Rice-fish culture, which means the simultaneous culture of rice and fish, is one of the best options to increase food production from limited land and is practiced in many countries in the world. Although many researchers and farmers believe that the rice yield is increased by fish farming in paddy fields, this hypothesis has never been fully tested. Here, we report ecological processes leading to higher rice yields in the rice-fish culture using crucian carp (Carassius complex), which have adapted to the paddy field ecosystem in Japan. We compare the rice-fish and rice-only plots in the experimental paddy field for biota, water quality, and rice yield. Coverage of duckweed and densities of zooplankton and benthic invertebrates in the rice-fish plots were lower than those in the rice-only plots, indicating that fish utilized them as food. NO3–N concentration in the rice-fish plots was higher than that in the rice-only plots, indicating that the increase in NO3–N concentration results from excretion of unutilized food nutrients by the fish. Consequently, rice yield in the rice-fish plots was 20% higher than that in the rice-only plots. The fertilizing effect of the fish excrement probably increased rice yield.
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