High Fish Stocking Density Weakens the Effects of Rice-Fish Co-culture on Water Eutrophication and Greenhouse Gas Emissions

Intensive aquaculture is widely responsible for serious negative environmental effects (e.g., water eutrophication and greenhouse gas (GHG) emissions) on water bodies and the atmosphere. Investigating the ecological measures used for mitigating these negative effects is urgent and necessary for the sustainable development of intensive aquaculture. In this study, a pot experiment was conducted to investigate the effects of rice-fish co-cultures on water nutrients and CH4 and N2O fluxes under different fish stocking densities. Results revealed that, compared with fish monoculture, rice-fish could significantly mitigate the water eutrophication through reducing nutrients N ( total nitrogen (TN), NH4+, and NO3-) and P (total phosphorus (TP) and PO43-) and chemical and biochemical oxygen demands (COD and BOD) under the equivalent fish stocking density. Rice-fish significantly mitigated N2O by 56.2% without significant change of CH4. Both N and P use efficiency of rice-fish were enhanced by 133.8% and 133.9% respectively compared to fish monoculture. However, the effect of rice-fish co-culture on water eutrophication and GHG emissions is highly influenced by the stocking density. High fish stocking density not only weakened the restoration of rice on water eutrophication but also enhanced the GHG emissions rather than yield. When the fish stocking density was raised by 50%, all the parameters of water eutrophication increased in the range of 21.7 to 117.3% and the yield-scaled GWP of rice-fish systems significantly increased by 341.6%. These results indicate that the ecological effects of rice-fish systems depend on fish stocking density. Specifically, a high fish stocking density weakened the mitigation of rice-fish co-cultures on water eutrophication and the GHG emissions of intensive aquaculture.