Light Penetration and the Muskies’ Environment

Illumination of the muskie’s environment is crucial for their being able to perceive objects using their eyes. Light from above the water crosses the water’s surface and bathes objects in the sub-surface world. Muskies perceive these things (lures, forage fish, cover) because light reflects from those items into the muskies’ eye.

Water clarity is therefore a crucial factor in a muskie’s environment. The water in our lakes, flowages, and rivers has all sorts of material suspended within it to give these water bodies their respective water color. This material consists of suspended inert particles, tannins, and plankton. All this suspended material tends to absorb and scatter light. In our above-water world, we are used to being able to see great distances due to the transparency of air. Muskies, however, live in aquatic environments that absorb and scatter light very significantly. In essence, muskies live in a perpetual fog bank where their range of vision is limited.

The way that water clarity is typically characterized is by making a measurement of the Secchi Disk depth. A Secchi disk is a disk with a diameter of about 8” that is painted in alternating black and white quarters.

The disk is lowered through the water to a depth where the black and white quarters are no longer distinguishable from one another. That depth is the Secchi Disk depth for that water body. When state agencies (like DNRs and water quality agencies) report water clarity numbers/depths, they are typically reporting their measurement of the Secchi Disk depth.

Let’s consider carefully what is happening to the light that is traveling to the Secchi Disk and being reflected back to us during this process of measuring water clarity. If we consider the disk being at the Secchi depth, the light must travel from the surface down to the disk, be reflected from the disk, and then return to us at the surface in order to be perceived by us. So if the disk was at a depth of 8 feet, the light is “extinguished” (from our point of view) after having traveled 16 feet through the water: 8 feet down and 8 feet back. That means that it takes 16 feet of travel through water to attenuate the light to a point where humans can no longer perceive the disk because it is too dim or indistinct.

There are a number of important conclusions that can be drawn from this Secchi Disk reading. First, the Secchi depth is related to a layer of water called the “photic zone” (pronounced FO-tick). The photic zone is the layer of water near the surface where there is sufficient light can for photosynthesis, the process that plants use to convert sunlight into energy. This zone is crucial because the growth of phytoplankton (plant-like plankton) within this zone is the basis for the food webs in our lakes: muskies feed on all sorts of creatures which themselves feed on plankton. Now, phytoplankton need more than just sunlight to engage in photosynthesis, they also need nutrients. Some of those nutrients could be suspended in the water column, but most of the time those nutrients are supplied by the bottom of the lake (just like for land-based plants). When the bottom of the lake is close enough to the surface to be within the photic zone, we have the recipe for lots of life processes to happen since any plants and plankton in this zone will have access to plenty of light (from above) and nutrients (on the bottom). This zone of a lake is called the littoral zone. Rooting plants like broadleaf cabbage, Eurasian watermilfoil, coontail, etc… will only take root in the littoral zone.  In fact, there is a widely accepted formula that makes a prediction of the maximum rooting depth for aquatic vegetation that is based on the Secchi depth, and it is quite accurate:

Rooting depth = (1.22 x Secchi Depth in feet) + 2.73 feet.

This basically defines the depth of the weed line in our lakes. If the bottom is at a depth greater than this rooting depth, the light intensity will be too low to initiate the growth of plant life from the bottom. This is why dark lakes typically have much shallower weed lines than clearer lakes. As an example, if a clear lake has a Secchi depth of 12 feet, this suggests a weed line of about 17 feet, while a dark lake with a Secchi depth of 3 feet suggests a weed line of about 6.4 feet. Of course, bottom composition also plays a significant role in determining where vegetation will grow. Sterile bottoms won’t produce plant life no matter how much sunlight they receive. Nutrient-rich bottoms like mud won’t produce plant life if they don’t receive significant sunlight at depth.

In addition to defining the photic zone, the Secchi depth also greatly affects the range at which predators can spot their prey. A Secchi depth of 5 feet means that the total distance light can travel and still be perceivable (to humans, at least) is 10 feet. That means if you were underwater trying to perceive an object that is 7 feet below the surface, that object would need to be no further than 3 feet away from you. That’s because the light from the surface that illuminates the object travels 7 feet to the object and can only travel 3 feet before it is too dim to be perceived by a human: 10 feet is all the further the light can travel before being “extinguished”.

Now let’s consider how muskies see underwater. Muskies are visual, low-light predators. By “visual predator” I mean that muskies typically use their vision to locate and initiate their hunt for prey. Scientific studies of muskie behavior have shown that muskies have a strong tendency to stalk their prey only after it has been visually located. In that controlled study, sound and vibration appeared to play little role in attracting a muskie’s attention. This was verified by studying blind muskies whose ears and lateral line systems were fully intact. The blinded muskies never began stalking behavior of any sort, but they would initiate strike behavior if prey items came within a body length or so of the muskie. Muskies that were blind and also had their lateral lines temporarily disabled didn’t initiate any hunting OR strike behavior, even though their ears were still fully intact and able to detect sound! The implication from this study is that vision is the main sense that muskies use in finding their prey; sound and vibration typically take a back seat. Note, however, that this study was done where muskies were given natural prey choices and had never been subjected to any angling, so the use of sound and vibration could still play a significant role for the muskies we fish for.

Now that we’ve established that a muskie is typically a visual predator, what about a muskie makes it a low-light predator? The eye of the muskie is adapted to be able to perceive very dim objects. One of these adaptations is the tapetum lucidum which gives a muskie’s eye the same ghoulish appearance as the eyes of wolves and walleyes.

Figure 2: The effect of the tapetum lucidum gives rise to the walleye’s "eyeshine”, as seen in this picture.

But any detection system that is very sensitive also has the issue of being easily over-stimulated. We humans have light-sensitive eyes as well. We have a couple adaptations that allow us to restrict the amount of light entering our eyes in bright conditions: eyelids and variable pupils! If light gets too bright for us, we can simply close our eyes. Muskies can’t do this.  But in addition to being able to close our eyes, our pupils involuntarily constrict (get smaller) under bright conditions to limit the amount of light that reaches our retina so that the light-sensitive detectors in our eyes don’t become over-stimulated (pupil dilation is the reverse: widening the pupil to allow in more light during low-light conditions). Muskies, on the other hand, cannot dilate or restrict their pupils in response to different brightness conditions. Muskies are stuck with their pupils just one size, just like you and I when we have our pupils dilated by the ophthalmologist. Recall your own discomfort in bright sunny conditions while your pupils have been dilated and you will know how muskies feel all the time under bright conditions!

The only way that muskies can reduce the intensity of the light they experience is to go deeper in the water column. The deeper a muskie is within the water column, the darker the environment, and the more of an advantage they have due to their low-light capability. But this is going to greatly influence the range at which muskies perceive their prey. Since muskies’ eyes are more light-sensitive than human eyes, they can likely detect dim objects at greater ranges. For argument’s sake, let’s say that muskies can detect light at 50% greater range than humans. That means that a Secchi depth of 5 feet for humans would be 7.5 feet for a muskie. This means that a muskie would be able to detect dim objects if the light travels no more than 15 feet through the water (down and back, remember!). If a muskie’s prey item is 10 feet beneath the surface of such a lake, they would only be able to visually perceive it at ranges of 5 feet and less. This suggests that when making deeper presentations, you would be well-served to place your lure in close proximity to muskies: keep your casts or trolling runs close together since muskies are less likely to investigate from large distances.

Remember that as light is attenuated, the colors of light that remain to be reflected at depth are different from the mix of colors we are used to observing in air. In air, a bait may appear yellow to our eyes because, of all the colors that mix to produce white light (ROY G BIV), it reflects the yellow light while absorbing the other colors. But if the water is absorbing the yellow light before it reaches the lure, there is very little yellow light left to be reflected by the lure. In that case, the lure will appear very dim underwater even if there is still plenty of the other colors of light to illuminate it.

There has been much discussion among muskie anglers about the importance of color, but one of the general rules that many follow is “bright day – bright baits…dark day – dark baits”. I think this rule works because of the importance of contrast. On a bright day, consider the background that a muskie is observing as they look up from shallow water: it is the dark blue of the sky overhead. A bright bait would show up very prominently against the dark blue background. Now consider the overhead background on a dark day: it is typically a white/gray cloud bank. What sort of color would show up prominently against this white/gray background? Something pretty dark, bordering on black, due to contrast.

For deep-water presentations, however, I would be more inclined to use colors that are as reflective as possible. Deeper in the water column, the color of the overhead background is not nearly as important as the brightness of the bait. Attenuation of light through the water column has filtered out much of the light that started its journey at the surface, so you want to reflect as much of the remaining light as possible. This is why I would suggest white (which reflects basically all remaining colors of the ROY G BIV spectrum) and reflective foils. Muskies are more likely to perceive these highly reflective lures at greater distances.

The muskie’s visual environment is quite a bit different than the visual environment we humans are used to. Water with suspended particles absorbs a great deal of light, complicating the hunting techniques of visual predators like muskies. But we can use our knowledge of how light propagates within the muskie’s environment to give ourselves an advantage when stalking these predators ourselves. Best of luck on the water!

Dr. Bob