The most serious threat to catfish in ponds is poor water quality. Water quality is not constant. It varies with the following factors:
A successful catfish producer must understand pond dynamics, the effect of catfish production on water quality, and management of water-quality problems.
Aspects of water quality of concern in catfish production include:
Temperature does not change very rapidly except in the case of small, shallow ponds. Dissolved oxygen, pH, and carbon dioxide levels change or fluctuate daily. Ammonia, nitrite, alkalinity, hardness, and chloride generally change slowly, although exceptions do occur under extreme conditions. Relatively inexpensive and easy-to-use chemical tests are available for checking these water quality factors. For information on how to order test kits, contact your county Extension office or the Extension fisheries specialists.
No two ponds are exactly alike. Pond color and water quality vary within a single pond from day to day. Adjacent ponds are seldom alike in their color, water quality, and the growth rate of the fish, even though they are stocked and fed at the same rates. These differences are not fully understood but may be related to soil conditions, algae (microscopic plants called phytoplankton), and bacterial populations of the pond.
Water temperature is one of the single most important factors in ponds. The metabolic rates of the plants, bacteria, and fish depend on the temperature. Catfish are warm water fish and perform most efficiently at warm temperatures (approximately 80 to 85 degrees F). At higher temperatures, respiration rates are high, feed conversion is poor, and overall growth is reduced. Channel catfish will die at temperatures above 96 degrees F.
Temperatures below the optimum range reduce metabolic rate, feed consumption, and growth. Very low temperatures impair the immune system and lower resistance to disease. Rapid changes in temperature, especially during hauling and stocking, stress fish and may reduce feeding and increase susceptibility to disease.
Algae are extremely important to catfish ponds. Algae produce most of the oxygen in the pond and remove most of the carbon dioxide and many of the nutrients. Algae also consume oxygen, produce carbon dioxide, cause pH to fluctuate, and release nutrients into the water as they die.
Algae populations change continuously, because different species nourish at distinct temperatures and under various pH and nutrient conditions. Algae populations, called "blooms" can die off and result in fish kills. The only way a fish farmer can become an efficient producer is to continuously monitor and keep records of bloom conditions, oxygen concentration, and other water quality factors.
Low dissolved oxygen is by far the most common water-quality problem in catfish production ponds. Ponds get oxygen from two sources: the air and photosynthesis. Oxygen diffuses into water from the air. Diffusion is a slow process unless it is aided by the action of wind or some type of mechanical agitation that mixes air and water together.
Most pond oxygen comes from photosynthesis. Photosynthesis is the process by which plants make food from carbon dioxide, water, nutrients, and sunlight. The by-product of photosynthesis is oxygen, which dissolves into the water. At night, no oxygen is produced and the respiration of the algae and fish and the decomposition of wastes by bacteria remove oxygen from the pond.
Under natural conditions, more oxygen is produced by photosynthesis than is removed by respiration, as it cycles up and down during the day. Figure 6 shows a general oxygen cycle for ponds during warm weather conditions.
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Figure 6. General 24-hour oxygen cycle in ponds. |
The amount of oxygen that will dissolve in water depends on the temperature, salinity, and atmospheric pressure. Salinity and atmospheric pressure are of little consequence in fresh water catfish production. Temperature, however, is an important regulator of dissolved oxygen levels in ponds.
Cold water holds or will dissolve more oxygen than warm water. Therefore, as temperature increases in the pond, less oxygen is available. The amount of oxygen that water will dissolve at different temperatures (saturation) is listed in Table 6.
Temperature, Degrees C--F |
Dissolved Oxygen, ppm |
Temperature, Degrees C--F |
Dissolved Oxygen, ppm | |
0--32 |
14.60 |
18 --64 |
9.45 | |
1 --34 |
14.19 |
19--66 |
9.26 | |
2--36 |
13.81 |
20--68 |
9.07 | |
3--37 |
13.44 |
21--70 |
8.90 | |
4--39 |
13.09 |
22--72 |
8.72 | |
5--41 |
12.75 |
23--73 |
8.56 | |
6--43 |
12.43 |
24--75 |
8.40 | |
7--44 |
12.12 |
25--77 |
8.24 | |
8--46 |
11.83 |
26--78 |
8.09 | |
9--48 |
11.55 |
27--80 |
7.95 | |
10--50 |
11.27 |
28--82 |
7.81 | |
11--52 |
11.01 |
29--84 |
7.67 | |
12--53 |
10.76 |
30--86 |
7.54 | |
13--52 |
10.52 |
31--88 |
7.41 | |
14--57 |
10.29 |
32--90 |
7.28 | |
15--59 |
10.07 |
33-92 |
7.16 | |
16--61 |
9.95 |
34--93 |
7.05 | |
17--62 |
9.65 |
35--95 |
6.93 |
The amount of oxygen that dissolves in water is very small compared to the oxygen concentration of the atmosphere. The atmosphere contains about 20 percent oxygen, or 200,000 ppm (parts per million). Water at saturation at 85 degrees F contains less than 8 ppm oxygen. Ponds can supersaturate with oxygen on sunny days when algae in the pond are very dense (heavy bloom). Very high concentrations of oxygen (twice saturation) during the day sometimes indicate that an oxygen depletion will occur that night.
Critically low dissolved oxygen concentrations can usually be predicted. Low levels occur because of one of the following:
Most low-oxygen problems occur between May and September. During this period, temperatures are warm, feeding rates are high, algae blooms are heavy, and fish are growing rapidly. All of these conditions can cause more oxygen to be removed from the pond at night than is produced during the day. Also, still and overcast days may reduce the amount of oxygen produced by wave action and by photosynthesis. This condition may promote an oxygen depletion. The result can be dead fish.
An oxygen depletion can also be caused by what is called a "turn-over. " In the summer, the surface of the pond heats up rapidly, forming a warm and less dense layer of water. This warm layer traps a cooler, denser layer of water beneath it. The pond is said to be "stratified" in this condition.
The two water layers do not mix with each other under normal conditions because of their differing densities. Oxygen is only produced in the upper warm layer and slowly becomes depleted in the lower layer because of bacterial and chemical action.
A cool front or a thunderstorm with wind and cold rain can cool the surface of the pond enough to make the two layers mix. The result is the dilution of the oxygen that was in the upper layer and an increase in demand for oxygen. The increased demand is usually both biological and chemical.
The algae usually die off under these conditions, causing rapid oxygen removal through bacterial decomposition. Turn-overs are a common cause of catastrophic fish kills in deep ponds (more than 8 feet).
Oxygen concentrations should be maintained above 4 ppm at all times if catfish are to grow well. Growth can be severely affected when oxygen levels remain below 4 ppm for extended periods. Stress caused by chronically low oxygen will lower resistance to disease.
Predicting Low Oxygen
Monitoring and predicting low oxygen is critical. Dissolved oxygen can be measured using either electronic or chemical methods. Electronic oxygen meters are relatively expensive but have become standard equipment on commercial catfish operations (Figure 7). Electronic oxygen meters require maintenance and calibration, but they are quick and accurate. Chemical dissolved-oxygen tests are accurate if directions are precisely followed, but they take several minutes to complete. For this reason, chemical tests are not recommended if more than three ponds are to be tested. Accurately reading color changes of the chemical method is difficult at night in poor light.
Graphic Projection Method. Low dissolved oxygen can usually be predicted using the graphic projection method. This method relies on the fact that oxygen generally declines at a constant rate throughout the night. Based on this steady decline, low oxygen can be predicted by graphing the rate of decline and projecting this decline until morning. To use this method, oxygen readings must be taken near dusk and 2 to 4 hours later.
To use this method, mark graph paper as shown in Figure 8, with oxygen concentrations along the Y axis (vertical). Mark time along the X axis (horizontal). Next, mark the two oxygen readings on the graph and draw a straight line through them to the X axis. If this line indicates that the oxygen concentrations will fall below 3 ppm before sunrise, then aeration will probably be necessary.
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| Figure 8. Graphic method of predicting oxygen depletions. |
One word of caution: this method only predicts low oxygen and gives the manager time to take appropriate action. It is not foolproof and still requires that oxygen be monitored to prevent unanticipated problems.
In Pond 1, the dissolved oxygen concentration at 8:00 p.m. is 12 ppm, and at 10:30 p.m. it is 6 ppm. Drawing a line through these two points indicates that the oxygen concentration will fall below 4 ppm between 11:00 and 11:30 p.m. Emergency aeration should begin before midnight.
In Pond 2, the dissolved oxygen concentration at 8:00 p.m. is 10.5 ppm, and at 11:00 p.m. it is 5.5 ppm. Drawing a line through these two points indicates that oxygen concentration will not fall below 4 ppm by sunrise. In this case, emergency aeration is probably not necessary.
Pond Record Method. Another method to predict low oxygen was developed from analyzing actual fish farm records. These records show that, if the oxygen concentration at dawn is 5 ppm or more and at dusk is the same as or greater than the day before at dusk, then no oxygen depletion will occur the upcoming night. But, if the oxygen concentration is less than 5 ppm and is less at dusk than it was the day before, then an oxygen depletion can be expected during the coming night. Figure 9 shows a sample graphic pond record which predicts that a nighttime oxygen depletion will occur.
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| Figure 9. Graphic pond record predicting that a nighttime oxygen depletion will occur because the oxygen concentration is less than 5 ppm at dawn and was less at dusk than the day before. |
Successful pond managers monitor oxygen every day at daybreak, at nightfall, and during the night throughout the growing season. Decreasing morning oxygen levels from day to day, low evening readings, and increasing supersaturation levels usually warn of upcoming problems. It is important to take readings at the same time and at the same location each day. In ponds larger than 5 acres, oxygen readings should be taken at two ends of the pond because oxygen may vary widely in the same pond. Keeping a chart (Figure 10) of daily oxygen readings will help you predict developing problems. Do not rely on memory. Maintain good records and use them.
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| Figure 10. Chart of daily oxygen readings. |