Cage Culture Technology for Silver Carp and Bighead Carp Fingerlings: Using Small and Medium-sized Reservoirs as Broodstock Bases

As the main drinking water source for Beijing, Miyun Reservoir plays a decisive role in the capital's economic development and social stability. In order to scientifically convert the nutrients and aquatic organisms in the water body of Miyun Reservoir, purify the water quality, and protect this basin of "water of life" for the capital, after the cancellation of cages@#@233@#@, the Miyun County government increased the amount of fingerlings stocked, releasing 485,000 kg, nearly ten million individuals, of silver carp, bighead carp, bream, grass carp and other fingerlings into Miyun Reservoir each year, aiming to reduce the nutrient load of the water body through fishery production. However, in the practice of fingerling release, some problems have emerged that need to be solved, namely: (1) the contradiction between release and fishing; (2) the change from autumn release to spring release of fingerlings, which is costly and leaves the source of fingerlings unprotected; (3) fingerlings purchased household by household and transported over long distances suffer damage, resulting in a relatively low survival rate of released fingerlings; (4) scattered pond culture of fingerlings for reservoir release occupies dozens of times the water area of cage culture, and healthy culture is difficult to control. Therefore, it is necessary to use small and medium-sized reservoirs as broodstock bases and to use cage culture technology for silver carp and bighead carp fingerlings to resolve the above contradictions, ensure the survival rate of released fingerlings, and effectively control the variety, quantity and quality of stocked fingerlings, so as to achieve the goal of "purifying water, boosting fisheries and enriching the people."

I. Necessity and Feasibility

1. Necessity. First, fish such as silver carp, bighead carp, common carp, grass carp and bream can effectively utilize plankton, benthos, aquatic plants, organic detritus and other substances, playing an important role in biological water purification, and meeting the need for fish fry required by Miyun Reservoir to improve its water purification capacity by biological methods; second, as a breeding base for Miyun Reservoir, it can produce and release fingerlings and catch market-size fish in a timely manner according to the growth patterns of the fish, effectively avoiding the losses caused by fingerlings being caught along with market-size fish, improving the survival rate of fingerlings, and meeting the need to resolve the contradiction between propagation and fishing; third, it meets the need to resolve the contradiction between fingerling release and fingerling production; fourth, it is an effective way to carry out healthy culture, reduce the water area used for culture, and improve ecological, social and economic benefits.

2. Feasibility. According to the fish productivity that the natural food organisms in Miyun Reservoir can provide, the variety and quantity of fingerlings to be released were determined. Starting in June 2006, Shachang Reservoir (a medium-sized reservoir) was used, with farmers in the reservoir area cultivating fingerlings in cages, to ensure the quantity and quality of fingerlings for propagation and release in Miyun Reservoir, thereby achieving the goal of "purifying water, boosting fisheries and enriching the people." The fish productivity of filter-feeding fish in Miyun Reservoir supported by plankton is about 19.8 kg/mu. Starting from the goal of water purification, the fish species structure was adjusted so that silver carp and bighead carp account for 70% of the filter-feeding fish yield, and the catch standard for silver carp and bighead carp was set at 1.5 kg/individual. Calculating with a survival rate rising from 7.5% to 20%, the release quantity of silver carp and bighead carp fingerlings is about 45 individuals/mu to 52 individuals/mu. Because the water level of Miyun Reservoir fluctuates greatly, quantitative analysis of emergent plants is difficult to make accurately, so the release quantity of fish such as grass carp and bream can only be estimated from the relationship among many years of release quantity, catch and fish growth rate. According to 20 years of catch statistics, the fish productivity of herbivorous fish is about 0.6 kg/mu to 0.8 kg/mu. With the catch standard for grass carp set at 1.5 kg/individual and for bream at 0.5 kg/individual, and the recapture rate calculated at 20%, the release quantities of grass carp and bream fingerlings are each about 3 individuals/mu to 4 individuals/mu. The existing water surface area of Miyun Reservoir is about 137,000 mu, the release quantity of silver carp and bighead carp fingerlings is about 8 million individuals, and the release quantity of grass carp, bream and black carp fingerlings is about 1 million individuals.

II. Materials and Methods

1. Cage setup. The water area of Shachang Reservoir is 2,250 mu, and the water quality meets fishery water quality standards. The cages were set up in a sunny spot in the west bay of the reservoir, where the water depth is more than 5 m. Single-layer closed cages were used, with cage specifications of 5 m x 5 m x (2-3) m; the cages were submerged to a depth of 1.5 m to 2.5 m, with 0.5 m exposed above the water surface. Total number of cages: 65 single-layer PE polyethylene warp-knitted knotless cages of 5 m x 5 m x 2 m with a mesh size of 0.8 cm; 135 single-layer PE polyethylene warp-knitted knotless cages of 5 m x 5 m x 3 m with a mesh size of 1.5 cm. The cage frame was a square frame made of 4 Chinese fir poles 5 cm to 10 cm in diameter bound together, with frame specifications of 5.2 m x 5.2 m. At each of the four corners and the midpoint of each of the four sides of the frame, 1 steel bar support 1.4 cm in diameter and 50 cm high was installed to support and fix the cage. The cages were arranged in a straight line, with 25 to 30 cages per row, 50 m between rows, and 1 m to 2 m between cages.

2. Summer fry. The summer fry of silver carp, bighead carp, grass carp, bream, black carp and other fish came from the Yukou Fish Farm in Pinggu District. The summer fry were placed in the cages from June 22 to 28, 2006, at a stocking density of 86,000 individuals/cage, namely: silver carp 52,000 individuals/cage and bighead carp 34,000 individuals/cage, totaling 50 cages; grass carp 130,000 individuals/cage, totaling 2 cages; bream 104,000 individuals/cage, totaling 1 cage; on July 10, black carp summer fry were stocked at 12,000 individuals/cage, totaling 1 cage. From August 8 to 15, after thinning, silver carp and bighead carp fingerlings were divided into 100 cages, grass carp fingerlings after thinning were divided into 4 cages, bream fingerlings after thinning were divided into 2 cages, and black carp remained in 1 cage, totaling 107 cages.

3. Feed and feeding. The feed for silver carp and bighead carp fingerlings required all raw materials to be ground to a fineness of more than 80 mesh, and the compound feed, after secondary grinding, to reach a fineness of 60 mesh. Grass carp, bream, black carp and other fish fingerlings were all fed opening micro-pellet feed. Feed quality requirements: when the fish body size was below 5 g/individual, feed containing 38% crude protein was used; when the fish body size was 5 g/individual to 15 g/individual, feed containing 34% crude protein was used; when the fish body size was 15 g/individual to 25 g/individual, feed containing 31% crude protein was used. Domestication feeding began on the second day after the fingerlings were placed in the cages. At first, they were fed 2 times a day, and domestication was successful after about 3 days; after the fingerlings gathered to feed, the number of feedings was gradually increased to 6 times a day, with each feeding lasting about 10 minutes per cage. During feeding, dry powder feed was mainly scattered, but in windy weather it could be mixed with water and fed. The feeding amount was flexibly controlled according to the different sizes of fingerlings, stocking density, fish growth, water quality and climatic conditions. Under normal conditions, the daily feeding amount was 3% to 8% of the total fish body weight.

4. Daily management. Keep the water quality in the cages fresh. After the fingerlings were placed in the cages, the cages were cleaned once every 7 days by striking them with tree branches. Frequently check whether the cages were damaged or had holes, to prevent fish from escaping. Measure water temperature, observe the feeding and activity of the fingerlings, adjust the feeding amount at any time as needed, and keep good culture records.