A Detailed Guide to Swimming Crab Farming Techniques and Methods
Swimming Crab Farming Techniques
Pond Conditions
For farming in the north, ponds of medium to small size, generally 10—20 mu, are suitable; choose ponds near sea areas with abundant water sources, no pollution, rich resources of small trash fish, shrimp and shellfish, and convenient water intake and drainage. Pond depth should be more than 5 m. The pond bottom is better with a muddy-sandy texture, and about 15 cm of sand should be spread on the pond bottom.
Preparation Work
Swimming crab farming mainly consists of three steps:
Step One: Tidying the pond: Tidying the pond mainly means removing excess sludge from the bottom, loosening and sun-drying the pond bottom, and also repairing supporting facilities such as pond embankments, sluice gates and water channels.
Step Two: Setting up obstacles: To make it easier for the crab seedlings to rest and hide, after the pond is repaired, obstacles such as cement pipes, tiles and bamboo tubes should also be placed on the pond bottom.
Step Three: Letting in water and cultivating bait organisms: Half a month after pond clearing, the sluice gate can be opened to let in water. To prevent some harmful organisms from entering the pond, seawater can be filtered through a 60-mesh silk bolting cloth. The pond water depth is generally kept at about 80 cm. After the pond is filled, chemical fertilizer should be applied to cultivate bait organisms, generally 1.5 kg/mu of N fertilizer and 0.3 kg/mu of P fertilizer, and the transparency of the pond water should be kept at about 30 cm.
Stocking of Crab Seedlings
1. Source and selection: Crab seedlings come mainly from two sources: harvesting from natural sea areas and artificial breeding. For seedling specifications, about 80 crabs/kg is better; the crab seedlings are required to be uniform in size, robust, with complete appendages, strong vitality, and a soft-shell rate of less than 10%.
2. Stocking time: We generally stock from late May to mid-June.
3. Water quality requirements: The water temperature should be above 18°C, the daily temperature difference should be less than 8°C, the salinity should be 18—32, the pH should be 7.8~8.6, the ammonia nitrogen content should be less than 1 mg/L of water body, the dissolved oxygen should be greater than 5 mg/L of water body, and the transparency should be kept at 30~40 cm.
4. Stocking density: For crab seedlings of generally 80-100 crabs/kg, the stocking density should be controlled at 1000~1500 crabs/mu.
Daily Management of Swimming Crabs
1. Water quality management: In July, change the water once every 2—3 days; in August—September, change the water once every 1—2 days; in October, change the water once every 4—5 days. The specific water exchange volume and interval should vary according to environmental factors such as weather and water temperature.
2. Feeding: The swimming crab (Portunus trituberculatus) is omnivorous, mainly feeding on fresh low-value fish, shrimp, shellfish, crabs, etc. The daily amount of bait fed should differ according to the size of the crabs. In the early stage of farming, feed about 8% of the crabs' total body weight; in the middle stage, 12%; in the late stage, 5%. When the water temperature drops to 10~15°C in autumn, the daily feeding amount should be 3% of the total body weight. Feed once in the morning and once in the evening, following the principle of feeding less in the morning and more in the evening. When the daily average water temperature is below 10°C, there is no need to feed bait.
3. Pond patrol management:
Patrol the pond once in the morning and once in the evening. During patrols, carefully observe the feeding and activity of the swimming crabs, pay attention to changes in the pond water, check the condition of residual bait in the pond, and promptly adjust the feeding amount and water exchange volume. Always pay attention to weather changes and do a good job of preparing for flood and typhoon prevention. Frequently check whether the blocking nets at the sluice gates and the embankments are damaged or have leaks; if problems are found, repair them as soon as possible.
Harvesting Operations
Swimming crabs should be harvested in late autumn; when the water temperature drops below 15°C and the swimming crabs no longer continue to gain weight, they should be harvested promptly. Timely harvesting of swimming crabs is the key to high quality, high efficiency and high yield. The harvesting time should be decided according to whether the swimming crabs have entered the roe crab stage, the selling price and weather conditions. Generally, it is best to arrange the harvesting time in December, and harvesting should not be done on rainy days. When only a small number of crabs are to be taken from the pond, they can be caught at night with crab cages or dip nets. When a large number is to be taken, fix net cage bags at the sluice gate and use the ebb tide to release water so that the swimming crabs flow with the current into the cage bags. When the water in the pond is drained to the lowest level, an iron rake can be used to slowly rake along the pond bottom. When a crab body is encountered, lift it up carefully and scoop it into a basket or bucket with a small dip net. The harvested swimming crabs should have their two chelipeds bound with rubber bands to prevent them from biting each other or escaping from the baskets or buckets.
Efficient Polyculture of Swimming Crabs
Controllable ecological polyculture technology is an effective way to improve the economic benefits of farming ponds, reduce farming costs, reduce farming risks and increase farming returns, and it is the goal that farmers have always pursued. The author summarizes several key points in the pond polyculture of swimming crabs and ridgetail white shrimp (Exopalaemon carinicauda) as follows:
1. Clear and tidy the pond; fertilizing the water is very important. Let water into the pond to 50 cm and disinfect with bleaching powder to kill copepods and eliminate harmful organisms, etc. Apply urea or fermented, disinfected chicken manure or pig manure so that the water color becomes yellowish-brown or yellow-green.
Swimming crab 2. Stocking of swimming crab seedlings and shrimp seedlings. Swimming crab seedlings are generally stocked in mid-to-late May; it is advisable to choose stage III young crabs raised in earthen ponds, stocking 1 kg per mu, and attention should be paid to selecting seedlings of uniform size and robust constitution. Ridgetail white shrimp seedlings are produced by self-reproduction of ovigerous shrimp. Locally, in late June, stock 1.5 kg per mu of ovigerous shrimp that are undamaged, respond sensitively to external stimuli and swim actively. Stocking during this period results in uniform emergence of young white shrimp and few diseases, which is beneficial to the growth of shrimp and crabs and convenient for production management.
3. Management of pond water to prevent “algae die-off” and hypoxia. After the pond takes in algae and changes water, stabilizing the water and detoxifying are most important. For ponds where conditions permit, it is recommended to change the water once every 5-7 days. After letting in water, disinfect the whole pond with a disinfectant. The introduction of bacteria and the quality of the bottom sediment are the key to successful farming. Farmers should regularly improve the pond bottom, preferably once a week, to prevent hypoxia and reduce the disease rate. Always pay attention to sudden weather changes and strictly prevent “algae die-off.” If the water color is too dense and the transparency is less than 30 cm, drain off the upper layer of pond water and add fresh seawater to inhibit algae reproduction. Chlorine dioxide or dibromohydantoin can also be used to disinfect the water body, and microbial agents can be applied to restore the water color. Strengthen daily inspection and recording, and turn on the aerators in time, especially in the evening and at night, to increase the dissolved oxygen in the pond water through the aerators and maintain a good water color.
4. Bait feeding. Adhere to many meals in small amounts, inspect frequently and observe more, make sure the bait is fresh and palatable, improve the conversion rate of the bait, reduce feces, and ensure that the shrimp and crabs eat their fill without waste. In the early stage of farming, biological preparations such as EM bacteria can be applied to decompose ammonia nitrogen, improve the pond bottom and prevent hypoxia and poisoning in the pond.
5. Remove male crabs in time. To reduce farming costs and increase farming returns, it is recommended that farmers fish out male crabs from early July to the end of August. This not only improves the farming density in the pond but also saves bait costs; moreover, in the swimming crab mating season, if there are too many male crabs, the swimming crabs in the pond will kill each other, affecting the economic benefits of the farmers.
6. Do a good job of fattening swimming crabs in autumn and winter. During the fattening period, organic calcium can be supplemented to increase the thickness and hardness of the swimming crab shell, thereby increasing weight. In the middle and late stages of grow-out, feed more low-value shellfish to promote the development of the crab gonads and the accumulation of nutrients.
7. Rotational harvesting of ridgetail white shrimp over 5 cm. During the grow-out period, according to market conditions, nets such as ground cages can be used to carry out planned rotational harvesting of ridgetail white shrimp for market, keeping the small and catching the large, selling them fresh and alive. This can control the density of ridgetail white shrimp in the pond, prevent hypoxia, and recover funds in time.
8. Disease prevention and control runs through the entire farming process. Adhere to “prevention first, combining prevention and control,” and maintaining a good pond water quality environment is the foundation of successful farming.