Commercial production in high-density catfish ponds requires aeration. Aeration strategies can be described as supplemental or emergency.
Supplemental aeration involves the nightly operation of aerators, regardless of the dissolved oxygen level, in an attempt to maintain oxygen concentrations above stressful levels. In supplemental aeration, aerators are run 5 to 7 hours per night beginning about midnight and ending about dawn.
Supplemental aeration appears to increase feed efficiency and total pounds of catfish produced over emergency aeration at moderate stocking and feeding rates (4,000 fish per acre and a maximum feeding rate of 50 pounds per acre per day). As stocking and feeding rates increase, supplemental aeration may not increase production.
Emergency aeration is used when the dissolved oxygen concentration drops to critical levels, when fish may die if not assisted. Emergency aerators must be available when using supplemental aeration.
Once emergency aeration is begun, it should continue until the oxygen level is above 4 ppm and the fish no longer gasp at the surface for air. This usually occurs after an extended period of aeration, after sunrise when photosynthesis beings, or when overcast cloud conditions break up during daylight.
Many types of aerators are commercially available. Aerators can be powered by electricity, diesel engines, or the power-take-off (PTO) of a farm tractor. The efficiency of an aerator can be determined from its ability to transfer oxygen into water. Aerators are rated in terms of pounds of oxygen transferred per horsepower per hour.
Most producers prefer stationary electrical aerators used for supplemental aeration. Electrical aerators are usually more efficient and less expensive to operate and maintain. As a general rule, 1 to 1-1/2 horsepower per surface acre is sufficient supplemental aeration and for some emergencies, except in extreme cases such as a bloom die-off.
In extreme cases, portable emergency aerators, like PTO-driven paddlewheels, are needed in addition to whatever stationary aerators are already in the pond.
Both electrical-paddlewheel (Figure 11) and pump-sprayer
aerators are efficient and effective emergency aeration devices for use
in ponds. Both agitate the water and create a current. The moving water
rapidly saturates with oxygen and the current (or waves) increases the absorption
of oxygen across the surface of the pond. The current is also important
in attracting fish to the aerated zone. Paddlewheels and pump sprayers can
be powered by PTO's, electricity, or diesel engines.
Spray- or vertical-pump surface aerators that lift water (Figure 12)
are usually not as efficient as paddlewheels or pump sprayers in emergency
situation. However, they may be useful in small ponds in maintaining oxygen
concentrations.
Propeller-aspirator pump aerators have high-speed propellers equipped with hollow drive shafts. As the propeller turns, it causes air to be drawn down the shaft and mixed into the water. These are not as effective as paddlewheel aerators, but they offer the added benefit of helping to destratify (break up temperature layers) deeper ponds. However, to destratify ponds effectively, they must be operated continuously. Propeller-aspirator aerators come in many sizes and, therefore, may be adapted to small ponds.
Diffuser aerators are operated by compressors or air blowers that release bubbles of air into the water. Diffusers are not very effective in most commercial catfish ponds.
Motorboats, twisting and turning at high speeds, have also been used for aeration in ponds. Large, tractor-powered rotary mowers, placed so that the mower blades agitate the surface water, have been used as well. However, the effectiveness of these methods is limited.
More detailed information on aerators and their efficiency can he found in Experiment Station Bulletin 584, "Evaluation of Aerators for Channel Catfish Farming," available from the Alabama Agricultural Experiment Station at Auburn University.
Stationary aerators should be placed where they will create the maximum circulation in the pond. In rectangular ponds, place stationary aerators in the center of the longest levee or side, with the discharge toward the middle of the pond. In this position, water is directed perpendicularly to the longest side and moves across the pond to create currents that reach most areas of the pond.
Placing aerators in the corner of the pond to direct water diagonally across the pond produces poor circulation. Locating fixed aerators in the middle of the levee will cause higher installation costs and may be inconvenient when aeration is needed at another location for harvesting operations. Portable aerators can be used during harvest.
Most aerators will not deliver adequate oxygen throughout the pond but will create oxygen-rich areas to which fish will be attracted and in which they can survive. Portable emergency aerators should be used before fish are stressed to the point that they cannot reach the aerated area. The best placement for an emergency aerator is in the area of the pond with the highest oxygen concentration. Fish will be gathered in this area.
If two aerators are needed, place them near each other (30 to 50 feet apart). This way, if one aerator fails, the other can hold the fish in the area and keep them alive until the problem is fixed. In single-aerator situations, fish will move into oxygen-poor areas in search of more oxygen. At that point, during a severe oxygen depletion, the fish may be dead by the time additional aeration is moved to the pond.
Fish cover the surface of the pond, particularly along the banks, when they are severely stressed from low oxygen. Place aerators in the areas where the most fish are congregated and try to attract them to the aerator. In a hill-type pond, fish will usually go to the shallow end in search of higher oxygen. Be prepared to operate an aerator in shallow water. Bankwasher aerators are effective at quickly providing oxygen to fish along the shoreline.
Most producers do not have enough paddlewheel or pump sprayer aerators for all ponds and, therefore, move them to ponds as they are needed. One portable aerator for every three to four ponds is adequate. If aeration requirements exceed the oxygen supplied by available equipment, then the ponds with the fastest-falling oxygen levels or the most valuable fish should be aerated first.
A portable paddlewheel or pump sprayer aerator can be difficult to situate in a pond properly without damaging it or the tractor. Before emergencies arise, try running aerators in several probable locations around each pond, so that placement becomes more or less routine. This is particularly important because most aerator maneuvering is done at night.
This section briefly describes some common methods of aeration. Additional information can be found in Southern Regional Aquaculture Center Publication No. 370, "Pond Aeration" and SRAC Publication No. 371, "Pond Aeration: Types and Uses of Aeration Equipment." These publications can be obtained from your county Extension agent or from the Extension fisheries specialists.
Well water. Pumping water from a well, stream, or adjacent pond with a high oxygen content is a good way to aerate in an emergency. Well water is often low in oxygen and must be splashed or sprayed before it enters the pond.
If well water is not available or not in sufficient quantity, then water from an adjacent pond or stream may be a good substitute. Water from streams or other ponds is not as desirable as well water because it can be a source of wild fish and disease.
To aerate ponds in this way, you need equipment that will pump at least 100 gallons per minute for each acre of pond. Drain some water from the pond bottom while adding water at the surface. This method is more effective than allowing excess surface water to pass through the pond standpipe or spillway.
Spraying. Water from the pond low in oxygen can
be sprayed into the air to add oxygen. Place pump intakes just beneath the
surface, not on the pond bottom. Discharge the water just a few feet above
the surface of the receiving pond.
A pump sprayer or relift pump powered by the PTO of a farm tractor (Figure 13) is an effective aerator for this situation. The discharge can he capped and slots cut in the sides to increase efficiency. Another modification is to mount the discharge manifold parallel to the surface of the pond and discharge in opposite directions down the pond bank (called a "bank washer" ). PTO-, electric-, and diesel engine-powered pump sprayers are commercially available.
Claims that chemicals such as potassium permanganate and phosphate fertilizers alleviate oxygen problems are unfounded.
The same factors that produce low dissolved oxygen concentrations in ponds also contribute to high carbon dioxide (CO2) concentrations. Carbon dioxide increases through the night because of respiration. Carbon dioxide levels can also increase rapidly after a algae bloom die-off.
Carbon dioxide interferes with oxygen uptake at the gills, so fish will show signs of oxygen stress even though oxygen readings may be in a safe range. A concentration of over 25 ppm of carbon dioxide in pond water is generally harmful to catfish and may cause death.
Aeration is the best way to help rid the pond of carbon dioxide and increase oxygen levels. Up to 100 pounds per acre of hydrated lime, Ca(OH)2, may be added in extreme cases to remove some of the CO2.
The pH is a scale on which the acidity (hydrogen ions) and alkalinity (hydroxide ions) of water is measured. A pH of 7 is neutral (balanced in H+ and OH - ions). Changes in the pH of a pond occur during a 24-hour cycle because of respiration.
Carbon dioxide from nighttime respiration reacts with water to form carbonic acid. Carbonic acid drives pH downward, making the water more acidic. During the daytime, pH moves upward (the water becomes more alkaline) because the carbon dioxide is removed for photosynthesis.
The optimum pH for catfish ponds is between 6.5 to 8.5. But in production ponds, pH can vary from 6.0 to 9.5 without severally stressing the fish.
The pH of the pond is usually checked only before certain chemicals are added or if ammonia levels are high. The pH of the pond affects the toxicity of chemicals like copper and ammonia. The pH of the pond water is strongly influenced by the pH of the pond mud and of the soils in the watershed.
The only way to modify pH in ponds is by adding lime, gypsum, alum, or bicarbonate. However, adding chemicals to alter pH should be done only in extreme circumstance.
Alkalinity is a measure of bases in water. These bases include hydroxides (OH-), carbonates (CO3 -2), and bicarbonates (HCO3-). They are related to, but not the same, as pH. Alkalinity acts as a buffer to absorb hydrogen ions and resist pH changes.
Hardness is a measure of divalent (+2) ions, mostly calcium and magnesium. In chemical tests, both are measured in ppm of calcium carbonate equivalence, which leads many people to think that they are the same.
If alkalinity and hardness are both derived from limestone soils, they usually have similar values. It is possible, however, to have water that is high in alkalinity and low in hardness and vice versa.
Alkalinity and hardness should be maintained above 220 ppm. Alkalinity can be increased by adding agricultural limestone, hydrated lime, quick lime, sodium bicarbonate, or sodium hydroxide. Generally, agricultural lime is the least expensive and most predictable chemical to adjust alkalinity.
More information on this use of agricultural lime can be found in Extension circular ANR-232, "Liming Fish Ponds." This publication is available from your county Extension agent or from the Extension fisheries specialists.
Alkalinity affects the toxicity of copper treatment in ponds. A fish farmer should check alkalinity before determining the rate for applying copper compounds. More information is found in Extension circular ANR-414, "Table For Applying Common Fishpond Chemicals," available from your county Extension agents or the Extension fisheries specialists.
Hardness can also be increased by the addition of agricultural limestone, hydrated lime, quick lime, gypsum, or calcium chloride. Low hardness can be a problem in catfish hatcheries. Hardness of hatchery water (pond or well) should be checked before the spawning season.
Catfish, like all other animals, produce nitrogenous wastes from the digestion of the proteins in their diet. Ammonia is the principal nitrogen waste product. It is excreted directly into the water from the gills and kidneys of the fish.
Ammonia is also produced from bacterial decomposition of the proteins from uneaten feed and from any dead animal or plant, including algae. About 2.2 pounds of ammonia is produced from each 100 pounds of feed fed.
Ammonia, once released into the pond, can be absorbed by algae or bacteria. Algae use ammonia as a nutrient for growth and reproduction. Certain aerobic (oxygen-requiring) bacteria use ammonia as a food source in a process called "nitrification."
Nitrification is an important process by which toxic nitrogenous wastes are decomposed. In the process of nitrification, bacteria of the genus Nitrosomonas convert (oxidize) ammonia to nitrite, and bacteria of the genus Nitrobacteria convert nitrite to nitrate. Ammonia and nitrite are both toxic to fish; nitrate is not.
Ammonia Toxicity. Ammonia in water dissolves into two compounds: ionized (NH4+) and unionized (NH3) ammonia. Un-ionized ammonia is extremely toxic to catfish, while ionized ammonia is relatively nontoxic. Un-ionized ammonia levels as low as 0.4 ppm can cause death. Reduced growth and tissue damage can occur at 0.06 ppm.
The ratio of the total ammonia nitrogen (TAN) in the un-ionized form depends on temperature and pH (Table 7). The amount of toxic un-ionized ammonia increases as temperature and pH increase. Under reasonable feeding rates and good water quality conditions, ammonia is seldom a problem.
Temperature in degrees C | |||||||||
pH |
16 |
18 |
20 |
22 |
24 |
26 |
28 |
30 |
32 |
7.0 |
0.30 |
0.34 |
0.40 |
0.46 |
0.52 |
0.60 |
0.70 |
0.81 |
0.95 |
7.2 |
0.47 |
0.54 |
0.63 |
0.72 |
0.82 |
0.95 |
1.10 |
1.27 |
1.50 |
7.4 |
0.74 |
0.86 |
0.99 |
1.14 |
1.30 |
1.50 |
1.73 |
2.00 |
2.36 |
7.6 |
1.17 |
1.35 |
1.56 |
1.79 |
2.05 |
2.35 |
2.72 |
3.13 |
3.69 |
7.8 |
1.84 |
2.12 |
2.45 |
2.80 |
3.21 |
3.68 |
4.24 |
4.88 |
5.72 |
8.0 |
2.88 |
3.32 |
3.83 |
4.37 |
4.99 |
5.71 |
6.55 |
7.52 |
8.77 |
8.2 |
4.49 |
5.16 |
5.94 |
6.76 |
7.68 |
8.75 |
10.00 |
11.41 |
13.22 |
8.4 |
6.93 |
7.94 |
9.09 |
10.30 |
11.65 |
13.20 |
14.98 |
16.96 |
19.46 |
8.6 |
10.56 |
12.03 |
13.68 |
15.40 |
17.28 |
19.42 |
21.83 |
24.45 |
27.68 |
8.8 |
15.76 |
17.82 |
20.08 |
22.38 |
24.88 |
27.64 |
30.68 |
33.90 |
37.76 |
9.0 |
22.87 |
25.57 |
28.47 |
31.37 |
34.42 |
37.71 |
41.23 |
44.84 |
49.02 |
9.2 |
31.97 |
35.25 |
38.69 |
42.01 |
45.41 |
48.96 |
52.65 |
56.30 |
60.38 |
9.4 |
42.68 |
46.32 |
50.00 |
53.45 |
56.86 |
60.33 |
63.79 |
67.12 |
70.72 |
9.6 |
54.14 |
57.77 |
61.31 |
64.54 |
67.63 |
70.67 |
73.63 |
76.39 |
79.29 |
9.8 |
65.17 |
68.43 |
71.53 |
74.25 |
76.81 |
79.25 |
81.57 |
83.68 |
85.85 |
10.0 |
74.78 |
77.46 |
79.92 |
82.05 |
84.00 |
85.82 |
87.52 |
89.05 |
90.58 |
10.2 |
82.45 |
84.48 |
86.32 |
87.87 |
89.27 |
90.56 |
91.75 |
92.80 |
93.84 |
Ammonia can become a serious problem, however, if:
Ammonia levels should be routinely checked each week and whenever an algae die-off occurs. High ammonia levels can occur at any time of the year, but they are most likely during the summer because of heavy feeding rates. Managing high ammonia levels is difficult. First, stop or reduce feeding rates and maintain good dissolved oxygen levels (ammonia damages the gills). Second, flush the pond if adequate water is available.
Nitrite Toxicity. Nitrite is also very toxic to catfish. Under normal conditions, nitrite does not accumulate to toxic levels. But it can reach toxic levels if bacterial decomposition (nitrification) is disrupted. Most nitrite problems occur during fall and winter, when sudden changes in pond water temperatures disrupt bacterial decomposition.
Nitrite passes through the gills of fish and attaches to hemoglobin of the blood, forming metheoglobin. Methmoglobin causes the blood to change in color from red to chocolate-brown. For this reason, nitrite toxicity is called "brown blood," check the gill color or cut off the tail of a fish and look for chocolate-colored blood.
Normal hemoglobin carries oxygen through the bloodstream, but methemoglobin cannot. Fish in this condition are under severe respiratory stress and will show signs of oxygen depletion. Nitrite toxicity is affected mainly by temperature, dissolved oxygen, and chloride ions. A nitrite concentration as low as 0.5 ppm can cause stress.
Nitrite concentrations can rise from 0 ppm to lethal levels in 2 to 3 days, so it is very important to test for nitrite regularly. Producers should check nitrite concentrations three times per week from August 1 to January 1 and throughout April and May. Checking nitrite one or two times a week is sufficient the rest of the year. Producers should also monitor nitrites closely after algae die-off.
Chloride ions (not chlorine) in the water can block nitrite from entering across the gills, protecting the fish from "brown blood." Research has shown that a minimum of 3 parts of chloride should be present for each part nitrite in the pond. Generally, a chloride to nitrite ratio of 5:1 or 6:1 is best.
Salt (sodium chloride) is commonly used to increase chloride concentrations in ponds. Calcium chloride, either anhydrous, has also been used for this purpose, but it is more expensive.
Some producers try to maintain chloride concentration at 30 ppm in ponds. Applying 45 pounds of salt in 1 acre-foot of water will bring the chloride level to 10 ppm. So, to achieve 30 ppm, 135 pounds (45x30) is needed for each acre-foot of water. In a 10-acre pond with an average depth of 4 feet (or a total of 40 acre-feet), a 30-ppm, chloride concentration would require the addition of 5,400 pounds of salt.
A more precise way to calculate the needed level of chloride is to measure the nitrite concentration and multiply it by 6.
Whenever nitrite levels rise, check chloride levels and add salt as needed. Flushing water through the pond can reduce nitrite levels but will also remove chloride ions. Watershed ponds lose chloride when they overflow. Test regularly and keep good records!
After the "brown blood" problem is corrected, watch the fish closely for bacterial infections. Bacterial infections often occur a few days after "brown blood" outbreaks.
Other Toxicity Problems. There are other potentially harmful chemical compounds that producers should consider.
Copper and zinc in small concentrations can be extremely toxic to fish. Galvanized equipment, such as pipes, containers, screens, and tanks used in holding and transporting fish may give up enough zinc to be toxic. Copper from algae treatment, pipes, and other equipment can also be toxic o fish in containers.
Catfish are very sensitive to chlorine. Water from city supplies should not be used for filling, hauling, or holding tanks.
Some pesticides are also toxic to fish. Fish in ponds built on cultivated watershed are always in danger of pesticide poisoning. Before stocking fish in these ponds, find out which chemicals have been used and their toxicity to fish.
Establish vegetative barriers between fields and ponds. Make sure that chemical applicators prevent chemical drift over ponds. Be aware that constant use of chemicals near ponds may eventually cause a serious problem.
In the future, one of the strongest selling points for aquaculture products should be their lack of chemical contamination. Keep dangerous chemicals away from your ponds and assure the consumer of the highest quality product.
Component |
Recommended Value or Range |
Dissolved oxygen |
4 ppm or more |
Carbon dioxide |
less than 20 ppm |
pH |
6 to 9.5 |
Total alkalinity |
20 ppm or more |
Total hardness |
20 ppm or more |
Un-ionized ammonia |
less than 0.05 ppm |
Nitrate |
less than 0.5 ppm |
Temperature change |
less than 5 degrees F as rapid change |