Farmers in Qingtian have kept carp in their flooded rice paddies for more than 1,200 years, where the fish eat the insects and weeds and fertilise the crop as they go. The system needs almost no pesticide and produces two harvests off one field.

In Qingtian County in southeast Zhejiang, farmers put roughly forty grams of carp fry into a rice paddy about a week after transplanting the seedlings, and they’ve been doing it for at least twelve hundred years. The fish live there for the next four months alongside the rice, eating whatever the paddy provides and depositing what they don’t retain into the mud, until the rice is ready to harvest and they are taken out with it. The carp come out at somewhere between three hundred and fifteen hundred kilograms per hectare depending on the year and the field, the rice yield is about the same as it would be under conventional monoculture, and the field has needed almost no pesticide and considerably less fertiliser than the equivalent paddy without fish. It’s one of the most efficient food-production arrangements ever devised, and it survives now mainly because it was recognised in time.

What the fish are actually doing down there

According to the Food and Agriculture Organization’s designation of the Qingtian rice-fish system as a Globally Important Agricultural Heritage System, the interaction between the two species does several distinct kinds of work at once. The fish eat planthoppers, aquatic larvae, algae, weeds, and whatever else they can find that isn’t the rice itself, which they leave alone. Their movement through the paddy oxygenates the water and softens the soil around the rice roots. Their waste feeds the rice as slow-release organic nitrogen. And the rice, in exchange, shades the water enough to keep it cool through the hottest part of the growing season, which is what allows an indigenous breed of common carp to survive in what would otherwise be an inhospitably shallow environment. There are six native carp breeds adapted to the system, in colours running from red through black and variegated, and twenty native rice varieties, many of them now threatened.

One field, two crops, and the pest control swims. The Aztecs ran a bigger version of the same trick — their fields sat in the lake, and the canals between them did irrigation, fertiliser and fishery all at once. We created a video about it on one of our sister sites – check it out below:

What happens when you take the fish out

The mechanism gets legible when you compare a field with fish to a field without them. A four-year field experiment in Qingtian County by Guo, Zhao and colleagues at Zhejiang University, published in eLife in 2022, ran side-by-side plots with and without carp under otherwise identical conditions and measured what changed. Weed biomass in the plots with fish came out between 45 and 52 per cent lower than in the monoculture plots, without a herbicide being used on either. Rice yield in the coculture plots was 9 per cent higher on average than in the monoculture. Stable-isotope tracing showed that roughly half of what the carp were eating was field-sourced rather than from the supplementary feed the researchers added, meaning the fish were foraging in the field for weeds, algae and invertebrates that would otherwise have been competing with the rice or attacking it directly.

The seminal earlier work on the same system, published in the Proceedings of the National Academy of Sciences in 2011 by Xie and colleagues at Zhejiang University, put the pesticide reduction at 68 per cent and the chemical fertiliser reduction at 24 per cent compared with rice monoculture, with rice yields statistically indistinguishable between the two systems. Video recordings from that study showed carp removing about 26 per cent of planthoppers by direct predation. Rice-monoculture yields at the same site had roughly the same output of rice, but no fish, and required chemical inputs the coculture didn’t.

The two-thousand-year history it sits inside

The twelve-hundred-year figure that the FAO uses for Qingtian specifically refers to documented continuous practice in that county, but the general Chinese tradition of putting fish in rice paddies goes considerably further back. Clay models unearthed from tombs of the Eastern Han Dynasty, which ran from 25 to 220 CE, include a miniature rice field with fish and a miniature pond containing six common carp, a soft-shell turtle, three frogs and five water chestnuts, which suggests the arrangement was well enough established by that period to be part of what someone wanted buried with them. What happened in Qingtian specifically, on the terraced fields of a hilly and mountainous corner of Zhejiang, was that the practice was refined over the following centuries into a stable local system, with an indigenous carp breed selected for tolerance of shallow warm water and a set of rice varieties selected for compatibility with the fish. Farmers in the region were still doing it, essentially unchanged, when the FAO looked in 2005 and made Qingtian the world’s first designated Globally Important Agricultural Heritage System.

Why it isn’t the default

The obvious question is why this arrangement, which produces two crops off one field with a fraction of the chemical input of a monoculture, hasn’t simply displaced the monoculture. The FAO’s own site notes that Chinese rice-fish farmland shrank from about 1.53 million hectares in 2000 to 1.48 million hectares in 2002, and the pilot village at Longxian in Qingtian has been steadily losing its working-age population to jobs abroad. The rice-fish system is labour-intensive in ways that machine-planted, chemically-managed monoculture is not, and the returns don’t compete with what a young person from Zhejiang can make in a European city. Most of Longxian’s household income now arrives as remittances from overseas rather than from the paddies themselves. The GIAHS designation in 2005 was partly a rescue effort, arguing that the system was worth preserving not for nostalgic reasons but because the ecological engineering encoded in it was globally valuable, and it seems to have worked in Qingtian specifically, where the local government has since built a rice-fish brand around it. Whether the same logic can hold up the practice elsewhere is a much harder question.

What the system is actually demonstrating

The interesting thing about Qingtian, from the point of view of anyone thinking about sustainable agriculture from scratch, is that it isn’t sentimental or ornamental. It’s a functioning piece of ecological engineering, refined over roughly forty human generations, that produces measurably more food per hectare with measurably less chemical input than the system currently being promoted as its replacement. The reason it works is that the two species aren’t competing for anything the field can only give one of them, and the by-products of each are what the other needs. The reason it’s disappearing isn’t that it’s been outperformed. It’s that it needs people willing to spend the labour, and the labour is worth more elsewhere. Whether we can put back what a monoculture cannot supply, once the last of the Qingtian farmers have retired, is a separate problem entirely.