How the Lake Turn Over Affects Fishing
by Josh Morris
I had a request from a reader to do a lake turn over article. I was glad to get the request but must admit this something I knew very little about. Like all fishermen I knew about the turn over and had a general idea. It is when the water from the surface gets cooler than the water under it making it denser and it sinks bringing the warmer water from underneath to the surface. I have seen fish kills happen in ponds when this happens. I know when it occurs fishing on Barren River Lake gets tuff. Also from now until the end of September we will all blame a bad fishing day on “the lake must be trying to turn over.” The actual factors of barometric pressure, fishing pressure, water temperature, and wind seem to take a back seat. In my research I learned quite a few things and hope this will help.
Starting in the spring and over the course of the summer, surface waters absorb a lot of the sun’s energy and can heat extensively, causing them to become quite buoyant. Winds and storms can cause some mixing and do add some oxygen; atmospheric oxygen is added by the air-water interaction to the oxygen produced within the water by aquatic plants. But there is too much difference in temperature between the surface water and that at depth to allow for complete mixing of all the water in the lake. Because of the density-temperature relationship, many lakes in temperate climates tend to stratify, that is, they separate into distinct layers. The middle layer, known as the thermocline, acts as an effective barrier to any mixing of the deeper waters. Toward the end of summer, the deep water becomes quite depleted of oxygen because no mixing has taken place.
As the days get shorter and cooler, and energy is transported away from/out of the lake, mixing becomes easier. The cooler water (with a higher oxygen content) at the surface begins to sink into and through the thermocline, forcing warmer and less dense water to the surface, eventually erasing the temperature stratification built up over the summer. At some point, the majority of the water in the lake reaches an approximately uniform temperature. Now, storms and sustained high winds can begin to perform the task of overturning and mixing all of the water in the lake — referred to as fall turnover. The deep water contains an abundance of decaying matter and sulfurous gases; when it reaches the surface, it produces a telltale odor that indicates the process has begun. Eventually the turnover mixes fresh oxygen into the entire lake mass, replenishing the deep waters with the life-giving stuff and cleansing the sulfurous fumes from the water, allowing fish to return to the depths where they will spend the winter months.
How this affects fishing on Barren River Lake is a large column of water devoid of oxygen moves to the surface, causing the fish to shut down and search for more oxygenated water. In most cases it is a gradual breakdown of the thermocline, so if the lake has turned over and the fish don’t seem to be where they were back off and look a bit deeper as the water with oxygen has become more dense as it cools and sunk to the bottom.
I do think as the water heats up in the summer the fish try to find a cooler spot, they will be in or near the thermocline. That is why the fishing is a bit easier up the river where there is some current and cooler water. We all know suspended fish on the main lake can be hard to catch. The turnover is a huge change to the fish’s habitat and opens up a new world of previously oxygen depleted water which makes the fish move. The good news is, post turn over fall fishing patterns begin to start up and great fishing is just around the corner.
I have referenced R. Karl for some of the scientific information I used. He has a very in depth article on thermoclines, water freezing and turn over at http://onthelake.net/fishing/turnover.htm
I have learned a great deal researching this article. I hope this helps you all catch more fish and understand what is going on during the turn over. One thing to watch is your depth finder. Once you know what to look for you can determine where the thermocline is and if it is starting to dissipate. This will help in finding fish.
Good luck and God Bless!