Fish Sensory Systems – Part 1

20/20 vision in an often dim world

A fish’s survival depends on its ability to sense its environment. Anglers, of course, are most concerned about feeding — how fish detect, select, and ultimately attack and consume food. We make the assumption that sensory systems involved in feeding are equally involved in catching fish with lures or real food impaled on a hook. This three-part series explores fish vision, taste, smell, hearing, and detection of water movement. Understanding the basics of how fish sensory systems work is relatively simple. The more difficult, but vital, part of understanding how fish use their sensory systems for feeding is comprehending the differences between the fish’s aqueous home and our aerial world.

Vision is our dominant sense. The same is true for most freshwater fish, exceptions being those that live in caves or in exceptionally and persistently turbid waters. So vision is a logical place to begin this study of fish senses.

An eye is an eye

The fish eye lacks a light-regulating iris but otherwise works like our eye. Light passes through a lens that focuses the image on the retina. The retina is a layer of densely packed light-sensitive nerve cells lining the inside of the eye. Many fish, and all freshwater game fish, have color vision. Different cells in the retina detect light and dark and different colors (different wavelengths of light). Fish are neither “near sighted” nor “far sighted.” The lens can be moved by muscles allowing the fish to focus on near and far objects. Fish visual acuity rivals and surpasses ours. Scientists have determined that some fish can detect a 4-inch object 50 yards away in clear water. Nice to know, but we don’t fish in water where you can see bottom where it is 150-feet deep. Let me put this in perspective — fish can see microscopic zooplankton that are essential food for the early life stages of almost all fish.

Fish have a field of vision that would make the best hunter jealous. Because the eyes protrude from the sides of the head, the fish can see forward and backward, up and down at the same time. The wide field of vision gives the fish binocular vision and allows them to judge distance.

If you are not into all the science, remember two things: 1. The fish we are trying to catch see the same colors we see. 2. Fish can see (detect objects) at about one tenth the amount of illumination we need to see.

Light in water

Vision is all about transmission of light, and light passes through water differently than air. Visible light is made up of a spectrum of wavelengths from red to violet. The different wavelengths are filtered differently by water. The short wavelength red light is absorbed near the surface. Longer wavelength green light penetrates farther. And blue light, with even longer wavelengths, penetrates still farther.

How far light penetrates in water also depends on surface conditions and material suspended in the water. Waves and turbulent conditions that disturb the otherwise flat surface of a lake or a stream reflect more light, so less light passes through the water column. Sediment and plankton in the water absorb and reflect light, so less light passes to deeper water.

The transmission of light in water follows well-proven physical laws, and these laws will help you understand what a fish is seeing. Lower a white object, like a spinnerbait or jig, into the water until it disappears. Now raise the white lure until you can just see it. Measure the distance from the water surface to the lure. Most humans can see a white object at about 1% of full daylight illumination. For you to see that lure, the light had to pass through the water, reflect off the lure, and pass back to the water surface. So, doubling the distance from the water surface to the lure gives a good approximation of the depth of 1% of surface light.

Most fish can see at about one tenth the amount of light we need to see. The physics of light in water predicts fish can visually detect an object at four times the depth at which your white lure disappeared. The amount of light needed for visual detection varies among fish, but do not expect vision to be a major factor in a fish detecting a lure at depths greater than four times the depth at which your white lure disappeared.

The tug on the end of the line is the ultimate determinant of sensory capabilities. And, as you will learn in the next parts of this series, fish use multiple senses to detect and consume prey. I don’t propose that understanding fish vision or light transmission in water will generate bites, but this information can go a long way to help you understand why a certain presentation worked. And it is that understanding that you can use to increase the probability of bites next time you are on the water.

If you can see a white lure 20 feet deep (very clear water)…

• Red light penetrates ~ 6 feet

• Yellow light penetrates ~ 19 feet

• Green light penetrates ~ 30 feet

• Blue light penetrates almost 40 feet

If you can see a white lure 5 feet deep (moderately clear water) …

• Red light penetrates ~ 1.5 feet

• Yellow light penetrates ~ 5 feet

• Green light penetrates ~ 7.5 feet

• Blue light penetrates almost 10 feet