By Dr. Hal Schramm

A fish’s survival depends on its ability to sense its environment. Anglers, of course, are most concerned about how fish detect, select, and ultimately attack and consume food, because they make the assumption that sensory systems involved in feeding are equally involved in catching fish with lures.

Bass are sight feeders, and vision is their dominant sense when light is sufficient. Vision is the angler’s dominant sense, too. The difference between bass anglers and their quarry is not how they see, but the environments in which they see — the angler’s aerial world versus the bass’ aqueous environment.

An Eye is an Eye
The fish eye lacks a light-regulating iris, but otherwise looks and works like our eye. Light passes through a lens and stimulates the retina — a layer of densely packed photoreceptive nerve cells lining the inside of the eye. Bass have color vision -- the retina contains cells for detecting light and dark (rod cells) and for detecting different wavelengths or colors of light (cone cells).

Muscles move the lens to allow the fish to focus on near or far away objects. Fish visual acuity rivals and surpasses ours. Bass can detect a 4-inch object 50 yards away in clear water, and they can see microscopic zooplankton.

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 and up-and-down at the same time, although they have a fairly wide blind spot below. The wide field of vision gives the fish binocular vision and allows them to judge distance.

Essentially, bass see what we see, but there is one very important difference: Bass can detect objects in darker conditions than we can. In fact, in low light conditions bass only need about one-tenth of the amount of light we need to see.

Light in Water
Vision is all about transmission of light, and light passes through water differently than through air. Light easily passes through air, as anyone who got sunburned from a star 93-million miles away can attest. How far light penetrates through water depends on surface conditions and the amount of 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 into the water column. Sediment and plankton in the water absorb and reflect light, again preventing light penetration. (The murkier the water, the less light passes through.)

A physicist would say light attenuates logarithmically in water. What this means to an angler is that you can predict the amount of light at different depths by measuring light near the surface. People who study this type of thing use a simple device call a Secchi disk to measure water clarity. You can accomplish the same thing with a white lure like a spinnerbait or jig. With your rod tip at the water surface, let the lure sink until it disappears, then reel in until you can just see it.

Measure the distance from the rod tip to the lure. Most humans can see a white object at about 1 percent of full daylight illumination. For you to see your 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 you a good approximation of the depth where 1 percent of surface light penetrates.

Since bass can see at about one-tenth the amount of light we need to see (0.1 percent of surface light), the logarithmic-attenuation relationship predicts that fish can visually detect an object at four times the depth at which your white lure disappeared. Beyond the depth of 0.1 percent of surface light, do not expect vision to be a major factor for a bass detecting a lure, particularly one dragged along the bottom.
how fish see
Color in water
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 penetrates the least, longer-wavelength green light penetrates farther, and blue light, with even longer wavelengths, penetrates still farther. Here are some guidelines for color in 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

Get the idea?

Seeing the Same Differently
Physics and physiology determine sensory capabilities, but bass psychology -- what the fish does with the visual information -- is another story altogether. I know a couple anglers who catch largemouth fishing big red worms in deep water. I don’t know what the bass are keying on, but I can guarantee you they aren’t seeing red. Nevertheless, the tug on the end of the line is the ultimate determinant of sensory capabilities, bass psychology included.