Fish Vision

A Presentation on What the Fish Sees of the Fisherman

Michael Phillips

Being an avid angler myself, I have often been perplexed by the fickle nature of the fish I am hoping to catch. As my skills progressed and my ability to cast improved it seemed as though the days of fishing without catching a thing were behind me. It turns out I was wrong and more times than I can count I have found myself walking back to the jeep with nothing to show for my efforts but wet feet and an empty stomache. "I was casting that fly right in front of that fish," I'd tell myself over and over, "I know it saw it." Well it turns out that even a concept as elementary and primitive as the relationship between a fisherman and his prey can be better understood through a scope of physics.

Before getting started a glance at the video below may give you a better understanding of the concepts were planning on discussing

Refraction of Light

It is a lengthy video but the segment beginning at 6 min. 45 sec. and ending at roughly 7 min. 15 sec. should suffice.

Refraction

The first process that is helpful in understanding this phenomenon of fish vision is refraction. Refraction was observed in a few of the activites done during the optics section in class. We first observed how light rays were affected after entering a glass block and a trapezoidal prism. We then discussed how our lines of vision were refracted by the use of convex and concave lenses. Well, just as light rays entering glass are refracted so are light rays entering water. This particular type of refraction; involving water that is, is one that bends a light ray entering the water to make it appear more vertical to eyes below the surface. So, if a ray is shown vertically down into the water the angle of refraction will be zero but as the light rays become more horizontal and move further away from the vertical axis the angle of refraction increase.

Seeing Up From Below

Due to this increase in the angles of refraction and the distinct shape of a fish eye a fishs view of the external world is confined to a circular permeter. Now the entire perimeter of the circle is made up of the horizon and anything that falls within the horizon falls within the circular view of the fish vision.A final factor to consider is that the deeper the fish lies in the water the smaller the cirlce of vision of the surface the fish will have, so depending on what type of fish you are looking to catch your casting target could be a 20 ft. diameter or a 2 foot diameter.

Diagram of Fish Vision and Refraction

By following the link above you can look at an image that demonstrates what was just discussed.
Both what vertical ray of light would appear to look like to a fish.
As well as how the rays change as they move further and further from the vertical axis.

Applicability

So what does all this mean applicably? Well, to start, the age old thought of casting a fly in front of a fish has been debunked. If the fly is ouside of the cirlce of the fish vision the fish will only be able to see what is submerged and in the case of a dry fly that will be little to none. Now even if an angler wants to be mildly bolder and cast a fly closer to the fish there are still issues to be considered. For example, if a fly is cast and lands within the cirlce of the fish's vision but is on the perimeter it will be distorted and potentially lost in the background. Remember, the permieter is a circular image of the horizon and often times in a fishing scenario this is going to be dominated by brush and trees; a backrground that would be easy to lose site of a fly in. So it is safe to say that if you find yourself fly fishing the best place to cast a fly would be right over a fish. Now, a different discussion for a different time would be to determine how the refraction of light into water distorts our view of where the fish truely lies. But as I mentioned earlier, if one were to figure that out, the casting game would be much easier.

Annotated Bibliography

Gray, Robert L. Physics Problems: Electricity, Magnetism & Optics. New York, NY,: Wiley, 1974 This source offered me little more than the class discussion on optics did. It was a in a sense a briefer summary of refraction. What it did not offer was the transition from refraction of glass to refraction of water or albeit light in water. The following website provided those helpful hints.

Borger, Gary. "Refraction 101." Gary Borger. 5 Jan. 2010. Web. 28 Oct. 2011. . Gary Borger is a fellow angler as well as an amateur physicist. Be that as it may, his website provided the same distinct answers that the Flying Circus of Physics book provided as well as a concise and very accessible image demonstrating the basic ideas behind my labor.

"Refraction of Light." YouTube - Broadcast Yourself. Manipal K-12 Technologies, 19 July 2010. Web. 28 Oct. 2011. . This video was rather long. Honestly I put it on here because in creating the html file I thought linking video was one of the coolest parts. Now don’t get me wrong this video, and particularly the segment I recommend on my page, is helpful in showing animatedly what I was hoping to convey.