In modern concentrated animal farms, a lot of manure arises which contains dissolved and undissolved organic material in large quantities. This secondary product can be used in plant cultures or in fish ponds. This utilization procedure has several steps. The organic material disposed in the pond is transformed by micro-organisms under aerobic conditions. The resulting organic material is nutriment for algae. Algae transform this inorganic material into organic plant material by using solar energy. The algal biomass thus resulting is food for the next consumption level for zooplankton.
Phyto- and zooplanktons together form plankton biomass which is the natural food for fish.
Utilization of organic fertilizers in fish culture has a long tradition. It is an accepted practice in Far Eastern countries where climatic conditions result in rapid metabolic processes.
Considering the allegation of Wohlfarth (1978) the manure can be a fodder replacing feed, it would be negligent not to exploit this source.
Only a part of the huge amount of pig manure accumulating in pig farms is utilized in the fields, so the excess, which contains high amounts of direct and indirect proteins (Moav et al., 1977), can be well used in increasing natural nutrient content of fish ponds.
Schroeder and Hepher (1975) found that manure load causes a profound change in the natural nutrient cycle of fish ponds, coming about by the organic matter decomposing activity of bacteria and protozoa. Micro-organisms getting into the water with the manure also become fish feed. Water bacteria of 20-30 m m size are also consumed by pelagic fish (Kuznetsov, 1977).
The optimal utilization of manure depends greatly upon the stocking structure. Yashow (1971) demonstrated that on the influence of feeding interaction there is a nutrient movement among the different specific zones in polycultural fish ponds. The findings of Wohlfarth (1978) unequivocally proved the yield stimulating nature of polycultural fish pond management. Leventer's (1981) studies also confirmed it when he proved that algal production is higher in polycultural than in mono cultural fish ponds. Put the yield is influenced by the way manure is used as well, for example, to preserve useful compounds by keeping them away from air. It is also important to ensure the proper balance between loading and decomposition of the manure in a pond: therefore, we followed the practice of daily disposal proposed by Moav et al. (1977) and Schroeder (1974).
2. MATERIALS AND METHODS
The aim of our experiment performed in 1980 was to find the optimal dose of manure ensuring the highest yield of fish as well as the best water quality. In 8 earthen experimental fish ponds of 0.17 ha each, we applied different manure loadings. The liquid pig manure from a nearby pig farm was rather diluted, due to the great amount of water used in the management regime. Dry matter content was about 1 percent. The composition of the manure was regularly analysed. The total nitrogen content was 8 g/l, while the total phosphorus was 1.2 g/l.
In each pond an identical stocking structure was used, with a dominance of silver carp. Considering the feeding competition between common and bighead carp, the stocking rate of the former was only 27 percent. The third member of the structure was grass carp. One and two year old silver and common carps were stocked in order to get an answer to the question as to which age group could better tolerate the new conditions.
In a follow-up experiment in 1981, the aim was to find the optimal stocking structure. Six combinations of stocking structure were employed in 8 experimental ponds, with 2 fish species only, silver and common carp, the former in varying numbers. The optimal amount of manure applied (with 1 percent dry matter content) was 1 035 m3 in 1980. Since the manure used had 4 percent dry matter content, this amount was 200 m3 in 1981. Nitrogen content was 8.6 g/l and phosphorus was 1.4 g/l. In the control ponds, inorganic fertilizer was applied containing only 150 kg/ha nitrogen and 20 kg/ha phosphorus.
The aim of the third year's experiment in 1982 was to compare the polycultural and bicultural systems. We wanted to get an answer to the question of which stocking structure was better in respect of water quality and fish production.
2.1 Short Description of Technology
The liquid manure from the pig farm was transferred in tank vehicles into a tank, from which it was pumped to the sprinklers through aluminium pipes and then sprayed into the ponds. Clean water for rinsing the pipes was taken from the water supply channels of the ponds. The pump had two forks, one for the manure and the other for cleaning water.
2.2 Equipment and Facilities
The CSN-301 type revolving pulley-pump has two main parts: a flexible rubber cylinder and a revolving metal pulley sucker. The pump is suitable to transfer pulpy, fibrous material of high, viscosity. By changing the rpm its carrying capacity can be controlled between 4.5 m3/h and 16 m3/h. The pump operates according to the volumetric displacement principle.
Two factors were considered when establishing the optimal dose of liquid manure applied. One was the highest fish yield, the other the quality of effluent water (Figure 1). Taking the yield of the control pond as 100 percent, we reached 109 percent when using 1 030 m3 liquid manure, which corresponds to a net yield of 2.4 t/ha. The net yields of the other treatments were far behind this value. Similar yields were obtained with 1 or 2 years old common carp, with the exception of one pond where the lack of feed was obvious, i.e. 560 m3 liquid manure was insufficient.
Both age groups showed very good growth rate at 1 030 m3 manure loading. The highest or lowest doses did not favour the growth rate of the 2 years old group.
Grass carp showed the best growth rate in ponds with a low amount of liquid manure. Due to good light conditions, the well growing macrophytes ensured the specific nutrient demand of grass carps, but the increase of manure loading did not result in increased yield at the same light conditions. There was a considerable mortality when we increased the manure loading, especially with the 1 year old silver carp. The loss with the 2 years old stock was far less. During the regular pilot harvests, we investigated the gill and general health condition of all the fish. No ill or deteriorated fish could be seen.
Based on experience obtained in 1981 (Figure 2/a), further work is needed to; find the optimal rates and stocking structure. A yield of 1 000 g fish was obtained only at a very low stocking rate (1 500/ha), which should be increased. While the weight of common carp was above 1 000 g in each combination at a stocking rate of 1 000/ha, the weight of grass carp decreased along with the increase of stocking rate (Figure 2/b). The highest net yield was obtained with the treatment resulting in the lowest individual growth rate. However, silver carps had not even doubled their original weight.
In 1982, the best net yield of 2.5 t/ha was obtained in a bicultural pond (Figure 3). The population was of good quality and healthy. Comparing the yield of the 4 bicultural and 4 polycultural ponds with identical stocking structure, the bicultural form proved to be better, with an average net yield of 2 064 kg/ha. The yield of the pond with polycultural stocking was 1 444 kg/ha.
The quality of effluent water did not show any difference with the two stocking structures applied.
Kuznetsov, Y.A., 1977, Consumption of bacteria by silver carp. J. Ichthyiol., 17:398-404
Leventer, H.,1981 Biological control of reservoirs by fish. Bamidgeh, 33:1
Moav, R. et al. 1977, Intensive polyculture of fish in freshwater ponds. 1. Substitution of expensive feeds by liquid cow manure. Aquaculture, 10:25-43
Schroeder, G.L., 1974, Use of fluid cow-shed manure in fish ponds. Bamidgeh, 26:84-96
Schroeder, G.L. and B. Hepher, 1975, Wastewater utilization in Israel aquaculture. Ann Arbor, Michigan, Michigan University
Yashow, A.,1981, Interaction between common carp and silver carp in fish ponds. Bamidgeh, 23:85-92
Wohlfarth, G.,1978, Utilization of manure in fishfarming. In Report of the Proceedings. Fish-farming and wastes: an International Conference sponsored by the Institute of Fisheries Management and the Society of Chemical Industry, Water and Environment Group. University College, London, 4-5 January 1978. Edited by C.M.R. Pastakia. Carlisle, Institute of Fisheries Management, pp 78-95
Figure 1. Relationship between fish yield and liquid manure
Figure 2 a. Fish yield in 8 earthen experimental fish ponds
Figure 2 b. Relationship between weight and stocking density of common carp
Figure 3. Fish yield in polycultural and bicultural systems