Understanding thermal stratification of water during the summer months can give us an idea of where to search for muskies. Many theories are out there concerning muskies and their behavior, but really the only facts that we know is muskies, as well as other fish species, need oxygen to breathe and food to survive. By getting an education of thermal stratification, we can learn how lakes develop several layers in them, where the oxygen is, how much oxygen is present depending on fertility of the lake, and how to use this information in our search for the elusive muskie.
During the summer months, the upper layer of water is warmed by solar energy. Due to the fact that water has a temperature dependent density, warmer water is less dense than colder water, causing a blanket of warm water to remain high in the water column. This layer of warm water is known as the epilimnion. The sun can only heat the water so far down into the water column, and the colder water beneath is referred to as the hypolimnion. The layer separating these other two is known as the metalimnion. The metalimnion is where the greatest difference in water temperature will occur and that is the thermocline.
In other words, the structure of the water column as determined by temperature is a reflection of the difference in water density. The warmer, less dense, water layer floats at the surface. The colder, denser water sinks to the bottom, and there is a zone of rapidly changing density in between and that is the thermocline. The true definition of a thermocline is a decrease in water temperature of greater than 1degree Celsius per meter.
Once a lake has stratified into the three different layers during the summer months, dropping a temperature probe down the water column will show you this. Normally, the temperature remains fairly constant for the first few feet, followed by a very slow drop in temperature until you hit the thermocline. Then the water temperature will drop very quickly in only a few feet. Once you pass the thermocline, the water temperature is more uniform but still slowly drops in temperature all the way to the bottom.
Does any of this really apply to muskie fishing? Well, maybe not yet, but we are getting there. Now we need to throw dissolved oxygen into the equation. Dissolved oxygen refers to the volume of oxygen that is contained in water. Oxygen enters the water by photosynthesis of aquatic biota and by the transfer of oxygen across the air-water interface. Photosynthesis only occurs in the upper layer of the water column where light can penetrate. Although warmer water holds less oxygen, it is the influx of oxygen from photosynthesis and diffusion from the atmosphere that keeps the upper warm layer of water well oxygenated no matter what lake we are referring to.
However, dissolved oxygen will vary considerably in the hypolimnion depending on the trophic status of the lake. What does trophic mean? In simple terms, it is a rating of how fertile a lake is and how much organic matter is in the water. Lakes that are considered very fertile are called eutrophic. Mesotrophic refers to lakes that are more mid-range in fertility levels, and oligotrophic refers to lakes that are very infertile. So depending on how fertile a lake is will determine how much oxygen will be present below the thermocline in the hypolimnion.
At the beginning of summer, and after the lake is thermally stratified, the hypolimnion contains sufficient levels of oxygen because colder water holds more oxygen than warmer water. However, as time goes on, an increased number of dead organic matter from the epilimnion will rapidly sink through the warmer, less dense water, and enter the hypolimnion. Once this dead organic matter, such as dead plankton, reaches the hypolimnion, microbial decomposers will decay this organic material. This process of decomposition removes oxygen from the water.
If the lake is very fertile (eutrophic), this process of oxygen removal is accelerated and the dissolved oxygen in the hypolimnion could be depleted by mid to late summer. This is due to a large amount of organic matter being present in the water, and consequently being decomposed after it dies.
In lakes that have a mid-range of fertility (Mesotrophic), oxygen levels may be reduced, but not depleted below the thermocline. In infertile waters (oligotrophic), not much microbial decomposition occurs and therefore there is plenty of dissolved oxygen below the thermocline.
Now we can start applying some of this to muskie fishing during the summer months. It is important to know the relative fertility level or status of the lake you are fishing. Therefore, you will have a good idea as to the severity of oxygen depletion below the thermocline. If there is substantial depletion of oxygen, then you can eliminate a large part of the water column in your search for muskies. As Dick Pearson refers to it, you have "boxed" them in.
Once a thermocline is established, the water from the epilimnion and the water from the hypolimnion are cut off from each other. There will be no mixing across the thermocline barrier. As the summer progresses, increasing oxygen depletion in the hypolimnion will not be replenished until fall turnover. Therefore, in eutrophic lakes, muskies will be limited to using the water at the thermocline and above due to oxygen restraints.
The biggest factor that you need to understand though is that almost every single lake will be different. No two lakes are truly identical in trophic status so every lake will have its own characteristics concerning stratification and oxygen levels. The depth of the thermocline will vary considerably due to water clarity. Even one body of water will vary from year to year as far as thermocline depth based on its water clarity. Water clarity will affect how much sunlight can penetrate the water, how much photosynthesis can occur, and consequently the depth of the thermocline.
Another factor that may affect the depth of the thermocline is known as the seiche effect. The seiche effect is a result of high winds out of the same direction for several days, making the water level actually rise on the downwind side of the lake. This affects the thermocline depth to where it becomes deeper on the downwind side, and shallower on the upwind side.
In I haven't confused you already, I might now. Not all lakes will stratify during the summer months. Shallow lakes or basins that are exposed to a lot of wind have difficulty in stratifying. The water column is constantly being mixed and a thermocline barrier is unable to form. Examples of this are Lake Erie, the main basin of Leech Lake, and Mille Lacs. For the most part though, temperate lakes in the Midwest that we chase muskies on will stratify, just not all of them.
Now brings up another important subject matter and that is preferred water temperature of muskies and their available forage. It has been documented that muskies prefer water temperatures ranging form 67-72 degrees Fahrenheit. However, they will leave their comfort zone to find food. Have you ever noticed that when muskies are sitting up in shallow water, or are on shallow rocks, you generally don't see high amounts of forage around? Believe me though; the forage usually isn't far away. Muskies may be sitting in shallow water simply because the temperature is of their liking.
Learning the preferred water temperatures of the available forage can be very useful. Ciscoes prefer a temperature of 55 degrees, where whitefish like water a little cooler, say 50-55 degrees. That is why these two forage species can only survive in oligotrophic lakes. They need deep, cool water during the summer months, and if there is no oxygen below the thermocline, generally they can't find that cool of water with out it being void of oxygen. Use this to your advantage though. I think the open water fisherman will be the next to break a world record. So many muskies never even inhabit the shallows and live in deep water nearly year round beefing up on oily ciscoes and whitefish. Until the fall spawn of ciscoes and whitefish, those giant muskies never see shallow water.
The majority of the time we are fishing for muskies, we are targeting them in their comfort zone. Shallow water that is warm. That is where they like to be, plain and simple. That doesn't mean that is where they are actively feeding. Their forage most likely inhabits totally different water, and that is where the muskies will leave their comfort zones to chase down a meal. Simply find out where the temperature in the water column suites the available forage, and that will be a high percentage depth to troll or open water cast.
I could babble on and on for days about this topic, but the main point of this article is for people to understand the characteristics of thermal stratification during the summer months. Understanding how different fertility levels of lakes determine the amount of oxygen depletion below thermoclines is a helpful tool in eliminating part of the water column. Also, a good understanding of preferred water temperatures for several different prey species could help narrow the search for muskie as well. We will probably never truly understand muskies and why they do what they do, but at least we can start narrowing it down.
Chad Cain has over 12 years working and fishing in the muskie industry. He owns and operates Chad Cain Muskie Guide Service fishing in Illinois and Minnesota. In 1996, Chad graduated from Southern Illinois University with a bachelors degree in Fisheries Biology. Chad is an avid seminar speaker, outdoor writer, and conservationist. He appears at most Muskie Shows during the off season. Chad owns and operates Musky Armor/Llungen Lures. To learn more about Chad please visit www.chadcain.com and www.muskyarmor.com