Abstract: Fish farming using net pens has become very widespread in the coastal areas of various countries in the world since the1970s. Now, it is one of the most important ways to obtain food resources from the coastal seas. This cultivated fishery, however, tends to cause eutrophication of the water and organic enrichment of the bottom sediment around fish farms, a problem that has yet to be addressed satisfactorily. In this study, we focus on the organic enrichment of the sediment below the fish farm, and develop techniques to treat the organically enriched sediment with biological activities such as feeding and reworking the sediment of a small deposit feeding polychaete, embers of the macrobenthic anim als in the organically enriched areas throughout the world. We created a mass-culture of this polychaete, and spread it on the organically enriched sediment below a net pen in a fish farm, in Amakusa, Kyushu, western Japan, in December 2003. Rapid population growth was observed after spreading the colonies, and it reached extremely high densities of approximately 130, 000indi./m2 in the sediment within three months. As the Capitella population increased, the organic matter content and AVS of the surface layers of the sediment decreased markedly. Thus, spreading the artificially cultured colonies of Capitella is an effective technique to treat the organically enriched sediment and to prevent further progress of the organic enrichment of the sediment below the fish farm.
