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The Importance of the Thermocline

Thermocline: A transition layer between deep and surface water

The thermocline is the transition layer between the mixed layer at the surface and the deep water layer. The definitions of these layers are based on temperature.

The mixed layer is near the surface where the temperature is roughly that of surface water. In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature.

The mixed layer and the deep water layer are relatively uniform in temperature, while the thermocline represents the transition zone between the two.

Why Worry About the Thermocline?

It is important to understand the importance of the thermocline if you want to find trout when water temperatures soar. The thermocline is a magnet for trout in the summer. It provides a sanctuary for trout where they find cooler water and oxygen. Trout are most comfortable in water temperatures between 50-65 degrees Fahrenheit. Mortality rates for trout caught in temperatures above 65 degrees increases exponentially. Although not part of the thermocline definition, oxygen is an important consideration for trout. They cannot live with less than 5 parts per million.

With knowledge of the thermocline and oxygen levels we can find where the fish are. With that understanding, we can make better decisions regarding when it is safe to fish and when to limit fishing.

Where trout live


Data for Oxygen/Temperature at Cedar

The thermocline layer is bounded on the top and bottom by temperature. The livable water for trout is determined by temperature at the upper level while oxygen saturation usually sets the lower limit. Trout will swim above and below the thermocline layer to feed. Early morning and late evening trout may move into the danger zone  near the surface to feed, however, they will return quickly quickly to the safe layer.

Bounded on the top, bottom and left by temperature and on the right where the temperature and oxygen cross, the green box shows the limits of the comfort zone. In the graph of the July data the sweet spot is roughly between 12 and 21 feet deep. This is where the trout are.

How do we know? Why do we care?

Measurements are taken several times a year by John Kornegay using equipment that measures water temperature and oxygen levels at three foot intervals. By locating the thermocline when it begins to form and tracking it through the season we can manage our fishery to best advantage. When surface temperatures become lethal we can selectively restrict fishing and when they moderate these restrictions are relaxed.

In July the surface temperatures in our quarries are lethal should a trout were to remain there. Fish are holding somewhere between 12-21 feet deep only moving into the danger zone to feed but then quickly returning to the comfort zone.

Fish caught at this time of year are subject to more stress. Larger fish fight hard and do not recover as quickly which leads to higher mortality rates. Using this information and with previous year's experiences John  and Owen Mitchell (Fisheries Chair) made the recommendation, which was accepted by the Board, to close Cedar to all fishing to protect our trophy population.  Although Pine and Birch have similar thermocline profiles, they were not closed. The fish are smaller and have easier access to deep water when released so it was decided to let them remain open for fishing.

It doesn't stop there!

Measurements and observations continue throughout the summer to make sure fishing in Birch and Pine is safe and to help determine when it is safe to reopen Cedar. The most recent measurements taken in August show the water continues to get warmer even though we have had plenty of rain and some cool nights. Last year, the water temperatures were much warmer and water levels were at drought levels. With accurate data and our experience we can better balance the need to maintain a healthy fishery with the opportunity to continue fishing.

We are fortunate to have a very robust thermocline layer compared to other lakes in Connecticut may be only 2-3 feet thick. We don't know why but aren't we lucky.