Detailed Explanation of Key Technical Points for Pond Culture of Mud Crab (Scylla serrata)

Introduction

The scientific name of the mud crab is Scylla serrata, commonly known as "chong xun" or "you mou." It belongs to the phylum Arthropoda, class Crustacea, order Decapoda, suborder Reptantia, section Brachyura, family Portunidae, genus Scylla, and is one of China's precious aquatic products. Its meat is delicious and unique, and it is extremely rich in nutrients. It has long been acclaimed as a delicacy at banquets, with high edible and medicinal value. In particular, the female crab with mature gonads after mating (red roe crab, high crab) enjoys the reputation of "ginseng of the sea" and is a premium tonic for postpartum women, the elderly, the young, and those with weak constitutions. The crab shell can be processed into chitin, which is a widely used industrial raw material. In China, the mud crab is mainly distributed along the coasts of Guangdong, Guangxi, Fujian, Taiwan, China, and Zhejiang provinces, with Zhejiang, Fujian, and Guangdong being the most abundant. Abroad, it is mainly distributed in the coastal waters of Japan, Vietnam, Thailand, the Philippines, New Zealand, the United States, the Indian Ocean, the Red Sea, and South Africa. The mud crab prefers to inhabit and live in estuaries where rivers and seas converge, in inner bays with buffered exchange between seawater and freshwater - intertidal mudflats and muddy-sandy flats. The mud crab is a swimming, crawling, and burrowing crab species. It generally hides in burrows during the day and emerges at night to forage in all directions. The mud crab is primarily carnivorous, preferring to seek out small miscellaneous bivalves, small snails, small fish, small shrimp, and small crabs. The mud crab is also a euryhaline marine crab. The suitable salinity for its culture is 7‰ to 33‰, with the optimum being 10‰ to 20‰. The suitable temperature range is 6°C to 35°C, with the optimum growth water temperature being 18°C to 25°C. At this time, the mud crab has strong activity and a robust appetite. It also has extremely strong resistance to aerial exposure. After a healthy mud crab leaves the water, as long as a very small amount of water remains in the gill chamber to keep the gill filaments moist, it can survive for several days. The mud crab undergoes 13 molts throughout its life (including 6 molts during larval metamorphosis, 6 growth molts, and 1 reproductive molt). In summary, the metamorphic development and the entire growth and life process of the mud crab are always accompanied by molting. Generally, juvenile crabs at stages 6 to 8 stocked in spring (April to May) can reach a body weight of 200 g to 250 g per crab after three and a half to four months of culture by summer (July to August). If juvenile crabs are stocked in autumn (September to October), they need to overwinter and be cultured until May to June of the following year to reach commercial specifications. Based on many years of production practice along the coasts nationwide, the adopted culture production methods have become increasingly diversified, including single-species culture in constructed crab ponds, mixed culture in enclosed ponds, low-dam high-net culture on tidal flats, hanging cage culture on stakes and poles, and cement pond culture, all of which have achieved good economic and social benefits to varying degrees. Here, only the pond culture techniques for mud crab are described. Crab Pond Construction Editor 1. Site Selection Mud crab culture ponds should be built in areas with no industrial or agricultural wastewater, no toxic substances, no pesticide or domestic sewage pollution, with fresh seawater, strong water exchange and self-purification capacity, stable physicochemical factors, large tidal range, calm inner bay waters, and a certain amount of land-derived freshwater inflow, in the middle-high tidal zone, with seawater specific gravity between 1.007 and 1.022, pH between 7.8 and 8.6, water temperature within 10°C to 30°C, transparency of 20 cm to 30 cm, chemical oxygen demand of 4 mg/L to 17.6 mg/L, and copper ion concentration not exceeding 0.488 mg/L. 2. Pond Bottom Substrate The soil quality of the crab pond tidal flat land affects both the burrowing and habitation of the mud crab and directly influences the firmness of the pond embankment. Therefore, homogeneous soil flats that retain water, do not seep, have high cohesive stability, and are not prone to collapse should be selected. 3. Pond Area The pond area for single-species mud crab culture is preferably 3 mu to 5 mu, and ponds for mixed shrimp-crab culture are more suitable at 10 mu to 20 mu. If the culture pond area is too large, it should be divided into several sub-areas using bamboo fences or blocking nets to facilitate graded (size-classified) culture of mud crabs of different sizes. 4. Pond Shape and Water Depth The pond shape is generally rectangular or square (the length-to-width ratio of rectangular ponds is 5:3 to 3:2, water depth 1 m to 1.5 m). The pond orientation is generally best with an east-west long axis and north-south short axis (but this is not mandatory). The pond bottom structure usually includes three types: flat bottom, slightly sloped bottom, and wok-shaped bottom. 5. Pond Flats and Pond Trenches Trenches should be excavated along the water flow direction at the pond bottom (central trench, ring trench, cross trench). The area ratio of flats to trenches is preferably about 3:1. The depth of trenches should generally be more than 0.5 m, the upper opening width of trenches should be 3 m to 6 m, the slope ratio of trench walls (sides) should be 1:2 to 1:2.5, and the main trench (central trench) should connect with the control sluice gate to facilitate convenient water intake and drainage regulation. A certain amount of open space should be reserved in the flats as "crab islands" for mud crabs to inhabit and hide, reducing mutual killing and injury. 6. Anti-Escape Facilities Anti-escape fencing must be installed along the inner side of the pond embankment perimeter (which can be made of thin cement boards, bamboo fences, hard plastic sheets, etc.). The height of the facilities should be more than 50 cm above the highest water level in the pond. The upper end of the facility boards and fence pieces should tilt slightly toward the pond interior, and the installation must be tight and firm to effectively prevent mud crabs from escaping. 7. Shelter Setup To prevent and reduce contact opportunities between crabs and allow them to molt and grow peacefully, some obstacles and shelters should be placed in the pond, such as inserting several rows of plum-blossom-pattern bamboo branches and tree branches with a spacing of 30 cm to 50 cm, and placing some ceramic pipes, cement culverts, bamboo tubes, brick and tile pieces, artificial caves, and other facilities as shelters. Setting up shelters can increase the space for mud crabs to hide and move about, achieve the purpose of dispersing and separating mud crab clusters to reduce encounters, and also allow mud crabs to freely choose suitable habitats in different seasons and growth stages. 8. Water Intake and Drainage Facilities The crab pond should be equipped with 1 to 2 adjustable control sluice gates to facilitate water exchange and counter-flow in the pond. To save construction costs, cement culvert pipe gate openings can also be installed. It is recommended that crab ponds with the means should design and install gates composed of upper, middle, and lower three-tier combination gates or upper and lower two-pivot block gates. This way, water can be exchanged layer by layer in a timely manner according to changes in the culture pond water body. However, for ponds with relatively high tidal levels, water pumps must be equipped to ensure convenient and safe water supply in different seasons and growth stages. Seedling Stocking Editor 1. Pond Clearing and Pest Elimination Pond clearing and pest elimination refers to removing all factors in the pond that are unfavorable to the growth and survival of mud crabs. The targets for removal include: organic sediments, organisms that prey on mud crabs, organisms that compete with mud crabs for food, organisms that damage pond facilities, and pathogenic organisms. Whether pond clearing and pest elimination is thorough is one of the important links related to whether healthy mud crab culture can be achieved and high yield and efficiency obtained, and it must be done conscientiously. (1) Silt Removal Before stocking, a comprehensive silt removal must be carried out in the pond, especially for old ponds. After the mud crabs are harvested, the silt and organic sediments should be promptly removed from the pond, and the gate should be opened to allow water in for repeated flushing. Then the accumulated water in the pond should be promptly drained, and the gate closed to expose the pond bottom to the sun, allowing residual organic matter to further undergo oxidative decomposition. While removing silt, pond maintenance work should also be done (embankments, gates, trench walls, anti-escape facilities, leak plugging, etc.). (2) Pest Elimination Within 15 to 20 days before stocking crab seedlings, chemical treatment should be carried out to eliminate harmful organisms in the pond. Commonly used chemicals include: Quicklime, which can not only kill miscellaneous fish, miscellaneous shrimp, parasites, and microorganisms, but also improve pond bottom substrate, increase calcium ion content in the water body, and promote smooth molting and growth of mud crabs. Its dosage is 400 g to 500 g per cubic meter of water body. After slaking with water, splash it evenly over the entire pond while hot or spread it dry. The chemical effect disappears after 10 days. Bleaching powder has a strong killing effect on protozoa and bacteria, so it can prevent diseases and also kill wild miscellaneous fish and other enemy organisms. The dosage is 40 g to 50 g per cubic meter of water body. Dilute with water, mix evenly, and splash over the entire pond. The chemical effect disappears after 1 to 2 days. Tea seed cake mainly kills fish and shellfish. The dosage is 20 g to 25 g per cubic meter of water body. Before use, crush the tea seed cake and soak it in freshwater for 24 hours. After diluting evenly, splash it together with water and residue. The chemical effect disappears after 2 to 3 days. Note that pond clearing with the above three chemicals should be carried out after 8 AM on sunny days to enhance efficacy. Try to drain the pond water as much as possible before pond clearing to save chemical dosage. For dead corners, pits, and inside caves of the crab pond, splash as evenly as possible. After the chemical effect of the pest-elimination pond-clearing chemicals disappears, seawater should be promptly introduced and the water depth maintained at 20 cm to 30 cm until 2 to 3 days before seedling release, when the water level in the pond should be raised to more than 1 m. 2. Crab Seedling Stocking (1) Crab Seedling Selection Currently, the sources of mud crab seedlings for culture mainly include two types: natural seedlings from sea areas and factory-based artificially bred seedlings (the technology for factory-based seedling production has not yet been fully popularized, and seedling sources are still scarce). Healthy seedlings should be selected that have strong bodies, hard shells, blue-green carapaces, uniform specifications, intact bodies without damage, complete ten legs (especially a pair of chelipeds and swimming legs must not be missing or damaged), quick responses, strong vitality, rapid crawling, and no pathological changes. (2) Seedling Specifications and Stocking Time The specifications of summer seedlings of the current year are generally juvenile crabs at stage III or above, which can be stocked in June to July. After 3 to 4 months of careful rearing, they can reach commercial edible crab requirements. The specifications of autumn seedlings of the current year are basically the same as summer seedlings of the current year, and can be stocked in September to October. After overwintering and being cultured until May to June of the following year, they can reach commercial specifications. Juvenile crab seedlings from late autumn of the previous year that have overwintered until this spring can be stocked from late April to mid-May. After more than 3 months of rearing, they reach commercial specifications by July to August. For the stocking of these three types of mud crab seedlings, if the stocking quantity at that time is insufficient, supplementary stocking can be done, but efforts should be made to keep crab seedling specifications consistent. (3) Stocking Density The stocking density (seedling release quantity) for mud crab grow-out should be reasonably determined and flexibly controlled according to the comprehensive actual conditions of each region (such as pond water depth and temperature conditions, water intake and exchange conditions, seedling resource conditions, bait resource conditions, production management level, and other factors). If the pond area for single-species mud crab culture is about 3 mu to 5 mu, the stocking density for grow-out in the current year is 1.5 crabs/sq m to 3 crabs/sq m, that is, 1000 to 1500 seedlings per mu (referring to summer seedlings). For ponds stocked with autumn seedlings, the stocking quantity can be appropriately increased, preferably 1500 to 2000 per mu. If the crab culture pond area is greater than 10 mu (including 10 mu), the seedling release quantity should be appropriately reduced, with summer seedling release of 800 to 1000 per mu. For shrimp-crab mixed culture ponds with an area of more than 20 mu, the crab seedling release quantity per mu should generally be controlled within 600 (referring to ponds mainly for mud crab culture with shrimp and fish as supplementary). Daily Management Editor 1. Feeding Techniques (1) Bait Types The mud crab is a carnivorous crab species, so carnivorous bait should be the main type, with plant-based bait as a supplement. Commonly used baits include small low-value bivalves such as red meat blue clams, Musculus senhousei, duck-bill clams, small blood clams, short-toothed clams, freshwater river clams, freshwater snails, as well as small miscellaneous fish, miscellaneous shrimp, and miscellaneous crabs. Some artificial compound feed for mud crabs can also be fed. There are many types of edible baits available, which can be selected according to local actual resource conditions. As long as they are non-toxic, harmless, and cheap baits that mud crabs like to forage for, they can all serve as guaranteed bait sources. However, the bait must be fresh. Moldy and spoiled bait must never be fed, otherwise it will seriously affect the healthy growth of mud crabs. (2) Feeding Ratio The feeding amount during the culture period should be flexibly controlled according to water temperature, tides, water quality, and the activity of mud crabs. For example, mud crabs feed voraciously when the water temperature is above 15°C, reaching a peak at 26°C. If the water temperature is above 30°C or below 13°C, their food intake significantly decreases. For example, along the Zhejiang coast (Wenzhou, Taizhou area), water temperatures in May to June and September to October are relatively suitable, and mud crab food intake increases, so more bait should be fed. During the high-temperature period of July to August, water temperature is relatively high, and before May and after October, water temperature is noticeably lower, so mud crab food intake weakens, and less bait should be fed. During spring tides or when the tide rises, mud crabs eat more; during neap tides or after the tide recedes, food intake is less. After water exchange during spring tides, water quality is good and food intake increases, so the feeding amount can be doubled compared to normal. If there is much rain, the pond water is turbid, or the weather is muggy, food intake decreases, and the feeding amount should be appropriately reduced. In cold weather, when the water temperature drops to around 10°C, mud crabs are less active or inactive, forage little or not at all, and attention should be paid to feeding less or not feeding. The food intake of mud crabs gradually increases with individual growth, but the ratio of daily food intake to their own body weight decreases as body weight increases. Generally, the relationship and percentage of daily feeding amount to individual mud crab size are: for crabs with a carapace width of 3 cm to 4 cm, the daily feeding amount is controlled at about 30% of body weight; at 5 cm to 6 cm, the daily feeding amount is about 20%; at 7 cm to 8 cm, the daily feeding amount is 15%; at 9 cm to 10 cm, the daily feeding amount is 10% to 12%; at above 11 cm, it is 5% to 8%. (3) Feeding Methods Bait should be evenly distributed around the perimeter of the crab pond and should not be placed in the center of the pond. Ponds with the means should preferably set up several feeding stations around the pond perimeter to better control the daily required feeding amount. Based on the mud crab's habit of hiding by day and emerging at night to forage, one-third of the daily feeding amount can be fed in the early morning and two-thirds in the evening. When the northwest wind rises around the Mid-Autumn Festival in August (referring to the lunar calendar), the temperature difference between the first and second halves of the night increases, and mud crabs become restless and prone to escape after midnight. It is best to appropriately add one extra feeding in the first half of the night, which can both prevent mud crabs from escaping and reduce mutual killing. In summary, feeding should be flexibly controlled in daily required amounts based on water quality conditions, weather changes, crab growth and molting conditions, and other factors. 2. Water Quality Management A good pond water quality environment is the basic guarantee for the growth and development of mud crabs. The mud crab must undergo multiple molts throughout its life to grow into an adult crab, and its molting activities all occur in the early morning or after midnight. If there is no fresh and good water quality or insufficient dissolved oxygen, mud crabs cannot successfully complete normal molting and growth, and may even die. Maintaining fresh and stable pond water with abundant dissolved oxygen makes molting easy for mud crabs and promotes fast growth, so water quality must be carefully managed. The basic content of good water quality management is water exchange and intake, water level control, regulation of water temperature and salinity, maintenance of suitable transparency, pH, and dissolved oxygen, and ensuring fresh and stable culture pond water. (1) Reasonable Water Exchange Water exchange is the most economical and effective way to improve the water quality environment. Daily water exchange should follow the principle of "small amount, high frequency," generally exchanging water once every 3 to 5 days, with more water exchange during spring tides, and the daily water exchange amount being 1/3 to 1/5 of the total pond water. During high-temperature periods, if the pond water quality is poor and the water outside the pond (open sea) is good, the water exchange amount should be increased to a ratio of 1/2 or 2/3, with specific adjustments made according to the actual conditions of the pond water. Note that the flow rate should not be too rapid and the flow volume should not be too large during water intake. In addition, when exchanging water, attention should be paid to understanding and monitoring changes in the external seawater quality. If the external seawater is found to be luminescent, highly viscous, have an odor, or contain red tide organisms, water should not be introduced. The normal water level should be maintained at around 1 m, and during high-temperature seasons, the water level can be raised to about 1.5 m to maintain the stability of the culture pond water temperature. (2) Salinity Adjustment When the seawater salinity in the pond is too low or too high, water should be exchanged promptly for adjustment. Especially during the rainy season with abundant freshwater or during typhoon and heavy rain storms, when the specific gravity of the pond seawater is too low, the upper gate board should be opened promptly to first discharge the low-specific-gravity water from the upper layer of the pond, and then intake high-specific-gravity seawater from the middle and lower layers at high tide. For ponds built on high flats where natural water intake is difficult, water pumps can be used to extract middle and lower layer seawater for adjustment. (3) Stabilizing Physicochemical Factors It is necessary to ensure that various physicochemical factors in the pond remain relatively stable. Before and after water exchange, the temperature and salinity of the seawater inside and outside the pond should be monitored at any time, especially before and after rainfall, to avoid excessive changes in temperature and salinity before and after water exchange. Pond water transparency should be controlled at 20 cm to 30 cm, and good water color (light yellow, yellow-green) and sufficient dissolved oxygen (greater than 3 mg/L) should be maintained, with pH controlled between 7.8 and 8.5. Once spoilage is found in the pond, it should be removed promptly. (4) Application of Water Quality Improvers Water quality improvers commonly used in mud crab culture mainly include photosynthetic bacteria and zeolite. Photosynthetic bacteria can effectively improve the pond ecological environment, prevent and reduce the occurrence of diseases, and greatly benefit the improvement of culture economic benefits. The method of use is to splash photosynthetic bacteria liquid at 15 ppm to 20 ppm over the entire pond 5 to 6 days before crab seedling stocking (bacterial liquid concentration of more than 1 billion per milliliter), and thereafter splash 6 ppm to 8 ppm every 10 to 15 days, which can be flexibly applied according to actual conditions. In order to timely understand and monitor the accurate production situation of the crab pond, in mud crab rearing management, in addition to strengthening scientific feeding and water quality regulation management, daily pond inspection work should also be done well. (1) Pond Inspection A sound daily morning and evening pond inspection system should be established and maintained. Inspection contents include gates, embankments, anti-escape facilities, water color, water level, mud crab foraging and living activity conditions, and whether diseased crabs appear around the pond perimeter. Especially during thunderstorms in midsummer and autumn, before thunderstorms, on windless and muggy evenings and before sunrise in the morning, after prolonged clear weather followed by heavy rain, and when pond water salinity changes abruptly, problems are most likely to occur, so pond inspection should be further strengthened to prevent mud crab escape and accidents. (2) Water Quality and Crab Growth Monitoring Persist in measuring the water temperature and specific gravity of the culture pond daily, especially after rain and before and after water exchange, with more frequent monitoring to timely grasp changes in water quality environmental factors. Throughout the entire mud crab culture period, randomly sample and measure the carapace width and body weight of mud crabs every 15 days. Through measurement, understand the growth of mud crabs in different seasons so as to timely adjust the feeding amount. 4. Overwintering Management Before overwintering, try to lower the water level in the pond to encourage autumn crabs to dig burrows on both sides of the middle and side trenches and at the pond bottom to overwinter. Before the arrival of cold air from the north, try to raise the water level to prevent autumn crabs from cold injury and affect overwintering survival rate. During the overwintering period, when the water temperature is below 10°C, there is no need to feed bait. When the water temperature is above 12°C, feed a small amount of bait. When the water temperature rises above 14°C, appropriately increase the feeding amount. For ponds with small areas and poor heat retention, shelters can be built with covers for overwintering. Disease Prevention and Control Editor Currently, research on the causes and prevention techniques of mud crab culture diseases is not yet thorough enough, so the principle of "prevention first, combining prevention and treatment, prevention more important than treatment, early prevention when no disease, early treatment when disease occurs" should still be adopted for prevention and control, trying to eliminate pathogenic factors. Starting from enhancing the constitution and self-resistance of mud crabs to reduce and alleviate the occurrence of diseases, achieve the goals of high quality, high yield, and high efficiency in mud crab culture. 1. Mud Crab Ciliate Disease and Filamentous Algae Attachment Syndrome This disease generally occurs in the larval stage of mud crabs and may also occur during the grow-out period. This disease has the characteristics of a long course and high cumulative mortality. (1) Symptoms of Diseased Crabs In the early stage of the disease, yellow-green and brown hair-like substances grow on the body surface. The crab moves slowly and responds sluggishly to external stimuli. The body surface feels slippery to the touch. Under a microscope, ciliate protozoa such as Vorticella and Epistylis, as well as green filamentous algae, can be observed. In the middle and late stages of the disease, the crab body is covered with thick attachments, causing gill filament damage and breathing difficulty, followed by secondary bacterial infection, leading to loss of appetite or even no feeding, stunted growth and development, weakened constitution and difficulty molting, with a mortality rate of about 20% to 30%. (2) Causes The main causes are excessive stocking density in the pond, excessive residual bait, severe pollution, and high organic matter content in the water, causing extreme eutrophication of the culture pond water, leading to massive reproduction of ciliates and filamentous algae. Especially during the high-temperature period from July to September each year, large numbers of ciliates and filamentous algae attach to the crab body, seriously affecting the normal growth and development of mud crabs. (3) Prevention and Treatment Methods ① Maintaining clean water quality is the most effective prevention and treatment method. Do regular water exchange, regulate water quality, promptly remove residual bait from the pond when draining, regularly splash chlorine preparations for disinfection, and regularly focus on monitoring ammonia nitrogen and pH. Once exceeding standards are found, immediately exchange water and splash 100 g/cu m to 150 g/cu m zeolite powder or 10 g/cu m to 20 g/cu m "Water Purification Treasure" over the entire pond. ② For affected ponds, zinc sulfate, copper sulfate, and ferrous sulfate (at a ratio of 8:5:2) can be mixed and splashed at a concentration of 1.2 ppm to 1.5 ppm for 3 consecutive days, and on the 4th day, splash "Rich Chlorine" at 0.3 ppm over the entire pond; or on the first day, use "Xianchongqing" at 1.2 ppm to 1.5 ppm for whole-pond splashing, and on the second day after water exchange, splash "Rich Chlorine" at 0.3 ppm over the entire pond. For severe cases, use once more after 7 to 10 days to kill the attachments on the crab body surface. After disinfection and killing of pests and diseases, mix antibiotics into the bait to make medicated feed and add 0.1% to 0.2% molting hormone, feeding continuously for 7 days as one treatment course, which can bring the disease under comprehensive and effective control. 2. Mud Crab Molting Failure Disease The occurrence of molting failure disease in mud crabs is one of the most serious cases during the entire mud crab grow-out period. This disease is mainly caused by sudden changes in the water environment and bacterial infection. (1) Symptoms This disease generally occurs mostly in autumn each year. When the first cold wave arrives, large numbers of mud crabs weighing about 150 g are susceptible to Vibrio infection, gradually deteriorating and dying during the process of being retained for continued culture without being harvested. The body fluid of diseased crabs is cloudy white, and the muscles at the base of segments are milky white (healthy crabs are azure blue). Breaking off the walking legs will exude white mucus, commonly known as "white awn disease." Affected ponds are highly prone to repeated infection, with a mortality rate of 30% to 80%, causing particularly serious impact on culture production. (2) Causes ① When the autumn water temperature is 22°C to 25°C, it is the most suitable condition for the reproduction of Vibrio and other pathogenic bacteria in the water body, while the decrease in water temperature reduces the vitality and disease resistance of mud crabs. Pathogenic bacterial infection is the fundamental cause of this disease. ② The disease is also related to the special physiological structure of mud crabs. In autumn, at water temperatures of 25°C to 28°C, it is the concentrated reproductive molting period for mud crabs, and the molting hardening time is generally longer (2 to 3 days), making them highly susceptible to bacterial infection. At this time, if the environment changes abruptly, or water quality and substrate quality are poor, it can also hinder male-female mating and cause massive deaths. (3) Prevention and Treatment Methods ① Maintaining good water quality is the most basic measure for disease prevention. During the culture process, external pollution sources and external pathogenic bacteria infection should be strictly prevented. ② Half a month before the disease occurs, use 25 ppm quicklime or 2 ppm bleaching powder for alternating disinfection every 7 days to improve water quality, kill pathogenic bacteria in the water body, and at the same time stimulate some mud crabs to molt early, encouraging molting periods between crabs to be staggered. ③ For affected ponds, use 0.33 ppm dichlorohydantoin or 0.8 ppm bromochlorohydantoin for whole-pond water disinfection for 2 to 3 consecutive days, combined with oral doxycycline for 5 to 7 consecutive days, which has good results. 3. Mud Crab Yellow Spot Disease Mud crab yellow spot disease generally occurs in juvenile crabs after stage VIII or in the adult stage. It is currently a relatively common disease in the culture process, with a long disease course and high mortality, second only to molting failure disease. (1) Symptoms In the early stage of this disease, yellow spots appear on the base of the chelipeds and the carapace of mud crabs, then rust-colored spots appear on the abdominal segments, or a yellow viscous liquid is secreted from the base of the chelipeds. The activity function of the chelipeds decreases, losing regular activity and feeding ability. The center of the spots on the abdominal segments is slightly depressed, appearing slightly reddish-brown. In the late stage, the ulcer spots expand and connect with each other to form irregularly shaped large patches, with relatively deep ulcers at the center and blackened edges. When the carapace is cut open for examination, light brown foreign objects the size of chili seeds can be seen in the gill area. (2) Causes There are many species of pathogenic bacteria causing this disease, and they are widely distributed. The direct cause of this disease is injury to the upper epidermis of the carapace during harvesting, transportation, and culture of mud crabs, allowing chitin-decomposing bacteria to invade. The frequency and infection rate of this disease increase with rising water temperature. The general occurrence time is mostly in seasons with relatively high water temperatures and abundant rainfall. (3) Prevention and Treatment Methods ① Feed fresh bait and more live bait such as bivalve blue clams, which can reduce the occurrence of this disease. ② Be careful in management operations during the catching, transportation, and culture production of crab seedlings to prevent injury to the crab body. Stocking density should not be too high. ③ Maintain fresh water quality. Before stocking, seedlings should be dipped in 200 ppm formalin solution for 2 to 3 minutes to kill parasites and pathogenic bacteria on the carapace surface. ④ Try to maintain stable salinity in the culture pond water, regularly use 25 ppm quicklime or 2 ppm bleaching powder for sterilization and disinfection, and splash tea seed cake extract over the entire pond monthly to stimulate mud crab molting and reduce disease occurrence. ⑤ For ponds that have already developed the disease, use 0.5 ppm dibromohydantoin for water disinfection for 3 consecutive days. At the same time, combined with oral compound ciprofloxacin at a dosage of 5 g per 100 kg per day for one consecutive week, which can prevent and alleviate the occurrence of the disease. 4. Mud Crab Yellow Water Disease Mud crab yellow water disease generally occurs from July to August and September to October of the following year, causing no small harm to mud crab culture production. (1) Symptoms It is named because the muscles liquefy into "yellow water" before death. The incidence rate at the turn of spring and summer each year is higher than in autumn. In severe cases, the mortality rate reaches over 70%, directly affecting the healthy growth of mud crabs. (2) Causes The main cause of this disease is poor water quality and substrate during high-temperature seasons, causing massive bacterial reproduction. The large temperature difference between day and night from July to October aggravates the occurrence of this disease. Whenever the peak season for shrimp culture disease occurs in spring and summer, mud crabs cultured in shrimp-crab mixed ponds (ponds), although resistant to low doses of white spot virus, when large numbers of diseased shrimp die and accumulate at the pond bottom, once live bait in the pond is in short supply, mud crabs that were previously infected with "yellow water" pathogen but had not yet developed the disease will quickly develop the disease after foraging on these dead shrimp, driven by large-dose white spot virus infection, and begin to die in large concentrated numbers. (3) Prevention and Treatment Methods Half a month before the disease occurs, use 25 ppm quicklime or 0.3 ppm dibromohydantoin for whole-pond splashing prevention, combined with oral compound ciprofloxacin at a dosage of 3 g per 100 kg of crab body per day for 3 consecutive days, which can prevent the occurrence of this disease. For ponds that have already developed the disease, continuous disinfection for 3 days, with oral compound ciprofloxacin at 5 g per 100 kg of crab body per day for 5 to 7 consecutive days, has good therapeutic effects. Commercial Harvest Editor 1. Harvest Time After 3 to 5 months of careful rearing in the pond, when juvenile mud crabs reach a body weight of 200 g to 250 g or more, harvesting can begin. The specific harvest time should be determined based on market conditions, actual conditions of pond culture in the middle and late stages of production, such as crab density in the pond, individual specifications, fatness, water temperature, water quality, substrate, bait, etc., comprehensively considering various factors before deciding whether to harvest early or late, how to harvest, and how many to harvest each time. However, the annual harvest time is from August to December (some are retained for overwintering and harvested from January to March of the following year). Mud crab culture and harvesting have their particularities, so harvesting must be carried out according to the principle of flexible operability: "stock seedlings once and harvest by thinning multiple times; stock seedlings multiple times and harvest multiple times; harvest large ones and keep small ones, harvest fat ones and keep thin ones," "harvest injured and diseased crabs first, harvest healthy crabs with growth potential later; harvest more when market conditions are good, harvest less when market conditions are poor." Only in this way can relatively good yield, quality, and satisfactory economic benefits be obtained. 2. Harvest Methods In the practice of mud crab culture production, the majority of farmers have accumulated considerable rich crab-catching experience. The main methods include: ① Based on the mud crab's habit of swimming upstream and gathering near the gate at high tide, attempting to escape, use dip nets to catch them. ② When water is taken into the pond at high tide, place crab cages at the gate, and crabs enter the cages while swimming against the current and are caught. ③ Take advantage of the favorable timing of forward and reverse flow generated during routine daily management water intake and drainage, and place multi-section spindle-shaped barbed nets to catch. ④ Following the mud crab's rule of being greedy for food and frequently moving out of burrows at night, feed bait to lure them into board nets for catching (light-assisted catching). ⑤ Drain the pond water to expose the flats, and use iron-toothed rakes to rake and catch, poke into burrows with hooks to catch, and catch by hand. The above fishing methods are all effective methods. However, catching should be done with care, especially when catching crabs by hand, being careful not to be pinched and injured by the crab general's pair of thick and powerful chelipeds. 3. Binding Methods After being caught, mud crabs should first be placed in containers holding green branches and leaves (such as reed stem leaves, etc.) or plastic buckets to prevent them from pinching and biting each other and causing injury. Then check each one individually, select fat crabs that meet commercial specifications, bind them, and put them into baskets (preferably specially made crab boxes). Thin crabs and small crabs that do not meet commercial requirements should be returned to the pond for further culture. If they cannot be immediately loaded and transported for sale, in hot weather, they should be stored in a cool and moist place. If caught in winter, they should be covered with frost-proof and warm-keeping covers. The straw rope used for binding crabs can be sourced locally. Generally, relatively cool saltwater grass is suitable in summer, while long-stem rice straw with warmth-keeping effect is used in winter. Plastic rope can also be used, which is both convenient and easy to bind. 4. Transportation in Transit ① Summer transportation. First, put the commercial crabs bound with saltwater grass or plastic flat filament rope into baskets, cover them, and immerse them in fresh seawater for several minutes to let the crabs spit out mud and take in fresh water, then they can be loaded onto vehicles. To prevent sun exposure and rain, the carriage should be covered with a highly breathable tarpaulin. For long-distance transportation, sprinkle moderately salty-fresh seawater several times daily in the morning, noon, and evening to keep the crab bodies moist. This can not only greatly improve the survival time and survival rate of mud crabs but also maintain better color. ② Winter transportation. Commercial crabs bound with rice straw or plastic rope should also first be put into bamboo baskets or plastic boxes for transport. In cold weather, spread straw and other warmth-keeping materials around the baskets or boxes to keep warm and prevent cold wind from entering. When placing them in baskets or boxes, the mouthparts of the mud crabs should face the center of the basket or box, with burlap sacks covering the top. It is best to choose daytime for driving. For long-distance transportation, sprinkle water (moderately salty-fresh) in the morning and evening daily to keep moist. In this way, commercial crabs can be stored for one week to half a month after leaving the pond.