1Graduate School of Biosphere Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8528, 2Ocean Construction Co. Ltd., Kurashiki, Okayama 711-0921, Japan
Artificial fish reefs have been deployed in the inner part of Ago Bay, where intensive pearl oyster culturing occurs, in order to clarify the carbon flow through the food web and mineralization of organic matter on the reefs. The reefs consist of steel frame and polyethylene mesh cages in which pearl oyster shells are stuffed. The tops of the reefs are situated in the middle of the water column under the pearl oyster raft so that trapped organic particles decompose in aerobic conditions. We monitored carbon content in the sediments under the reefs in addition to benthic fauna and fish, for ca. 7 months during 2006-2007. Budget analyses revealed that decomposition of organic matter by macrobenthos respiration was 16-61% and removal of organic matter through the food web, in terms of feeding by top predators (e.g., fish), was 27-72% from summer through fall. In winter, however, the removal rate of organic matter decreased to 20-70% of that in the warm season. In conclusion, artificial fish reefs help reduce the organic matter in aquaculture areas via decomposition occurring in the water column and enhance the growth of top predators in the bay.