Water Temperature: Top to Bottom and Beginning to End

Water Temperature: Top to Bottom and Beginning to End

Musky Shop April 29, 2024

Article by Dr. Bob

Water temperature is one of those pieces of data that we can use to help us understand muskie behavior. One of its uses is to make a rough guess about where muskies are within their seasonal progression. There are many examples of this. For instance, conventional wisdom suggests that muskies conduct their spawn when water temperatures reach the mid-50’s. Water temperatures rising into the 70’s suggest muskies are in their summer peak pattern. When water temperatures fall into the high-60’s in the early autumn, anglers look for muskies in the shallows. As water temperatures fall into the 40’s and 30’s, muskies tend to position along steep breaks (depending on forage movements). These are all examples of using temperature as a factor when “reading the water.” Of course, water temperature is only one facet of the muskie’s environment, so one should be careful of ascribing too much weight to it. Other factors like photoperiod (the duration of daylight) are also crucial for determining where muskies are in their seasonal progression, but water temperature definitely has a place when developing your strategy.

Apart from water temperature being a good stand-in for timing within muskies’ seasonal progression, surface temperatures are also a reasonable (but incomplete) indicator of muskie activity levels. Being cold-blooded, muskies’ metabolic rate is closely tied to the temperature of their environment. Higher water temperatures force a muskies’ metabolic “furnace” to run hotter, demanding that they feed more actively and more frequently. Yet there is an important detail that many anglers either ignore or don’t notice: muskies don’t necessarily live at the depth of your sonar’s temperature sensor. Water temperature varies significantly with depth, so surface temperatures only give hints about a muskie’s thermal environment. This is why I employ a temperature sensor that works at depth (e.g. the Fish Hawk TD). Such a sensor allows me to measure the temperature from the top of the water column to the bottom. The difference in data-gathering potential provided by such a sensor is the same as the difference between traditional sonar (which gives you information only beneath the boat) and side-imaging sonar (which gives you information at much larger horizontal ranges from the boat). So…what can you learn with this expanded capability?

The first thing you learn when using a temperature-depth sensor consistently is that the top 5-10 feet or so within the water column has a highly variable temperature. By “highly variable” I mean its temperature can change from hour to hour depending on atmospheric conditions: sunny/cloudy, warm/cold air temperatures, windy/calm. The part of the water column just below that level tends to be much more stable. Even during a prolonged warm spell that sends surface temperatures soaring upward by 5 degrees Fahrenheit or more in just a few days, the water beneath that volatile surface layer might only increase in temperature by a few tenths of a degree.

How deep the volatile surface layer extends beneath the surface depends on the body of water. For darker waters with less light penetration, the volatile layer near the surface doesn’t extend as far downward as it would for bodies of water that are very clear. Thus, the volatile surface layer on a stained lake may be the top 5 feet, while the surface layer on a very clear lake might extend as deep as 15 feet. This will also affect the rate at which these water bodies warm and cool during the season. Lakes in which only the top 5 feet of water is easily susceptible to light absorption will tend to warm more quickly, whereas a lake where the top 15 feet of water is susceptible to light absorption will have to warm three times as much water to get the same temperature change. This is one of the reasons clear lakes tend to warm more slowly in the early part of the season compared to darker lakes.

Direct measurements of sub-surface water temperatures are extremely important during the early part of the muskie season. Right after ice out, water temperatures in a lake tend to be very uniform in the upper-30s, top to bottom. As time goes on, sun exposure will warm the upper layers. The smaller the depth of light penetration (ie in darker lakes), the more quickly will the surface layer warm. This is especially true when there is very little thermal mixing between layers like on windless days. When the wind blows, however, the surface temperature can fall precipitously. This is not because the wind directly cools the surface water. Rather, it is because the wind pushes the warm surface water to the side of the lake that the wind is blowing toward (this is also an object lesson in how wind-driven current affects the muskie’s environment!). With the warm surface water peeled away by wind, the cooler layers underneath remain behind and become the new surface layer. This is why surface temperatures in the early part of the season can vary so significantly. If you are looking for warm water in the early season, look to the windy side of the lake at the beginning of a blow. However, if the wind has been blowing for a while, it will have also blown cool water there, muting the effect.

This effect is less pronounced on clearer lakes. In clearer lakes, greater light penetration will deposit warmth within deeper layers of the water column. While the clearer lakes warm more slowly, they tend to warm more evenly. In either case, you should think carefully about muskies’ metabolism during this period. The surface temperature could read in the 70s on a calm day in the early season, but if muskies are 10 feet down off a break line, they could be situated in water that is 50F or less!

Another process that is important for heat being distributed within the water column is the process of heat exchange between adjacent layers of water. While the main driving force behind the warming of the lakes is sunlight, the upper layers of water also give up some of their heat to adjacent lower layers, even when the sun is not out. This can be observed, day to day, using a temperature-depth sensor as well. Even when sunlight doesn’t reach the deeper layers of the lake, those deeper layers do still tend to warm up from heat exchange with adjacent layers above them.

By early to mid-summer, the water temperature profile of most deep lakes will become stratified. Stratification refers to the stable stacking of layers within the water column based on their temperature. Nearest the surface is the volatile region that is most affected by interactions with the air and weather. Below that the water temperatures are more stable from hour to hour and day to day. This stable region is generally cooler than the surface, and as you go deeper in the water column within this more stable layer the water gets a little cooler (but not much). Continuing downward, you eventually reach a region in the water column where the water temperature drops very rapidly with depth. This thin layer of rapid temperature drop is called the thermocline. Below this thermocline is a layer of cold water, in the 50’s or upper 40’s (even in summertime!). Since this cold water never mixes with the air at the surface, it typically becomes depleted of oxygen, making it a region that active fish won’t persist within for very long. However, I have a theory that fish that have a low metabolic rate (because of being in cold water!) have less need for oxygen, so fish CAN certainly survive for some time beneath the thermocline if they choose.

The table below shows a temperature profile taken from a mid-depth area in a deep clear lake. This profile was taken in the late afternoon on a hot, sunny day in the first week of August

From experience, I know that on this lake the top 15 feet of the water column varies significantly in temperature due to atmospheric conditions. Below 15 feet, the water temperatures are relatively stable throughout the summer. Note the shift in temperature from 35ft (74.0F) to 45ft (58.4F). That range of depths is the thermocline.

An interesting transition begins in mid-August here in the northern tier. The thermocline descends significantly around this time. You may not notice that the surface temperatures change their behavior at all, but the angle of the sun’s ray’s changing makes a huge difference in how temperatures get redistributed beneath the surface. Eventually, the surface temperatures will consistently be close to the temperatures in the stable zone that’s above the thermocline. That’s when surface temperatures over the deeper regions of the lake will stop having wild variations from day to day.

Once the thermocline has dissipated in a section of the lake, that area of the lake is subject to vertical mixing. This mixing will occur whenever surface temperatures fall below the temperatures of the water near the bottom. This vertical mixing often happens when surface temperatures are in the mid-60s or even the low-70s (but it varies from lake to lake and from year to year). I recognize that conventional wisdom states that turnover occurs when surface temperatures are in the mid-50s and that it is a very disruptive event that occurs over a few days, maybe a week. This conventional wisdom is in direct contradiction to my careful observations of water temperature profiles. The only time that a disruptive mixing of layers happens over just a few days is when surface temperatures crash rapidly due to a very severe extended cold front. Otherwise, the process of turnover takes multiple weeks and is fully complete, for all practical purposes, by the time surface temperatures reach even the lower 60s.

By way of an example, consider a snapshot of a mid-clarity lake that has a maximum depth in the mid-30s. On this lake the thermocline sets up in early summer at about 17 feet. By the end of August, the thermocline has dissipated, having descended all the way to the bottom even in the deepest basin in the lake. The temperature profile typically looks something like: 72F at the surface, 70F at 10 feet down, 67F at 20 feet down, and 62F at 35 feet down. Note there is no thermocline; just a steady gradual decrease in temperature from top to bottom. In a couple of weeks, partway through September, the whole basin was at about 66F from top to bottom; thermal mixing is complete. Of course, it doesn’t always happen exactly like this. Sometimes we have warm spells or cold fronts so that water temperatures change in fits and starts. But the general trend of what I have described is what happens most of the time. For deeper lakes, this process takes longer, but if muskies are only using the parts of the lake that have depths less than 35 feet, most of the acreage of those lakes will have completed the process of getting their temperatures mostly uniform from top to bottom by the end of September.

There is a use for keeping track of these temperature profiles. One of the most prominent patterns that muskie anglers enjoy is the early fall “shallow slide”, when big muskies invade the shallows in mid-August and early September. But why do they do this? I’ve been carefully observing the timing of this “shallow slide” for several years now, and while it is very difficult to predict when it will happen based on surface temperatures (which swing wildly), predictions based on sub-surface temperature profiles taken in deep basins are much more reliable. My theory is that when the thermocline begins to descend and dissipate in mid-August or early September (for the northern tier), muskies and other forage fish that used the stable-temperature layer as a home base in mid-summer tend to shift their home base toward shallower haunts. This is because once the weeks-long process of turnover begins with the dissipation of the thermocline, muskies and forage fish that had been pelagic in the summer months abandon these areas over deep water because they have become thermally disrupted. They move to shallow structure along shorelines to avoid this instability.

That this “shallow slide” occurs at a time when surface temperatures in the shallows happen to match muskies’ preferred temperature range (in the mid-60’s to low-70’s) accounts for why muskies seem to be particularly aggressive at this time of year. This theory predicts that the early fall shallow-water pattern would be strongest in lakes that stratify and less pronounced in waters that do not stratify (eg. in very shallow lakes or in rivers with significant current). Since almost all of my experience is with water bodies that stratify, I welcome anyone to contact us here at Musky 360 if they’ve experienced a strong “shallow slide” pattern on a non-stratified water body!

My thought on the traditional “funk” that muskies and other fish species experience when water temperatures fall into the mid-50s is that it doesn’t have much to do with turnover, which is typically already complete by the time surface temperatures drop into the 50’s. Rather it is simply a time of transition where muskies and other species are adjusting to the new normal of “non-preferred” temperatures. Muskies, for example, appear to prefer water temperatures in the mid- to upper-60s, so when the water temperatures fall below their preference, they need some time to acclimatize. Not to anthropomorphize muskies too much, but it may be similar to when we humans get our first taste of really cool weather in fall. Temperatures in the 50’s feel cold to us when we are used to more moderate temperatures in the mid-70s. After a week or so of cooler weather, however, we feel comfortable in the 50s. The same is likely true for muskies. This can also explain why anglers can still have success in very shallow water during the transition time that conventional wisdom calls “turnover”: shallow water is more susceptible to temperature changes so muskies can use “sun-baked” shallows to get their last experience of warmer water for the season.

As lakes cool down in late fall, there is a very interesting effect on water density that profoundly affects the temperature environment. Water achieves its maximum density at a temperature of 39F. This means that once the surface water achieves a temperature of 39F, that water will sink to the bottom of the lake, displacing all other water upward. This is why surface temperatures seem to pause in their descent at a temperature of 39F: all the water warmer than 39F gets pushed to the surface to be cooled to 39F. Before surface temperatures can drop below 39F, practically every drop of water in the lake also needs to be 39F. When this finally happens and the surface temperature drops below 39F toward ice-up temperature of 32F, the warmest water in the lake will be on the bottom of the lake and it will be 39F. This 39F water at the bottom will be insulated from heat exchange with colder air at the surface by all the water between the bottom and the surface. My theory is that this is why bottom-related presentations are so important in the very late fall. The bottom is where the warmest water is during this time. What is more, the mixing that has occurred during and after turnover has allowed even the deepest layers of water to be oxygenated at the surface before sinking down when it reaches 39F. That means that bottom-feeding forage like suckers, bullheads, and redhorse will be in well-oxygenated water that happens to be the warmest water in the lake. That’s not even to begin the discussion of how cisco and whitefish congregate for their spawn at about these same water temperatures. Consider carefully the feeding opportunities muskies have in the late fall, much of them due to how water temperature influences their environment.

Hopefully this has given you some food for thought about how water temperature at depth can affect muskie behavior. Try to put that information to use next time you are wondering what muskies are doing, and you might tip the odds in your favor. Best of luck on the water!

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