Rice–fish system

A rice–fish system is a rice polyculture, a practice that integrates rice agriculture with aquaculture, most commonly with freshwater fish. It is based on a mutually beneficial relationship between rice and fish in the same agroecosystem. The system was recognized by the FAO in 2002 as one of the first Globally Important Agricultural Heritage Systems. The benefits of rice–fish systems include increased rice yield, the production of an additional (fish) crop on the same land, diversification of farm production, increased food security, and reduced need for inputs of fertilizer and pesticide. Because fish eat insects and snails, the systems may reduce mosquito-borne diseases such as malaria and dengue fever, and snail-born parasites such as the trematodes which cause schistosomiasis. The reduction in chemical inputs may reduce environmental harms caused by their release into the environment. The increased biodiversity may reduce methane emissions from rice fields.

History The simultaneous cultivation of rice and fish is thought to be over 2,000 years old. Ancient clay models of rice fields, containing miniature models of fish such as the common carp (Cyprinus carpio), have been found in Han dynasty tombs in China. The system originated somewhere in continental Asia such as in India, Thailand, northern Vietnam and southern China. The practice likely started in China since they were early practitioners of aquaculture.

Carp were probably among the first fish used in rice–fish systems. Wei dynasty records from 220 to 265 AD mention that "a small fish with yellow scales and a red tail, grown in the rice fields of Pi County northeast of Chengdu, Sichuan Province, can be used for making sauce". Liu Xun wrote the first descriptions of the system, with texts written during 900 AD in the Tang dynasty. Rice–fish systems may have evolved from pond culture in China; one theory proposes that the practice started when farmers decided to place excess fry in their ponds and found the results beneficial. The practice may have developed independently from China in other Asian countries; there is evidence that it spread from India to neighbouring Asian countries over 1,500 years ago. The practice slowly gained popularity among farmers, and by the mid-1900s, over 28 countries on all continents except Antarctica used rice–fish systems. Historically, the common carp was the most commonly used fish, with the Mozambique tilapia (Oreochromis mossambicus) in second place. As the practice spread throughout the world, new species were adopted. For example, Malaysia introduced the snakeskin gourami (Trichogaster pectoralis) and Egypt used the Nile tilapia (Oreochromis niloticus). An early study, in Jiangsu Province in 1935, found that placing black carp (Mylopharyngodon piceus), grass carp (Ctenopharyngodon idella), silver carp (Hypophthalmichthys molitrix, 'silverfin'), bighead carp (Aristichthys nobilis) and common carp together with rice was beneficial. Rice–fish systems were traditionally low maintenance, growing additional animal protein alongside the staple food, rice. The space used for fish–rice systems in China grew from 441,027 hectares (1,089,800 acres) to 853,150 hectares (2,108,200 acres) and the production increased dramatically, going from 36,330 tonnes to 206,915 tonnes between 1983 and 1994. In 2002, the rice–fish system became one of the first Globally Important Agricultural Heritage Systems to be recognized by the FAO.

Principle

Mutualism

Rice and fish form a mutualistic relationship: they both benefit from growing together. The rice provides the fish with shelter and shade and a reduced water temperature, along with herbivorous insects and other small animals that feed on the rice. Rice benefits from nitrogenous waste from the fish, while the fish reduce insect pests such as brown planthoppers, diseases such as sheath blight of rice, and weeds. By controlling weeds, competition for nutrients is decreased. CO2 released by the fish may be used in photosynthesis by the rice. The constant fish movements allow for the loosening of the surface soil which can:

Improve oxygen levels by increasing the amount of dissolved oxygen. Consequently, the activity of microorganisms is increased and they generate more usable nutrients, which will allow an increased nutrient uptake for the rice. Increase mineralization of the organic matter. Optimization of nutrient release in the soil. Promote fertilizer decomposition and therefore fertilizer effectiveness. Better root development of the rice. Soil fertility is improved by the integration of fish, whose manure is a fertilizer recycling organic matter, nitrogen, phosphorus and potassium. The inclusion of fish in rice-fields helps to maintain soil health, biodiversity, and productivity. The aquatic diversity in rice–fish systems includes phytoplankton, zooplankton), soil benthic fauna and microbial populations; all of these play a role in enhancing soil fertility and sustaining long-term production. However, benthic communities may be disturbed by constant grazing by the fish.

Application as a polyculture

Rice–fish systems are polycultures based on the potential for mutual benefit. To put this into practice, channels are added in the previously flat rice fields to allow the fish to continue growing even during rice harvest and dry seasons. Before creating the rice field, the field is treated with 4.5–5.25 tonnes per hectare (2.0–2.3 short ton/acre) of organic manure. Organic manure is applied again during the main growing season, with about 1.5 tonnes per hectare (0.7 short ton/acre) applied every 15 days. This provides nutrients for rice and the added cultures of plankton and benthos that feed the fish. During the main growing season, supplementary feeds complement the plankton and benthos culture and are used once or twice a day. The supplementary feeds include fish meal, soybean cake, rice bran and wheat bran. Fish are stocked at a rate between 0.25 and 1 per square metre (1,000–4,000/acre). Unwanted fish or invasive species can threaten the mutualistic relationship between rice and fish, and therefore reduce productivity. For example, in the integrated Rice–Swamp Loach Aquaculture Model, catfish, snakeheads (Channa argus) and paddy eels (Monopterus albus) are considered as unwanted species. Predatory birds are a threat; bird netting can be used to protect the fish.

Benefits

Economic

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