I took this Humminbird screenshot Thursday afternoon. The weekend before, I had been out on this same lake and immediately noticed the same thing on the graph, something not normally seen this time of year. I also immediately thought I knew exactly what was going on, as I've seen this happen on this lake and others before. Turned out I was right. Here's the deal.
As you can see, almost all the fish are stacked into the top 12' or so of the water column, especially at the bottom of that band of water. For those that guessed thermocline, you have the right idea, but not quite the right answer. So our waters are definitely starting to heat up as suggested by the 81 degree surface temps, but it is still early in the year in the big scheme of things. Yes, the thermocline is starting to develop, but I don't think many anglers quite fully grasp the entire concept around this. Much of the online bassing sites and articles don't help in this regard.
The thermocline is basically equivalent to the middle layer of the water column, also known as the metalimnion. It is in this layer that you get the quick drop in water temps. Technically, a thermocline is defined as the area of water with the fastest drop in temperature, usually at least 1 degree Celsius per meter, or just over half a degree F per foot. The upper layer of water is the epilimnion, and the lowest layer of water is the hypolimnion. In these two bands of water, the temperature stays relatively stable regardless of depth. As such, the thermocline can be a pretty wide band of the water column, especially early in the year when we are first getting separation of layers. That is what is currently happening on most Indiana reservoirs at the moment.
As the year goes on, that middle layer will get squeezed and become smaller and smaller, and also get pushed to the shallower side. In the screenshot, as on most other Indiana waters, the bottom layer of the thermocline is beginning to form around that 20-25' range. This bottom level of the thermocline, the one most commonly seen on sonar units, is due to the settling of particles, aquatic organisms, and the like. Right now, the top of the thermocline is forming around 10-15' on these same waters. Normally, the warm water fish such as bass, bluegill and catfish will tend to stay near the upper level of the thermocline or shallower, while the cool water fish like trout, stripers, pike and walleye will tend toward the bottom layer of the thermocline. So why are pretty much all the fish in this screenshot staying around that 12'-13' depth band without any being graphed deeper?
The answer is the lake I was on had received an algae treatment about a week and a half ago. As the algae have started to die off and settle, their decomposition has started consuming all the oxygen in the water. To confirm this, I borrowed a YSI ProODO dissolved oxygen meter and went out on the lake to test/confirm this. Sure enough, at the surface, there was 8.75 ppm of oxygen, and the water was about 110% saturated. This oversaturation occurs during the day while the plant life is producing oxygen as part of it's photosynthetic process. It actually puts more oxygen into the water than what that water at the same temperature would hold otherwise. But as we lowered the probe down to about the 10' level, the oxygen dropped to 5.8 ppm and a 71% saturation level.
Slowly, we dropped the probe down a few more feet to the 13.5' level, and the oxygen level continued to dive. At that point we were down to just 1.75 ppm and only 22% saturation. Biologists generally consider 2 ppm as the lower level of oxygen content that can safely sustain fish life for any extended length of time. Once you hit the 15' level, for all intents and purposes, oxygen was gone from the water column. So the reason you don't see any fish pushing further down into the developing thermocline, even though those temperatures are perfectly suitable for fish life, is because they have been stripped of oxygen. This was not the case last year, where at this same time, you could easily find fish down to 20'-22' of water or more.
So the mystery is solved. This exact situation can occur on any of our reservoirs where algae treatments take place. For those who might remember over a decade ago, the initial large treatment at Eagle Creek Res. did the same thing, not only wiping out the oxygen levels to a worse degree than this, but also lead to the death of a lot of fish that were copper sensitive (copper in some form is the active ingredient in many algae applications). That experience, plus a lot of research by local universities, have led to a change in how we now treat these algae outbreaks in most of our state waters.