高村 典子 環境庁国立公害研究所, 主任研究員 (80132843)
前田 広人 滋賀県琵琶湖研究所, 主任研究員
岩田 勝哉 和歌山大学, 教育学部, 教授 (10031816)
小長谷 庸夫 三重大学, 生物資源学部, 教授 (30024821)
MAEDA Hiroto Senior Researcher, Shiga Prefecture Lake Biwa Institute
TAKAMURA Noriko Senior Researcher, National Institute of Environmental Studies
KONAGAYA Tsuneo Professor, Faculty of Biological Resources
IWATA Katsuya Professor, Faculty of Education, Wakayama University
The production processes in the Dianshan Lake type fish culture which is one of a Chinese integrated fish culture techniques, and shows an extremely high yield, had been considered as follows. Faeces egested by grass carp (Ctenopharyngodon idella) and a Chinese bream (Megalobrama amblycephala) which are given macrophytes harvested from the lake, is decomposed, and plays a role as an important supplier of inorganic nitrogen and phosphorus. The nutrients being continuously regenerated, accelerate phytoplankton production, resulting a good food condition for zooplankton and phytoplanktivorous fishes. Consequently, zooplanktivorous fishes are also given a good food condition. However, it is hardly understood that the pond produces such a large amount of fishes only by the processes described above. Hitherto, roles of micro-organisms on fish production have never evaluated in the researches on this type of pond. Thus, we have started the present study on the hypothesis that micro-organisms
smaller than 10 mum, including bactetria, picoplankton, nanoplankton, and protozoa, play additional roles on the fish production through food webs from them to the fishes.
The research work was made from September 12 to October 13 at the Qingpu Fish Farm. The area of the pond is approximately 0.5 hectare and 2.5 m deep. The concentration of NH_4-N and NO_3-N was very high and was classified as hypertrophic. PO_4-P was extremely low throughout the studying period. However, judging from the P/N ratio of phytoplankton, phosphorus was not to be a limiting factor. Since the regeneration rate of phophorus from the bottom was several times higher than those observed in lakes, the phosphorus supply was supposed to be sufficient for the phytoplankters.
The concentration of bacteria was as high as those in a eu- or hypertrophic water. Particularly, the bacteria showed the highest density in the lower layer where suspended particles were also highest. Among phytoplankers larger than 10 mum were scarce, probably due to the strong feeding pressure of planktivorous fishes. Nano- (2-10 mum) and picoplankton (0.2-2 mum) possessed 70-87% of the total amount of chlorophyll of the phytoplankton assemblage. That of picoplankton observed in the pond was one order higher than those reported so far, and its photosynthetic activity was higher than those of the remainders.
The cells of the macrophytes in the intestines of the grass carp and Chinese bream were rarely damaged, and their tissue surfaces were attached with bacteria. Particularly, those of the former herbivor were covered by larger bacteria than those on the faeces, in a greater number than those on the faeces. The result suggests that the bacteria grow in number in the intestine, and are utilized as food by the fish. The amount of organic-carbon and -nitrogen, and amino acids per unit of dry weight of grass carp faeces hold in a laboratory tank, increased day by day significantly. The SEM observation indicates that attached particles on the macrophyte tissue in the faeces are scarce at the initial stage, and 6-8 days later the faeces became a large fragment densely attached by diatoms (Cocconeis placentula dominated), bacteria, and unidentified matters. Numerous identical particles were also found in the intestines of bighead carp (Aristichthys nobilis) and a phytoplanktivorous crucian carp (Carassius auratus cuvieri). Because the faeces of an omnivorous crucian carp (Crassius auratus) contained a large amount of chlorophyll b(25-49% of the total pigments), it is highly possible that the carp also utilize the organic particles as food.
The particle size in the intestine of silver carp (Hypophthalmichthys molitrix) is much smaller in comparison with that of bighead carp, and the faeces contained only a little amount of chlorophyll b. These facts indicate that the silver carp ingest mostly phytoplankton. In addition, the result of analysis of chlorophyll a contained in the ingested algae suggests that the pigment is not reduced into inactive ones during the intestine passage.
The photo-synthetic activity per unit amount of chlorophyll a of the silver carp faeces was estiamted to be higher than that of the bighead carp faeces, and to be approximately 50-100% of that shown by the pond phytoplankton. It is noticeable that the activity increases markedly when the faecal bellets are kept in a dark condition for 24 hours (1.5 to 4 times higher than that of the pond algae in the silver carp and 1.5 times in the crucian carp).
Through this study, the world of microbes smaller than 2 mum in the pond was unveiled to some extent. The microbes and attached diatoms form a community on a faecal fragment. The fragmental community can be ingested by the planktivorous fishes, and nourish them. All these are new findings. It is highly possible that such processes as these are related to acceleration of the poud productivity. The next step for this study is a quantitative evaluation of the processes in the whole system.