Abstract
Fish farming using net pens in Japanese coastal waters has become increasingly common over the past two decades. In this new style of fishery, however, large amounts of material are discharged from the net pens. The increased organic input to the bottom immediately below the net pens tends to result in organic enrichment of the sediment. This enrichment causes a catastrophic environmental disturbance on the bottom during summer, resulting from hypoxic bottom water conditions and the development of reduced conditions in the sediment. In March, 1993, we assessed the chemical conditions of the bottom sediment and the composition of the Benthic community in a cove where fish farming has been taking place since 1973. In this cove, assessments of the benthic conditions have been conducted since 1968, prior to the start of fish farming. We compared the results of the study in 1993 with the earlier data to describe the f aunal changes in the benthic community, caused by the organic enrichment of the sediment. The most serious organic enrichment of the sediment occurred only at the areas adjacent to the fish farms. The benthic f aunal community, however, changed dramatically in the whole area of the cove after the onset of fish farming. Various molluscs predominated prior to the start of fish farming. As the sediment became organically enriched, the abundance of molluscs markedly declined, their biomass decreased, and the molluscan community species composition was extremely simplified. In 1993 only three species of small polychaetes (Capitella sp. 1, Pseudopolydora paucibranchiata, Euchone sp.) and one species of amphipod, Aoroides columbiae, predominated in the Benthic community in the most organically enriched areas. These species were very rare or not found at all in the cove prior to the start of fish farming. Thus, the organic enrichment of the sediment caused by fish farming for two decades has resulted in drastic changes in the benthic faunal community of the cove, to the extent of the replacement of molluscs by previously rare species.
