How Blue Gold works out a farm
Two questions, answered in order: can a farm go here, and what would a hectare of it give back.
In short
- 400 rafts
- Fit on a hectare of favourable water: 12 × 12 ft bamboo rafts of Kappaphycus alvarezii, the seaweed nearly all Indian farms grow.ICAR-CMFRI, Johnson et al. 2023
- 30.4 t dry
- Seaweed that hectare gives in a year, after keeping back seed for the next crop.
- ₹18.9 lakh
- Left over a year once rafts, seed, labour and capital are paid, at ₹95 a kilo dried. Enough work for about 9 families.
- 22.2 t CO₂
- Taken up by that crop in a year. Sold as food or carrageenan, as nearly all of it is today, none of it stays out of the air for long.
Where a parcel can go
ICAR-CMFRI surveyed India's coast for seaweed farming and named the sites where the water suits it: calm, shallow sea within a kilometre of the lowest low tide. Blue Gold draws that rule on the map as the gold band, and only water inside it counts.
| Step | What the map does |
|---|---|
| Sites | 274 of the 317 sites in CMFRI's survey could be placed on the map from their names and descriptions. |
| Near the shore | Sea within 1 km of the coastline. The low-tide line isn't mapped for all of India, so the high-water line from OpenStreetMap is used, which keeps the band on the shoreward, safer side. |
| Near a site | Within 3 km of a surveyed site. Water far from any site may be fine, but nobody has checked it. |
| Not land | Land cut out using OpenStreetMap's land polygons, accurate to about 10 m. Pieces under 5 ha are dropped: too small for a farm. |
| Your parcel | What you draw is clipped to the band. The part outside turns orange and isn't counted; the part inside is filled with rafts, 400 to a hectare. |
The result is 108 stretches of water covering 1,12,140 ha, around 264 sites. That is more than the 23,970 ha CMFRI itself counted as potential, because the band marks where you can draw, not where a farm is certain to work: it is the search area, and the site visit decides.
One hectare, step by step
Every figure the map gives is this chain, multiplied by the hectares you drew and scaled by the yield slider.
| Step | Working | Result |
|---|---|---|
| Rafts | CMFRI's spacing for 12 × 12 ft rafts | 400 a hectare |
| Wet harvest | 400 rafts × 760 kg a year (four crops of 1,000 kg a raft, less 240 kg kept back as seed) | 304 t |
| Dried | × 10% dry weight | 30.4 t |
| Carbon | × 19.9% carbon in the dry weight | 6.05 t C |
| CO₂ taken up | × 44 ⁄ 12, the weight of CO₂ per unit of carbon | 22.2 t |
| Sales | 30.4 t × ₹95 a kilo, the average farm-gate price for dried seaweed | ₹28.9 lakh |
| Cost | 400 rafts × ₹2,500 a year, capital included | ₹10.0 lakh |
| Net income | Sales less cost | ₹18.9 lakh |
| Families | 400 rafts ÷ 45, the number one person can tend | 8.9 |
Prices and costs are CMFRI's, in 2023 rupees. The map rounds rafts down to whole ones; this table doesn't.
Three carbon numbers, never added
Seaweed is often sold as a carbon sink. The honest answer depends on what happens to the harvest, so Blue Gold keeps three numbers apart.
- Taken up: the carbon built into the crop in a year, 22.2 t CO₂ a hectare. Real and measurable. This is the figure usually quoted as "sequestration".
- Stored: the part of that carbon still out of the air after 100 years. It depends entirely on the end use, shown below.
- Avoided: emissions saved when seaweed replaces something else, such as a synthetic fertiliser. Blue Gold gives none, because it would need a named substitution to count.
| What happens to the harvest | Carbon kept 100+ years | t CO₂ a hectare a year |
|---|---|---|
| Food and carrageenan (today's market) | 0% | 0.0 |
| Biostimulant or animal feed | 0% | 0.0 |
| Compost into soil | 5% | 1.1 |
| Biochar | 23% | 5.1 |
Biochar: 35% of the seaweed's carbon ends up in the char, and 65% of that lasts a century (IPCC 2019, low-temperature char). Compost: an order-of-magnitude figure. Sinking seaweed in the deep sea isn't included: it isn't done or verifiable on India's shallow shelf.
When it pays
The map's two sliders are the two things a farm least controls: how well the seaweed grows against CMFRI's reference farm, and what dried seaweed sells for. Net income for one hectare a year; the reference farm in bold:
| Price, dried | 40% yield | 60% yield | 80% yield | 100% yield | 120% yield |
|---|---|---|---|---|---|
| ₹50 a kilo | −₹3.9 lakh | −₹0.9 lakh | ₹2.2 lakh | ₹5.2 lakh | ₹8.2 lakh |
| ₹70 a kilo | −₹1.5 lakh | ₹2.8 lakh | ₹7.0 lakh | ₹11.3 lakh | ₹15.5 lakh |
| ₹95 a kilo | ₹1.6 lakh | ₹7.3 lakh | ₹13.1 lakh | ₹18.9 lakh | ₹24.7 lakh |
| ₹120 a kilo | ₹4.6 lakh | ₹11.9 lakh | ₹19.2 lakh | ₹26.5 lakh | ₹33.8 lakh |
| ₹140 a kilo | ₹7.0 lakh | ₹15.5 lakh | ₹24.0 lakh | ₹32.6 lakh | ₹41.1 lakh |
At ₹95 a kilo a farm covers its costs from 35% of the reference yield. At ₹50 it needs 66%. A disease year can take it far below either: when ice-ice disease struck in 2013–14, India's output fell from 1,490 to 40 tonnes dry.
What it can't tell you
- Whether your water suits seaweed this year. A site still needs checking: salinity 28–38 ppt, 26–31 °C, water clear to 2–6 m, and shelter from heavy waves.
- Whether you may farm there. Permissions come from the state fisheries department, and near the Gulf of Mannar's coral reefs, only native species within the biosphere reserve's rules.
- Disease, cyclones, theft or grazing fish. The reference farm is a good year on a good site.
- Other species or methods. The model is Kappaphycus on bamboo rafts; monoline and tube-net farms are counted in raft equivalents.
Maplore seaweed model v0.1, 26 September 2026. The map and this page run the same code; change a parameter and both change.
Sources
| What | From |
|---|---|
| Farm, yield, costs, price | Johnson B. et al. (2023) Seaweed farming technologies. ICAR-CMFRI. eprints.cmfri.org.in/17847 |
| Sites and potential area | Johnson B. et al. (2020) Preliminary estimates of potential areas for seaweed farming along the Indian coast. ICAR-CMFRI, MFIS 246. eprints.cmfri.org.in/14936 |
| Coastline and land | OpenStreetMap contributors, land polygons (ODbL) |
| Biochar durability | IPCC (2019) Refinement to the 2006 Guidelines, Vol. 4, Annex 4 |
| Stored versus taken up | Troell et al. (2022) and Hurd et al. (2022), reviews of seaweed and carbon |
| Disease collapse | Mishra and Mantri (2026), Frontiers in Aquaculture. doi.org/10.3389/faquc.2026.1833554 |