Understanding Fish Vision
November 27, 2017
In a sense, fish are hard-wired to bite. Their reactions to external stimuli are based on electrical messages passed from diverse sensory systems to the brain, via a vast nervous system. And especially in winter, vision is their most important sense.
Fish vision is complicated by their existence in water, a medium that ranges from what's called "gin clear" to tea-stained or even murkier. But while water color can change quickly in open-water conditions, due to rain or dam releases, water under ice tends to be stable, and often the clearest of the year. Once covered, wind no longer stirs sediment, and particulate matter drifts to the bottom. Moreover, ice and snow cover block a large portion of the sun's light, limiting photosynthesis by planktonic algae and aquatic plants. Plankton blooms are rare, further clearing the underwater environment.
Under ice, sight-feeders, including most key winter species, rely on vision even more than in open water. And while this makes them susceptible to our many artificial lures and artful presentations, water clarity gives them a clear line of view on a lure that's more or less hanging in place, not scooting along on a trolling pass. No wonder fish can get finicky, especially by midwinter.
Years ago, in a fish physiology class, my professor began a lecture on fish vision by projecting a foggy scene on the screen. "Think of how fog restricts your visual field," he began. "In a pea-soup situation, you can barely see 30 feet in front of you, so it's unsafe to drive. In a forest, even huge trees are obscured. But as you walk closer to them, trees come gradually into view. Standing next to one in a dense fog, you can make out every detail of its bark.
"Fish live in perpetual fog, sometimes thin, sometimes thick. Our visual field can extend miles on a clear day, but in the clearest waterway, maximum visual distance is 100 to 150 feet and that's rare. In some situations, it's a matter of inches." Absorption and scattering of light by water reduces and changes the light available for fish to see and define objects. And the more stain and particles present in water, the greater the effect.
How Fish Eyes Work
Structurally, fish eyes are similar to ours. Both have an outer cornea, adjustable iris, lens, and a retina that contains rods and cones, the visual cells of the eye. When we dive under water, our vision is poor without a face mask, which provides a layer of air between or eyes and water. That layer is essential because of the difference in density between air and water. The greater the difference in density between two mediums, the greater the angle of refraction or bending of light. When light hits our eyes in air, refraction occurs at the cornea and the lens then focuses it on the photosensitive cells on the retina, located at the back of the eye.
Unlike the rather flat lenses of humans and other mammals, fish eyes have round lenses that protrude outward, giving them greater peripheral vision. A round lens is optimal for bending light. This spherical lens also is dense, with a refractive index of about 1.67, the highest of any vertebrate animal. These characteristics enable it to bend light so it can be focused on the retina. Their protruding lens gives fish their "bug-eyed" appearance, but also provides a much wider field of vision than ours. So, under water, many fish can see better than a human diver. Physiologically, fish can thus be said to "make the best of a bad situation."
We focus light on our retinas by automatically changing the shape of our lens. But to maintain a refractive index of 1.67, a denser and more rounded lens is required. Without the ability to change its shape, fish instead alter the position of the lens, much like the way the lens of a single lens reflex (SLR) camera focuses by moving closer and farther from a subject.
Because of the visual limitations of their world, fish typically swim with the lens pushed forward by specialized muscles, since this position provides the best vision of rather close objects in front of them. To see more distant objects, the lens is pulled back by other muscles. The degree of this process, called accommodation, varies greatly among fish species, primarily based on their environments and their role. Sharks, for example, typically swim with their lens retracted, giving them an optimal view of distant surroundings, helpful for an ever-cruising predator.
Many of our freshwater gamefish have rather well-developed retractor lentis muscles and a high degree of accommodation, meaning they may feed by spotting a potential prey item at some distance, then approaching it as their lenses are moved forward for the best up-close look at it. The eyes of popular gamefish are designed for the sharpest three-dimensional focus on objects in front of the fish. So fish turn their body to face a potential prey item, sometimes tilting for the closest inspection.
Veteran ice anglers and those who make use of underwater cameras readily recognize this behavior. On sonar, a flicker on the outer edge of the screen becomes brighter, then looms red within inches of a bait. Or on an LCD screen, a line moves diagonally from the bottom toward one's lure, then slows and stops. On a camera screen, I've watched the barely perceptible shadow of a pike or big bass gradually approach the camera lens over 10 seconds or more, then peer at it. In the case of pike, you're sometimes assaulted by the sight of teeth and an esophagous.
Walleyes and saugers also have a reflective area on their retinas called the tapetum lucidum that gives them their baleful appearance and allows anglers to spot their eyeballs with a spotlight. This feature increases the sensitivity of the eye in low light by reflecting ambient light back over the rod cells so vision is improved. The tapetum is most developed in the lower portion of the retina, so in dim light, walleyes likely see objects above them better than below.
Because many species feed in both light and darkness, they have a mechanism of light and dark adaptation. In fish, the lens extends through the pupil, so they cannot adjust to light levels by contracting and dilating the pupil as we do. Instead, they undergo a synchronized shift of the rods, cones, and dark pigment granules. In bright light, pigment granules migrate toward the outer segments of the rods and cones, while the rods move back into the pigment for protection from the bright rays while cones move toward the light to enhance color vision. In low light, on the other hand, rods move toward the light and cones move away. This process of adaptation takes 30 to 60 minutes to complete.
What Do Fish See?
The well-developed eyes of fish contain sensory cells in the retina to form detailed images, including some level of color vision for most freshwater predators. But eyes are merely receptors. The brain then must interpret these images. It assembles bits of information on movement, shape, color, and more, then creates some sort of composite image that informs the brain of possible actions — flee from a large predator; move a little closer to inspect potential prey; ignore a waving plant stalk; launch an attack; and so on.
Humans have large portions of our large brains devoted to analyzing visual images and remembering. Veteran anglers may be able to view and identify dozens of rather similar-looking spoons. But fish brains are relatively tiny. It seems likely that even top visual predators like pike and perch "see" only simplified images like basic shapes, sizes, motions, and color highlights of familiar objects and organisms. They likely can't detect details such as lure brand or precise coloration, nor can they typically associate this shape with an unpleasant experience, such as being caught.
But their perceptive shortcomings don't mean that fish can't see small details. In laboratory settings, with clear water and a rather featureless environment, various species of gamefish have been trained to detect even slight differences in color and shape, though trial and error, and inspired by the reward of food, or else the punishment of some negative stimulus.
Fortunately for us anglers, fish in natural environments likely can't duplicate such feats of learning and memory, given that they're driven primarily by a need to seek food and avoid attack, typically without much time to ponder a decision. So we can sit over a school of crappies and catch one after another on a tiny spoon or soft plastic lure. The lure's mediocre imitation of a natural prey, and the sight of their schoolmates being hauled away sometimes fail to lessen the action. And on many walleye lakes, anglers continually pound favorite spots with old favorite lures, catching fish regularly. Such situations can give the power of fish cognition a bad reputation. Instead, anglers often find optimal conditions and can take advantage of the competitive nature of schooling fish, especially since natural prey can be limited in winter.
On other days, we locate fish and try all manner of novel artificials, livebait additives, and our most finesseful presentation, yet are spurned. This can make anglers think fish are getting too "educated." But instead, particularly in midwinter, reduced oxygen and the lower metabolic needs of fish can, at times, rob even big predators of an appetite.
The presence of visual cone cells strongly suggests that our common North American gamefish discriminate colors. And lab experiments have verified color vision in several fish species with good numbers of cone cells.
Walleyes, for example, have two sets of cone cells in their retinas: twin and single cones. The large twin cones contain a visual pigment most sensitive to wavelengths of light we call orange-red. Their less abundant single cones have a pigment most sensitive to green. So, they should have best discrimination between red and green. Eelpout have very few cone cells, suggesting their color vision is minimal, and of course they're most active after dark.
But in addition to the physiological aspects of fish eyes, one must consider what light is available to them in their habitats. When sunlight strikes ice and snow, a great deal is reflected. Light that passes through is then transmitted downward, weakening as it gets deeper and is absorbed and scattered. The clearest water absorbs violet and red wavelegths more than intermediate wavelengths, and blue wavelengths reach deepest. That's why underwater images from the deep ocean are saturated with blue.
Most lakes are far from clear, due to phytoplankton, dissolved matter, and suspended particles. In fertile waters, green is common, and reservoirs often show brown. In lakes with moderate plankton levels, blue and violet light are strongly absorbed while green and yellow wavelengths become dominant in deeper water. And in muddy water, red wavelengths are transmitted deepest.
For a fish to most easily detect an object, contrast between that object and the environmental background is beneficial. A lure is more easily seen when it contrasts, and that's affected by the direction a fish is looking: up, down, out into open water, or into a mass of vegetation. So, using the walleye as an example again, a reddish or orange lure should be most easily seen against a greenish lake background or near vegetation. In a murky river backwater, contrasting hues of black and purple or blue should show best, while reds become harder to see. A chartreuse jig and black softbait, or a spoon with pink on one side and a glow or orange tone on the back should be good choices, too.
On ice, fish vision matters, often a lot, but that doesn't preclude the importance of scent and taste, vibration detection, and sound. Flavorful offerings like eurolarvae, waxworms, crappie minnows, flavored lures, or a sucker minnow can count for a lot. And while anglers think of the scent or flavor of natural baits, it's also their lifelike motions that can't be fully replicated, despite our efforts.