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

广岛湾每年生产约2万吨鲜肉重的牡蛎，约占日本牡蛎总产量的60%。从生态学的角度来看，牡蛎具有很高的过滤能力，可能在河口和沿海生态系统的物质循环中发挥重要作用。本研究的目的是估计广岛湾养殖牡蛎处理的碳、氮和磷的数量，并评估从木筏上悬挂的养殖牡蛎对该海湾中这些元素循环的作用。为了实现这些目标，除了实地调查外，还建立了广岛湾牡蛎的生理和生物量模型。野外调查结果表明，夏季和秋季牡蛎筏对颗粒的吸收和释放速率均高于冬季。牡蛎悬浮培养对颗粒物质中的C: N: P原子比和沉降颗粒的影响不显著，但对培养区水体中物质浓度和沉降颗粒质量有显著影响。用生理模型模拟的牡蛎鲜肉重（由估计的生长量求和）与观测值吻合较好，说明该模型适合模拟牡蛎的生理活动和生长。利用该生理模型对筏下养殖牡蛎的过滤速率和生物沉积速率进行了估算，结果与筏上的观测结果吻合较好。利用基于牡蛎生长、死亡变化和收获损失的生物量模型对广岛湾养殖牡蛎的生物量进行了估算。结果表明，水质参数可用于估算牡蛎的实际生长和累积死亡率。利用生物量模型估算了整个研究期间海湾内养殖的活牡蛎总数，并与生理模型一起用于估算牡蛎加工的C、N和P的量。结果表明，碳、氮和磷的过滤量分别为9360、1560和210 g m-2年-1。过滤后的物质中约有42%以生物沉积物质（粪便和假粪便）的形式释放，其中31 ~ 48%被代谢。过滤后的碳、氮、磷分别有7%、19%和20%作为牡蛎收获。以广岛湾北部为研究区，评价了牡蛎悬浮培养对土壤碳、氮、磷循环的影响。本研究结果表明，牡蛎在海湾中碳、氮、磷的循环中起着重要作用。此外，结果还表明，牡蛎的大量加工在海湾中保留了大量的物质（即4.7吨N和1.0吨P -1）。在从海湾向陆地的物质转移和能量转移方面，本研究结果表明，广岛湾牡蛎养殖的能量转移和可转移物质c、N、P均高于其他渔业。少

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