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Limnology 101- by Rob Brewer |
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| Most of us are familiar with the term “eliminating water”, yet often our elimination tactics involve nothing more than fishing an area. The whole idea behind eliminating water is to identify water that’s not worth fishing in the first place. Armed with an understanding of the bass’ liquid environment and how the seasons affect it, eliminating water becomes quite simple. The first step to effectively eliminating water is a basic understanding of limnology. For those unfamiliar with the term, limnology is the science that deals with the physical, chemical and biological properties of fresh water. Let’s begin by dissecting a Tidewater lake in summer when there’s three distinct layers in the lake. These layers are the Epilimnion (top), the Metalimniom or thermocline (middle) and the Hypolimnion (bottom). Let’s look at each layer in more detail while it’s still summer time. Epilimnion- this top layer contains the warmest water in the lake. It’s warmest at the surface, but as you penetrate deeper into it, the temperature gradually drops. This layer contains plenty of oxygen and just about ALL of the bass at this time of year. Metalimnion- (Thermocline) this thermal barrier separates the epilimnion from the hypolimnion. This band of water rapidly drops in temperature in just a few feet or less. The thermocline varies in depth and width (distance between epilimnion and hypolimnion) on any given lake. Keep in mind that the thermocline may not be at a uniform depth throughout the same body of water. For example, a windy day may slant the thermocline, making it deeper on the windblown side and shallower on the opposite shore. The thermocline has diminishing levels of oxygen the deeper in it you go. The thermocline often contains a surprising amount of particulate debris and sediment constantly suspended in it. This suspended debris is what enables you to see the thermocline on your sonar unit. Finding the depth of the thermocline enables you to eliminate all water beyond that depth. Hypolimnion- this bottom layer of the lake consists of the coldest water in the lake and tends to have a more uniform temperature throughout. This layer is usually void of bass because it lacks sufficient oxygen. This is due to all the decaying organic matter typical of the fertile lakes in our region. Not only does this decaying process consume the oxygen, but also it also produces methane as a byproduct and further makes the layer even more intolerable for bass. Imagine it’s a hot July day. You and your partner are beating the banks with worms and spinnerbaits, occasionally hooking a runt here and there, but the bite is tough. The two of you discuss a possible deepwater bite and motor to a hole 25’ deep. One of you rigs a Carolina rig, the other a Mann’s 20 plus. You begin fishing and after a half-hour without any action, you begin second guessing your move. Eventually, you head back to the bank and stick with what worked. Had you a better understanding of limnology, you would have realized your lure choices were doomed. Those lures worked too deep, they plied the waters of the hypolimnion where there are no bass in our lakes. Had you found the thermocline and fished some structure at that depth, sooner or later, you’d connect with fish. Finding this magic depth is the first key in unlocking a deepwater bite. Once found, it’s map and graph time. By now your probably asking, how do I find this thermocline thing? There are a few ways to find it, some easier than others. The most common way is with a sonar unit. You’ll need to turn off the automatic settings and turn up the gain (sensitivity). Often, you’ll pick up a very minute, but distinct line at a given depth. Some find it and mistake it for interference. Another way with a graph is to motor around marking fish. Note the depths of the deepest fish you’re spotting. Probably the most accurate way is to construct a limnograph (Pg 60, Jul/Aug 2000, Bassmaster). A last resort is for you and your partner to fish two different depth working diving plugs. Start at 8’ and 10’ and gradually work deeper lures until you connect. Keep in mind that thermoclines are usually shallower in murky/stained water than they are in clear water lakes. Our tidewater lakes remain in this stratified condition until late fall when the water begins to cool. As the surface temperature cools, it becomes denser and begins to sink. Bass are still occupying the epilimniom, but don’t relate to the thermocline as much due to the cooler water throughout the epilimniom. As the water continues to cool, the thermocline compresses and becomes a thin band. Eventually the epilimnion will cool to the same temperature as the thermocline. Once this happens, the thermocline ruptures and fall turnover takes place. After the thermocline has ruptured, all the trapped particles and toxic methane are released into the entire lake. The water becomes very turbid and oxygen levels are at an all time low. Turnover is very stressful for bass and other species. Fishing is very tough now. So tough, that you’re better off in a tree stand. Bass can be anywhere in the lake since there’s no longer a hostile hypolimnion. Turnover is one period where we should welcome cold fronts. You want the winds to come and mix things up. This will help stabilize conditions. Once the bass adjust, they will begin feeding again. If you find you must fish during turnover, try the headwaters or any underwater springs you know of. Once winter is upon the lake, It will again stratify, but not to the extent it did in summer. Remember water is densest at 39.2 degrees Fahrenheit. Let that be your clue to finding warm water. If the surface temperature is 36 degrees, you know it’s warmer in the depths, likewise, if the surface temperature is 41 degrees; it’s colder in the depths. Winter bass are usually in deep water due to the stability of conditions there. Often, they stack up in just a few areas of the lake for whatever reason. Find this area and adjust your presentation to the slowed metabolism of the fish and you can have a real productive day. Just how cold the water gets will dictate whether or not there will be a spring turnover. If the surface temperature goes below 39 degrees then a spring turnover will commence as soon as the water again warms to its densest temperature. If the surface temperature never dips below 39 degrees, the lake will not experience spring turnover and will slowly begin its stratification process until it has gone full circle back to where we began this article. Hopefully, this has given you some insight that will help you effectively eliminate water according to the season you’re fishing. Remember, fishing is life. Rob Brewer |
