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About Temperatures

Temperature can often direct an angler to the location of possibly the best fishing spot. It is important to understand temperature in relation to thermal fronts. A thermal front is described as any interface between water masses of significantly different temperatures where relatively rapid water temperature changes occur. These fronts are often abundant with aquatic life, including trout and salmon. Understanding thermal fronts may help anglers see how the movements and locations of salmonids may relate to these fronts.


There are three types of thermal fronts.

Thermal Bar - As water reaches 39 degrees Fahrenheit it sinks to the bottom, because water is densest at 39 degrees Fahrenheit. This creates lake turnover. This zone of water that warms and then sinks is the thermal bar. It is a surface-to-bottom vertical "wall" that is located where the 39 degree Fahrenheit water temperature first occurs on the surface as one moves away from shore. It moves slowly offshore in the spring.

How can one find the thermal bar?

  • Coast Watch Lake Surface Temperature Reporting System
  • Boat's surface thermometer
  • Look for sharp water color changes
  • Look for ripples or changes in turbidity
  • Look for collections of debris (insects, feathers, dead forage fish)

Spring Thermocline - An inclined zone of rapidly declining water temperatures. It is represented by a strata of water temperature change between 46 and 42 degrees Fahrenheit. It is both a surface are and a sub-surface layer which is characterized by changes in measures of water quality, biological productivity, turbidity, and current.

Thermal Break - Randomly occurring surface temperature gradients. The break gradients may range from 3-10 degrees Fahrenheit over a relatively short trolling distance. These breaks tend to be the most productive fishing areas at the sharpest breaks.


Another important aspect of temperature is the winter-summer transition which usually occurs from about April to mid-June. This transition is a period of time where the frozen or near freezing surface waters, with underlying warmer waters, change to a body of water having warmer surface water temperatures underlaid by cooler water temperatures. The Great Lakes warming processes and features vary slightly in degree and timing due to the differences in the lakes' water volume and regional climate. Temperature is less of a fish-catching factor as summer becomes fall. This is because fish enter river mouths and many move upstream.

Some related links:
Marine Forecasts for all the Great Lakes
Our Great Lakes
NOAA Great Lakes Environmental Research Lab
Images of the Great Lakes
National Survey of Fishing, Hunting, and Wildlife Recreation
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