Fish rearing in cages is a relatively recent activity in China, which is becoming more widespread. This farming system is carried out in different environments, either still on an experimental scale, or already on an industrial scale. Since fingerling production using this system has already been dealt with (Section 5.4), this section will deal with cage production of food-fish. The main data collected during the study tour are given in Table 36.
The district of Xishui (Hubei) has been developing fish culture in floating cages in the Bailianhe Reservoir since 1976. In May 1980, more than 7 ha of cages (approximately 2 400 units) were being farmed by 300 persons, 5.1 ha for food-fish production. A future expansion of this type of farming over a further 5 000 m2 was already being planned.
The cages consist of a floating bamboo frame to which is attached a net bag made of polyethylene (Figures 60 and 61). Unit capacity is 56 or 64 m3. The mesh is relatively small (maximum 2.5 cm). The frame usually lasts 2 years and the net 3–4 years.
Figure 60 Extensive fish culture in floating cages, Bailianhe, Hubei
Table 36
Cage production of food-fish
| Location | Bailianhe Reservoir (Xishui, Hubei) | Dianshan Hu (Qingpu, Shanghai) | Qiantang Jiang Tributary (Hangzhou, Zhejiang) | |||||
| 1. | Rearing cages | floating | fixed | floating | ||||
| - | unit area | 28–32 m2 × 2 = 56–64 m3 | 30–85 m2 × 2 = 60–170 m3 | approx. 30 m2 × 2 m | ||||
| - | mesh, cm | 1.5–2.5 | 1.25 | |||||
| - | density, cages/ha of water | approx. 8(240 m2/ha) | approx. 46(1 360 m2/ha) | |||||
| 2. | Stocking | |||||||
| - | size/unit weight | 13 cm/25 g | 200–250 g | 50+ g | ||||
| - | density, specimens/m2 | 25 | 9–18 | 20 | ||||
| - | rate, kg/m3 | 0.30–0.35 | 1–2.25 | 0.50 | ||||
| - | polyculture (%) | silver carp | 85 | grass carp | 94 | grass feeders | 70 | |
| bighead carp | 10 | bighead carp | 2 | plankton feeders | 30 | |||
| grass + black carps | 4 | crucian carp | 2 | |||||
| Wuchang bream | 1 | X. argentea | 2 | |||||
| 3. | Diet | natural | floating granules | granules + fodder | ||||
| 4. | Duration of rearing | 12–18 months (growth 6 months/year) | 6 months | 10 months | ||||
| 5. | Yield | |||||||
| - | size/unit weight | 0.5+ kg | 1–1.5 kg | 0.1–1.5 kg | ||||
| - | rate, kg/m3 | 5.6–7.5 | 9 (max. 13.5) | 7.5 (max. 11.3) | ||||
| 6. | Production | |||||||
| - | total kg/m3 | 5.3–7.5/year | 8 (max. 11.3) | 7 (max. 10.8) | ||||
| - | monthly average, kg/m3 | 0.44–0.63 | 1.33 (1.90) | 0.70 (max. 1.10) | ||||
| - | annual, t/ha | 2.5–3.6 | 41–58 | |||||
Figure 61 Bailianhe Reservoir, Hubei: on-the-spot cleaning of a floating cage net using a pressurized jet of water
The farming site is usually chosen to allow 2 m of water under the cages in all seasons. Since this type of farming is based essentially on natural feeding, the floating cages are anchored in areas of the reservoir which are rich in plankton, at a distance of 30 m from each other. They occupy an average of 240 m2/ha of the water area, at a rate of 8 cages/ha.
Polyculture is practised with the silver carp as the principal species (85 percent), in combination with grass and black carps (4 percent) and Wuchang bream (1 percent), to control the organisms growing on the net. To start with an average number of 25 fingerlings weighing around 25 g (13 cm) are stocked/m2 of cage. Since the annual growing period is only six months (May to October), it takes 12–18 months to produce food-fish weighing at least 0.5 kg.
During rearing, no supplementary feed is provided. Annual average production is relatively low - 11.2–15 kg/m2. This type of extensive fish culture is nevertheless considered advantageous in view of the small outlay involved (cost of net: Y 80) and the low cost of production. The food-fish costs Y 0.5/kg to produce and is sold to the State at Y 0.78/kg, which is considered a very satisfactory profit. Based on current costs, it is estimated that maximum profit would be obtainable with a farming area of 2 000 m2/person.
The only operation which must be done periodically is the cleaning of the nets. This is done on the spot, using a motor operated pump and jet of pressurized water (Figure 61). In summer this operation must be repeated every 6–10 days, while in winter, at 1–2 month intervals.
The district of Qingpu (Shanghai) has been conducting experiments on cage farming in the Dianshan Hu since 1979 (Section 9.5). The ‘Liberation’ People's Commune is in charge of these farms (Table 15).
A shallow area (maximum 3 m) of the lake was cut off by a protective bamboo screen. Fixed cages are arranged in this area in rows 12 m apart (Figure 62). In each row the cages are 4 m apart. A staggered row arrangement is also being experimented. The total surface area of the cages is 2 490 m2 for 65 units. They cover an average of 1 360 m2/ha of water at a rate of approximately 46 cages/ha.
Figure 62 Experimental fish farming in cages in the Dianshan Hu, district of Qingpu, Shanghai. Protected enclosure and watchman's house
The fixed cages consist of a nylon net bag, attached to a number of bamboo poles (Figure 63). The depth of water in the cages never exceeds 2 m and the net is therefore 0.5–1 m from the bottom. Three sizes of cage are being tested: 30.5 – 36 – 85 m2.
Figure 63 Dianshan Hu, Shanghai: fixed cages for intensive fish polyculture
Polyculture is practised where the grass carp is the principal species (94%), which is combined with bighead carps (2%), crucian carps (2%) and Xenocypris argentea (2%). These last two brousing species are used to control net fouling.
Large fingerlings (200–250 g) are stocked in April at three different densities (9 – 12 – 18 specimens/m2). The fish are fed floating granules (daily ration 2% biomass) and aquatic grasses. The granules are made of soy bean cake (50%), colza cake (30%), barley or wheat bran (15%) and fish meal (5%). They are manufactured locally at a price of Y 0.44/kg and their nutritional quotient in the cages is relatively low (Qu 2). Harvesting takes place after about six months of rearing and fish weighing 1–1.5 kg are taken. The only disease observed has been enteritis in grass carps.
Net cage output in 1979 was 16.1 kg/m2 on average. Grass carps are therefore produced at a cost of Y 1.50/kg and are sold for Y 1.60/kg, which is a very small profit margin. The cost price includes the cost of fry, the cost of the cage, which is amortized in three years (e.g., a cage measuring 36 m2 × 2 m = 72 m3 costs Y 180), the cost of artificial feed and labour. As regards the latter, it is estimated that an average of five persons per mu of cages (667 m2) are required for stocking, making granules, daily feeding, cage maintenance, supervision, harvesting, etc.
The Hangzhou Experimental Centre of Aquatic Products (Zhejiang) is carrying out experiments on intensive fish culture in floating cages, in a tributary of the Qiantang Jiang (Figure 64). The depth of water varies between 3 and 4 in, depending on the season.
Figure 64 Fish farming in cages in the Qiantang River, at the Hangzhou Experimental Centre, Zhejiang
The farm covers a total area of 2 000 m2 of cages of a depth of 2 m, i.e., a total capacity of 4 000 m3. Carp polyculture (grass, silver, bighead, common carps) is practised in combination with Wuchang bream and Sarotherodon mossambicus. The fish are fed on fodder and rice bran or wheat bran granules.
Fingerlings weighing 50 g are stocked at a density of 20 specimens/m2. After ten months rearing, food-fish of variable individual weights are harvested: tilapia 100–250 g; Wuchang bream 500 g; grass carp 1–1.5 kg; other carps 0.5–1 kg. The average harvest is 15 kg/m2, although a maximum of 22.5 kg/m2 was obtained in the past.
It must be mentioned here that fingerlings (Section 5.4) have already been produced in navigable channels in Zhejiang province. Similar technology could be applied to food-fish production (Figure 65).
Figure 65 Fixed cages in the channels in the district of Shaoxing, Zhejiang. Artificial feed is distributed in the tray inside the main frame
