Understanding the Thermocline Impact on Bass

Thermocline, a summer consideration, is the transition layer between the warmer less dense water near the surface and colder denser water below that has less oxygen. In other seasons the water does more mixing providing oxygen to lower in the water column. Lake stratification is the term to describe all three layers with the thermocline being the layer between the warmer, less dense water and the colder, denser water. 

Why is Understanding the Thermocline Impact on Bass Important?

It is important to understand the thermocline to:

  • Bass are Above the Thermocline: In the summer, especially late summer, when fishing deep, it is important to identify what maximum depth to which bass will reside based on the thermocline. Some suggest bass will not go below the thermocline in part due to the reduced oxygen.
Understanding The Thermocline Impact on Bass
  • Oxygen Level: In the summer as the surface water gets warm the deeper water below the thermocline does not circulate with the water above resulting in depleted oxygen in deeper water. The thermocline is sometimes referred to as “the dead zone”.
    • No Bass below Thermocline: This results in the bass and baitfish staying above the thermocline. In deeper water the bass will suspend above the thermocline.  Some suggest that being the bass’ ideal location when combined with water temperature considerations.
  • Depth: In the summer the thermocline becomes prominent and depends on the lake water clarity. It  can be between 20 or 30 feet, sometimes deeper. Lakes with clear water will have lower thermocline depth.
  • Fall Transition: The water cooling in the fall results in the thermocline going deeper until it disappears completely which is referred to as the “fall turnover”. Some suggest bass become less active at the fall turnover but others suggest any reduction in bass activity is due to reduced temperature not the turnover itself.
    • Strong Winds : Albeit rare, strong sustained winds over deep water can bring the low oxygen water that was below the thermocline to shallow shoreline water.
  • Structures: Some suggest bass will seek structures that are above but near the thermocline. 
  • Water Bodies with No Thermoclines: There is no thermocline in rivers, reservoirs with sufficient current, or shallow lakes, especially shallow lakes with high winds.
Figure 1: Thermocline at 22 Feet
Figure 2: Thermocline at 22 Feet
Additional Considerations
  • Depth Identification:
    • Electronics: [Figure 1 and 2] Sonar bounces back off colder water thus visible with electronics. Some suggest going to the deepest part of the lake and turning up the sensitivity of the 2D sonar unit to identify the thermocline depth.
    • Observation: If bass are not being observed or caught below a certain depth in the summer it could be due to the thermocline. 
It's About the ALLURE™

Fishing Deep Lures
Targeting bass that are just above the thermocline. 

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