You will find that when you get that sudden change in water temperature in just a foot or two that the amount of Dissolved Oxygen will also drop right below the thermocline. The lake water is STRATIFIED and it's this stratification that forms the boundary between the hot water and the colder water and this is where you will find the OXICLINE as well.


The epilimnion is where you will find the most fish. Below the thermocline you have the hypolimnion and that's where the water is devoid of dissolved oxygen during the hot summer months when the lake is stratified by temperature.

An easier way to find the thermocline is to just watch the depth finder and not the depth where most of the fish are located in the hot summer months. Fish will suspend above the thermocline and you can determine the thermocline by just watching to see where the fish suspend.

Also remember that different species of fish have different dissolved oxygen requirements. Rough fish like catfish or carp may not need as much dissolved oxygen as a Small Mouth Bass.

If you have 5 ppm of Dissolved Oxygen most fish can survive and lay eggs in this type of environment.

Cold water holds more dissolved gases so therefore colder water will hold more dissolved oxygen. But in the summer months when the lake is stratified the cold bottom water can be devoid of dissolved oxygen. The reason is that the bacteria in the lake bottom consume all the dissolved oxygen as they feed on the dead algae that falls down though the water column. And since the lake is not mixing oxygen from the surface to the lake bottom the lake bottom becomes devoid of dissolved oxygen. So that's why you won't find many fish down in that colder deeper water. And there is not enough sunlight reaching the deeper bottom areas so there is not plants growing. Remember that green plants produce oxygen when they undergo photosynthesis. Fish can be found in the shallow waters where there is a lot of green plants growing as the plants give them oxygen and shade from the sun.

I've been out on my lake trolling crank baits at depths up to 10 to 12 ft and catching a lot of Largemouth Bass. The thermocline on these lakes runs around 25 ft during this time of the year.

I use a YSI dissolved Oxygen meter and temperature probe to find the thermocline. This is the same equipment that the COE guys use to develop the DO and Temp data for the COE web sites.


Quote Originally Posted by Grumpy View Post
Tom, you are absolutely right. I didn't want to get into that in my original post but I also check the change in temperatures to get the probable location of the thermocline. HOWEVER, over the years I have been doing this, I have found that it is much easier to see the drop in oxygen content then the drop in temperature. Both ways are correct and will help to eliminate a lot of unproductive water by not fishing deeper than thermocline/depleted oxygen depths. As Peter said, "To know where fish will not be is huge."

Grumpy