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Rt970701.jpg (96555 bytes)The Scientific Angler


Dissolved Oxygen Dynamics and Bass: The Summertime Blues


By Ritch Taylor


"This summer has been hot, really hot. I just can’t get comfortable unless I am in front of the air conditioner," how many times have you heard that this summer? We all tend to not like the heat, especially by the end of the summer. But, how do you think the fish feel. Especially those big largemouth bass in Lake Ray Roberts or any North Texas lake when the surface water temperature hits 90 degrees. Fortunately for us, we can still find adequate air to breathe and we get to escape into a refrigerated house or building. But the fish, they must try to find the right mix of oxygen in the water, adequate water temperature, and cover to be as comfortable as possible. Plus, if they do want another meal, the fish have to make sure their prey like the conditions chosen.

As mentioned, oxygen is a must for fish. So, we will begin by developing the idea of dissolved oxygen in reservoirs, move on to how dissolved oxygen is influenced during summer, and then analyze how these changes may direct our summertime fishing patterns. However, as any angler knows, sometimes the bass defy our thinking and will do things and be places unexpected for the persistent conditions. It is our fishing mission though to assess the local conditions, determine how these conditions may dictate baitfish and bass movement, and then predict areas that will potentially hold fish. Putting all the pieces of the puzzle together, including the water quality (dissolved oxygen) pieces, is most of the fun in fishing. Especially when you get it right.

The first part of the water quality puzzle to understand is dissolved oxygen. Oxygen is vastly important to understanding fish movement patterns and location. It is probably the most important parameter because so many biological processes depend on it. Dissolved oxygen or DO is the oxygen dissolved in the water. This is the oxygen available for bass and other fish to breathe in through their gills. Dissolved oxygen can be put into the water by surface wave action (our beloved jet skiers), diffusion from the air, and by photosynthesis of algae and plants. It is a critical water quality component of any fresh water reservoir, stream, river, or stock tank, and especially to the fish. If we were to take a similar volume of air and water it may surprise you, but the water would have less than 5 percent of the oxygen in the air. You may ask, "Man, how do those bass breathe?" It’s all in there construction. Specialized gills for extracting small amounts of dissolved oxygen. The gills provide a vast surface where blood runs near the water exchanging carbon dioxide for precious oxygen.

When we take a close look at dissolved oxygen we must understand that it has a low solubility in water. Meaning not much can be dissolved in water. This dictates that it must either be continually replenished or it will be depleted by the respiration of all those thousands of fish, zooplankton, algae, and bacteria that live in the water. The solubility of oxygen or amount of oxygen that can be dissolved in water is affected by water temperature and the atmospheric pressure. This is important because as the water gets warmer or the pressure drops it holds less and less oxygen. Of course the opposite is also true, and in winter the cold lake water can hold much more dissolved oxygen.

The influence of temperature, especially hot summer temperatures, is twofold. First, as we just discussed, hot water holds less oxygen. Less oxygen means less dissolved for the bass and prey to breathe. Second, warm temperatures mean the reservoir water will eventually stratify as it does every summer. By stratify I mean the lake becomes layered into distinct zones as shown in Figure 1. An upper zone of warmer mixed water (epilimnion), a middle zone of changing water temperatures (thermocline), and a bottom layer of cool water (hypolimnion). We might just think all those fish would just go down and be cool and happy in the bottom layer or hypolimnion. Well the catch 22 is that those bottom layers usually have no light penetration, don’t get mixed with upper layers, and are rapidly depleted of available oxygen. Furthermore, as summer progress the oxygenated layers may shrink to thinner layers. All this happens because warm water is lighter than cool water and once solar heating takes place in the summer the warm surface layers of the lake tend to just float on top and the cool layers get trapped below. Light penetration is further reduced by algae productivity and sediment loads during rainfall events. It all adds up to a limited zone in the water column that bass and baitfish can be found. Interactions such as algae production of oxygen can tremendously influence the amount of dissolved oxygen in the water. At times during daylight hours the algae may produce conditions that result in more oxygen than is physically possible according to the temperature and pressure, this is termed supersaturated conditions or when the solubility exceeds 100 percent. The flip side is at night these algae respire and drastically reduce the levels of oxygen. So, levels of dissolved oxygen are controlled by many complicated factors which all influence our fishing. In some cases oxygen depletion in the upper layers can drastically alter our fishing by causing a fish kill.

How do all these conditions play out on area lakes? It depends. It depends on the light penetration, the degree of stratification, temperatures, pressure, the size and shape of the lake, the operation of the lake, and on and on. The bottom line is that the stratification of a reservoir varies from one reservoir to another and from one area of the lake to the other. Typically the deeper light penetrates the thicker the epilimnion or the deeper adequate levels of oxygen are in a reservoir. So, when on the lake, notice how far down you can see your lure. Notice when this changes. Learn to use your depth finder to locate the thermocline. Typically bass are going to be like us. Hibernating deep during the day to the coolest spot possible (note: this can all change at night). But in the case of a bass this spot must have adequate oxygen. Therefore, in summer this spot is typically just above the thermocline to the surface. It then stands to reason that fishing waters that are less than adequate is useless. In this case, knowing where the thermocline is limits the depths you need to search for baitfish and bass or other gamefish. Now, we have not talked about measuring oxygen, but that is another ball of wax that we can get too later. It is not difficult but a good instrument to measure oxygen can be a bit costly, about $500 or more. But, I guarantee you that more and more anglers are going to learn to use water quality measurements to their advantage to locate bass. It just stands to reason.

The key to finding fish during summer, as just stated, is first knowing where adequate oxygen exists. For bass and most gamefish the level needs to be about 3 milligrams per liter (mg/L) or greater. Don’t worry so much about the mg/L; know this is the unit of measurement for dissolved oxygen. Just like a football field is measured in yards and for that matter a spool of fishing line. Scientists measure dissolved oxygen all the time. It is one of the key water quality parameters of any reservoir or water resources study. Through testing and time fisheries scientists have determined the levels of oxygen fish can live in. The minimum needed by gamefish, such as a bass, is about 3 mg/L and above. Below 3 mg/L the fish seem to not live very well. Levels less than 3 mg/L will typically be below the thermocline. So, always begin fishing from the thermocline to the surface. Next, baitfish is of course a key. But baitfish exists in a location because of food and environment. Again, limit your search to the zone above the thermocline. Typically, if you see a shad flip on the surface the school is below and maybe just above the thermocline. So get on that sonar and look. The bass will not be far behind.

To end we will take a look at one of our area lakes for an example. Lake Lewisville, in the DFW area, is a busy, busy lake with many boaters, lots of development on its shores, and some good fishing at the right times and places. If you want to see differences in stratification go near the dam and compare this to water in the Old Lake Dallas arm. Major differences. The water near the dam will be clearer, it will have a deeper thermocline, and it can have fish living deeper. On the other hand, if you look at the water in the Lake Dallas arm and in the area of the marinas there you will notice the differences. Less visibility, just dip a white spinnerbait in the water and see how deep it is when it disappears (this is a trick of the pros), and a shallow thermocline. Water quality measurements taken on Lake Lewisville in the Old Lake Dallas arm indicate that the thermocline is shallow and that most fish will be 12 feet or shallower. People comment that the fish on Lewisville stay shallow. Well the reason may just be in the water quality. The fish are limited to near the surface due to a shallow thermocline or a shallow zone of adequate dissolved oxygen. It’s a fact that at Ray Roberts the thermocline is at least 35 feet or deeper in some areas. Why, the fish are this deep or deeper.

Knowledge about dissolved oxygen and water quality will not always result in catching fish but it will make you a better angler and a more scientific one at that. We have to remember that the fish live where they do for specific reasons and understanding just one more piece of the puzzle gets us that much closer to catching fish. It also gives us an idea of how we might better protect and preserve these valuable resources for future anglers.

God Bless,

Ritch Taylor

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Last modified: February 13, 2002