**1. Introduction**

Integrated rice field aquaculture evolved from rice–fish coculture, and is playing a more and more important role in China, as the largest aquaculture producer in the world. It is also becoming one of the main freshwater aquaculture systems, in addition to ponds, lakes, reservoirs, streams, and other aquaculture systems, amounting to 10.52% of China s freshwater aquaculture production and 33.71% of the aquaculture area in 2020 [1] (Figure 1). China, the most populous country, is short of land resources and is also the biggest consumer of rice; therefore, food security is a priority of the Chinese government. The development of aquaculture by digging more ponds is strictly controlled by law. Therefore, integrated rice fields are getting more and more attention as a resource for increasing aquaculture production because of their properties of water conservation and using less land. This practice is normally adopted by individual small-scale farmers as an ideal use of land and an easy source of cheap, fresh, and convenient animal protein. It not only promotes sustainable agricultural and aquaculture development and farms and livelihoods, but also reduces poverty [2–4]. Consequently, during the past 40 years, both the production and area of integrated rice field aquaculture in China have significantly increased from 0.02 million t and 0.35 million ha in 1982 to 3.25 million t and 2.56 million ha in 2020, respectively [1,5] (Figure 2). Advances in ecology research, policy support, and aquaculture techniques for integrated rice–fish aquaculture are probably the main contributors to this achievement.

**Citation:** Yuan, J.; Liao, C.; Zhang, T.; Guo, C.; Liu, J. Advances in Ecology Research on Integrated Rice Field Aquaculture in China. *Water* **2022**, *14*, 2333. https://doi.org/10.3390/ w14152333

Academic Editors: Xiangli Tian and Li Li

Received: 26 May 2022 Accepted: 24 July 2022 Published: 28 July 2022

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**Figure 1.** Comparison of freshwater aquaculture production (**a**) and area (**b**) between rice fields and other main water bodies in China [1].

**Figure 2.** Changes in the aquaculture production and area in rice fields in China from 1950 to 2020 [1,5].

Integrated rice field aquaculture in China includes 25 different models [4]. In this paper, we systematically reviewed the advances in ecology research for the three most important systems in China, namely the rice–fish, rice–crab, and rice–crayfish coculture systems [6]. We also introduced the prospects for ecology research on integrated rice field aquaculture. This information could assist individual small-scale farmers to make better use of rice field space to produce safer aquatic and rice products at a lower cost and help aquaculture scientists to further study the ecology of this industry.

For this review, we searched for Chinese articles in the China National Knowledge Infrastructure database and English articles in the Web of Science database in 2022. We collected as many relevant pieces of literature as possible, and we only selected and referred to important published articles and books. Most of the information that was extracted included productive data (such as density, area, yield, survival rate, feed, and pesticide usage), aquaculture models, and advances in ecological research.

#### **2. Advances in Ecology Research on the Rice–Fish Coculture System**

The rice–fish coculture system is the most ancient integrated rice field aquaculture system in the world [2], and many fish species have been chosen for coculture, such as grass carp (*Ctenopharyngodon idella*), common carp (*Cyprinus carpio*) and its diverse strains, goldfish (*Carassius auratus*) and its diverse strains, swamp loach (*Misgurnus anguillicaudatus*), silver carp (*Hypophthalmichthys molitrix*), bighead carp (*Aristichthys nobilis*), and tilapias (*Oreochromis* sp.) [7,8]. Furthermore, ecology research in this system not only accelerates the development of the rice–fish aquaculture industry, but also lays the foundation for the formulation and development of other rice field integrated aquaculture systems.

#### *2.1. Establishment of the Rice–Fish Symbiosis Theory*

Ni [9] studied the population interactions between rice and fish in the coculture system based on biological and ecological traits in the late 1970s and early 1980s and put forward the Rice–Fish Symbiosis Theory in 1981. Ni and Wang suggested that rice is the main body in the rice field ecosystem and is also the dominant population, which absorbs solar energy, carbon dioxide, water, and other nutrients, and produces organic materials by photosynthesis and rice and straw for stakeholders. Moreover, plenty of weeds, phytoplankton, and photosynthetic bacteria in the field also conduct similar energy conversion, transportation, and storage as rice, but they also compete with rice for fertilizers, water, space, and solar energy without providing useful products for people [10]. Thus, the clearance of weeds in rice fields, which results in the loss of fertility and solar energy for rice, is imperative. Furthermore, plankton, bacteria, and other microorganisms are usually flushed away by irrigation, directly or indirectly causing the loss of fertility and solar energy [10]. Fish culture in rice fields can partially compensate for this loss. Fish in rice fields can directly or indirectly utilize weeds, zoobenthos, plankton, and detritus, decreasing competition with weeds for fertilizers, and utilizing the food and energy that cannot be utilized by rice [10]. In addition, nutrient loadings from fish can also provide nutrients for rice and plankton, and the CO2 released by fish can also be utilized by rice, weeds, and algae. Additionally, the fish may loosen the surface soil and improve the oxygen condition of the soil to promote the mineralization of organic matter and the release of nutrients [9]. In this ecosystem, rice and fish complement each other and both play an active role, promoting the cycle of materials inside the field, directing beneficial energy flow to both the rice and fish, and efficiently recycling the materials and energy in this coculture ecosystem [10].

The Rice–Fish Symbiosis Theory, although a qualitative description in theory, provides a theoretical base for the Chinese government to adopt specific extension policies for the development of this traditional practice with a long history. In 1983, the Health Commission of China listed fish culture in rice fields as an important measure for killing mosquitoes, and the Ministry of Agriculture, Husbandry, and Fisheries held the first nationwide "On Site Experience Exchange Conference on Fish Culture in Rice Fields" in Wenjiang County, Sichuan Province. Additionally, in 1984, the National Economic Commission of China listed fish culture in rice fields as a national technical development project and extended this technique to 18 provinces; and in 1987, the technical extension of fish culture in rice fields was accepted into the National Harvest Project and State Key Agricultural Extension Project. Then, in 1990, the Ministry of Agriculture held the second nationwide "On Site Experience Exchange Conference on Fish Culture in Rice Fields" in Chongqing [2]. Thus, the breakthrough of ecological theories for integrated rice field aquaculture won national policy support and accelerated the development of fish culture in rice fields in the 1980s (Figure 2).
