Muskies, Light, and Reflection
Have you ever wondered why following muskies always seem to high tail it away from you when they get about 20 or 25 feet away from the boat? The simple answer is because they spot you, but why didn’t they see you before that? By digging a little bit into how light behaves at the water’s surface, we might be able to answer these questions and give a little insight into how muskies view the world above the surface.
How Do We (and Muskies) See?
The reason that we see anything is because light reflects from the subject. This reflected light travels in practically all directions away from the subject. Some of that light ends up in your eye, allowing you to perceive it. So when we see a muskie below the surface of the water, what is happening is that light from the sky shines down on the muskie and some of the light that is reflected from that muskie ends up traveling into your eye, crossing the boundary between the water and the air along the way. Muskies perceive the world in the same way. Light from the sky illuminates you and is reflected. Some of that reflected light heads toward the water, crossing the water’s surface and ending up in the muskie’s eye.
The tricky part is that when light passes from air to water or from water to air, the direction of travel of the light changes. This process of changing the direction of light propagation is known as refraction. You’ve probably noticed this when viewing a straw through a glass of water. The straw appears broken at the surface of the water. The reason is that the light coming from the part of the straw that is underwater is passing from water to air on its way to your eye, and that light appears to be coming from a slightly different direction than the light that comes from the part of the straw that isn’t submerged.
This means that our brains get fooled into thinking that the light is coming from a direction it isn’t really coming from. We use this same principle to build things like eyeglasses, magnifying lenses, and all sorts of other devices to enhance (or correct) our vision.
How can this affect our fishing though? Consider the case where you are about to net a muskie for a friend. The fish is under the water’s surface a bit, so you’re going to have to scoop downward to get beneath the fish to net it. How deep should you aim? The answer is: “Deeper than you think!”
The light coming from the muskie toward your eye has had its direction changed by the air-water boundary because of refraction. When the light goes from water into the air, it gets bent toward the surface. This will result in you perceiving that the muskie is shallower than it really is! A careful calculation shows that you’d perceive it to be ¾ as deep as it is in reality: if the fish is 4ft below the surface, you’d perceive it as being only 3ft below the surface. If you believe your eyes and scoop 3ft down, you’ll miss it. To actually land that fish (and save your friendship), use science: aim well below where you think the bottom of the fish is!
The Muskie and You
Now let’s consider how muskies perceive the world above the surface of the water. This is relevant to us because we are a part of that above-surface world, and if a muskie can see you there is a possibility of spooking that fish.
When light comes from above the water’s surface, it could strike the water’s surface from any angle, from light that comes from directly overhead straight toward the surface… to light that comes in very low, almost grazing the surface of the water. Every one of those light rays gets bent away from the surface in the process as it goes into the water. Light from overhead gets bent very little, but light grazing the surface gets bent a lot. In fact, light that comes in almost grazing the surface gets bent to an angle of 41 degrees from horizontal. This is crucially important for how muskies perceive the above-water world. Everything they can see above the water appears to them to be coming from an angle between 41 degrees above level, up to 90 degrees above level (i.e. directly overhead). They will never see ANYTHING from above the surface of the water if they don’t lift their sightline at least 41 degrees above level. You can test this for yourself by submerging below the surface of a swimming pool and looking around, noticing how high you need to look to see objects above the surface. This effect I am talking about isn’t about how a fish’s eye works, rather it is about how light interacts with the water surface; it works the same for us as it does for fish!
But that’s not all. Light that comes from low angles to the water’s surface has a larger percentage of that light reflected, not even passing beneath the water’s surface where the muskie’s eye could pick it up. This is sort of like skipping rocks off the water’s surface. We all know that the best way to skip a rock is to throw it as close to parallel to the water’s surface as possible. That’s how you get the water’s surface to deflect the rock. Light is similar: the closer it is to parallel to the water’s surface, the larger fraction of that light will be reflected rather than transmitted into the water. It is only the light that is transmitted into the water that is available for muskies to use to see the above-water world.
Now let’s put it all together. You’re standing on your boat, reeling in your bucktail. A muskie starts to follow your bait from pretty far off. How do they follow? A little behind and below the bait, of course. The muskie scrutinizes the bait, directing its sightline nearly horizontal but maybe a little bit upward. Meanwhile, you’re in your boat maybe 3-4 feet above the surface of the water. The muskie is 50 feet away horizontally from your boat when it starts to follow mid-cast. The light from you hits the surface of the water at a very shallow angle to the horizontal. Most of that light gets reflected off the surface like a mirror with only a little bit of that light going below the surface. You are practically imperceivable to the fish. She’s looking mostly forward toward the bait, so she’s looking in the wrong direction to see that light coming from you. Plus, you’re not sending much light to her under the surface since most is getting reflected.
But as she gets closer to you, the angle of the light coming from you gets steeper to the horizontal. More light is now getting to the muskie below the surface. She still might not be looking in the right direction, though, since she needs to look more vertically to perceive you (above the 41-degree line!). Still, you’re in the background and your image is getting more perceivable as less light gets reflected and more ends up transmitted into the water.
Now she’s 20 feet from the boat. The angle of that light coming from you toward the water is now pretty steeply downward. Very little of the light from you is getting reflected; most is going right into the water (very tough to skip a rock throwing it almost straight down, right?). You are definitely perceivable at this distance. She moves below and behind the bait to strike it. After all, that’s where the blind spot is for all her prey. But as she does so, her sightline moves to about 40-45 degrees above the horizontal. And there you are in the background: perfectly seeable, whereas before you were very tough to notice.
Will she see you and run for it, or will she ignore you somehow and go for the kill? Part of that is up to you and part of it is up to the fish. Anglers with a good boatside game have some ways to move the muskie’s sightline away from the angler. If you sweep the bait off to the side with a long rod, you can direct the muskie’s sightline laterally away from you. If you lower your rod tip as you come into your figure eight, you can keep the muskie’s sightline low and under the boat. Or you can try to keep the muskie’s focus on the bait rather than you in the background, often by speeding up the bait so that the muskie must ignore the background clutter (i.e. you!) to maintain visual contact with their prey item. But when you attempt to keep the muskie’s focus on the bait, you need to do so in a way that keeps your body as motionless as possible; any extraneous movement is likely to draw the muskie’s attention to you. An important thing to know is at what point in your retrieve should you be engaging in this muskie misdirection. Some careful calculations show that an angle of 10 degrees from the surface is where about 65% of the light is transmitted into the water rather than being reflected. If you are about 4 feet above the water, the distance to the muskie for which this happens is about 23 feet. I would strongly consider making your muskie misdirection maneuver before that to minimize the chance that they can see you. Whatever method you use to re-direct the muskie’s attention, you now have an idea of the reason why muskies have such a hard time seeing anglers at greater distances from the boat, even when water is quite clear, calm, and transparent.
Best of luck on the water!
Dr. Bob