A method of lining up 'rafts' floating seaweed (rong biển) in Vietnamese shrimp farming ponds is spreading. Forty to fifty units are placed in a single pond, and the seaweed takes up the nitrogen and ammonia in the water to keep the water quality in order, while the grown seaweed is sold as a feed ingredient for an extra 50–60 million dong a year in side income. Already run across more than 100 hectares, it is drawing attention as a low-carbon model that lowers dependence on chemicals. For Japanese seafood buyers who champion sustainable sourcing, this is a rare case in which 'environmental compliance runs on revenue, not price,' and it can serve as a basis for a premium for Japan.
Forty to fifty seaweed rafts floating in the pond take up ammonia and nitrogen compounds and curb toxic gases at the bottom
The mechanism is simple. In intensive shrimp farming, ammonia and nitrates from leftover feed and shrimp waste build up in the water, and toxic gases such as hydrogen sulfide accumulate at the pond bottom. Float a seaweed raft here and the seaweed absorbs these nitrogen compounds and organic matter as nutrients for its growth, lowering the load in the water. It is close to the image of floating a living biofilter in the pond.
In the Vietnamese practice, 40 to 50 rafts are lined up per pond. By taking in the excess nutrient salts, the seaweed is said to curb the deterioration of water quality and the shrimp disease that comes with it. This 'shrimp × seaweed' combination is known academically as integrated multi-trophic aquaculture (IMTA), and studies in various countries have accumulated showing Gracilaria and Ulva to be effective at absorbing nitrogen and phosphorus. The Vietnamese field has, in effect, implemented that principle with low-cost floating rafts.
The grown seaweed sells as a feed ingredient for 4–5 million dong a month, becoming 50–60 million dong a year in side income
The biggest reason this method is spreading is that the environmental measure does not end as an out-of-pocket cost. The seaweed grown to purify the water can be sold as is to feed makers as a raw material for livestock feed. The farmer's take comes to about 4–5 million dong a month, or about 50–60 million dong a year.
Converted to Japanese yen, 50–60 million dong a year comes to roughly ¥290,000–350,000 (at ¥1 ≈ 170 dong; about US$2,000–2,400). The amount itself is not large, but what matters is that this is revenue born from a by-product of cleaning the water, separate from the main business of harvesting shrimp. Water-quality management had been a cost center until now, but the seaweed raft turns it into a small source of profit.
| Item | Conventional intensive shrimp farming | With seaweed rafts |
|---|---|---|
| Water-quality management | Relies on water exchange, chemicals, and mechanical aeration | Seaweed absorbs nitrogen and ammonia |
| Chemicals | Large input volumes | Dependence can be reduced |
| Additional income | None | 50–60 million dong a year (about ¥290,000–350,000) |
| Carbon | Large feed-derived emissions | About 2.5 tonnes reduced per tonne of seaweed feed |
| Japanese buyers' evaluation axis | Centered on price and size | An environmental story can be added on top |
The calculation that one tonne of feed cuts about 2.5 tonnes of CO2 becomes a bargaining chip for ESG sourcing for Japan
The original article holds that, per tonne of feed made from seaweed, about 2.5 tonnes of carbon dioxide emissions can be cut compared with conventional feed ingredients. Seaweed absorbs CO2 as it grows and, unlike soybeans or corn, requires no land conversion or long-distance transport, so it works to push down the feed's carbon footprint.
This is the crux for Japanese buyers. Domestic food retail and foodservice face year-by-year stronger demands to disclose Scope 3 (indirect emissions including from suppliers), and shrimp is watched nervously as an item with large emissions from freezing and farming. The story of 'shrimp from a farming pond that lowers dependence on antibiotics and chemicals and even cuts the feed's carbon' becomes a differentiator in sourcing that steps beyond mere cost competition. In Vietnam, the government and NGOs are advancing certification and support for low-emission shrimp, and such on-the-ground accumulation can form the basis of a premium for Japan. On the institutional side that underpins export credibility, a traceability reform that eases the requirements for obtaining area codes and shifts to post-inspection-led oversight is advancing in parallel.
Whether antibiotics can be reduced depends on the design of stocking density and seaweed harvest frequency
Here one should look calmly. The seaweed raft absorbs nutrient salts in the water to curb disease, but it is not itself magic that brings antibiotics to zero. The size of the effect depends on the operational design — the shrimp stocking density, the number and placement of rafts, and how frequently the seaweed is harvested to carry nutrient salts out of the system. Leave the seaweed floating in the pond untended, and withered seaweed could instead become an organic load.
In other words, do not jump to 'seaweed raft = no medicine needed'; density management, harvest cycle, and water-quality monitoring must be viewed as a set. If the Japanese side is to use it as a sourcing basis, it is safest to cross-check against antibiotic usage data and third-party certification. As with the case of combining rice fields with shrimp and giant freshwater prawn to raise the value of the land (Giant freshwater prawns co-cultured in Thanh Hoa's paddies raise the value of the same land 1.5–3 times), the effect of composite farming is decided by the design of how things are combined.
What Japanese sourcing staff should check next is the seaweed's species, sales channel, and traceability
If you source shrimp or seaweed from a producing area using this method, or encourage a supplier to take up a similar effort, there are three points to check.
- The seaweed's species and use: Local reporting does not identify the species. Whether it is Gracilaria-type or Ulva-type changes whether it suits feed or food and agar raw material. If food grade, there may be a chance to open a separate sales channel as an ingredient for Japan, like sea grapes (Vietnamese sea grapes earn organic certification: will they become an option for imports to Japan?）。
- The reality of the sales channel: The premise of the side income is 'that a feed maker buys it.' Whether a buyer exists in the region greatly changes how reproducible the model is.
- Backing for the emissions reduction: If the figure of 2.5 tonnes per tonne is to be used in a sourcing context, confirm the calculation scope and whether there is third-party verification. Citing the number on its own is best avoided.
Environmental compliance becomes the farmer's revenue, and its by-product reinforces the sourcing side's environmental story. The seaweed raft in shrimp farming shows, at a not-small scale of over 100 hectares, that sustainability and cost can coexist. If expansion to several thousand hectares is realized, it becomes an option Japanese seafood sourcing cannot ignore either.
