As a result of its low investment, easy setup, minimal risk, and high profitability, river crab farming in greenhouses has become a popular project for civilian investment in the coastal areas of East Jiangsu and Eastern China. Through extensive field research and practical experience, the author has developed improved techniques for raising river crabs in greenhouse environments. The following is a summary of the key improvements.
1. Enhancement of Infrastructure Technology
The scientific design of infrastructure is crucial for stable and high-yield seedling production. Currently, there are three main shortcomings in the construction of nursery ponds.
1.1 Pond Size
Most nursery ponds currently have an area of 40 to 50 square meters, with some reaching up to 80 square meters. While this size helps maintain stable water quality in the early stages, it leads to several issues after the Z4 stage. First, water exchange becomes difficult, and the effectiveness of water changes is limited. Second, juvenile crabs suffer from severe molting problems, especially during the Z5 to M stages, increasing the risk of disease outbreaks. In short, larger pond sizes lead to more challenges. The author recommends reducing the pond size to 20–30 square meters for better control and efficiency.
1.2 Bottom Treatment
It is commonly believed that soil has good dirt-absorbing properties. However, studies show that while soil can initially absorb pollutants, it lacks the ability to break them down. Over time, the soil becomes saturated and loses its effectiveness, leading to poor water quality in later stages. To address this, the author suggests covering the bottom of the pond with color stripe film. This material allows waste to settle directly on the surface, making it easier to remove. Additionally, the rough texture of the film promotes biofilm formation and prevents pH fluctuations caused by lime used in ground preparation.
1.3 Aeration System Optimization
In traditional setups, air stones are placed 20 cm above the pond bottom. However, this creates an oxygen-depleted zone below the stones, where residual food and waste accumulate. This can lead to harmful gas buildup, which may damage newly molted larvae during the Z5 stage. By placing air stones directly on the color stripe film, the entire water column becomes well-oxygenated. It is recommended to install 2–3 air stones per square meter to ensure optimal aeration.
2. Improvements in Nursery Management
2.1 Water Treatment
Bleaching powder is often used to disinfect water and reduce nitrite levels. However, it can also generate harmful compounds like ammonium amines under high temperatures and may inhibit algae growth. Instead, the author recommends using polymerized aluminum salts at (10–15) × 10â»â¶ concentration to clarify the water, followed by methylene blue at 1 × 10â»â¶ to oxidize nitrites. Finally, adding 50–70 × 10â»â¶ concentration of activated clay particles helps adsorb ammonia and hydrogen sulfide. All treated water should be filtered through a 300-mesh sieve before being introduced into the nursery.
2.2 Temperature Control
Maintaining a water temperature of 21°C during the Z1–Z2 stages and gradually increasing it to 24°C after Z2 helps improve larval vigor and development speed. The author suggests delaying the start of the nursery season from mid-April to late April in Rudong, Jiangsu, to take advantage of higher water temperatures and better growth conditions.
2.3 Feeding Practices
With rising costs, efficient feeding is essential. Although artificial bait is cost-effective, it is less palatable than live brine shrimp and can easily cause contamination. The author recommends feeding every two hours, using a small number of feedings and increasing the frequency of feeding mother assistance to aid digestion. Larvae in the Z4, Z5, and M stages require higher feeding rates, and the amount should be adjusted based on their condition.
2.4 Water Quality Management
Water quality is easier to manage in the early stages but becomes more challenging as feeding increases and sediment accumulates. Regular water changes, dirt removal, and the use of photosynthetic bacteria are effective methods. The best time for water changes is around 3 a.m., when the sediment is most effectively cleaned. The author recommends suctioning dirt once during Z1–Z3 and daily during Z4 and Z5. Immobilizing photosynthetic bacteria with PVA can enhance their performance and improve water quality.
2.5 Lighting Conditions
Greenhouse nurseries often use color films to create shaded environments, which may not provide sufficient light. The author suggests using fluorescent lights to supplement illumination, which helps promote algae growth and reduces oxygen debt and ammonia levels, especially during nighttime.
2.6 Disease Prevention
Viral diseases are often overlooked but can cause large-scale losses. Studies have shown that unexplained mass mortalities in nurseries are sometimes due to zebra virus infections, which are resistant to conventional treatments. Using acyclovir baths and incorporating the drug into bait can effectively control such outbreaks.
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