In 2004, the county initiated demonstration projects for 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 maximized pond productivity but also improved water quality and disease control.
**I. Pond Conditions and Escape Prevention Facilities**
The ponds used for this system are typically 5 to 10 mu in size, with a silt layer at least 10 cm deep. Before stocking, the pond should be dredged and disinfected using 15-20 kg of tea cake or 100-150 kg of quicklime per acre. After disinfection, the pond is left to dry under the sun for several days, then the sediment is turned over and exposed again for 10 days before new water is introduced. Before filling, 250-500 kg of organic manure is applied near the water inlet as a base fertilizer. A plastic net is installed around the pond to prevent wild fish from entering. Approximately 500 kg of live snails are placed on the water surface to serve as natural feed for the crabs. Aquatic plants like water peanuts and duckweed are also planted, covering about one-third of the pond's surface area. Each pond is equipped with 1-2 aerators (3 kW each) and an automatic feeder (90 W) to ensure efficient management.
**II. Anti-Escape Measures**
To prevent escape, high fences made of polyethylene nets are installed at pond entrances and exits, standing 1.6 meters tall with 0.4 meters buried underground. These are secured with bamboo rafts. For crabs, a plastic film wall 70 cm above ground and 50 cm below is used, also supported by bamboo rafts. The corners are rounded to avoid crabs climbing up and escaping.
**III. Stocking of Seedlings**
Eel seedlings are sourced from natural stocks in the county’s coastal areas, with sizes over 4 cm. They must be healthy, with intact scales and active movement. Carp seedlings are acclimated to fresh water before being stocked. Before introduction, they are bathed in 5% saltwater for 8-10 minutes. Stocking density is about 250-300 per acre. Crabs are stocked at 160-200 per kg, after formalin treatment (25 ml/m³), at 500 per acre. Grass carp are stocked at 2-3 per kg, 80 per acre. White bream and crucian carp are stocked at 10-12 per kg, 200 and 50 per acre respectively.
**IV. Aquaculture Management**
Feeding is done using grass carp pellets, initially manually and later with an automatic feeder. Feeding follows the “three looks†and “four fixed†principles—adjusting amounts based on fish size, season, and activity. Two daily feedings are scheduled: 9–10 am and 3–5 pm. The feeding rate varies between 2% to 6% of the total fish weight. Snail supply is monitored regularly to ensure sufficient natural feed.
Pond management includes regular inspections, early aeration during hot weather, and routine checks for escaped crabs or abnormal fish behavior. During storms or typhoons, additional precautions are taken, including immediate water exchange and aeration if needed.
**V. Disease Control**
Common diseases include parasitic infections and fungal infections (like "watery mildew"), with recent cases of hemorrhagic disease. Parasites such as ticks and nematodes can cause internal damage. To prevent these, the water is regularly treated with bleaching powder, quicklime, or antifungal agents. Fresh water is added periodically to maintain good water quality and reduce disease risk.
**VI. Economic Benefits**
In areas like Old Dam Harbor and Lixiahe Town, the survival rate of squids reached 90%, with an average yield of 500–750 grams per individual. At a market price of 12–18 yuan per kilogram, the output value per mu was 2000–3000 yuan. After deducting costs for feed and seedlings, the income per mu was more than 1000 yuan higher compared to previous monoculture practices.
**VII. Summary**
Squid plays a crucial role in maintaining water quality by consuming organic debris, algae, and small organisms. This natural filtration helps reduce disease incidence among other species like grass carp and crabs, lowering medication costs and promoting eco-friendly aquaculture. The integration of squid into existing fish ponds does not interfere with original production, provided proper anti-escape measures are in place. When young carp grow slowly, it’s advisable to raise them in separate ponds to increase their size and improve future yields.
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