人工中層海底を用いたカキ養殖場底質への有機物負荷軽減策の検討  [in Japanese] Artificial midlayer seafloor : simple and new devices to reduce organic loads from oyster rafts to the sediment  [in Japanese]

近年,カキ養殖楊の底質悪化が問題となっている。そこで,カキ養殖から海底への有機物負荷を軽減するために養殖施設下の酸素条件の良い中層に人工的に分解層(人工中層海底)を設け,そこで好気的な分解を促進することを試みた。人工中層海底の材料は,カキ殼,竹炭,懸濁物吸着マットの3種類とした。その結果,いずれの人工中層海底でも奸気的分解が行われていることが確認され,最も効果の高かったカキ殼製では設置から69日間に,カキ筏から底質への沈降有機物負荷の約6.6%を軽減する効果があった。

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.