Key Technical Points for Water Quality Regulation in River Crab (Chinese Mitten Crab) Farming in Southern Jiangsu
A Brief Discussion on Water Quality Regulation in River Crab Farming in Southern Jiangsu
This post was last edited by fishfirst on 2009-11-27 09:23
□By Yang Xiuhua, Wuxi Lvshui Zhiyuan Biotechnology Co., Ltd.
The river crab is hailed as "a delicacy of the waters, a fine dish for banquets," and is deeply favored by consumers at home and abroad. Under the current new situation of developing high-quality, high-efficiency, high-yield fisheries, the river crab farming industry has shifted from "raising big crabs" in pursuit of yield to "raising large crabs" and "raising eco-crabs" centered on quality and efficiency. Small-sized, poor-quality crabs are increasingly unwelcome to consumers, while large-sized, eco-friendly crabs not only sell well but also command an absolute price advantage. However, China's river crab farming techniques are still in an exploratory stage, progressing from rough to refined, and many production technical challenges remain to be studied and solved.
As the saying goes, "To raise fish, first raise water." This applies equally to raising crabs: a pond of good water raises a pond of good crabs. Adult crab farming is mainly carried out in freshwater, and the period from the beginning of each year to the end of August is the critical period for adult crab farming. During this period, proper water quality regulation is the key to annual yield. In July of this year, the author participated in after-sales service in Gaochun, Nanjing, and took part in production work at some farming ponds. The following is a summary of water quality regulation for adult crab farming, which it is hoped will be helpful to crab farmers in future production.
I. Regulation of Conventional Physical and Chemical Water Quality Indicators
1. pH value. The pH value of pond water is an important indicator for judging water quality. It generally needs to be controlled between 7.5-8.5. If too high or too low, it is directly harmful to aquaculture and may even cause death. When the pH value is below 7.0, it weakens the oxygen-carrying capacity of the blood of aquatic animals, causing physiological hypoxia in crabs. Quicklime can be used to splash the whole pond regularly to raise the pH value of the water body, but care should be taken to avoid splashing during the molting period. The pH value of crab ponds will rise because algae carry out photosynthesis and consume large amounts of carbon dioxide, causing bicarbonates in the water to decompose, producing carbon dioxide and hydroxide ions, consuming acidic substances, and causing the pH value to rise. A pH value above 9.0 may cause corrosion of the gills of river crabs, causing them to lose respiratory capacity and die, and as the pH value rises, the toxicity of ammonia nitrogen also increases. To lower the pH value of the water body, first observe the water color, control the amount of algae, and promptly remove organic matter. When the pH value is too high, it is recommended to use 2-3 jin of acetic acid per mu of water surface at 1 m depth, or alum for adjustment.
2. Dissolved oxygen. Maintaining sufficient dissolved oxygen in the water can inhibit chemical reactions that produce toxic substances and transform and reduce the content of toxic substances (such as ammonia, nitrite and hydrogen sulfide). The dissolved oxygen required by crabs is 5-8 mg/L, with a minimum above 4 mg/L. Although mild hypoxia will not necessarily kill river crabs, it can cause restlessness, accelerated respiration and slowed growth; at 2.5 mg/L they begin to surface for air, and at 1.5 mg/L suffocation and death occur. When oxygen is deficient, oxygen-increasing agents need to be applied in a timely manner. During crab farming, excessively high dissolved oxygen generally does not occur, and excessively high dissolved oxygen has little effect on bottom-dwelling crabs.
3. Ammonia nitrogen. This includes the total amount of non-ionic ammonia and ionic ammonia. Almost all algae can rapidly and preferentially utilize them. The disadvantage is that nitrification consumes dissolved oxygen, and non-ionic ammonia in particular has strong toxicity to crabs; even at very low concentrations it can damage gill tissue. Generally, the ammonia nitrogen concentration in ponds should not exceed 0.1 mg/L. Below 0.05 mg/L indicates that the water is relatively lean and needs timely fertilization to cultivate high-quality biological bait; above 0.2 mg/L, biological preparations such as Bacillus can be used to reduce ammonia nitrogen content.
4. Nitrite. Nitrite is generally an intermediate product produced during the conversion of ammonia nitrogen to nitrate. When the conversion of ammonia nitrogen to nitrate is blocked, the intermediate product nitrite will be produced in the water body. The presence of nitrite causes the ferrous protein in the blood of river crabs to be oxidized into methemoglobin, thereby inhibiting the oxygen-carrying capacity of the blood. Long-term exposure to a high-nitrite environment affects their feeding and growth. It generally needs to be controlled below 0.1 mg/L, and water bodies with excessive levels must promptly use nitrifying bacteria-type degrading substances.
5. Hydrogen sulfide. Hydrogen sulfide is more toxic to crabs than to conventional fish. Hydrogen sulfide is generally produced at the bottom of ponds, and the bottom is precisely the habitat of crabs. Therefore, crab farming must strictly control the content of hydrogen sulfide, requiring its content to be below 0.02 mg/L.
II. Fertilizing the Water
River crab stocking requires fertilized water for pond entry, and during the farming process a certain fertility must be maintained. One must never raise crabs in lean water. This is very important for increasing yield and market size. The advantages of raising crabs in fertilized water are as follows: (1) Fertilized water can increase the number of benthic animals. Through fertilization, natural bait in natural crab ponds can also be increased, so that river crabs can find food everywhere, eat their fill, and eat well. (2) Fertilized water can promote the growth and reproduction of plankton, improve the photosynthetic capacity of the water body, provide sufficient dissolved oxygen for the water body, promote the molting and growth of river crabs, and especially increase the growth multiple of river crabs after molting. (3) Fertilized water can promote the reproduction of plankton, effectively purify the water body, and reduce the content and harm of harmful substances.
1. Conditions for fertilizing water
(1) When the water is clear and transparency is too high, fertilization should be carried out appropriately. Compound fertilizer and base fertilizer should be sufficient; inorganic fertilizer should be moderate, and efforts should be made to promote the use of monocalcium phosphate. (2) After heavy rain, when the water is whitish and turbid, it may be due to the influence of pests. It is recommended to kill the pests first, then fertilize and regulate the water. (3) When aquatic plants are not vigorous and turn yellow and withered, it is recommended to use some fertilizer that strengthens roots and protects grass, so that the withered grass can regain vitality and provide a better living place for crabs. (4) Topdressing should be flexibly determined according to water quality conditions, following the principle of "small amounts, many times."
2. Analysis of reasons why water is difficult to fertilize. The reasons why water is difficult to fertilize despite repeated fertilization may include the following: First, the amount of base fertilizer applied is insufficient, and the nitrogen, phosphorus and other elements needed for algae growth are insufficient. Among nitrogen and phosphorus elements, the effective content of phosphorus is especially important, while nitrogen is relatively abundant in the water body. However, phosphorus in the water body is easily adsorbed and precipitated by other substances and becomes ineffective, thus causing an imbalance in the nitrogen-to-phosphorus ratio, so that the effective nitrogen in the water cannot be fully utilized. At low temperatures, the content of diatoms in the water body is particularly high. Therefore, in low-temperature seasons, when the water temperature is below 15°C, consideration should be given to strengthening the application of effective silicon. Other nutrient salts needed by phytoplankton are generally not very lacking in water and may be disregarded. It is recommended to use organic fertilizer as base fertilizer.
Second, biochemical factors exceed standards. When the pH value is on the high side (greater than 9), it is generally very difficult to fertilize the water. The bottom can first be improved, and then some acidic biological preparations can be used to regulate the water and stabilize the pH value; when there are many types of zooplankton in the water body (rotifers, cladocerans, copepods, etc.), pests can first be killed and then biological water-fertilizing preparations used to fertilize the water; when there is a lot of residual chlorine in the pond, quicklime can first be used for neutralization and then biological water-fertilizing preparations used; when there is a lot of heavy metal residue, it is also difficult to fertilize the water, and detoxification can first be carried out, followed by biological water-fertilizing preparations and fertilizers.
Third, improper fertilization. An inappropriate fertilization ratio will lead to two situations: the water becomes too fertile or cannot be fertilized. The correct fertilization ratio is: nitrogen-to-phosphorus ratio of (3-4):1. An unreasonable fertilization sequence will also make it impossible to fertilize the water. The correct sequence is to apply phosphorus fertilizer first, then nitrogen fertilizer. After applying phosphorus fertilizer, quicklime should not be added.
Finally, a lack of algae species in the water body also makes it difficult to fertilize the water. New water should first be added to a depth of 10-20 cm, and then biological fertilizer containing algae species should be used. If too many snails are added at one time, they will consume too much microorganisms in the water, making the water clear and lean. Snails can be added in batches. For newly opened ponds, base fertilizer should be used first, then biological fertilizer containing algae species.
III. Treatment of Moss
Moss is a type of filamentous algae. It has strong reproductive capacity and causes great harm. When it reproduces in large amounts, it absorbs large amounts of nutrients, making the water body clear and lean. When it dies in large amounts, it releases large amounts of toxins, easily causing hypoxia and harming river crabs. Therefore, when moss occurs in a pond, it should be treated promptly. Some farmers have used copper sulfate for treatment. Although the moss was killed, it caused casualties among aquatic plants and crabs, with more loss than gain. At present, there is still a gap in the market for drugs that can truly kill only moss without causing casualties to aquatic plants and crabs. There are only drugs such as polymeric copper and chelated copper, which are somewhat better than copper sulfate, but still cause relatively large casualties to aquatic plants and crabs.
The author's experience is: the growth of moss requires sunlight, and moss lives at the bottom of the water. By fertilizing the water, increasing water fertility, reducing transparency, and weakening light, while also applying bottom-improving substances that sink to the bottom and cover the surface of the moss, directly reducing its light exposure and eliminating photosynthesis, the moss can be killed while the corpses are decomposed. Two farmers once tried this on ponds of 30 mu each, with good results.
IV. Algal Blooms
"Algal bloom" is a natural phenomenon in freshwater, caused by the explosive reproduction of blue-green algae. When an "algal bloom" breaks out, the water generally appears blue or green. The main cause of "algal blooms" is eutrophication of the water body. In Jiangsu, Taihu Lake has become a headache for the government precisely because of blue-green algae outbreaks.
When blue-green algae break out, secondary metabolism produces microcystins, which are toxic to crabs. After algae die in large amounts, the process of decay and decomposition also consumes dissolved oxygen in the water, making the water body seriously malodorous. Dead algae also damage the liver through the food chain, have carcinogenic effects, and directly endanger human health.
When blue-green algae occur in crab ponds, drugs such as chelated copper can first be used to kill the blue-green algae. After 3 hours, sodium thiosulfate can be splashed to detoxify, while Bacillus can also be used. This method can effectively resolve blue-green algae outbreaks, but care must be taken not to cause hypoxia. Because the cost is relatively high, ordinary farmers, especially those with large water areas, cannot accept it.
V. Scientific Feeding Management
Feed quality is the material guarantee for raising large, good crabs. In March, when the water temperature reaches above 10°C, start feeding bait to crabs. The bait mainly includes animal carcasses and their slaughter offal, mussel meat, snail meat, small shrimp, etc., as well as wheat, grain, cakes, and tender aquatic or terrestrial grasses. Feeding must adhere to the principle of a reasonable combination of fine, green and coarse feed. From April to May, aquatic plants in the pond sprout, biological bait multiplies, and the newly stocked crab seeds are small, the water temperature is low, and the feeding intensity of river crabs is not high, so less animal feed can be fed and more green and coarse feed can be given. From June to October, river crabs gradually grow larger, the water temperature rises, and the feeding intensity of river crabs is high. Especially from August to October, river crabs fatten and accumulate roe, and body weight increases significantly, so more animal feed should be fed. Feed once each day around evening, placing the feed in shallow water at the pond edge, with multiple feeding platforms set up. Each feeding amount should be about 10-20% of the total weight of crabs, and should be appropriately increased or decreased according to season, water temperature, weather, feeding and growth conditions.
In addition, the following links should also be noted in actual operation: (1) Around Qingming, more than 300 kg of fresh live snails, clams, mussels, etc. should be put in per mu as the base bait for the whole year. (2) Feeding should achieve fixed time, fixed location, fixed quality and fixed quantity. (3) When manually preparing mixed feed, it is best to make the feed into noodle-like or ball-like shapes that are convenient for river crabs to eat. (4) In the early and middle stages of river crab growth, mainly feed wheat, pumpkin, grain, cakes, etc. During this period, it is advocated to feed malt (feeding malt can ensure it will not rot and deteriorate for 1-2 days), and other feed should preferably be cooked before feeding. (5) In its lifetime, the river crab must undergo multiple molts. The increase in body size, changes in morphology, and regeneration of broken limbs all form after each molt. Molting runs through the entire growth and life process of the river crab. Therefore, molting hormone should be appropriately fed at ordinary times, and the feeding time of molting hormone and feed should be appropriately separated; in addition, during the molting period of river crabs, new water should be frequently injected and exchanged, maintaining slight flowing water, which can effectively stimulate river crab molting. (6) Throughout the entire farming cycle, quicklime should also be splashed appropriately. On the one hand, it purifies water quality; on the other hand, it increases the calcium content in the water body in preparation for multiple molts.
VI. Scientific Regulation of Water Quality
Healthy crab farming has very high requirements for the quality of the water environment, and water quality must be scientifically regulated. The water quality indicators require transparency of 35-40 cm, dissolved oxygen above 5 mg/L, pH value of 7.5-8.5, and ammonia nitrogen concentration of 0.2-0.5 mg/L. The above water quality indicators can be clinically tested by means such as water quality test kits, so as to take some measures (mainly simulating the natural ecological environment) to carry out necessary, beneficial and timely regulation of the water environment.
At the end of February and the beginning of March, during the initial stocking period of crab seeds, the water depth should be controlled at 0.6-0.8 m; from May to July, the water depth should be 0.8-1.0 m; from August to October, the water depth should be controlled between 1.0-1.2 m. In spring and autumn, change the water once a week, deepening by 30-40 cm each time. When changing water, drain and fill at the same time, and the temperature difference of the exchanged water should not exceed ±5°C. If there is a lot of silt at the bottom of the pond, low transparency, darker water color, deep black carapace on the river crab's back, water rust appearing on the abdomen, and yellowing at the ends of the walking legs, use 7.5-15 kg of fresh quicklime per mu, dissolve it into water and splash the whole pond, continuously for 2-3 times, with an interval of 5-7 days each time. This can regulate water quality, increase the calcium concentration in the water, and benefit the growth of river crabs and changes in crab shell color.
At the same time, the following precautions should also be noted: (1) Change water in a timely and appropriate amount to improve the environment and lower the temperature. The suitable molting water temperature for river crabs is 25-28°C. (2) Quicklime should not be used excessively, heavily or frequently. If used frequently, water hardness will be relatively high, dissolved oxygen will decrease, and phosphate concentration will decrease. (3) Maintain a certain fertility of the pond water to promote the reproduction and growth of bait organisms such as snails, clams and mussels. (4) Implement crab-fish polyculture. Crab-fish polyculture can fully utilize the water body space, improve feed utilization, purify water quality, and effectively improve pond water quality conditions. However, crab farming should be the main focus, and a small number of silver carp, bighead carp, grass carp, bream and tilapia may be appropriately stocked, but bottom-dwelling carnivorous fish such as common carp and black carp must not be stocked. Attention should be paid to preventing escape.
The crab market situation is not very stable, but as long as scientific and technological crab farming is strengthened, variety quality is improved, water areas are rationally utilized, comprehensive benefits are increased, and market demand is met, the market prospects are still very broad!
□By Yang Xiuhua, Wuxi Lvshui Zhiyuan Biotechnology Co., Ltd.
The river crab is hailed as "a delicacy of the waters, a fine dish for banquets," and is deeply favored by consumers at home and abroad. Under the current new situation of developing high-quality, high-efficiency, high-yield fisheries, the river crab farming industry has shifted from "raising big crabs" in pursuit of yield to "raising large crabs" and "raising eco-crabs" centered on quality and efficiency. Small-sized, poor-quality crabs are increasingly unwelcome to consumers, while large-sized, eco-friendly crabs not only sell well but also command an absolute price advantage. However, China's river crab farming techniques are still in an exploratory stage, progressing from rough to refined, and many production technical challenges remain to be studied and solved.
As the saying goes, "To raise fish, first raise water." This applies equally to raising crabs: a pond of good water raises a pond of good crabs. Adult crab farming is mainly carried out in freshwater, and the period from the beginning of each year to the end of August is the critical period for adult crab farming. During this period, proper water quality regulation is the key to annual yield. In July of this year, the author participated in after-sales service in Gaochun, Nanjing, and took part in production work at some farming ponds. The following is a summary of water quality regulation for adult crab farming, which it is hoped will be helpful to crab farmers in future production.
I. Regulation of Conventional Physical and Chemical Water Quality Indicators
1. pH value. The pH value of pond water is an important indicator for judging water quality. It generally needs to be controlled between 7.5-8.5. If too high or too low, it is directly harmful to aquaculture and may even cause death. When the pH value is below 7.0, it weakens the oxygen-carrying capacity of the blood of aquatic animals, causing physiological hypoxia in crabs. Quicklime can be used to splash the whole pond regularly to raise the pH value of the water body, but care should be taken to avoid splashing during the molting period. The pH value of crab ponds will rise because algae carry out photosynthesis and consume large amounts of carbon dioxide, causing bicarbonates in the water to decompose, producing carbon dioxide and hydroxide ions, consuming acidic substances, and causing the pH value to rise. A pH value above 9.0 may cause corrosion of the gills of river crabs, causing them to lose respiratory capacity and die, and as the pH value rises, the toxicity of ammonia nitrogen also increases. To lower the pH value of the water body, first observe the water color, control the amount of algae, and promptly remove organic matter. When the pH value is too high, it is recommended to use 2-3 jin of acetic acid per mu of water surface at 1 m depth, or alum for adjustment.
2. Dissolved oxygen. Maintaining sufficient dissolved oxygen in the water can inhibit chemical reactions that produce toxic substances and transform and reduce the content of toxic substances (such as ammonia, nitrite and hydrogen sulfide). The dissolved oxygen required by crabs is 5-8 mg/L, with a minimum above 4 mg/L. Although mild hypoxia will not necessarily kill river crabs, it can cause restlessness, accelerated respiration and slowed growth; at 2.5 mg/L they begin to surface for air, and at 1.5 mg/L suffocation and death occur. When oxygen is deficient, oxygen-increasing agents need to be applied in a timely manner. During crab farming, excessively high dissolved oxygen generally does not occur, and excessively high dissolved oxygen has little effect on bottom-dwelling crabs.
3. Ammonia nitrogen. This includes the total amount of non-ionic ammonia and ionic ammonia. Almost all algae can rapidly and preferentially utilize them. The disadvantage is that nitrification consumes dissolved oxygen, and non-ionic ammonia in particular has strong toxicity to crabs; even at very low concentrations it can damage gill tissue. Generally, the ammonia nitrogen concentration in ponds should not exceed 0.1 mg/L. Below 0.05 mg/L indicates that the water is relatively lean and needs timely fertilization to cultivate high-quality biological bait; above 0.2 mg/L, biological preparations such as Bacillus can be used to reduce ammonia nitrogen content.
4. Nitrite. Nitrite is generally an intermediate product produced during the conversion of ammonia nitrogen to nitrate. When the conversion of ammonia nitrogen to nitrate is blocked, the intermediate product nitrite will be produced in the water body. The presence of nitrite causes the ferrous protein in the blood of river crabs to be oxidized into methemoglobin, thereby inhibiting the oxygen-carrying capacity of the blood. Long-term exposure to a high-nitrite environment affects their feeding and growth. It generally needs to be controlled below 0.1 mg/L, and water bodies with excessive levels must promptly use nitrifying bacteria-type degrading substances.
5. Hydrogen sulfide. Hydrogen sulfide is more toxic to crabs than to conventional fish. Hydrogen sulfide is generally produced at the bottom of ponds, and the bottom is precisely the habitat of crabs. Therefore, crab farming must strictly control the content of hydrogen sulfide, requiring its content to be below 0.02 mg/L.
II. Fertilizing the Water
River crab stocking requires fertilized water for pond entry, and during the farming process a certain fertility must be maintained. One must never raise crabs in lean water. This is very important for increasing yield and market size. The advantages of raising crabs in fertilized water are as follows: (1) Fertilized water can increase the number of benthic animals. Through fertilization, natural bait in natural crab ponds can also be increased, so that river crabs can find food everywhere, eat their fill, and eat well. (2) Fertilized water can promote the growth and reproduction of plankton, improve the photosynthetic capacity of the water body, provide sufficient dissolved oxygen for the water body, promote the molting and growth of river crabs, and especially increase the growth multiple of river crabs after molting. (3) Fertilized water can promote the reproduction of plankton, effectively purify the water body, and reduce the content and harm of harmful substances.
1. Conditions for fertilizing water
(1) When the water is clear and transparency is too high, fertilization should be carried out appropriately. Compound fertilizer and base fertilizer should be sufficient; inorganic fertilizer should be moderate, and efforts should be made to promote the use of monocalcium phosphate. (2) After heavy rain, when the water is whitish and turbid, it may be due to the influence of pests. It is recommended to kill the pests first, then fertilize and regulate the water. (3) When aquatic plants are not vigorous and turn yellow and withered, it is recommended to use some fertilizer that strengthens roots and protects grass, so that the withered grass can regain vitality and provide a better living place for crabs. (4) Topdressing should be flexibly determined according to water quality conditions, following the principle of "small amounts, many times."
2. Analysis of reasons why water is difficult to fertilize. The reasons why water is difficult to fertilize despite repeated fertilization may include the following: First, the amount of base fertilizer applied is insufficient, and the nitrogen, phosphorus and other elements needed for algae growth are insufficient. Among nitrogen and phosphorus elements, the effective content of phosphorus is especially important, while nitrogen is relatively abundant in the water body. However, phosphorus in the water body is easily adsorbed and precipitated by other substances and becomes ineffective, thus causing an imbalance in the nitrogen-to-phosphorus ratio, so that the effective nitrogen in the water cannot be fully utilized. At low temperatures, the content of diatoms in the water body is particularly high. Therefore, in low-temperature seasons, when the water temperature is below 15°C, consideration should be given to strengthening the application of effective silicon. Other nutrient salts needed by phytoplankton are generally not very lacking in water and may be disregarded. It is recommended to use organic fertilizer as base fertilizer.
Second, biochemical factors exceed standards. When the pH value is on the high side (greater than 9), it is generally very difficult to fertilize the water. The bottom can first be improved, and then some acidic biological preparations can be used to regulate the water and stabilize the pH value; when there are many types of zooplankton in the water body (rotifers, cladocerans, copepods, etc.), pests can first be killed and then biological water-fertilizing preparations used to fertilize the water; when there is a lot of residual chlorine in the pond, quicklime can first be used for neutralization and then biological water-fertilizing preparations used; when there is a lot of heavy metal residue, it is also difficult to fertilize the water, and detoxification can first be carried out, followed by biological water-fertilizing preparations and fertilizers.
Third, improper fertilization. An inappropriate fertilization ratio will lead to two situations: the water becomes too fertile or cannot be fertilized. The correct fertilization ratio is: nitrogen-to-phosphorus ratio of (3-4):1. An unreasonable fertilization sequence will also make it impossible to fertilize the water. The correct sequence is to apply phosphorus fertilizer first, then nitrogen fertilizer. After applying phosphorus fertilizer, quicklime should not be added.
Finally, a lack of algae species in the water body also makes it difficult to fertilize the water. New water should first be added to a depth of 10-20 cm, and then biological fertilizer containing algae species should be used. If too many snails are added at one time, they will consume too much microorganisms in the water, making the water clear and lean. Snails can be added in batches. For newly opened ponds, base fertilizer should be used first, then biological fertilizer containing algae species.
III. Treatment of Moss
Moss is a type of filamentous algae. It has strong reproductive capacity and causes great harm. When it reproduces in large amounts, it absorbs large amounts of nutrients, making the water body clear and lean. When it dies in large amounts, it releases large amounts of toxins, easily causing hypoxia and harming river crabs. Therefore, when moss occurs in a pond, it should be treated promptly. Some farmers have used copper sulfate for treatment. Although the moss was killed, it caused casualties among aquatic plants and crabs, with more loss than gain. At present, there is still a gap in the market for drugs that can truly kill only moss without causing casualties to aquatic plants and crabs. There are only drugs such as polymeric copper and chelated copper, which are somewhat better than copper sulfate, but still cause relatively large casualties to aquatic plants and crabs.
The author's experience is: the growth of moss requires sunlight, and moss lives at the bottom of the water. By fertilizing the water, increasing water fertility, reducing transparency, and weakening light, while also applying bottom-improving substances that sink to the bottom and cover the surface of the moss, directly reducing its light exposure and eliminating photosynthesis, the moss can be killed while the corpses are decomposed. Two farmers once tried this on ponds of 30 mu each, with good results.
IV. Algal Blooms
"Algal bloom" is a natural phenomenon in freshwater, caused by the explosive reproduction of blue-green algae. When an "algal bloom" breaks out, the water generally appears blue or green. The main cause of "algal blooms" is eutrophication of the water body. In Jiangsu, Taihu Lake has become a headache for the government precisely because of blue-green algae outbreaks.
When blue-green algae break out, secondary metabolism produces microcystins, which are toxic to crabs. After algae die in large amounts, the process of decay and decomposition also consumes dissolved oxygen in the water, making the water body seriously malodorous. Dead algae also damage the liver through the food chain, have carcinogenic effects, and directly endanger human health.
When blue-green algae occur in crab ponds, drugs such as chelated copper can first be used to kill the blue-green algae. After 3 hours, sodium thiosulfate can be splashed to detoxify, while Bacillus can also be used. This method can effectively resolve blue-green algae outbreaks, but care must be taken not to cause hypoxia. Because the cost is relatively high, ordinary farmers, especially those with large water areas, cannot accept it.
V. Scientific Feeding Management
Feed quality is the material guarantee for raising large, good crabs. In March, when the water temperature reaches above 10°C, start feeding bait to crabs. The bait mainly includes animal carcasses and their slaughter offal, mussel meat, snail meat, small shrimp, etc., as well as wheat, grain, cakes, and tender aquatic or terrestrial grasses. Feeding must adhere to the principle of a reasonable combination of fine, green and coarse feed. From April to May, aquatic plants in the pond sprout, biological bait multiplies, and the newly stocked crab seeds are small, the water temperature is low, and the feeding intensity of river crabs is not high, so less animal feed can be fed and more green and coarse feed can be given. From June to October, river crabs gradually grow larger, the water temperature rises, and the feeding intensity of river crabs is high. Especially from August to October, river crabs fatten and accumulate roe, and body weight increases significantly, so more animal feed should be fed. Feed once each day around evening, placing the feed in shallow water at the pond edge, with multiple feeding platforms set up. Each feeding amount should be about 10-20% of the total weight of crabs, and should be appropriately increased or decreased according to season, water temperature, weather, feeding and growth conditions.
In addition, the following links should also be noted in actual operation: (1) Around Qingming, more than 300 kg of fresh live snails, clams, mussels, etc. should be put in per mu as the base bait for the whole year. (2) Feeding should achieve fixed time, fixed location, fixed quality and fixed quantity. (3) When manually preparing mixed feed, it is best to make the feed into noodle-like or ball-like shapes that are convenient for river crabs to eat. (4) In the early and middle stages of river crab growth, mainly feed wheat, pumpkin, grain, cakes, etc. During this period, it is advocated to feed malt (feeding malt can ensure it will not rot and deteriorate for 1-2 days), and other feed should preferably be cooked before feeding. (5) In its lifetime, the river crab must undergo multiple molts. The increase in body size, changes in morphology, and regeneration of broken limbs all form after each molt. Molting runs through the entire growth and life process of the river crab. Therefore, molting hormone should be appropriately fed at ordinary times, and the feeding time of molting hormone and feed should be appropriately separated; in addition, during the molting period of river crabs, new water should be frequently injected and exchanged, maintaining slight flowing water, which can effectively stimulate river crab molting. (6) Throughout the entire farming cycle, quicklime should also be splashed appropriately. On the one hand, it purifies water quality; on the other hand, it increases the calcium content in the water body in preparation for multiple molts.
VI. Scientific Regulation of Water Quality
Healthy crab farming has very high requirements for the quality of the water environment, and water quality must be scientifically regulated. The water quality indicators require transparency of 35-40 cm, dissolved oxygen above 5 mg/L, pH value of 7.5-8.5, and ammonia nitrogen concentration of 0.2-0.5 mg/L. The above water quality indicators can be clinically tested by means such as water quality test kits, so as to take some measures (mainly simulating the natural ecological environment) to carry out necessary, beneficial and timely regulation of the water environment.
At the end of February and the beginning of March, during the initial stocking period of crab seeds, the water depth should be controlled at 0.6-0.8 m; from May to July, the water depth should be 0.8-1.0 m; from August to October, the water depth should be controlled between 1.0-1.2 m. In spring and autumn, change the water once a week, deepening by 30-40 cm each time. When changing water, drain and fill at the same time, and the temperature difference of the exchanged water should not exceed ±5°C. If there is a lot of silt at the bottom of the pond, low transparency, darker water color, deep black carapace on the river crab's back, water rust appearing on the abdomen, and yellowing at the ends of the walking legs, use 7.5-15 kg of fresh quicklime per mu, dissolve it into water and splash the whole pond, continuously for 2-3 times, with an interval of 5-7 days each time. This can regulate water quality, increase the calcium concentration in the water, and benefit the growth of river crabs and changes in crab shell color.
At the same time, the following precautions should also be noted: (1) Change water in a timely and appropriate amount to improve the environment and lower the temperature. The suitable molting water temperature for river crabs is 25-28°C. (2) Quicklime should not be used excessively, heavily or frequently. If used frequently, water hardness will be relatively high, dissolved oxygen will decrease, and phosphate concentration will decrease. (3) Maintain a certain fertility of the pond water to promote the reproduction and growth of bait organisms such as snails, clams and mussels. (4) Implement crab-fish polyculture. Crab-fish polyculture can fully utilize the water body space, improve feed utilization, purify water quality, and effectively improve pond water quality conditions. However, crab farming should be the main focus, and a small number of silver carp, bighead carp, grass carp, bream and tilapia may be appropriately stocked, but bottom-dwelling carnivorous fish such as common carp and black carp must not be stocked. Attention should be paid to preventing escape.
The crab market situation is not very stable, but as long as scientific and technological crab farming is strengthened, variety quality is improved, water areas are rationally utilized, comprehensive benefits are increased, and market demand is met, the market prospects are still very broad!