Useful Data:

Thermocline and Dissolved Oxygen Explained:

     This is an attempt to explain two concepts – thermocline and dissolved oxygen and how they
relate to fish populations.    The thermocline is a thin but distinct layer in a large body of fluid
(e.g. water, such as an ocean or lake, or air, such as an atmosphere), in which temperature changes
more rapidly with depth than it does in the layers above or below.  The temperature gradient is relatively steep at the
thermocline depth relative to the equivalent gradient above and below that depth.
    
Thermoclines are normally a large factor in oceans but they can also be observed in lakes. In colder
climates, this leads to a phenomenon called stratification. During the summer, warm
water, which is less dense, will sit on top of colder, denser deeper water, with a thermocline separating them. 
Because the warm water is exposed to the sun during the day, a stable system exists, and very little mixing of warm
water and cold water occurs, particularly in calm weather. One result of this stability is that as the summer wears on,
there is less and less oxygen below the thermocline, as the water below the thermocline never circulates to the surface,
and organisms in the water deplete the available oxygen.

     The thermocline is of primary interest to fisherman as this is the best starting place for locating
walleyes and other warm water species.  The intersection of the thermocline with hydrographic features such as
points, bluff walls with trees and other types of structures are important starting locations when first graphing for fish or bait fish locations.  

     As winter approaches, the temperature of the surface water will drop as nighttime cooling dominates heat transfer.
A point is reached where the density of the cooling surface water becomes greater than the density of the deep water,
and overturning begins as the dense surface water moves down under the influence of gravity. This process is aided by wind or
any other process (currents for example) that agitates the water.  
     
     This turnover process usually begins when the surface water temperature reaches the low 60’s
and is completed when the surface water temperature reaches approximately 55 degrees(F.).  Evidence that
turnover has begun will be the appearance of ribbons of bubbles in long strings across the surface of the lake
or reservoir.  Once turnover is completed fish can now utilize the entire water column since oxygen and
temperature barriers no longer exist.

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