Lake Turnover – Part 1

Summer and Fall

   It’s a common excuse during the fall “The fish aren’t biting because the lake is undergoing turnover”. I’ve even used it myself a few times. To help give you an idea of what techniques you should be using I will explain Lake Turnover in a simple manner and how it affects fish. I will start in the summer and work my way through the seasons until the next summer. I’m going to split it into two posts because there is a lot of information to cover.

In the US, during the summer months most lakes will be stratified into two distinct layers of water.  The top layer is warmer, well oxygenated (due to plants and wind-wave action), and contains the majority of aquatic life during the summer. The top layer is the lighter because the water is less dense than the cooler water below it. These two layers are separated by a zone known as the thermocline (a sharp change in temperature).    Below the thermocline is a zone of water that does not have enough oxygen to support most fish. This zone lacks oxygen because the top and bottom layer do not mix and the available oxygen is quickly used up by bacteria and detritovores decomposing organic matter on the bottom. The bottom layer is denser than the top and the sharp gradient between the two layers is actually what keeps them from mixing.

In the summer months, the fish are forced to stay shallow because of the lack of oxygen below the thermocline. As an angler, you can really use this to your advantage by eliminating deeper areas of structure that are located below the thermocline. You can sometimes see the thermocline on a color fishfinder, but if you can’t….. More turbid lakes usually have a shallow thermocline 15-25 and clear lakes usually have a deeper one 35-60.

As fall approaches, the air temperature begins to drop and the top layer of water begins to cool. Eventually the top layer of water approaches the same temperature as the bottom layer of water. This erodes the thermocline, the density and temperature differences that existed between the two layers are gone. Without the thermocline the two layers of water begin to mix, the degree of mixing is determined by the wind and the volume of water. This is the time of year when you can have fish patterned in one place and they vanish overnight.  What causes this to occur? Remember the two layers of water, the bottom layer is low in oxygen and it mixes with the top layer. When these two layers mix it can lead to a variety of problems for fish: low oxygen can kill them, the lake can become very turbid (cloudy from the bottom layer’s debris), toxins that were stored on the bottom can be released, and many other water quality problems can occur. It can be a real nightmare for the fish and the angler trying to find them.

So, what can you do? Luckily, in most cases, the whole body of water won’t turn over at one time. This can be due to the topography of the lake or the amount of water exposed to the prevailing wind. The shallow areas of the lake will experience the turnover a little sooner than deeper areas of the lake and many times lakes in the same geographic area won’t undergo turnover at the same time because of differences in the topography between the lakes themselves. During this time I try to cover a lot of water and try to find the fish, if the turnover has occurred then the area the fish have to hide in will be drastically increased. Don’t be afraid to load up and try a different region of the lake if the end you’re on isn’t producing.   After the turnover the entire water body will be uniform in temperature from the bottom up. This gives the fish lots of room to roam because the entire water column is oxygenated. This allows fish to move into 30 ft of water where previously you wouldn’t find them any deeper than 15 or 20 ft. If you have any questions about turnover during this time of the year feel free to ask!

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