Where Not To Fish
Blog by: Lloyd Tackitt , Texas 11/13/2014
At any given time of the year, from 50% to 80% of the water in a lake does not contain enough dissolved oxygen to support fish life - this of course varies by time of year and water temperature, etc. There is no adaptation for a fish to the amount of dissolved oxygen (DO), too little or too much is lethal. So why fish in those areas? Mostly it happens because without a specific instrument to measure the DO, the fisherman just isn't aware he or she is fishing in water than can not possibly contain fish. These instruments can be purchased from the usual suspect list of sport equipment outfitters, some for under $50.
Survivable DO levels for fish generally run (depends on the exact species so look your's up on-line) from 5 to 13 parts per million (ppm), with the optimum range running from 9 to 12 ppm of DO. Below 3ppm DO and fish die of oxygen starvation, above 13ppm and they will experience oxygen poisoning. So ideally you're looking for water that contains the 9 to 12 ppm range. This may make up a large portion of the lake, depending a lot on time of year, water temperature, wind conditions, lake contour and orientation and size and so on.
One way to go about this is to get an oxygen meter and spend the first hour or so checking oxygen levels in the places you would normally fish. You may eliminate non-productive areas and if you do you won't be wasting your time fishing where the fish are not.
You can further refine this fish location search by also measuring the water's ph levels. These levels will vary with location and depth. Fish blood averages about 7.6, slightly alkaline, and they will seek a ph level in the water as close to that as they can find within their normal movement range. When the water ph is close to their blood ph fish are better able to withstand stress and to use the oxygen in the water.
Fish aren't likely to travel long distances to seek out a perfect ph level, they can survive a range of 6.7 to 9.6, but they will move short distances to get to the most comfortable level. If there is water in their immediate area that is between 7.5 and 7.9, that's where they'll go to.
The procedure for measuring ph levels is to lower the meter's probe into the water taking a reading at the surface and then at one foot intervals down to the bottom. Generally you'll discover a "break-line" at some depth, a point where the ph changes rapidly. For instance the readings may fluctuate back and forth by one tenth of a unit and then at the break-line you may note a full one-unit change. This is significant since the ph scale is logarithmic.
The beauty of this break-line is that the break-line is the depth at which a majority of fish will congregate - and are most active. So, with an oxygen meter you can eliminate fish-less areas from your efforts and with a ph meter you can determine the most likely depth to find fish where there is enough DO to support them.
Using those two key pieces of information you can then add a third to triangulate in on the fish - search for contours or structure that your species tends to like and you now have a three part plan for where to find and catch fish. Optimum dissolved oxygen, ph break-line, and cover - these can bring you to the fish and keep you from fishing where they aren't.