Impact of fish and pearl farming on the benthic environments in Gokasho Bay: Evaluation from seasonal fluctuations of the macrobenthos

Abstract

In order to clarify the influence of mariculture on the benthic fauna, samples of the macrobenthos were collected from Gokasho Bay, where intensive fish culture and pearl oyster culture have been carried out. Monthly samples collected from the fish farm and pearl farm sites during June 1995 to July 1996 revealed that the community structure of the two sites showed distinct differences with seasonal fluctuations. At the fish farm site, azoic conditions were found from July to November; after December, the diversity increased markedly through successive recruitments of small-sized species such as the polychaetes Capitella sp. and Pseudopolydora paucibranchiata, and the amphipods Aoroides spp.; macrofaunal density, biomass and species richness peaked from March to April. At the pearl farm site, a higher diversity, including larger-sized species, and no clear seasonal fluctuations in abundance was found, and the community structure was similar to that at the control site. These results show the large impact by fish farming on the macrofauna, whereas pearl farming causes less effect on the benthic fauna. It is suggested that the difference in the level of organic input between the two sites results in the differences in the dissolved oxygen content of the bottom water, sulfide content of the sediments and, subsequently, the macrobenthic assemblages.