Analysis of the Causes of Low Yield in River Crab Farming and Methods to Increase Production
The river crab is the most important freshwater crab species in China. Its flesh is tender, its taste is delicious, and it is extremely rich in nutrients. Because the river crab has strong adaptability and a wide farming range, the scale of farming has increased rapidly in recent years, bringing good economic benefits to farmers. However, some river crab farming ponds have shown low yield, and some have even suffered serious losses. So what are the causes of low yield in river crab farming? What should be done? How can production be increased?
I. Causes of low yield in river crab farming
1. Poor control of seed stock quality
Poor control of seed stock quality plants hidden risks for later production.
2. Impact of unfavorable weather
If sustained extreme high temperatures (average temperature above 32℃) and little rain occur from July to August, the accumulated heat in the pond water body will rise sharply, which will seriously affect the molting and growth of river crabs. High temperatures will also cause aquatic plants to rot and water quality to deteriorate, resulting in large numbers of dead crabs.
3. Inadequate production management
This is most prominently shown by improper water quality regulation measures, leading to outbreaks of blue-green algae; unreasonable planting and management of aquatic plants, causing a major decline in the pond's ecological function; unscientific feed feeding, worsening water quality deterioration; and improper use of drugs, causing river crab deaths.
II. How to increase production in river crab farming
1. Develop ecological crab farming
Carry out desilting and renovation of aging crab ponds, build independent water intake and drainage systems where conditions permit, and equip them with aeration facilities. Mixed planting of multiple aquatic plant species such as tape grass, Hydrilla verticillata, Elodea, and hornwort can meet the needs of river crabs for aquatic plants at different growth stages. Adopt mixed and supplementary farming models with multiple species such as crabs, fish, and shrimp to increase the biodiversity of crab ponds, make full use of the water body, and improve the comprehensive utilization rate of feed.
2. Strictly control the quality of crab seed stock
As far as possible, breed crab seed stock yourself or purchase it nearby from seedling bases with a good reputation, and try not to use out-of-town crab seed stock purchased over long distances. Crab seed stock should be uniform in size, robust in constitution, agile in crawling, complete in appendages, smooth and undamaged on the body surface, and free of attached dirt. Before the crab seed stock is put into the pond, it should be selected one by one, and poor-quality ones should be strictly kept out of the pond to improve the survival rate. The stocking density should be appropriate. Do not simply pursue yield; instead, base decisions on market demand and improve size and quality.
3. Strengthen production management
(1) Water quality regulation
Throughout the farming period, always keep the water quality fresh and rich in dissolved oxygen, with transparency controlled at 20-40 cm, dissolved oxygen above 5 mg/L, and pH at 7.5-8.5. For water level, adhere to the principle of “shallow in the early stage, deep in the middle stage, and stable in the later stage”; that is, maintain a shallow water level in the early stage (www.nczfJ.com/) to raise water temperature and promote molting of river crabs; maintain a deep water level in the middle stage, especially in the hot summer and autumn, to keep water quality fresh and dissolved oxygen sufficient; and keep the water level stable in the later stage.
Generally, in spring and autumn, change the water once every 15 days, each time to a depth of 10-20 cm; from June to September, change the water once every 5-10 days, each time to a depth of 10-20 cm, first draining the old water in the pond and then adding new water. In hot seasons, add new water to a depth of about 20 cm every day to lower the water temperature and ensure a good growth environment for river crabs. When the water quality of the crab pond is poor, water should be changed promptly or other measures taken to improve water quality. From May to September, apply microbial preparations once every 10-15 days, and then fertilize appropriately according to water quality conditions. In addition, apply quicklime once every 15-20 days at a rate of 10-15 kg/mu·m, slake it into a slurry, and splash it evenly over the whole pond to regulate water quality and increase the calcium ion content in the water, promoting normal molting and growth of river crabs.
(2) Feeding management
Throughout the farming process, scientific feeding should be carried out to ensure balanced nutrition, mainly feeding river crab compound feed, snails, and chilled fresh fish, with a small amount of plant-based feed such as wheat, soybean meal, and corn added as a supplement. Daily feeding should “grasp one method, adhere to one principle, and highlight one focus”; that is, the scientific feeding method of “four fixed and four observations”, the principle of “combining concentrated and green feed, and matching meat and vegetables”, and the focus of “concentrated feed in the early stage, green feed in the middle stage, and meat feed in the later stage”. The specific operation is: on the premise of ensuring that the feed is fresh and palatable, the daily feeding amount depends on the weather, water temperature, and the feeding situation of river crabs.
In the early farming stage (March to June), use soybean cake, wheat flour, silkworm pupa powder, fish meal, or chopped small fish, crushed snails, and clam meat as raw materials, mix them into balls for feeding, or mainly feed starter compound feed. In the middle farming stage (July to mid-August), mainly feed concentrated and green feeds such as soybean cake, corn, wheat, potatoes, and pumpkin. In the later farming stage (late August to November), appropriately increase the proportion of animal-based feed (animal-based feed accounts for 50%) or feed compound feed specially formulated for river crabs in the later stage. Under normal circumstances, the daily feeding amount is 1%-2% of the body weight of crabs in the pond from March to April, 3%-8% of the body weight of crabs in the pond from May to July, and 8%-10% of the body weight of crabs in the pond from August to October. The number of feedings is 1 time/day in the early stage and 2 times/day in the middle and later stages, with 30% fed in the morning and 70% fed in the evening. Concentrated feed and fresh live feed should be fed alternately every other day or every other meal, evenly placed in shallow water areas. Persist in checking the daily feeding situation, and it is appropriate for the feed to be consumed within 3-4 hours after feeding.
(3) Aquatic plant management
Ensure that aquatic plant density is reasonable. When growth is excessive, carry out “topping” treatment, and properly stock duckweed in summer. From February to March, do a good job of fertilizing the water, and appropriately control the water level to raise water temperature and promote rooting and sprouting of aquatic plants.
4. Strengthen disease prevention and control
Diseases to which river crabs are susceptible should be prevented early and treated with scientific drug use. Do not rush to seek treatment when disease occurs, causing unnecessary losses. Strict pond clearing and disinfection, stocking robust crab seed stock, maintaining good pond water quality, and feeding fresh high-quality feed and other ecological farming technical measures can effectively prevent the occurrence of river crab diseases. In hot seasons, splash microbial preparations over the whole pond once every 10-15 days, and alternately apply quicklime to improve the water quality and bottom sediment of crab ponds and ensure healthy growth of river crabs.
After river crabs develop diseases, for bacterial diseases, high-efficiency and low-toxicity disinfectants such as bromochloro hydantoin, chlorine dioxide, or iodine preparations can be evenly splashed over the whole pond, and appropriate amounts of norfloxacin, allicin, vitamin C, immune polysaccharides, etc. can also be mixed into the feed; for ciliate diseases, drugs such as ciliate killer and shell cleaner can be dissolved in water and evenly splashed over the whole pond. In addition to taking corresponding treatment measures, attention should also be paid to repairing the water body of the crab pond to ensure a good growth environment for river crabs. The specific method of water body repair is: 2-3 days after achieving good results in treating river crab diseases, first splash a water quality protection and detoxification agent over the whole pond, and then according to water quality conditions appropriately apply biological fertilizer and an appropriate amount of microbial preparations once more.