How do you clear a pond for aquaculture? A complete guide to pond clearing methods
I. Quicklime
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Sources and main components of quicklime
Any natural rock with calcium carbonate as its main component, such as limestone, chalk, dolomitic limestone, etc., can be used to produce lime. In the production of quicklime, natural rock whose main component is calcium carbonate is calcined at a temperature of 1000-1100°C, and after the carbon dioxide released by decomposition is removed, the resulting product with calcium oxide (CaO) as its main component is quicklime.
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The principle, usage and dosage of quicklime for pond clearing
[Principle of pond clearing with quicklime]
After quicklime contacts water, it generates calcium hydroxide and releases a large amount of heat. Calcium hydroxide is a strong base and can raise the pH of the pond water above 11 in a short time. During pond clearing, if the pond water pH can be raised above 11 and maintained for more than 2 hours, the purpose of killing harmful organisms and pathogenic microorganisms can be achieved, and it can effectively kill wild trash fish, frog eggs, tadpoles, aquatic insects, etc.
[Methods of pond clearing with quicklime]
·Method 1: Dry pond clearing
First drain the pond water, or leave 5-10 cm of water, and use 50-75 kg of quicklime per mu, increasing or decreasing according to the amount of sludge at the pond bottom. When clearing the pond, first dig several small pits in the pond bottom, then put the quicklime in to dissolve, and immediately and evenly splash it over the whole pond before it cools.
The next morning, it can be turned over once with a rake, the purpose being to mix it with the sludge and give full play to the pond-clearing and disinfecting effect of quicklime.
·Method 2: Pond clearing with water
For each mu with a water depth of 1 m, use 125-150 kg of quicklime. Usually the quicklime is put into a small boat, and after it dissolves, it is immediately splashed over the whole pond while still hot.
After pond clearing, once the medicinal effect disappears after 7-9 days, fish fry can be stocked. With this method, there is no need to add new water before stocking, which prevents the danger of wild trash fish and pests entering the pond with the water, so the disease-prevention effect is better than that of the dry pond clearing method, but the amount of quicklime used is larger, and the labor cost and overall cost are higher.
[Other effects of quicklime during pond clearing]
During pond clearing with quicklime, some other effects are often easily overlooked. Quicklime can not only clear the pond, but also supplement calcium ions in the water body and increase the hardness of the water body, which is especially important in shrimp and crab farming. In addition, because the main raw material of quicklime is natural rock, it can also effectively supplement trace elements in the water body, all of which are necessary for the growth of aquatic animals and plants.
[Precautions when using quicklime]
① In saline-alkali ponds in the north, using quicklime to clear the pond will increase the alkalinity of the pond bottom and cause the problem of excessively high pH after stocking. In this case, it is recommended to use bleaching powder instead.
② Because quicklime readily absorbs carbon dioxide and water from the air and deliquesces. When purchasing quicklime, it is best to choose lump lime. Purchases should be arranged according to production needs, and too much should not be stored at one time or for too long, so as to avoid deliquescence forming calcium carbonate and becoming ineffective.
II. Tea seed meal
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Sources and main components of tea seed meal
Tea seed meal, also known as tea seed cake, with the alternative names tea bran and tea dregs, tea seed cake. It appears as purple-brown granules and is the cake meal pressed from the residue left after oil is extracted from the fruit of wild Camellia oleifera. It is a commonly used fishery input in aquaculture pond clearing in China for killing wild trash fish.
The principle by which it poisons fish is that tea seed meal contains 12-18% saponin. Saponin, also called saponin glycoside, is a hemolytic toxin that combines with heme in the blood cells of aquatic animals, causing blood cell breakdown and death of the aquatic animals. The tolerance of shrimp and crabs is more than 40 times that of fish. The main reason fish, shrimp and crabs have different tolerances to tea seed meal is that their blood pigment compositions differ. Fish blood pigment contains iron-containing heme, while the blood pigment of shrimp and crabs is copper-containing hemocyanin. Saponin mainly acts on heme. It can kill wild trash fish, frog eggs, tadpoles, snails, leeches and some aquatic insects, and also shows certain toxicity to shellfish, sandworms, etc. in the water, but has no killing effect on pathogenic bacteria or aquatic plants.
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Usage and dosage for pond clearing
Pond clearing with water is recommended, with a water depth of 30 cm-50 cm, so that a small boat holding the soaked tea seed meal liquid can be moved easily and splashed evenly over the whole pond. When the water depth is 30-50 cm, the dosage per mu is 20-30 kg, adjusted according to how many loaches, rice field eels and other aquatic animals that burrow into the mud are in the pond. When using tea seed meal to clear a pond, the tea seed meal needs to be crushed first and soaked for 24 hours, so as to release the effective pond-clearing component (saponin) in the tea seed meal. The medicinal effect generally lasts 7-10 days. Fry can be stocked only after the medicinal effect disappears.
If there are many loaches, rice field eels and other aquatic animals that burrow into the mud in the pond, fill the whole pond with water to 0.15 m one or two days before pond clearing and let it soak. After the loaches and rice field eels emerge from the bottom mud, use tea seed meal to clear the pond, and the killing effect will be more thorough.
Because tea saponin is easily soluble in alkaline water, adding 1.5 kg of quicklime per 50 kg of tea seed meal during use gives a better effect.
In addition, tea seed meal is rich in crude protein and various amino acids and other nutrients. After pond clearing and filling with water, it helps plankton reproduce in large numbers and is a good organic fertilizer as a basic bait organism. For ponds with little sludge and poor bottom substrate, it can play a role in fertilizing the water.
III. Bleaching powder
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Principle of pond clearing
Bleaching powder is a mixture of calcium hydroxide, calcium chloride and calcium hypochlorite, and its main component is calcium hypochlorite [Ca(ClO)2]. It is made by bringing slaked lime into full contact with chlorine gas, and the exothermic chemical reaction is as follows:
Bleaching powder appears as a white or grayish-white powder or granules, has a pronounced chlorine odor, is very unstable, is highly hygroscopic, and readily decomposes under the action of light, heat, water and ethanol. Bleaching powder dissolves in water. Bleaching powder generally contains about 30% available chlorine. After bleaching powder contacts water, it produces extremely unstable hypochlorous acid. The chlorine atoms and oxygen atoms produced by the decomposition of this hypochlorous acid, through oxidation and chlorination, produce a powerful sterilizing effect, so it has a strong killing effect on pathogenic microorganisms in aquaculture water bodies, and can also kill aquatic insects, tadpoles, snails, wild trash fish and some freshwater mussels.
Because bleaching powder readily absorbs moisture and deliquesces, it needs to be purchased according to the needs of aquaculture production during use. It is not advisable to purchase too much at one time, so as to avoid decomposition and loss of effectiveness due to long storage time, which leads to a decrease in available chlorine content.
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Usage and dosage
After dissolving bleaching powder in water, immediately splash it over the whole pond. It is recommended to drain the pond water to 30-50 cm before pond clearing, so as to make it easier to move a small boat containing the bleaching powder solution, and after the bleaching powder is fully dissolved, splash it over the whole pond.
Check 3-5 days after pond clearing. If the medicinal effect has completely disappeared, fish fry can be stocked. Before stocking, it is best to test the water to confirm that the medicinal effect has disappeared.
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Precautions during use
(1) The pond-clearing effect is greatly related to the environment. It is suitable to choose a sunny day for pond clearing, as the water temperature is then higher, the effect of the drug is stronger, and the pond-clearing effect can be improved. If there is prolonged low temperature during pond clearing, the pond-clearing drug is difficult to degrade, easily settles at the pond bottom, is adsorbed by sand and soil, and its slow release can easily lead to the death of the stocked fry.
(2) Bleaching powder should be placed in a wooden or plastic container, fully dissolved with water, and then evenly splashed over the whole pond. The residue must not be poured into the pond. Bleaching powder should not be placed in metal containers, otherwise it will corrode the container and reduce the medicinal effect.
(3) Safety protection measures should be taken when applying bleaching powder. Operators should wear masks and rubber gloves, and the applicator should apply the drug from the upwind side to avoid the drug blowing into the face with the wind, causing poisoning and clothing contamination and corrosion.
(4) If the available ingredient content of the bleaching powder used is less than 30%, the application amount of bleaching powder should be appropriately increased. If the bleaching powder has already deteriorated and become ineffective, it should be prohibited from use.
IV. Pyrethroids
In aquaculture, they are mainly used to kill aquatic insects and the like that were not completely killed during the previous pond clearing or that entered the pond when water was added before stocking. They have contact and stomach toxicity, causing acetylcholine to accumulate at nerve synapses, abnormal permeability of nerve cell membranes, and inhibition of nerve transmission, causing insects and larvae to become excited, paralyzed and die.
When national-standard pyrethroids are used for pond clearing, the dosage is 5-10 times the dosage used in normal treatment of parasitic diseases, and can effectively kill wild trash fish, shrimp, crabs and crustaceans, aquatic insects, etc. in the pond.
Crustaceans such as red swamp crayfish, river crabs and freshwater shrimp are relatively sensitive to pyrethroid drugs, and pyrethroids take a long time to dissipate, so their use very easily causes drug residues. In East China, many farmers have the habit of using pyrethroids to soak ponds. During river crab farming, many farmers report that in river crab culture ponds that have been cleared with pyrethroids, the incidence of "water turtle" is higher than in ponds not cleared with pyrethroids. Therefore, it is not recommended to use pyrethroids to clear ponds for shrimp and crab culture ponds. After fish fry ponds are cleared with pyrethroids, relevant detoxification products must be used before stocking, and fish fry must be used to test the water. Fry can be stocked only after confirming there are no problems.
V. Comparison of various pond-clearing drugs
In the process of using quicklime, bleaching powder and tea seed meal to clear ponds, quicklime has a relatively broad range of killing wild trash fish and harmful organisms during pond clearing, and after pond clearing it easily forms a peak of plankton, which can promptly provide biological bait for the fry stocked in the pond, making it the first choice among pond-clearing drugs.
Although tea seed meal pond clearing helps the formation of a plankton peak in the pond water, its range of killing harmful organisms is relatively narrow, and it basically has no killing effect on pathogenic microorganisms or shrimp and crabs.
The range of bleaching powder in killing harmful organisms and wild trash fish is close to that of quicklime, but because it itself has a certain algae-killing effect, after pond clearing it is not conducive to the formation of a plankton peak.
Pyrethroid pond clearing is mainly used to kill harmful organisms such as aquatic insects in fry ponds. It itself has relatively large residues, and the drug itself also runs counter to the increasingly advocated concepts of environmental protection and healthy aquaculture. Therefore, in the process of clearing shrimp and crab culture ponds where pyrethroids are not suitable for pond clearing, it is recommended to use quicklime for pond clearing with water to kill harmful organisms in the pond water, and not to add water before stocking, so as to avoid the entry of harmful organisms such as aquatic insects.
Reposted from the official website of Daya Agriculture