FISH CULTIVATION IN NATURAL LAKES AND RESERVOIRS

In China the lakes are categorized as large, medium and small on the basis of surface area. Large lakes are those that exceed 100 000 mu in area; medium lakes are from 10 000 mu to 100 000 mu; while small lakes are from 1 000 mu to 10 000 mu. Bodies of water less than 1 000 mu in area are considered as ponds. The study group visited one reservoir, two large natural lakes, one medium and one small lake. These lakes were of relatively shallow depth, the maximum depth averaging 4 m, while the shallowest ones were of 2 m average depth.

All the lakes, including the largest one, Taihu, were used for fish culture and were under careful management. The smallest one, Tanghu, was managed more like a large fish pond, except that it is not drained or fertilized.

The stocking of selected species of fish, usually big head, silver carp, black carp and grass carp, is a common practice. Wuchan fish, common carp, crucian carp and bream may also be stocked. Emphasis is now on stocking of large-sized fingerlings which have a higher survival rate than smaller ones. Fish 15-20 cm in length and weighing over 50 g (over five months' old) are preferred for stocking. The stocking rate in Paitan Lake is about 150 fingerlings/mu. Eradication or control of predatory fish is done in the medium- and small-sized lakes. Several blocking devices are used to prevent the escape of stocked fish into connecting rivers and streams. The water level in the lakes is maintained at around 3 m. A form of integrated production is practised even in lakes.

For example, in Paitan Lake, fish are cultivated in deep water, lotus in shallow water, rice around the edge of the lake and mulberry trees on the dikes. The income from the lake has been appreciably increased by this. The income from sideline occupations is over 20 percent of the total income of Yuan 262 000 from the lake.

In the small and medium-sized lakes (Paitan and Tanghu) fishing is done only by the state farms which manage them, while in the larger lakes fishing is open to the members of the communes that control the lake. In all cases, fishing is done by the same group of people who are responsible for stocking and management of the lakes. Such collective ownership and management ensures a highly responsible approach to the utilization of resources.

In large lakes such as Taihu, closed seasons (March to May) are observed to protect brood stocks during their breeding season and to ensure enhanced recruitment to fish populations. For those species which attach their eggs to grasses or vegetation, clumps of floating grass are anchored over large portions of Taihu, to provide favourable breeding sites and thereby increase the chances of successful spawning.

Fishing gear has been improved to facilitate harvesting lakes. Special methods have been developed to concentrate fish in portions of the lake for easy harvesting. Harmful fishing methods such as the use of poisons and explosives are forbidden. Mesh-size regulation is enforced to ensure that fish below 1 jin in weight escape capture.

In Taihu there is a provincial-level Lake Management Committee, which includes representatives of the communes and fishing brigades around the lake, and technical personnel from universities and other institutions. The task of the Committee is to review available information on the lake, including the results of culture and capture, and to use it as a basis for the management of its fisheries. In general, stocked fish under 1 jin are not allowed to be caught. In at least some areas there is now a tendency to reclaim shallow portions of lakes into large ponds for intensive fish farming.

The Hubei Hydrobiological Research Institute carries out research on the limnology and fish production in Tanghu. This work, which includes studies of ichthyomass, primary production and carrying capacity of the lake, has indicated that the lake has a potential production of 2 million jin per year, whereas the actual production in 1977 was 1.3 million jin. It has also been found that the stocking of large-sized fingerlings increases the recapture rate of stocked fish from 1 to 20 percent. In 1977, 95.5 percent of all the fish caught in Tanghu were stocked fish, only 4.5 percent belonging to natural stocks in the lake. Thus the value of culture operations has been clearly demonstrated.

Among the problems encountered in the management of small- and medium-sized lakes is the control of predatory fish such as Siniperca chautsi, Erythroculter mongolicus, and Elopicthys bambusa. Some of these predators appear in schools during the spawning season and advantage is taken of this known habit to capture them by intensive seining. Destruction of spawning grounds is another measure widely adopted for the control of predators.

Fish production per unit area from the Chinese lakes varies with the size of the lake as in other countries. This is clear from Table 6 which summarizes some of the available data on stocking and annual yields of the four lakes visited by the study group. The smallest lake, Paitan, has the highest yield of 250 kg/ha, while the largest, Taihu, has the lowest, 53 kg/ha. Fish culture is admittedly the main source of increased fish production from lakes and reservoirs in the country.

The management of freshwater reservoirs is similar to that of small-sized lakes, except that feeding, manuring and stocking rates approach the standards adopted in fish ponds, if the reservoir is small.

Only one reservoir, the Mei-chuan Reservoir, was visited by the study group. It has an area of 2 500 ha or 37 500 mu, with minimum arid maximum depths of 7.5 m and 19.5 m, respectively. The annual yield was 100 t in 1977, which gives an average yield of only 40 kg/ha. This rain-fed reservoir, although built for irrigation, also provides water for 70 mu of fish ponds and generates power for local use. Fry are reared in the fish ponds and from these 12 million fry are stocked in the reservoir annually. About 1.2 million jin of grass and pig manure are applied as fish feed and fertilizer in the reservoir and associated ponds. The main problem faced in fish production in this reservoir is the low survival of stocked fish (5-7 percent). The fingerlings stocked are only 9-12 cm and fall easy prey to predatory fish.

Bismark Bangali
BSC  in Agrotechnology

  Khulna University.