Thermal Stratification: Water Science for More Fish on the Lake

fishing northern Minnesota lake in canoe

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Anglers are sportsmen, and like all sportsmen, they are always looking for ways to improve their game. Most of the time anglers look for new equipment, whether it’s a new type of fly, lure, or bait, or a new trolling motor or fish-finder. However, knowledge is often the most beneficial tool.

Many anglers are purists, preferring the more naturalistic approach of understanding their prey and environment to reel in a catch. Purists are my favorite kind of outdoorsmen, the Dundee’s of the natural world, so good at their craft they may as well be coaxing the fish to jump into the net with magical forest powers.

This article is about one of the little talked about facts of something that occurs in the inland waters we fish – mainly freshwater lakes and reservoirs, although it can also apply to rivers with deep pockets.

Besides understanding the behavior of fish, it is just as important to understand the behavior of the water in which they live, and water is an odd duck with its own personality.

So let’s dive in.

Water Science and Lake Fishing

Water is something that you won’t hear a lot of people talking about in-depth unless that person is a physicist, chemist, or naturalist. Easily overlooked and almost always taken for granted, water is the single most abundant molecule on earth.

While we humans live on rocky continents you could also say that we live on an ocean world. This is a great thing, as most scientists will tell you that the abundance of liquid water is the whole reason life magically, or spontaneously, generated itself here.

They still haven’t quite figured out how life arose from non-life. The jury is still out on that one, however, if you’d like a good read while out on the boat, check this book out.

In the process of trying to reverse engineer the creation of life on this planet, they have studied water extensively as you might imagine. What they have found is really quite neat, and one of those traits of water in a large body is a significant boon for anglers of all stripes.

Thermal Stratification and Fish Behavior

What is important for anglers is this natural phenomenon called thermal stratification, which means ‘layering by temperature.’ It is important for anglers because fish are incredibly sensitive to changes in water temperature and can go into shock with an abrupt change of only a few degrees up or down.

In large inland bodies of water such as lakes, understanding this property of water will help you understand where fish are going to be during certain times of the day. And what angler could not use this kind of information?

First, though, let’s understand a basic fact about fish. Most game fish have a very small brain, the size of your pinky nail, small, and their operating system is called the ‘reptilian brain.’

Which essentially goes something like this: Eat. Sleep. Reproduce. Stay Alive.

So they naturally have this tendency to swim into temperatures that are the most conducive for their survival. It is not something they have to think about, it’s purely instinctive.

Since we are products of our environment and fish being no different, they follow these patterns of depth that are ultimately determined by the thermal stratification of the body of water they are swimming in.

In large lakes or large enough ponds, the water naturally separates into three different layers of temperature known as:

  • The Epilimnion, which is the top layer of water. The epilimnion is constantly being mixed by the currents created by the wind as it blows across the surface of the water and the warmth of the sun
  • The Metalimnion is a thin layer of water that acts as a buffer between the epilimnion and the hypolimnion. This layer can easily be felt by a diver as the temperature difference is often 20 degrees Fahrenheit lower than the epilimnion.
  • The Hypolimnion is the third and bottom-most layer of water. This layer of water is generally found to be at a temperature of 40 degrees Fahrenheit.

These layers of water do not mix due to the natural properties of water and heat. As the sun heats the epilimnion, it causes the molecules of water to become less dense and this density is what causes the separate layer of thermal stratification.

Bizarre, but you can see the same effect of liquid density and separation by going to a bar and asking for a Black and Tan. It’s a nice visual for the effect because a stout or porter is less dense than a pale ale, causing visually distinct layers in the glass.

The layers of water in thermal stratification act in the same way but the density of water is directly affected by the temperature of the water itself.

How Thermal Stratification Helps Anglers

using thermal stratification to find fish

Knowing how water functions in this way will help anglers understand where fish will be during different times of the day in the lake.

Though the principle applies to just about all lake species, let’s use rainbow trout, for instance, to help us understand how it works.

Rainbow trout, in particular, prefer temperatures between 55 and 65 degrees Fahrenheit. So let’s say you start fishing at dawn and are getting good hits on your lines near the surface and closer to shore. After a break at noon, you get back to fishing these same spots but can’t get a bite to save your life.

This is because the morning sun has heated the water past the trout’s comfort zone and they have migrated to a cooler area of the pond.

All fish are incredibly sensitive to temperature, and a little-known fact is that water transfers heat 3 times faster than air, so they are always going to be more reactive to temperature than you will be.

Rainbow trout will go into shock when the temperature reaches around 75 degrees. So when the temperature of the epilimnion rises to around 70-75 degrees, trout will migrate to cooler and deeper waters to continue feeding.

So as the day moves on and the water becomes warmer, you will want to start fishing deeper waters and try to find the metalimnion. When the epilimnion temperature reaches 75 degrees Fahrenheit the metalimnion will have a temperature of 55 degrees Fahrenheit, which is the preferred temperature for trout.

This layer also may not exist close to shore simply because the water is not deep enough for this temperature contrast. If the shoreline is shallow enough, sunlight will reflect from the sand underneath and also contribute to heating the water.

Do Your Own Experiments

If you get some time to just do some of your experimentation, you can purchase a fishing thermometer and check the temperatures of your favorite fishing holes yourself.

Various laws of thermal transfer could help you get an approximation from your couch, but a lake or pond is far from a closed system and there are simply too many variables involved to give you a formulaic solution. So your best bet is to be out on the water with a thermometer.

However, the insight you do have is that the metalimnion is going to be significantly cooler than the epilimnion by about 15-20 degrees and that these two-layer do not mix.

Meaning that simply fishing deeper is not going to put you in the right temperature zone to find fish, but fishing at the exact right depth is what you will be trying to do.

Unless you have a thermometer that sinks and can relay the temperature, this will be a kind of hit-and-miss experiment. But hey, that’s part of the excitement of fishing, isn’t it?

Well, I hope this explanation of thermal stratification helps you to understand its effect on your favorite fish’s behavior and equip you with knowledge you can use to form strategies will have catching fish throughout the day on almost any lake.

Let us know what you think. Do you use thermal stratification to help you catch more fish?

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