A thermocline is an underwater layer where the temperature decreases rapidly as you go deeper. Colder water is dense so it sinks to the lake bottom while warmer water is lighter and floats above. Thermocline depths tend to be consistent for the same calendar month year to year except for very shallow lakes, where they might not form at all. Thermocline location is important to fishermen because fish tend to favor depths near the boundary. You can find thermoclines using several techniques separately or in combination.
Gather Lake Information
Talk to other sportsmen in the area to obtain information on the best fishing “holes” in the lake of your choice. Ask them if they know at what depth the thermocline is, and how much it changes between summer and winter.
Get a bathymetric map of the lake that shows the underwater depths. Look on the Internet. Your lake map may be online like 2,700 of Michigan’s lakes. The map will not give you the thermocline directly. Eliminate shallow spots as thermocline candidates. Use this method in a process of elimination rather than as a direct way to find thermocline.
Check with university or government monitoring stations like the USGS and Army Corps of Engineers. Both have temperature profile data available for some larger lakes.
Anchor your boat where you wish to “sound” for temperature, then attach an underwater thermometer to a weighted rope and lower it over the side. Use fishing sinkers for the weights: a few ounces should be sufficient. Release one arm's length worth of rope. Leave the thermometer in place for two minutes, then retrieve it and note the reading. Repeat the process, adding one arm's length to the rope each time, until you reach the bottom.The thermocline will be between the two readings that change the most.
Choose a fish sounding device to find the thermocline. Read the instructions for your model. Displays and controls vary.
Use the lake map or directions obtained from others and guide the boat over the location of choice. Take multiple readings within a small area around your preferred spot to eliminate a false sounding. Anchor in place before fishing as the thermocline may vary with water depth.
Items you will need
- Fish sounding device
- Internet connection
- Personal flotation device
- Underwater thermometer
- Try getting the information from other anglers or the Internet first. It's much easier.
- Always wear a personal flotation device when boating. Avoid the lake when it is windy and bad weather threatens.
- Grand Valley State University: Seasonal Lake Stratification
- Grand Fishing Report: Thermocline Impact on Grand Lake Fishing
- Michigan Dept. of Natural Resources: Lake Maps by County
- University of Texas: Current LTTS Weather Readings (Lake Travis)
- Use a fish sounding device to find the thermocline. Read the instructions for your model and anchor the boat over the location of choice. Take multiple readings within a small area around your preferred spot to eliminate a false sounding.
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