is little ultraviolet light, UV vision becomes less useful.
Recent research shows that many fish can also sense polarized light. Regular light vibrates in all directions perpendicular to its direction of travel. Polarized light vibrates only in one plane. When light is reflected off many nonmetallic surfaces, including the ocean surface, it is polarized to some degree. Light reflected off baitfish scales, for instance, is polarized, and polarizing vision enhances the contrast between almost transparent animals and their background – meaning fish that can detect polarized light have an advantage in finding food. Another possibility is that having polarizing vision can let fish see objects that are farther away, perhaps three times the distance. If this speculation is correct, it may explain why some fish seem to feed more aggressively under very low-light conditions, such as dawn or dusk.
What Does All This Mean for Fishing?
Due to Snell’s window, a fish can see through an area of the surface which has a diameter of about twice that of its depth. So a fish at 5 feet of depth will be able to see through a circular window of about 10 feet in diameter above her. Additionally, the fish can see objects above the water far to the side of the window, due to refraction. Outside of the window, the fish will see a mirror of the bottom structure. Potentially useful information when stalking fish in clear water.
Some anglers maintain that the choice of color is critical, while others say it’s irrelevant. There is evidence to suggest that both points of view may be correct. Picking the appropriate color(s) will, under certain conditions, improve your chances of attracting fish, but in other situations, the color of your lure is of limited value or no importance whatsoever. The first thing to realize is that the color of your lure in the water is almost always different from what it is in the air. Different colored lures may be equally effective or ineffective simply because they are similar in color at the depth the fish see them. Try to consider what the colors in your lure will look like at the depth you are fishing, and choose appropriately. Also remember that what our human eyes see may be different than what a particular species of fish will see. Fun Fact: sharks are color blind.
As colors disappear in the depth of the water column, contrast and flash will become more important, especially for fish that feed by looking up toward the surface. If you are fishing in deep water, the motion, noise, or disturbance your lure makes might be much more important than its color. If the ability to sense polarized light helps fish hunt, then lures with irregular surfaces that reflect more polarized light should be more attractive to such fish.
Under the right conditions, fluorescent colors can be seen for considerable distances. An object that is fluorescent emits light of a longer wavelength after absorbing light of a shorter wavelength, such as blue, green, or ultraviolet (so cloudy days are optimal since the visible spectrum is diminished but UV light is still strong). For example, fluorescent yellow appears as bright yellow when exposed to blue or ultraviolet wavelengths. Because of this unique characteristic of fluorescent colors, their color does not fade as quickly when they are fished deeper.
While most fish have an adequate sense of vision, it’s usually not as impressive as their sense of hearing, smell, or ability to detect vibrations through their lateral lines. A fair bit of surface noise (talking, music, etc.) mostly bounces off the surface of the water, but noise from banging on the bottom of the boat or stomping on the shoreline conducts extremely well. Sound, especially lower-frequency sound, can travel quite far with very little loss of signal, and fish hear exceptionally well. Some fish can detect scents with extreme sensitivity, at concentrations of as little as a teaspoon of liquid in a lake 13 feet deep, half a mile wide, and a mile long. Fish often use these super senses to initially perceive their prey, and then use their vision only in the final attack.
In order to change a color, it is enough to change the color of its background.
~Michel Eugene Chevreul
Where I learned about color, light, and sight, and you can too!
Woods Hole Oceanographic Institution
American Museum of Natural History
Georgia State University - Dept of Physics & Astronomy
University of Hawaii at Manoa
Underwater Photography Guide
The Scientific Fisherman
BBC Earth News
Polarized Light in Animal Vision: Polarization Patterns in Nature
by Gabor Horvath, Gábor Horváth, Dezsö Varju, G. Horváth
Evolution's Witness: How Eyes Evolved
by Ivan R. Schwab, Richard R. Dubielzig, Charles Schobert
by Michael F. Land, Dan-Eric Nilsson