What Do Fish See?   

The eternal question underlying fly fishing is, Why do fish eat the fly? There have been friendly debates, outright arguments, and a few world wars over the last few centuries about this question. Only the fish know the answer, which is the charm of the sport.

Fish psychology may be beyond our scope, but one thing we can analyze is a fish's vision. First of all, fish do see in color. Both 'rods' and 'cones' are present in fish eyeballs. The three chemicals that allow humans to see our seven-color spectrum are also present, as well as a fourth common to most predators that permit them to see into the ultraviolet range. It's very important to understand this. Two flies that look identical to us can look very different to fish because they see colors we don't.

Fish's eyes (at least the fish we fly fish for) are set in the sides of their heads. That means they do not have 'binocular vision' as people do; Their depth perception is extremely poor and most of them have a semi-blind spot straight ahead. This is alleviated by the shape of their retina (why do you think 'fish eye' lenses are called that?), which allows for some overlap, but gives them poor distance vision and excellent close vision (a trait they share with me). I believe this is why fish don't stray far from their feeding lanes: They simply have trouble seeing food at a distance.

We must also consider the fish's environment. Some twenty-five years ago, Vince Marinaro published "In The Ring of the Rise", the definitive work on the subject to this day. We all know the speed of light is slower in water than in air, but he used underwater photography to show how it would affect a trout's vision, not ours. From my snorkeling experience, I was already aware of the reflectivity of the surface of the water, which looks like a mirror from underneath. Until his book, however, I'd not known about a fish's 'window', just above its head, a cone-shaped area where it can see through the surface film. The closer the fish is to the surface, the smaller the diameter of the 'window'. Everything beyond the window is hidden by the mirror effect. Fish can only positively identify surface food inside the window, another good reason for those narrow feeding lanes.

A final observation to remember is how much light there is underwater. The mirror effect is caused by sunlight. The bottom of the body of water will, depending on its composition, reflect this sunlight to a greater or lesser extent. A bottom of dark decaying vegetation would not reflect much, but one of sand would reflect a lot. High reflective qualities will cause the bottom to be reflected in that mirror, further diminishing the fish's ability to see through it; And, in some cases, providing a visual chaos that is a poor background for the fish to see underwater food against. However, this much light will also show up the colors of objects both suspended and in the surface film, a fact we should make special note of. It penetrates the water, lighting suspended objects like sunbeams. This supports the ancient wet-fly fishing adage, "Bright day, bright fly". Against the cluttered background of, say, a bottom of light-colored stones reflected in the mirrored surface, natural colors would be well camouflaged; But bright colors and flashy materials would show up admirably.

In low-light conditions, it's easy to see why the wet-fly adage continues: "Dark day, dark fly". No appreciable light is reflected off the stream bottom, so a drifting fly would be silhouetted against a more tranquil background. Also, in lower light the 'rods' of the eye do most of the work, so, as in a black-and-white photograph, contrast becomes the key factor. Black is the best for contrast. White also works, but not as well. And in these modern times, we have day-glow colors that gather and reflect even small amounts of light, so those fluorescent colors are also good on dim days.

Fluorescent colors have the added advantage of reflecting ultraviolet. Remember, most predators can see into the ultraviolet range. We can't even imagine what the color values are to them, but we can observe and deduce what attracts them. As an old fly-tier, I have the disgusting habit of scrutinizing road kills. I've noticed, at night, that on fresh ones the blood fluoresces. Experimentation has convinced me that there is an ultraviolet marker in fresh blood. It makes sense that predators would both be able to see this color and be attracted by it. I believe that this phenomenon accounts for the fabulous attraction of the color chartreuse. It may not look anything like blood to us, but to a creature that can see into the ultraviolet range, it very well might. I sometimes consider digging through my box of old hippie stuff and installing the black light over my tying bench!

When it comes to surface food, fish may spot the dimpling in the surface of the mirror where the insect touches the surface film some distance away. This distance factor is important, because it explains why several dozen hackle points can be mistaken for six insect feet. I suspect they see this as 'something different', the way I spot deer and birds in the woods. Their attention is caught, then when the object comes closer they can determine what it is. Some insects are small and light enough that not even their feet penetrate the surface when they drift into the fish's window. On these flies, wing color and shape become very important, and floatability. Other insects are plumper, and their bodies penetrate the surface film. This includes terrestrials, spinners, and some duns. Anything that penetrates the surface film will be clearly lighted by reflection from the bottom, unless the bottom is quite dark. That means the belly color of the fly becomes very important, most probably the trigger that tells selective fish this object is food. This specifically includes terrestrials, which are traditionally considered to be seen by the fish only as silhouettes. This is true in dim light, but not in sunlight over light stream bottoms. I've seen enough selectivity to color by trout eating ants to confirm this idea. Many tiers tie beetles and 'hoppers to imitate carapace colors, but the fish never see these. What do they look like from the bottom? I attempt to match that color, with good success by the only judges that matter: The fish. 

Considering the fish's point of view not only increases our chances of success in terms of catching fish, but in terms of understanding them. It brings us that much closer to the real, intangible goals of fly-fishing and tying.

--Rabbit Jensen--