Aquaculture, 71 (1988) 301-312 Elsevier Science Publishers B.V., Amsterdam -
301 Printed in The Netherlands
Pen Culture in Wuli Lake, Jiangsu, China SHEN PEIRONG’ and XU BINCHEN’ ‘Asian-Pacific Regional Research and Training Centre for Integrated Fish Farming, Wuxi (China) ‘Freshwater Fisheries Research Centre of the Chinese Academy of Fisheries Sciences, Wuxi (China)
(Accepted 26 October 1987)
ABSTRACT Shen Peirong and Xu Binchen, 1988. Pen culture in Wuli Lake, Jiangsu, China. Aquaculture, 71: 301-312.
Two trials were run in which small fingerlings of Chinese carps and tilapia were reared to marketable-sized fishes in one year. The pen was made of single-layer polyethylene nets without nodes. In 1985, the pen area was 4.8 mu (15 mu= 1 ha) and the average yield was 305.75 kg/mu with a marketing rate of 98.7%. In 1986, the area was 4.5 mu and the average yield was 793.5 kg/mu with a marketing rate of 90.23%.
China has vast inland water resources including lakes. The total water surface of lake in China is about 111.3837 million mu, of which 28.0478 million mu are suitable for fish culture (Wang Hongdao, 1987). But only 9 328 700 mu has been utilized so far for fish farming, which accounts for 33.26% of the culturable water surface. The fish production in lakes, however, is very low, only 17.56% of the average ponds production of 102 kg/mu. Therefore, there is a great potential for the development of intensive fish farming in lakes, in particular through pen culture. Statistics show that the total pen area for fish culture in Jiangsu Province in 1986 was around 200 000 mu. Mainly the traditional monoculture system is practised, that was to rear 2-year-old fingerlings of lo-17 cm and sometimes marketable fish as well. In the traditional practice, bamboo fence and fishing nets were commonly used for the construction of a pen, which always caused the problem of high investment and maintenance and shortage of seed supply. In order to promote the optimal utilization of pen space and natural food available, and to improve the traditional practice in pen culture, two trials were
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conducted in Wuli Lake, Jiangsu Province, from 1985 to 1986. In these two trials, small fingerlings of various fish species, after being cultured in ponds for 25-30 days (Lei Huizhen, 1981)) were transferred into the pen and cultured to marketable-sized fish in one year through the polyculture system. In addition, large fingerlings were also produced for the following production cycle. The improved technology has the advantages of a shorter culture period, lower stocking amount and production cost, and quicker economic returns. The results obtained may provide a basis for further research on the development of lake resources and the improvement of pen culture technology in China and elsewhere. Moreover, they may also provide a better technical package for farmers to apply. MATERIALS
Wuli Lake was selected as the experimental site. Then pen was constructed 30 m away from the lake bank and 40-50 m away from the main channel (Fig. 1) . The bottom is comparatively flat, with humus and sandy clay. The water level is relatively stable and there is a slow water current. In 1985, the pen area was 4.8 mu (15 mu= 1 ha) and total stocking of fingerlings was 120.51 kg (on average, 25.11 kg/mu). The experimental period was 153 days (from 17 June to 16 November). In 1986 the pen area was 4.5 mu. The stocking of fingerlings of various fish species and different size was carried out separately from 10 March to 14 August. The total stocking weight was 377.8 kg (on average, 83.95 kg/mu). During the experiments, daily measurements of the ecological parameters such as water temperature, depth, oxygen, transparency and pH were made at 08.00 h. Oxygen was determined with a YSI-57 Oxygen Meter and pH was measured by titration. Water flow rate was determined several times per month. The pen for the trial was made of single-layer polyethylene nets without nodes. The mesh size was 1 cm and the height was 4 m. These dimensions were specially chosen based on the previous records of water depth (average annual water depth 1.51 m), flow rate, waves, fish species and other factors. There was a cover-net with a height of 1.0 m and mesh size of 2.5 cm attached to the pen head line to prevent fish escaping. The head line was fixed with 500-g plastic floats at a distance of 0.5 m. The bottom line was fixed with iron chains weighing 11 kg/m. Bamboo was also used to fix the pen which was either in a round or elliptical shape. In 1985, fry of common carp (Cyprinus carpio), crucian carp (Carassius auratus), blue tilapia (Oreochromis uureus ) , and grass carp (Ctenophuryngodon idellus) were first reared in ponds for 25-30 days and then transferred into the pen for further culture. Fingerlings of silver carp (Hypophthalmichthys molih-ix) and bighead carp (Aristichthys nobilis) with a body weight of 107 g and 52 g respectively were stocked directly into the pen. In 1986, silver carp and
Fig. 1. (a) Traditional construction Wuli Lake, Jiangsu. China.
of a pen of bamboo. (b ) Pen as used during the 19186 trial in
bighead carp, as well as wuchang fish (it4egalobrama anblyocephula) were also stocked in addition to the above species to better utilize the natural organisms in the water. Common carp and crucian carp fingerlings were reared in ponds for another 10 days because fish purchased were too small. Fish were randomly sampled and measured once per month using a cast net or a small seine. Fish body condition was calculated based on the Fulton formula (Shanghai Fisheries College, 1982): K=W/
Where W=weight (g) and L=length (cm). Wild fish were eradicated or frightened to escape by beating the water before setting up the pen. The pen was further netted before stocking the fingerlings. In the first 7-10 days after stocking, fingerlings of grass carp and wuchang fish were fed with duckweeds ( Wolffia arrhizas, Lemm), and common carp, crucian carp and tilapia were supplied with soyabean cake paste. Later on, a feeding platform, either round or rectangular, was set up with dimensions of 80 cm diameter or 60x40~ 10 cm, respectively. The platform was hung on bamboo and was 50 cm above the pen bottom, but it could be moved with a change of water depth in the lake. When the feeding platform was established, feeds such as soaked wheat and cake paste were put on it, at a feeding rate of 5-6% of total fish body weight. Grass was put into a feeding frame of bamboo. The main types of feeds used in the trials were bean cake, wheat and pelleted food, and a certain amount of green fodder was supplied. In general, the feeding was properly adjusted every 7-10 days, based on the water temperature, season, fish growth and feeding status. RESULTS The results indicate that the ecological parameters of the water of Wuli Lake were very favourable to the growth of fish (Table 1) . The average oxygen content was 7.5 ppm and the maximum was 9.5 ppm; pH was 6.5-7.5; average water depth was 1.6 m, with a minimum of 1.25 m; average water temperature was 20.6”C and transparancy, 38.9 cm. The details of stocking and harvesting for 1985 and 1986 are shown separately in Tables 2 and 3. In 1985, stocking of fish began on 17 June. All fish were harvested on November 16 and the experimental period was 153 days. A total of 25 601 fingerlings of various species was stocked with a total weight of 120.51 kg. On average, the stocking number was 5333 fingerlings/mu and the weight was 25.11 kg/ mu. At the end of the trial, a total of 4892 fish was harvested with a production of 1467.6 kg. The average fish yield was 305.8 kg/mu with a marketing rate of 98.7%. Common carp, crucian carp, blue tilapia, silver carp, bighead carp and grass carp with mean stocking sizes of 1.18, 1.48, 0.67, 107, 52 and 1.34 g in-
305 TABLE 1 Monthly changes of ecological parameters in the pen Mar.
Water depth (m)
Water temperature (C” 1
creased up to 321.7,172.6,174,621.5,468.7 and 153 g after 153 days of culture. Their total body weight increments were 66.4,35.7,22.4,5.0,2.5 and 1.4 times, respectively. In 1986, stocking of fish began on 14 June and all fish were harvested on 15 November. The experimental period was 152 days. The total stocking was 64 800 fish of various species of different sizes with a total weight of 377.8 kg. On average, the stocking number was 14 402 per mu and the stocking weight was 83.95 kg/mu. At the end of the trial, a total of 10 935 fish was harvested with a production of 3568.52 kg. The average yield was 793.5 kg/mu with a marketing rate of 90.23%. The body weight of common carp with initial stocking sizes of 4.45 and 1.85 g increased to 705 and 335 g and their body weight increments were 98.3 and 5.98 times, respectively. For crucian carp, weight increased from 4.16 and 2.0 g to 261.4 and 108.8 g, the increments being 38.24 and 7.57 times; wuchang fish increased from 1.30 and 50 g to 85.6 and 675 g with body weight increments of 14.86 and 12.3 times; tilapia, silver carp, bighead carp and grass carp increased from 0.7, 72.3, 71 and 5.0 g to 321.5, 675, 890 and 63 g, with body weight increments of 31.3, 7.25,10.97 and 3.55 times, respectively. Grass carp did not grow well because the stocking size of 1.34 g was too small and mortality was high in 1985. Although the stocking size of grass carp was increased to 4.19 gin 1986, a sudden tornado on 6 August damaged four bamboo stakes used for fixing nets and, as a result, a large quantity of grass carp escaped. The average daily increments of body weight of different species are shown in Fig. 2. The daily body weight and body length increments of the different species changed with the season, water temperature and feeding rate. The highest peak of daily body length increment usually appeared prior to the max-
Common carp Crucian carp Silver carp Bighead carp Grass carp Blue tilapia Wild fish
17 Jun. 17 Jun. 3 Jul. 3 Jul. 13 Jul. 4 Jul.
11.83 8.13 47.95 39.05 13.21 0.34
5500 448 751 9854 500
1.18 1.48 107 52 1.34 0.67
Total weight (kg)
Stocking and harvesting in the pen of 4.8 mu in 1985
2083 1146 93 15.7 2053 104.2
2.46 1.64 8.89 8.14 2.75 0.39
Stocking (per mu)
16 Nov. 16 Nov. 16 Nov. 16 Nov.
321.7 172.6 621.5 468.7 153 174
2441 1683 384 216 124 44
785.5 290.5 238.9 101.2 19 7.6 25
Total weight (kg)
24.4 30.6 85.7 45 1.2 8.8
Catch rate (%)
509 351 80 45 26 9
163.6 60.5 49.8 21.1 4 1.6 5.2
Catch (per mu)
66.4 35.7 5 2.5 1.4 22.4
Times of body wt. _ increment
Common carp Common carp Crucian carp Crucian carp Wuchang fish Wuchang fish Bighead carp Silver carp Blue tilapia Grass carp Grass carp Wild fish
Stocking (per mu)
Stacking and harvesting in the pen of 4.5 mu in 1986
Catch (per mu)
Times of body wt.
Body weight lncremen t (gl 6 5
5 5 6.0 5.0
Fig. 2. Daily increment of body weight of fishes in 1986.
TABLE 4 Condition coefficient of fish reared in the pen ( 1986 )
Jun. Jul. Aug. Sep. Oct. Nov.
2.4 2.8 3.1 2.7 3.3 2.5
1.5 2.3 3.9 4.1 4.9 4.1
2.8 3.1 4.1 4.6 4.8 4.9
1.1 2.2 3.6 2.8 3.3 3.2
1.05 0.90 1.17 1.08 1.17 1.18
1.05 1.35 1.17 1.21 1.20 1.38
Blue tilapia 7.77
imum daily increment of body weight. In this trial, the body weight of common carp increased considerably every month after stocking. The maximum average daily increment of body weight was 5.8 g in September, but it declined afterwards. The maximum daily increments of body weight of crucian carp, wuchang fish and tilapia were 3.9 g in August, 1.2 g in September and 2.2 g in September, respectively. The average daily body weight increment of grass carp was 3.73 g in August. Silver carp had two high peaks of daily body weight increment and one high peak of daily body length increment. The maximum daily increment of body weight of silver carp was 6.1 g in July and 4.72 g in September and its maximum daily increment of body length was 1.78 mm in July. On the contrary, bighead carp had one high peak of daily body weight increment and two high peaks of daily body length increment. The maximum daily body weight increment of bighead carp was 6.6 gin July and its maximum daily increment of body length was 2.2 mm in May and 2.3 mm in July. In these two trials, commercial food was supplied. In 1985, the total consumption of commercial food was 2903.5 kg with a food conversion ratio of 2.16. In 1986, the total input of commercial food was 6774 kg, supplemented with 1500 kg of submerged aquatic weeds and 400 kg of duckweed on a fresh weight basis. The food conversion ratio of commercial food was 2.14 and that of wild celery ( Vullkneria spiralis) and duckweed was 100 and 20, respectively, on a fresh weight basis. Wuli Lake has good water quality and was rich in natural organisms. The average annual biomass of phytoplankton was 14.61 mg/l with the dominant species Cryptomonas uvata, Crucigenia tetrapediu, Merisemopedia tenuissima and Amphora oualis, and the average annual biomass of zooplankton was 3.05 mg/l with the dominant species being cladocerans, copepods and rotifers (Dalin Fisheries College, 1982). The condition coefficient of the different fish species varied with water temperature, feeding rate and other factors. The maximum condition coefficient occurred from October to November but declined with the decrease of water temperature, and fish growth slowed down accordingly. Table 4 shows that the maximum condition coefficient of grass carp, wuchang fish, crucian carp, common carp, silver carp and bighead carp was 3.3, 3.6, 4.9, 4.9, 1.18 and 1.38, respectively. It was found in these trials that fish reared in the pens were less susceptible to disease than in ponds because the ecological conditions were more favourable for growth. Only grass carp was affected with enteritis and that was controlled by the application of medicated food once a day for 6 days. The common chemical used was sulphaguanidine at a rate of 10 g/100 kg of fish. The amount was reduced by 50% from the 2nd day. The catching rate was closely related to the stocking sizes of the different species of fish (Table 5). Fish with an initial stocking size below 2 g had a low
TABLE 5 Stocking sizes and catching rate of different fishes in 1986 Species
Common carp Common carp Crucian carp Crucian carp Wuchang fish Wuchang fish Blue tilapia
4.45 1.85 4.16 2.0 50.0 1.3 0.7
2000 5500 3000 3000 300 7000 1900
1247 177 1825 1111 274 1579 133
62.4 3.21 60.8 13.9 91.3 22.6 7.0
catching rate. For example, the catching rate of common carp, crucian carp, wuchang fish and tilapia was 3.21 13.9, 22.6 and 7%, respectively. On the contrary, those fishes with an initial stocking size above 4 g all had a higher catching rate: common carp, 62.4%; crucian carp, 60.8%; and wuchang fish, 91.3%. Thus, on the basis of these trials, the stocking sizes of fish should be above 45 g in body weight and 5-7 cm in body length for culturing in lake pens. DISCUSSION
The main aim of our study was to rear small fingerlings of different species of fish to marketable-sized fish in one year in the pen. Therefore, the selection of healthy fingerlings was very important. Common carp and crucian carp are bottom feeders which have the characteristics of strong adaptability and fast growth. Blue tilapia was recently introduced into China and it grows well after local acclimatization. Blue tilapia reared in the pen fed on filamentous algae attached to the net and so maintained water exchange. In 1986, fingerlings of common carp, crucian carp and tilapia all reached marketable sizes at the end of the trial. The production was 1533.12 kg comprising 44.13% of the total output. Grass carp and wuchang fish, which are herbivorous, live in the mid and lower water layers. Polyculture of different species of various sizes not only increased fish production, but also produced large fingerlings for the next cycle of production. Meanwhile, water space and food available in the pen could be fully utilized. The production of these two species accounted for only 8.8% of the total output, due to the natural disaster. For the better use of natural organisms available in the water column, it was very beneficial to mix in silver carp and bighead carp so as to increase fish production and the economic returns per unit area. The total production of silver carp and bighead carp was 1457 kg, comprising 40.83% of the total out-
put. But it may not be neccessary to stock filter-feeding fish such as silver carp and bighead carp if the lake water is poor in quality and natural organisms. Fish growth is not only related to their genetic makeup, but is also associated with the environmental conditions such as oxygen, water temperature, pH, water depth and biomass of natural organisms. Generally speaking, Chinese carps have optimal growth and feeding rate and the lowest food conversion ratio when they are reared at a water temperature of 25-32 “C and oxygen content of 4.5-5.5 ppm. During the trial, the water temperature from June to September (the optimal growing period for fish) ranged from 25 to 29°C; therefore, all species of fish grew well. Pen culture in lakes is a semi-artificial ecosystem which is very profitable for fish farming. In order to increase the production of high-quality fishes, fish which take artificial feed should be stocked as the target species and reared with advanced technology as practised for fish culture in ponds. In general, the following proportions of species are recommended for stocking in a polyculture system: common carp 15%, crucian carp 20%, blue tilapia 5-8%, wuchang fish 20%, grass carp 30%, and silver and bighead carps 7-10%. In addition, pen culture should be started as early as possible so as to extend the growing period of fish and, in turn, increase fish production. In Wuxi, stocking should be done before early July, otherwise, a high mortality of fish will occur due to the difficult transportation of fingerlings in hot weather. Because of the natural disaster in 1986,lO 000 grass carp fingerlings with a body weight of 5 g were restocked on 14 August. Although the distance of the transportation was not too far, 832 fingerlings died due to hot weather, a mortality of 8.32%. Since the grass carp fingerlings were in poor health, their mortality was still high after stocking. The final harvest rate was only 28.21% of the total number stocked. The stocking size in the pen should be suitable. Stocking of too small fingerlings affects the survival rate and production. There are always slight waves in lakes, which may cause fish to get exhausted by continuously swimming against water currents. As polyculture of different species with various sizes is practised, very small fish will not grow well and sometimes may be eaten by predatory fish due to their poor abilities to feed and escape from their enemies. It is very obvious that good management is a prerequisite for the success of pen culture in lakes. Fingerlings transferred from stagnant ponds need a certain time to adapt themselves to the new environment with slow water currents. In order to promote fish growth and adaptabilities, suitable food should be applied as early as possible in accordance with their feeding habits. In the first 7-10 days, duckweeds were applied around the pen for grass carp, crucian carp and blue tilapia. But 4-5 kg of fish meal were mixed into every 100 kg of soaked bean cakes to promote the feeding rate of fish. As fish grow, they are attracted to take food around the feeding platform. This reduces the waste of
food, the feeding status can be checked and disinfection and treatment of fish diseases can be carried out. A feeding frame was set up especially for the application of grass or green plants which otherwise might be spread over the water surface of the pen by waves. Another main task of management is to discourage water rats which often damage nets when eating dead fish. Dead fish should be netted out timely. The escaping of fish should also be taken into consideration and prevented. In general, nets under the water surface need to be checked once or twice every month. Through our 2-year trials, it was found that pen culture in lakes with polyculture of various species of different sizes not only increased fish production and produced large fingerlings for the next production cycle, but also promoted the optimal utilization of favourable conditions of lake water for fish farming. These experimental results may provide a scientific basis for the further development of lake resources in China and elsewhere. ACKNOWLEDGEMENT
We thank Prof. Zhu Linggen and Prof. Peter Edwards for reading of the manuscript and Dr. S.J. de Groot for his valuable suggestions. We also thank Mr. Zhou Enhua for his active participation in this work and translation of the paper.
REFERENCES Dalin Fisheries College, 1982. Freshwater Biology. Agricultural Publication Corporation, Beijing, pp. 11-76. Lei Huizhen, 1981. Fish Culture in Ponds. Shanghai Scientific Publication Corporation, pp. 156157. Shanghai Fisheries College, 1982. Ichthyology and Culture of Marine Fishes. Agricultural Publication Corporation, Beijing, 292 pp. Wang Hongdao, 1987. The analysis and assessment, of water quality in lakes of China. Oceanol. Limnol. Sin., 18 (1): 12-14.