1Graduate School of Biosphere Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8528, Japan, 2Hiroshima Fisheries Experimental Station, Ondo, Hiroshima 737-1207, Japan
Oyster culture is intensively carried out in Hiroshima Bay, Japan, causing massive sedimentation of oyster faeces/pseudofaeces which degrade the sediments under the oyster rafts and sometimes induce oxygen-depleted water through decomposition of the organic matter. In the present study, we tried to reduce the organic load from the cultured oysters deposited on the seafloor by hanging an artificial midlayer seafloor that traps some of the sinking organic particles before they reach the seafloor. The artificial seafloors are suspended in the midlayer so that the organic matter decomposes in the aerobic condition. Three kinds of artificial seafloor which were made of oyster shell, bamboocharcoal, and particle-filtering mat were tested. Monitoring for 69 days showed that the number of benthic animals increased and decomposed organic matter along with bacterial decomposition on the artificial seafloors. Budget analyses of organic matter revealed that the artificial midlayer seafloor made of oyster shell was most efficient to reduce the organic load from the cultured oysters above, showing the highest decomposition rate of 6.6% in 69 days. The devices proposed in this study will support the future sustainability of oyster culture by accelerating the natural self-purification ability.