The composition of causative organisms of Aoko (cyanobacteria bloom) in Lake Mikata have been confirmed to be alternately dominated by Microcystis and Planktothrix genera. These two species are already known to have different growth characteristics to the same salinity. Thus this paper attempts to express the succession processes of two cyanobacteria that appear in Lake Mikata, in terms of a numerical model. In the culturing experiments for the tolerance to salinity, M.aeruginosa proved that it could not be grown in a salinity of more then 5 psu. This phenomenon also coincided with the observational results in the north end of Lake Mikata where the salinity is more than 5 psu. However, P.agardhii was observed all over Lake Mikata. Based on the results of culturing experiments, the inside natural growth rate (μ), environmental carrying capacity (K) and interspecific competition coefficient (γ_<i,j>) were formulated. In addition, we compared the numerical experiment results with a box model with field observational results that proved the appropriateness of a succession model on the basis of the observational results and competition model.

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The mega-infaunal community of an artificial tidal flat in Mikawa Bay was investigated for two years in order to evaluate its biological production and water purification function. A total of 47 species were identified, which was comparable to natural tidal flat in Mikawa Bay. The dominant species were the bivalves, such as Mactra chinensis, Ruditapes philippinarum and the crustaceans, Portunus trituberculatus. During the first six months of the research period, the abundant biomass of bivalves at the artificial tidal flat resulted in higher removal rate of particulate organic nitrogen, which fell within the range of a natural tidal flat in Mikawa Bay. However, the biomass of bivalves drastically decreased in accordance with the appearance of diving ducks and mass fishery. This study suggests that the biomass of mega-infaunal community in artificial tidal flats may drastically decrease by predation or fishery if the areas of created tidal flats are small.

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This study describes an experimental attempt to measure the behavior of vertical longline gear used in the Japanese diamond squid (Thysanoteuthis rhombus) fishery. The gear comprised a 100-m main line with a weight attached to one end, a buoy attached to the other end, and two jigs at the end of branch lines (8- and 15-m long) attached to the main line at the weight. After the gear was released at the surface, the weight sank first, followed by the two jigs, and reached its maximum depth in 3.5〜4 minutes. The positional relationship between the weight and jigs changed frequently during the descent and after the depth of the weight leveled off. Fishing depths were similar to the length of main line, but when the current was strong, the jigs ascended up to 20〜30 m, which suggests the actual fishing depths can be much shallower than the length of line used.

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Concerning the appropriate plan and site for the creation of fishing ground. function of water purification. network between tideland and zostera bed and present ecological value of site should be considered based on material cycle. The achievement of sustainability and effective productivity of the created fishing ground and the balance of ecosystem around the created fishing ground are realized through the imitating natural ecosystem analyzed by material cycle. The accuracy of ecological index. theoretical intelligibility. calculating profit, convenient measuring and effective management method are needed for management of bivalve fishing ground by fisher and fisheries cooperative association. There are obtained studying material cycle.

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The sudden decrease of catch of corbicula calm in Lake Hinuma was caused by overfishing of corbicula, progress of desalination and formation of anoxic waters. In order to achieve both restoration of corbicula fishery and water quality conservation, the environmental assessment in brackish lake was carried out by ecosystem model that assess the function of purification and biological production. The effect and influence on the water route making in Hinuma River as case study was examined. By the water route making in Hinuma River, the seawater exchange was promoted and the salinity in Lake Hinuma was risen. On the other side, the reduction of dissolved oxygen and the rise of nutritive salts density was caused in the upper Lake Hinuma. The rise of dissolved oxygen and the reduction of nutritive salts density was caused from the lower Lake Hinuma to Hinuma River. The rise of corbicula resource was caused in Lake Hinuma. The reduction of corbicula resource was caused in Hinuma River.

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Lake Ogawara, located in Northern Japan, is a brackish lake with the surface-area of 63km^2. The maximum depth is about 25m, and approximately 12psu of salt water exists in a lower layer under the depth of 20m throughout the year. Lots of Corbicula Japonica are living in the lake, and about 2500 tons are taken out as fishery product. We develop an ecological model to consider the biochemical processes of the bivalve, that calculates water temperature, salinity, chlorophyll-a, inorganic nitrogen and phosphorous, organic nitrogen and phosphorous, COD and dissolved oxygen. We make one-dimensional calculations to clarify the seasonal variabilities in water qualities and nutrients cycle, and use a three-dimensional hydrodynamic model coupled with the ecological model to clarify the spatial distributions of water qualities. It is found that an increase of nutrient supply from the lower layer, caused by the intense vertical mixing, makes phytoplankton blooming active, and that the spatial distribution of the phytoplankton greatly depends on the habitat distribution of Corbicula Japonica. The calculation results for the nutrients cycle show that the amount of phosphorous consumed at primary production is more dependent on the biochemical and physical processes occurred in the lake than the inflow load supply. About 20% of primary production is ingested by Corbicula Japonica, and its purifying function is about 30% for COD. Because Corbicula Japonica stores nitrogen internally, the concentration of total nitrogen in the lake water is restrained and decreases in October in spite of the time of the maximum primary production.

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In order to discuss the optimum future management plan for sustainable fishery, the numerical ecosystem model has been applied to Lake Saroma, which is one of the major sources of farmed scallops in Japan. The model includes the pelagic ecosystem, the benthic ecosystem and the scallop growth model. These factors were dynamically linked each other in the model. This structure provided the model with an ability to predict future water quality and production of scallops with sediment changes. As the preliminary step of discussion about the future management plan, the material flows during/after the spring bloom period in the lake were evaluated using the model. Then, the influences of the scallop aquaculture on the material cycle were evaluated quantitatively by comparing the cases-with and without the scallop aquaculture. The results show the followings. 1) The spring bloom had a great influence on scallop juvenile's growth, and the subsequent lower trophic level period didn't have a bad influence on the growth due to the rational farming cycle. 2) Since the farmed scallops kept 30% of N in the pelagic system in their body and were finally harvested, the aquaculture generally purified the water quality in the lake. However, the aquaculture would generate worse water quality at the bottom in the summer. Because the aquaculture has also the function like a pump that transfers the particle organic matter from the water to the sediment, the increased organic matter in the sediment would cause more release of nutrients and consumption of DO in the summer.

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It is known that the short-necked clam play an important role on the matter cycle in the tidal flats in Mikawa Bay. Therefore, even when modeling suspension feeder that the short-necked clam represented was described in detail based on the indoor experiment results and applied to some tidal flats, excellent reproducibility was obtained. However, we could not obtain the good reproducibility about the Mesocosm data set reproduced by an existing model and the parameter set applied to the tidal flats. Therefore, it verifies and accuracy of an existing tidal flat ecosystem model has improved by using the continuous data set obtained with Mesocosm, and the in existence data set of Isshiki tidal flat in Mikawa Bay calculated in the past was calculated again, and both were compared. In the results of the calculation by the modified model, a higher primary production and a faster matter cycle in the tidal flats are expressible compared with a past model, and, the characteristic of the dynamic matter cycle in the tidal flats is being reproduced. It is become clear that the ecosystem model and the parameter to who improved it by using the data of Mesocosm being able to reproduce the biomass of a different annual in a real sea area, and being able to do the nitrogen circulation analysis.

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The purpose of pearl farming is the pearl production that is a jewel. The healthy growth of pearl shell is important for production of high quality pearls. Therefore the environmental evaluation of pearl farming fishing ground was carried out as follows. Firstly the environmental items of pearl farming fishing ground such as water temperature, salinity, tidal flow, nutritive salts, feeds density, growth & survival rate of pearl shell and so on were examined. Next the relation between growth of pearl shell and quality of cultured pearl were examined. Thirdly the quality assessment of cultured pearl which used HEP (Habitat Evaluation Procedure) method was suggested. Fourthly the ecosystem model that considered pearl farming was applied to Asou bay in Tsushima Island, and was verified its appropriateness on the basis of pearl faming examination.

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