Impact of fish net pen culture on the benthic environment of a cove in South Japan

Abstract

Over the past three decades, fish net pen culture has been developed in the coastal waters of Japan. However, serious organic pollution of the surrounding water and sediment followed the development of this new style of fishery. This paper reports a typical example of the environmental disturbance that has occurred due to the organic pollution resulting from fish net pen culture. Since fish cultivation was initiated in 1973, oxygen depletion of the bottom water and extremely reducing conditions within the sediment have occurred during the summer. These environmental disturbances resulted in unusual dynamics of the benthic communities including defaunation in the summer and recolonization of the azoic areas from autumn to the next spring. Significant changes in the benthic fauna were observed between benthic communities before and after organic pollution of the sediment. Molluscs were originally the most dominant benthic fauna. As organic pollution of the bottom sediment progressed, the molluscs were replaced by polychaetes. Likewise, total biomass of the benthic fauna decreased markedly over the past two decades. Prior to contamination, the organic matter deposited on the sediment was decomposed by the action of bacteria, ciliates, meiofauna, and macrofauna. The addition of organic material resulting from fish culture has overwhelmed these decomposition mechanisms, and conditions have worsened as organic matter is continuously added to the sediment. Contamination of marine sediment by the net pen cultivation of fish sets up a vicious cycle: the addition of exogenous sources of organic material increases the need for a more vigorous decomposition process while actually creating conditions that limit the rate of the necessary decomposition activity. The sustainable development of fish net pen culture will require the creation of a new technology that efficiently promotes the decomposition of organic matter deposited below the pen nets.