How to fatten mud crabs? A detailed guide to practical farming techniques

The mud crab (Scylla serrata Forskal), or green crab for short, is a species whose sexually mature females, because both sides of the carapace are filled with "red roe," command a higher economic value in mariculture. Fattening mud crabs is therefore a good way to improve the market quality of the crabs and increase the income of farmers. Mud crab fattening refers to the process of taking female crabs of about 200 g (already mated) and thin, weak water crabs and, through about 20-40 days of intensive feeding, raising them into roe crabs with mature ovaries filling the carapace and meat crabs. The whole rearing process is as follows:
　　
Selection of individual mud crabs:
Throughout the fattening process, crabs should be accurately identified and reared separately according to whether the female has already mated and how mature the gonads are.
1. Unmated females, also called white crabs. They are generally smaller, with a grayish-black abdomen. When observed under light, no obvious ovary outline can be seen near the anterolateral margin of the cephalothorax. Such crabs can only be raised as meat crabs and cannot be directly raised into roe crabs.
2. Newly mated females are generally larger. Under light, a crescent-shaped ovarian gland can be seen along the lateral margin of the cephalothorax. After another 30-40 days of rearing, such crabs can become roe crabs.
3. Flower crabs: newly mated females whose ovaries gradually develop after 15-20 days of rearing become "flower crabs." The ovaries of flower crabs have not yet extended to the edge of the carapace; after another 15-20 days of rearing, they become roe crabs.
4. The individuals stocked must be healthy and strong with intact appendages. For female crabs, parasitic Sacculina must be removed. This is because female crabs parasitized by Sacculina suffer greatly affected ovary development and cannot be raised into roe crabs.
Selection of fattening ponds:
The fattening pond should be chosen near the mid-tide line in a sheltered inner bay, so that the water level can be controlled according to production needs, and a freshwater source should be available so that water quality can be adjusted at any time; there should be no sewage pollution nearby. We generally use a "field"-shaped pond, which consists of four small ponds joined into the character "field", with a total area of 1-5 mu. In the center of this pond, a small square pond 1.5 m wide is set up, with a sluice on each of its four sides connecting it to the four crab ponds, and a water channel is provided so that water is introduced through the small pond; by exploiting the mud crab's habit of swimming against the current, the crabs automatically move upstream into the central small pond for capture. The bottom substrate is best made of mud and sand.
Stocking:
The water temperature most suitable for mud crab fattening is between 18-25 degrees, when the crabs feed heavily, have vigorous metabolism, and grow relatively fast. Therefore, mud crab fattening is generally carried out from August each year to May of the following year. Stocking density should be determined according to season, feed, and other conditions, and is 3-5 crabs/m3. It is advisable to stock males and females separately to prevent male crabs from interfering with the growth of female crabs.
Daily management techniques:
(1) Feeding. The mud crab is a benthic crustacean living in the intertidal zone, with the foraging habit of hiding by day and emerging at night; its diet is mainly carnivorous, with omnivory secondary. In production, feeding is mainly based on low-value freshwater and marine shellfish, such as Lan clam, short-toothed clam, river snail, and freshwater mussel; before feeding, the shells are generally crushed. In addition, as needed, a small amount of trash fish, shrimp, crab, and some nutritionally complete compound feed may be given. Generally speaking, in April-May and August-September, the daily feeding amount should be about 10% of the total body weight of the mud crabs, and from late September to March of the following year it can gradually be reduced to about 5%. Generally, during spring tides the feeding amount can be appropriately increased, while during neap tides the opposite applies; in hot, muggy weather and when water quality deteriorates, the feeding amount should be reduced.
(2) Control of the water quality environment.
Taking necessary measures to control water quality well is an important part of mud crab fattening management techniques, especially in places where the water quality conditions in the sea area are poor and water intake and drainage are not very convenient.
1. Changes in water salinity.
Different seawater salinities greatly affect the growth of mud crabs. The optimum salinity is generally between 13-26‰, although normal farming can also be carried out in estuarine sea areas with a salinity of 6-8‰. Mud crabs are relatively poor at adapting to sudden salinity changes. During heavy rain, a large amount of fresh water rushes into the crab pond, causing a sudden drop in pond water salinity and easily triggering "white hair disease" in mud crabs. At this time, the upper freshwater layer should be drained off or salt blocks should be hung to compensate; and after the rain, dibromohydantoin should be splashed over the whole pond at 0.2 ppm for water disinfection to prevent disease outbreaks.
2. Controlling changes in water temperature.
The water temperature most suitable for mud crab fattening is between 18-25 degrees. When the water temperature rises to about 33-35 degrees, mud crabs show obvious signs of discomfort. They crawl around the embankments everywhere, trying to escape. At this time, the water level should be kept at about 1.3-1.5 m, and if conditions permit, a slight flow-through state should be maintained. When the temperature is below 7 degrees, mud crabs stop feeding completely and enter a dormant state to get through the unfavorable environment.
3. Changes in dissolved oxygen in the water.
After many years of observation, when the dissolved oxygen in the water is greater than 2 mg/l, mud crabs live normally and grow well; when the dissolved oxygen level drops to 1 mg/l, mud crabs respond slowly, stop feeding, and exhibit "floating head" or even death. When hypoxia occurs in the pond, organic matter at the bottom undergoes anaerobic decomposition, causing excessive accumulation of non-ionic ammonia (NH3-N) and poisoning the mud crabs. During molting, mud crabs have slightly higher dissolved oxygen requirements than usual, so water should be changed frequently, generally once every 3-4 days, to keep the water clean and the dissolved oxygen sufficient.
4. Changes in water quality factors.
pH is a comprehensive indicator reflecting the state of water quality. Generally speaking, the pH of a mud crab pond should preferably be kept between 8.0-8.5. In addition, the transparency of the pond water is also an important factor affecting the success of fattening. According to our experiments, the transparency of the pond water should preferably be kept at 20-25 cm. Within this range, mud crabs can move normally and have relatively fewer opportunities to kill one another. When transparency drops to 15 cm, if measures are not taken in time, sudden water quality changes are easily triggered and cause mud crab death.
5. Pond patrols.
During the farming period, pond patrols must be carried out frequently to observe the water color, water level, and activity of the mud crabs. Because mud crabs have the habit of escaping harsh environments, when water quality changes or oxygen in the water is insufficient, they will climb out of the water and escape over the embankments, so anti-escape work is very important. When such situations are found, corresponding remedial measures should be taken immediately, such as turning on aerators, changing water, and splashing Yishuibao over the whole pond at 0.40 ppm to improve water quality.
6. Disease prevention.
During the mud crab fattening period, changes in weather and temperature are generally great, so water quality also changes greatly, and mud crabs are very prone to disease. Farmers must regularly prevent disease. Biological enzyme activity additives and immune polysaccharides, among others, are added to the feed to improve the body's immunity; disinfectants such as Qiangke 101, Qiangke 202, and dibromohydantoin are used to improve water quality conditions and maintain a good water environment. When disease occurs, it should be handled through correct diagnosis and active treatment. (For detailed disease treatment plans, see issue 9 of China Fisheries, 2002.)
7. Harvest.
After the mud crab seedlings are stocked, if managed properly, they can be harvested and marketed in 20 days. Harvestable crabs must be "full of roe and plump with meat." For female crabs, the mature ovaries have filled the serrations of the cephalothorax; at this time roe accounts for 80-90%. The catching method is similar to that for general crabs. During transport after being bound, water should be sprinkled frequently to keep the gills moist and ensure that live crabs reach the market.