Can the co-cultivation of rice and fish help sustain rice production?

Rice is the main component in the daily diets of about 3 billion people1, many of whom live in developing areas and cannot afford high prices for rice2. Future global food security and the precarious livelihoods of the world’s poor depend on a reliable supply of rice. How to meet the increasing demand for rice without increasing environmental cost is a great challenge for the world rice farming2. Rice fields can provide habitat for “fish” (the term ‘fish’ in this article refers to a wide range of aquatic animals including carp, crab, crayfish, soft-shelled turtle, and others)3,4. The coupling of rice culture with fish production (also referred to as a rice–fish system or an RFS) has long been practiced in many rice-growing areas because it provides both rice grain and an aquatic protein source3,4. Because raising fish in rice fields can often reduce the use of pesticides for rice by reducing the incidence of diseases, insect pests, and weeds, and can also reduce the use of chemical fertilizer-nitrogen (N) through the complementary use of feed-N between rice and fish, RFSs have been considered an important way to help sustain local rice farming5,6,7,8. The Food and Agriculture Organization of the United Nation and other international organizations have been attempting to develop RFSs in the rice-growing areas of the world in order to help secure food supplies and improve rural economies5,6,9. Researchers recently proposed that RFSs could be useful for expanding freshwater aquaculture and increasing water productivity in some Asian countries (e.g., Bangladesh and Indonesia)10,11. In China, the development of RFSs has also been listed as a national strategy to simultaneously produce rice and freshwater fish12,13. The Ministry of Agriculture of the People’s Republic of China has been trying to help farmers improve traditional RFSs and develop intensive RFSs with high fish yields12,13 in order to increase rice-field productivity and farmer income. Since 2010, China has had the largest RFS area in the world12,13. Traditional RFSs use only small amounts of fish feed and keep the fish in only a small area that is referred to as a “fish refuge”; as a consequence, fish yields in traditional RFSs are relatively low, and rice yield is not reduced5,7,14. Intensive RFSs, in contrast, are large-scale operations that use relatively high quantities of commercial “fish” feed and large fish refuges to achieve high fish yields and significant farmer profits12,13. This suggests two important and related questions regarding intensive RFSs. Are rice yields lower in intensive RFSs than in rice monoculture? Can the intensive RFSs help sustain rice production? From the perspective of national food security, it is essential to determine whether the development of intensive RFSs threatens rice production. In this study, we undertake a survey to assess rice yields in various types of RFSs across five rice-growing areas with different climates in China. We discuss the implications of our results for the development and management of intensive RFSs with the goal of sustaining rice farming in China and other countries. Because the raising of fish in rice fields could promote rice growth and benefit rice yield, and because the raising of fish requires that some percentage of each rice field be devoted to a fish refuge, we expected that rice yield could change in response to changes in fish yield and that rice yield might be reduced if fish yields exceeded some threshold. We therefore hypothesized that there is some maximum level of fish yield at which rice yield is not harmed. We tested this hypothesis with a farm and farmer survey that included one type of traditional RFS (rice-carp) and five types of intensive RFSs (rice-carp, rice-crab, rice-crayfish, rice-loach, and rice-turtle [soft-shelled turtle]). The survey covered 56 counties in 14 provinces or cities (Fig. 1a). A traditional RFS is defined here as a small-scale farming system without application of commercial fish feed, and an intensive RFS is defined as a relative large-scale farming system with application of relatively high quantities of commercial fish feed. Figure 1 Performance of rice and fish yields in the sampled traditional and intensive RFSs.