Biological Production and Nitrogen Use Efficiency in a Water-Sharing and Water-Saving System Combining Aquaculture and Vegetable Hydroponic Cultivation

Aquaponics, a biological production system that combines land-based aquaculture and hydroponic cultivation of plants, is a water-sharing and water-saving system that is expected to be a sustainable food production system with water and nutrient resource circulation in agricultural and fisheries fields. The balance among feeding, fish density, and plant absorption capacity was investigated to obtain fundamental data for sustainable aquaponic systems. To clarify the effects of feeding rates on biological production and nitrogen utilization efficiency, fish and plant growth performance and nitrogen flow were evaluated in an aquaponic system that combined loach aquaculture with lettuce hydroponic cultivation. Test groups with different feeding rates and different fish densities were set. As a result, the fertilizer components in loach excreta contributed to plant growth, and the growth rate of lettuce plants tended to be greater than that of control hydroponic cultivation without fish. However, there was no difference in lettuce growth at feeding rates of 0 to 2 g d−1/system, but above 2 g d−1/system, the growth of lettuce plants was suppressed due to an overload of excreta. The yield of loaches increased with increasing daily feeding rate per system, but a minimum feed conversion ratio was detected. The NO2− concentration increased with increasing daily feeding rate per system and amount of excreta. The nitrogen use efficiency did not change at feeding rates ranging from 0 to 1.5 g d−1/system. In this feeding rate range, 80% of NUE in aquaponics was due to NUE in the plant hydroponic cultivation subsystem. However, above 2 g d−1/system, nitrogen use efficiency decreased with increasing daily feeding rate per system. A feeding rate of approximately 1.5 g d−1/system maximized biological production while maintaining high nitrogen utilization efficiency. In conclusion, a balance among feeding, fish density, and plant absorption capacity is essential to maintain a sustainable aquaponic system for sustainable fish and plant production as a food production system, saving water and chemical fertilizer.