In 2004, the county initiated demonstration projects on polyculture systems in freshwater ponds, combining catfish, river crabs, and grass carp. Following two years of trials, the results were highly promising. This approach not only improved fish health but also boosted productivity and economic returns for local farmers.
**Pond Conditions and Escape Prevention**
The pond size is typically between 5 to 10 mu (approximately 0.3 to 0.7 hectares). The silt layer at the bottom should be more than 10 cm thick, so regular dredging is necessary. To disinfect the pond, 15-20 kg of tea cake or 100-150 kg of quicklime per acre is applied. After disinfection, the pond is left to dry under the sun for several days, then the sediment is turned over and exposed again for about 10 days before new water is introduced. Before filling the pond with water, a base fertilizer of 250-500 kg of manure per acre is added near the water inlet. A plastic net is installed to prevent wild fish from entering. About 500 kg of fresh snails are placed on the water surface to serve as natural food for the crabs. Aquatic plants like water peanuts and duckweed are also planted, covering roughly one-third of the pond's surface area. Each pond is equipped with 1-2 aerators of 3 kW and an automatic feeder of 90 watts to ensure efficient management.
**Escape Prevention Facilities**
To prevent escape, especially for fast-moving species like eels and crabs, high fences are installed around the pond entrances and exits. These include polyethylene nets that are 1.6 meters high and extend 0.4 meters into the mud. Bamboo rafts are used to secure the nets. For crabs, a plastic film wall is constructed, standing 70 cm above ground and buried 50 cm underground, supported by bamboo rafts. The corners are rounded to prevent crabs from escaping by hitting the edges.
**Stocking of Seedlings**
Eel seedlings are sourced from the natural population in the county’s coastal areas, with a minimum size of 4 cm. They are carefully selected for strong vitality, intact scales, and normal swimming behavior. Carp seedlings are acclimated to freshwater before being stocked to improve survival rates. Before stocking, they are briefly immersed in a 5% saltwater solution for 8-10 minutes. The stocking density is about 250-300 fish per acre. Crabs are stocked at 160-200 per kilogram, after being treated with formalin at 25 ml/m³, at a rate of 500 per acre. Grass carp are stocked at 2-3 per kilogram, with 80 per acre, while white bream and crucian carp are stocked at 10-12 per kilogram, with 200 and 50 per acre respectively.
**Aquaculture Management**
Feeding is done using grass carp pellets, gradually transitioning to automated feeding. Throughout the rearing period, feed is given following the "three looks" and "four sets" principles—adjusting the amount based on fish size, season, and activity. Feeding occurs twice daily, at 9-10 am and 3-5 pm. On sunny days, more feed is provided; on rainy or cloudy days, less. Fish that show normal activity receive more feed, while those showing signs of stress get less. Snail availability is monitored to ensure a steady supply of natural food.
Pond management includes regular monitoring to prevent issues like floating heads and flooding. Aerators are used during hot weather, and two daily inspections are conducted to check for problems. Anti-escape facilities are reinforced, especially during storms. If any unusual fish behavior is observed, emergency measures such as adding fresh water and turning on aerators are taken immediately.
**Disease Control**
Common diseases include parasitic infections and fungal infections, with recent cases of hemorrhagic disease reported. Parasites like ticks and nematodes can cause internal damage, while injuries lead to fungal infections. Regular water disinfection using bleaching powder, quicklime, or antifungal agents is essential. Water is refreshed when needed to maintain a clean environment and reduce disease risk.
**Economic Benefits**
From the old dam harbor area and Lixiahe Township, polyculture systems have shown a squid survival rate of 90%. Each year, the average weight reaches 500-750 grams, with yields exceeding 160 kg per acre. At market prices of 12-18 yuan per kilogram, the output value per mu ranges from 2000 to 3000 yuan. After deducting costs for feed and seedlings, the income per mu exceeds 1000 yuan compared to traditional monoculture methods.
**Summary**
Squid play a vital role in maintaining water quality by consuming organic debris and algae, acting as natural "scavengers." This helps reduce disease occurrence in other species like grass carp and crabs, leading to healthier aquatic products and lower medication costs. The integration of carp seedlings into existing ponds does not interfere with original production, provided proper anti-escape measures and timely fry placement are maintained. When young carp grow slowly, it’s advisable to rear them in separate ponds to enhance their size and yield in the following season.
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