STUDY ON THE SUSTAINABLE OYSTER CULTURE IN HIROSHIMA BAY WITH SPECIAL REFERENCE TO THE ROLE OF OYSTER CULTURE ON NITROGEN AND PHOSPHORUS CYCLES

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Hiroshima Bay produces oysters of about 20,000 tons of fresh meat weight annually, which is ca.60% of the total oyster production in Japan. From the ecological point of view, oysters may play an important role on material cycling in the estuarine and coastal ecosystems due to the high filtering capacity. The objectives of the present study are to estimate the amounts of carbon, nitrogen and phosphorus processed by cultured oysters in Hiroshima Bay and to evaluate the role of the cultured oysters suspended from the rafts on the cycling of these elements in this bay. In order to achieve these objectives, in addition to the field investigation, the physiological and biomass models of oyster in Hiroshima Bay were developed.The results from field investigation showed that the rates of particulate uptake and release at an oyster raft were higher in summer and autumn than in winter. The suspended oyster culture did not significantly affect C : N : P atomic ratio in particulate matter and sink … More ing particles, but significantly affected the concentration of materials in water column and the mass of sinking particles in the culture area. The simulated fresh meat weight of an oyster (summed from the estimated growth) using physiological model was in good agreement with observed values, indicating that this model is suitable to simulate the physiological acitivities and growth of oyster. With the physiological model, the filtration and biodeposition rates by cultured oysters under the raft were estimated and these results also showed a good agreement with the results observed at the raft.Biomass of the cultured oysters in Hiroshima Bay was estimated using the biomass model, which is based on the variation of growth and mortality of oysters and the harvested loss. Our results showd that water quality parameters could be used for the estimation of actual growth and cumulative mortality of oysters. With the biomass model, total number of living oysters cultured in the bay was estimated throughout the study period and was used together with physiological model for the estimation of the amounts of C,N and P processed by oysters. The results revealed that filtered C,N and P were 9,360,1,560 and 210 g m-2 yr-1, respectively. About 42% of the filtered materials were released as biodeposit materials (feces and pseudo-feces), and 31 to 48% of them were metabolized. The amounts of 7,19 and 20% of the filtered C,N and P were harvested as oysters. The role of suspended oyster culture on C,N and P cycles was evaluated for northern Hiroshima Bay.The results from the present study revealed that oysters play a significant role on the cycling of carbon, nitrogen and phosphorus in the bay. In addition, the results also revealed that high amount of materials (i.e.4.7 ton N and 1.0 ton P d-1) were retained in the bay by the significant processing of oysters. In term of the removal of materials from the bay to the land and energy transfer, the results of the present study demonstrated that the eneregy transfer and removable materials i.e.C,N and P through oyster culture in Hiroshima Bay are higher than those through the other fisheries. Less