Eel Anguilla japonica: Toward Zero Emission

Abstract

The development of a closed recirculating aquaculture system that does not discharge effluents would reduce a large amount of pollutant load on aquatic bodies. In this study, eel were reared in a closed recirculating system, which consisted of a rearing tank, a foam separation unit, a nitrification unit, and a denitrification unit. The foam separation unit has an inhalation-type aerator and supplies air bubbles to the rearing water. The gross eel weight increased 2.8–24 kg, and the magnification was 8.6 times. Then, the feed conversion ratio [(growth wet weight)/(feed dry weigh)×100] was 67%, which is equal to that obtained by culture farms. The maximum fish density in the rearing tank became 44 kg-fish/m³, which is about seven times higher than that of conventional eel culture farms in Japan. The survival rate under the congested experimental conditions was 91%. The foam separation unit maintained oxygen saturation in the rearing water above approximately 87%. Furthermore, fine colloidal substances were absorbed on the stable foam formed from eel mucus and were removed from the rearing water by foam separation. Ammonia oxidation and the removal of suspended solids were accomplished rapidly and simultaneously in the nitrification unit. The ammonia concentration and turbidity were almost kept at less than 1.0 mg-N/L and 10 units, respectively. When the denitrification process was operated, nitrate that accumulated in the rearing water was reduced to 10 mg-N/L. The sludge was easily recovered from the nitrification and denitrification tanks, and the components were found suitable as compost. Based on these results, the intensive aquaculture of freshwater fish such as eel can be achieved using a closed recirculating system without emission.