Dr. Hal Schramm

If you pound the shoreline all day long and never put the boat over water more than four feet deep, you probably won’t care about this information. But if you want to catch more bass year-round, you need to learn to fish deeper water, and one key to deepwater bassin’ is knowing how thermal stratification positions fish.

Essentially, stratification creates the thermocline. It’s driven by temperature, but is a result of the density of water.

Two simple principles of physics make stratification almost self-explanatory:

1. The breaking point for water density is 39 degrees. Water grows more dense as it cools below that temperature, and less dense above that mark.

2. Heavy (more dense, cooler) water sinks. Light (less-dense, warmer) water floats.

 In early spring (March in the South and ice-out in the North), all of the water in a given lake is the same temperature – and therefore the same density – top to bottom. As the water warms throughout the spring, it also gets less dense. At some point, rapid warming of the upper part of the water column creates a density difference between the upper and lower portions of the column, with warmer (less dense) water floating on the cooler (more dense) water.

The warmer upper layer (called the epilimnion) is separated from the colder layer (the hypolimnion) by a narrow layer – the thermocline. As the weather continues to warm, the difference in temperature between the two layers gets even greater. A 20-degree difference during late summer is not uncommon. thermo

Ok, so the thermocline separates a layer of warmer, less dense water from a layer of colder, more-dense water. So what?

Why does the thermocline and a body of water’s stratification make a difference in bass fishing? It’s not the density of the water, or the temperature. It’s really all about oxygen.

Most oxygen in the water is produced by phytoplankton -- microscopic algae suspended in the water column. Phytoplankton in the epilimnion get plenty of light and keep that layer oxygen-rich. Very little light is normally available in the hypolimnion, the layer below the thermocline, so oxygen isn’t produced, and what little there is normally is depleted by the decomposition of dead things that sink to the bottom.

Little or no oxygen means the water below the thermocline is a dead zone. No oxygen, no fish.

In Part 2 of “Understanding The Thermocline,” we’ll examine how you can use this information to boost your summertime bass catches.