California water body net enclosure method

Net fish farming in large and medium-sized water bodies, such as rivers and lakes, is an effective method for cultivating fish in open waters that are suitable for aquaculture. This technique is especially useful for raising larger species of fish or for large-scale fish farming in areas with smaller water surfaces. Below is a detailed introduction to the technology of net-based fish farming: **1. Construction of the Net Enclosure** The location of the net enclosure should be on a flat and firm bottom with some silt. It is typically placed in open water areas of lakes, near the mouths of lakes, or in gentle river channels. The area should have some submerged vegetation or benthic organisms, with a water flow between 0.05 m/s and 0.2 m/s. Water quality must be clean, with a depth of less than 4 meters and a stable water level. The design of the net enclosure is usually circular or elliptical, covering an area of 0.2 to 0.6 hectares (3 to 10 mu). The structure includes wall nets, stone cages, bamboo poles, and crossbars. The wall net is generally double-layered, with a spacing of 1 to 5 meters between the layers. A fish recovery device is installed in the middle to monitor any damage to the inner net. The mesh size depends on the species being farmed, typically 2.5 to 3 cm, with a horizontal shrinkage factor of about 0.65 to 0.70. The height of the wall net should account for the maximum water depth plus wave height. If climbing species like crabs or jumping fish are present, an additional 50 to 80 cm inward protrusion is added at the top. The wall net is secured with strong ropes, usually 5 mm in diameter, and connected to a gabion at the base. The gabion is filled with stones and weighs 10 to 12 kg per meter. Bamboo poles are used to support the net and resist wind and waves. They should be driven into the mud up to 1 meter above the high water level, spaced 1.5 to 3 meters apart. On the windward side, an extra support bar is added every 6 meters. Crossbars are placed at the top and bottom of the poles, approximately 50 cm from the top or bottom. **2. Fish Stocking** Fish species stocked in the net enclosures depend on the local food availability. In lakes with low transparency and high plankton content, oysters and squid are commonly raised. In estuaries with abundant water, grass carp, head gudgeon, and Pengze crucian carp are often stocked, along with some crabs. Benthic organisms like snails and maggots can also be introduced. To control unwanted fish, a small number of salmon may be added. Additionally, yellow algae and small fish like silver carp are stocked to help remove algae from the net walls. Stocking sizes vary depending on the species. Grass carp are typically stocked at 20–25 grams, head gudgeon at 10–15 grams, and common carp at 5 grams. Herring are usually stocked at 100–200 grams. Fish should be raised for one year to reach market size, with minimum weights of 50 grams for grass carp, 250–300 grams for head gudgeon, 50–60 grams for common carp, and more than 50 grams for other species. The stocking ratio depends on the type of culture. In semi-intensive systems, main food-eating fish like grass carp and common carp make up 40–60% of the stock, while eels and squid are around 5% each. In intensive systems, the ratio is adjusted based on feed type. Stocking density varies by environment. In rivers, it's about 40–150 kg per 667 m², while in lakes, it ranges from 100–200 kg per 667 m² for areas under 0.6 hectares, and 50 kg per 667 m² for larger areas. Squid and summer fish are stocked at about 20 per 667 m², and crabs at 100 per kilogram. **3. Feeding and Management** Feeding is crucial for increasing production. Artificial feed is commonly used, and the type of feed depends on the local resources. For example, grass carp and head gudgeon can be fed with aquatic plants, while snails and worms can be used for mollusk-eating species. Feed should be given according to the "four settings" principle: location, timing, quality, and quantity. Feed stations should be placed in areas where fish are concentrated, with floating rafts or submerged tables. Feeding is usually done in the morning and afternoon, with coarse feed first, followed by concentrate. Feed should be soaked, germinated, or crushed as needed. The amount of feed depends on fish weight, water temperature, and dissolved oxygen levels. Regular maintenance is essential. Debris should be removed, and the net walls should be cleaned when necessary. Damaged nets should be repaired promptly, and the gabions checked for blockages. During flood seasons or strong winds, extra attention should be paid to the structure. Harvesting can be done in batches using different methods, such as pulling nets for grass carp, using seine nets for head gudgeon, or electric pulse fishing for all species. This ensures efficient and sustainable fish farming.

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