Macrobenthos as Biological Indicators to Assess the Influence of Aquaculture on Japanese Coastal Environments

To delineate patterns in the gradients of macrobenthic communities around fish farms and to generate site-selection guidelines for sustainable fish farms, a quantitative survey was conducted in August-September 1998 and in February 1999 in fishfarming areas along the coast of Kumano-nada, central Japan. The biomass peaked in sediments containing 1.2 mg=g of total nitrogen or 9.2 mg=g of total organic carbon, suggesting that these values are indicative of possible aerobic mineralization of the loaded organic wastes. Macrobenthos was scarcely found in sediments with acid-volatile sulfides (AVS) >1.9 mg S=g, which suggests critical AVS conditions for maintaining sustainable and environmentally “friendly” aquaculture. An index, “ED,” which serves as an index for bathymetry of the sampling site, was devised. ED was highly correlated with four community parameters (biomass, density, species diversity, and species richness) and with five environmental factors (sediment total nitrogen, total organic carbon, total phosphorus and acid-volatile sulfides, and bottom-water dissolved oxygen). Compared with small-scale farms, environmental deterioration and decline in macrofauna were obvious in large-scale farms (_600 t=yr of fish production) that were located in inner parts of embayments and that had comparatively high ED values. However, in areas near open waters and that had comparatively low ED values, the impacts of aquacultural activities were not clear. These findings suggest that the variation in macrobenthos and environmental factors found in the study area was attributable principally to the area’s bathymetry and, secondly, to aquacultural activities. Seven assemblages, recognized on the basis of cluster analysis, were placed in a gradient of ED versus fish production. These assemblages were classified into three groups: a group characteristic of a healthy zone, a group characteristic of a cautionary zone, and a group characteristic of a critical zone. As fish production increased, the location of the assemblage characteristic of the cautionary zone shifted to more open, deeper areas (lower ED values). An azoic-site group was composed of large-scale farms in a stagnant basin. The community type of the macrobenthos in the vicinity of open-water, net-pen fish farms is an indicator of the condition and assimilative capacity of the benthic environment in that area and can be used to evaluate the suitability of the site for continued fish farming. The index of embayment degree, ED, can be used as a simple and effective substitute indicator for this evaluation.