Science1 publisher2 min readPublished
Fish stocked in Senegal's rice paddies lifted grain yields by a quarter
Farmers in the lower Senegal River valley took more rice and a crop of fish out of the same fields. The pilot's evidence on schistosomiasis is a count of snails, and infection in people has yet to be reported.
The Scientist · Science desk
What happened
- Researchers stocked rice paddies in Senegal with Nile tilapia, which fertilize the crop with their feces, and African bonytongue, which eat the freshwater snails that release the schistosomiasis parasite.
- Adding the fish raised rice yields by 25% and produced an average net benefit of at least $380 a year for participating rice-farming households, according to the pilot's lead author.
- More than 40 farmers across multiple villages took part in the pilot and the follow-up work, and the pilot paper appeared in Nature Sustainability with Emily Selland of Notre Dame as lead author.
- Fields stocked with the predator fish held fewer of the snails that transmit the parasite than fields without them.
- Tilapia raised at Gaston Berger University in Saint-Louis are heading to AfricaRice research paddies, where similar studies will run under controlled conditions before any scale-up.
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Why it matters
- constraint Because the disease evidence stops at snail numbers, the approach cannot yet be compared on cost per infection averted with drug distribution, which is judged on people who test positive.
- cost The farmer sells the fish, so part of the recurring cost of snail control sits inside farm revenue instead of an annual health budget that has to be refunded each year.
- decision Adopting this means a rice farmer also runs a small aquaculture operation, a job nobody in these villages has done before, and uptake will turn on whether the fish income holds without study support.
- precedent AfricaRice's controlled paddies now set the bar the idea has to clear, and the case for scaling will be settled by replication under managed conditions.
The snail is the intermediate host. Freshwater snails common in the paddies release the parasites that cause schistosomiasis, and the illness can run from itchy skin to organ damage or death [8]. Medicine treats the infection but does not stop a farmer who wades back into the same water from catching it again [8]. Moussa Ndiaye, an 80-year-old farmer in the village of Lampsar, has had it three times, and the sons who worked his fields contracted it too [9].
Snail density is the disease measure here, and it sits one step upstream of the outcome a control programme is judged on, which is infection in people [5]. A count of hosts is not a measurement of transmission, though fewer hosts is consistent with less of it. The yield and income findings come straight off farm output, so they stand on their own footing [4].
The pilot ran on farmers' own fields, not on research plots [2]. Ali Ibrahim, regional representative of AfricaRice, said that "when you conduct an experiment in a rural setting with farmers directly, there are many parameters you can't control" [14]. Fish are not traditionally kept in paddies in the region, so the study was also asking farmers to learn a second crop [16].
The World Health Organization estimates that 254 million people require preventive treatment for schistosomiasis and that 94% of those needing it live in Africa, which works out at roughly 239 million people [6][1]. Transmission has been reported in 79 countries [7].
On the day AFP visited Ndiaye's field, the combine filled 20 sacks of rice at 80 kilograms each, about 1.6 tonnes [10][3]. Several days later he took more than 350 large tilapia and three African bonytongue out of the same paddy [10]. The bonytongue is the snail eater, and it was under 1% of the fish he landed [2]. How many of each species went in is not reported.
Abdoulaye Diop, 62, said he had "noticed that fields containing fish are consistently greener than those without them", and he put the profit from his fish into sheep and poultry [13]. Emily Selland of the University of Notre Dame, lead author of the pilot paper, told AFP: "It's not just a one-hit punch, we're really tackling a lot of challenges" [3][11].
What to watch
- Whether the AfricaRice controlled-paddy trials reproduce the 25% yield gain outside farmer-managed fields.
- Whether the follow-up research reports human infection or reinfection rates in stocked villages, not just snail counts.
- Whether hatcheries can supply African bonytongue in the numbers a district-scale stocking programme would need.