Seaweed farming orkelp farming is the practice ofcultivating andharvestingseaweed. In its simplest form farmers gather from natural beds, while at the other extreme farmers fully control the crop'slife cycle.

The seven most cultivatedtaxa areEucheuma spp.,Kappaphycus alvarezii,Gracilaria spp.,Saccharina japonica,Undaria pinnatifida,Pyropia spp., andSargassum fusiforme.Eucheuma andK. alvarezii are attractive forcarrageenan (agelling agent);Gracilaria is farmed foragar; the rest are eaten after limited processing.^[1] Seaweeds are different frommangroves andseagrasses, as they are photosynthetic algal organisms^[2] and are non-flowering.^[1]

The largest seaweed-producing countries as of 2022 areChina (58.62%) andIndonesia (28.6%); followed bySouth Korea (5.09%) and thePhilippines (4.19%). Other notable producers includeNorth Korea (1.6%),Japan (1.15%),Malaysia (0.53%),Zanzibar (Tanzania, 0.5%), andChile (0.3%).^[3][4] Seaweed farming has frequently been developed to improve economic conditions and to reduce fishing pressure.^[5]

TheFood and Agriculture Organization (FAO) reported that world production in 2019 was over 35 million tonnes. North America produced some 23,000 tonnes of wet seaweed. Alaska, Maine, France, and Norway each more than doubled their seaweed production since 2018. As of 2019, seaweed represented 30% of marineaquaculture.^[6] In 2023, the global seaweed extract market was valued at $16.5 billion, with strong projected growth.^[7]

Seaweed farming is acarbon negative crop, with a high potential forclimate change mitigation.^[8][9] The IPCCSpecial Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" as a mitigation tactic.^[10]World Wildlife Fund, Oceans 2050, andThe Nature Conservancy publicly support expanded seaweed cultivation.^[6]

The earliest seaweed farming guides in the Philippines recommended the cultivation ofLaminaria seaweed and reef flats at approximately one meter's depth at low tide. They also recommended cutting off seagrasses and removing sea urchins before farm construction. Seedlings are tied to monofilament lines and strung between mangrove stakes in the substrate. This off-bottom method remains a primary method.^[11]

Long-line cultivation methods can be used in water approximately 7 meters (23 ft) in depth. Floating cultivation lines are anchored to the bottom and are widely used inNorth Sulawesi,Indonesia.^[12][13] Species cultured by long-line include those of the generaSaccharina,Undaria,Eucheuma,Kappaphycus, andGracilaria.^[14]

Cultivation in Asia is relatively low-technology with a high labor requirement. Attempts to introduce technology to cultivate detached plant growth in tanks on land to reduce labor have yet to attain commercial viability.^[11]

A bacterial infection calledice-ice stunts seaweed crops. In the Philippines 15 percent reduction in one species appeared in 2011 to 2013, representing 268,000 tonnes of seaweed.^[6] The spread of ice-ice disease is strongly associated with increasing seawater temperatures.^[15]

Seaweed is an extractive crop that has little need for fertilisers or water, meaning that seaweed farms typically have a smaller environmental footprint than other agriculture or fedaquaculture.^[16][17][18] Many of the impacts of seaweed farms, both positive and negative, remain understudied and uncertain.^[19][16]

Nonetheless, many environmental problems can result from seaweed farming.^[19] For instance, seaweed farmers sometimes cut downmangroves to use as stakes. Removing mangroves negatively affects farming by reducing water quality and mangrove biodiversity. Farmers may removeeelgrass from their farming areas, damaging water quality.^[20] Seaweed farms are routinely placed on top of seagrass meadows, particularly across Southeast Asia and the Western Indian Ocean, and numerous negative impacts occur.^[21]

Seaweed farming can pose a biosecurity risk, as farming activities have the potential to introduce or facilitateinvasive species.^[22][23] For this reason, regions such as the UK, Maine and British Columbia only allow native varieties.^[24]

Farms may also have positive environmental effects. They may support welcomeecosystem services such asnutrient cycling, carbon uptake, and habitat provision.^[25]

Evidence suggests that seaweed farming can have positive impacts which include supplementing human diets, feeding livestock, creating biofuels, slowing climate change and providing crucial habitat for a marine life, but must scale sustainably in order to have these effects.^[26] One way for seaweed farming to scale at terrestrial farming levels is with the use ofROVs, which can install low-cost helical anchors that can extend seaweed farming into unprotected waters.^[27]

Seaweed can be used to capture, absorb, and incorporate excess nutrients into living tissue, aka nutrient bioextraction/bioharvesting, is the practice offarming and harvesting shellfish and seaweed to remove nitrogen and other nutrients from natural water bodies.^[8][28]

Similarly, seaweed farms may offer habitat that enhancesbiodiversity.^[22][23] Seaweed farms have been proposed to protect coral reefs^[29] by increasing diversity, providing habitat for local marine species. Farming may increase the production of herbivorous fish and shellfish.^[5] Pollinac reported an increase inSiginid population after the start of farming ofEucheuma seaweed in villages in North Sulawesi.^{[13][19][22][23]}

In Japan the annual production ofnori amounts to US$2 billion and is one of the world's most valuable aquaculture crops. The demand for seaweed production provides plentiful work opportunities.

A study conducted by the Philippines reported that plots of approximately onehectare could produce net income fromEucheuma farming that was 5 to 6 times the average wage of an agriculture worker. The study also reported an increase in seaweed exports from 675 metric tons (MT) in 1967 to 13,191 MT in 1980, and 28,000 MT by 1988.^[30]

About 0.7 million tonnes of carbon are removed from the sea each year by commercially harvested seaweeds.^[31] In Indonesia, seaweed farms account for 40 percent of the national fisheries output and employ about one million people.^[6]

The Safe Seaweed Coalition is a research and industry group that promotes seaweed cultivation.^[6]

Seaweed farming has had widespread socio-economic impacts in Tanzania, has become a very important source of resources for women, and is the third biggest contributor of foreign currency to the country.^[32] 90% of the farmers are women, and much of it is used by the skincare and cosmetics industry.^[33]

In 1982Adelaida K. Semesi began a programme of research into seaweed cultivation inZanzibar and its application resulted in greater investment in the industry.^[34]

• 
  Zanzibar's seaweed growers face a changing climate. Here, a farmer tends to her farm in Paje, on the southeast coast of the island.
• 
  Mwanaisha Makame and Mashavu Rum, who have been farming seaweed on Zanzibar island for 20 years, wade through the low tide to their farm.
• 
  The seaweed grows underwater for 45 days. When it reaches one kilogram it is picked and dried, then packed in bags to be exported to countries like China, Korea, and Vietnam. There, it is used in medicines and shampoos.
• 
  The farmers have a lot of problems due to climate change. Two decades ago, 450 seaweed farmers roamed Paje. Now, only about 150 farmers remain.
• 
  Mwanaisha holds up a healthy clump of seaweed. Then she holds up seaweed the farmers will not be able to use. A hard white substance grows on it—ice-ice disease, caused by higher ocean temperatures and intense sunlight.
• 
  The seaweed farmers learned how to make soap from their seaweed at the Zanzibar Seaweed Center, a business that started as an NGO in 2009. At their homes, they mix water, ground seaweed powder, coconut oil, caustic soda, and essential oils in a large plastic tub.
• 
  Later in the week, the seaweed farmers will sell their finished soaps in Zanzibar town or to regular local customers. As seaweed levels decline, they have found a way to increase the value of their work.
• 
  The finished product—a bar of seaweed soap

Farmed seaweed is used in industrial products, as food, as an ingredient in animal feed, and as source material forbiofuels.^[35]

Seaweeds are used to produce chemicals that can be used for various industrial, pharmaceutical, or food products. Two major derivative products arecarrageenan andagar. Bioactive ingredients can be used for industries such aspharmaceuticals,^[36]industrial food,^[37] andcosmetics.^[38]

Seaweed cultivation in the open ocean can act as a form of carbon sequestration to mitigate climate change.^[9][53] Studies have reported that nearshore seaweed forests constitute a source ofblue carbon, as seaweed detritus is carried into the middle and deep ocean thereby sequestering carbon.^{[10][9][54][55][56]}Macrocystis pyrifera (also known as giant kelp) sequesters carbon faster than any other species. It can reach 60 m (200 ft) in length and grow as rapidly as 50 cm (20 in) a day.^[57] According to one study, covering 9% of the world's oceans with kelp forests could produce "sufficientbiomethane to replace all of today's needs in fossil fuel energy, while removing 53 billion tons of CO₂ per year from the atmosphere, restoring pre-industrial levels".^[58][59]

Seaweed farming may be an initial step towards adapting to andmitigating climate change. These include shoreline protection through the dissipation of wave energy, which is especially important to mangrove shorelines. Carbon dioxide intake would raisepH locally, benefitting calcifiers (e.g. crustaceans) or in reducing coral bleaching. Finally, seaweed farming could provide oxygen input to coastal waters, thus counteringocean deoxygenation driven by risingocean temperature.^[9][60]

Tim Flannery claimed that growing seaweeds in the open ocean, facilitated by artificial upwelling and substrate, can enable carbon sequestration if seaweeds are sunk to depths greater than one kilometer.^[61][62][63]

Seaweed contributes approximately 16–18.7% of the total marine-vegetation sink. In 2010 there were 19.2 ×${\displaystyle {10}^6}$ tons of aquatic plants worldwide, 6.8 ×${\displaystyle {10}^6}$ tons forbrown seaweeds; 9.0 ×${\displaystyle {10}^6}$ tons for red seaweeds; 0.2 ×${\displaystyle {10}^6}$ tons of green seaweeds; and 3.2 ×${\displaystyle {10}^6}$ tons of miscellaneous aquatic plants. Seaweed is largely transported from coastal areas to the open and deep ocean, acting as a permanent storage of carbon biomass within marine sediments.^[64]

Ocean afforestation is a proposal for farming seaweed forcarbon removal.^[9][65] After harvesting seaweed is decomposed intobiogas (60%methane and 40%carbon dioxide) in ananaerobic digester. The methane can be used as a biofuel, while the carbon dioxide can be stored to keep it from the atmosphere.^[59]

Similarly, the NGO Climate Foundation andpermaculture experts claimed that offshore seaweed ecosystems can be cultivated according to permaculture principles, constitutingmarine permaculture.^{[66][67][68][69][70]} The concept envisions using artificial upwelling and floating, submerged platforms as substrate to replicate natural seaweed ecosystems that provide habitat and the basis of a trophic pyramid for marine life.^[71] Seaweeds and fish can be sustainably harvested. As of 2020, successful trials had taken place in Hawaii, the Philippines, Puerto Rico and Tasmania.^[72][73] The idea featured as a solution covered by the documentary2040 and in the bookDrawdown: The Most Comprehensive Plan Ever Proposed to Reverse Global Warming.

Human use of seaweed is known from theNeolithic period.^[4] Cultivation ofgim (laver) inKorea is reported in books from the 15th century.^[74][75] Seaweed farming began in Japan as early as 1670 inTokyo Bay.^[76] In autumn of each year, farmers would throwbamboo branches into shallow, muddy water, where the spores of the seaweed would collect. A few weeks later these branches would be moved to a riverestuary. Nutrients from the river helped the seaweed to grow.^[76]

In the 1940s, the Japanese improved this method by placing nets of synthetic material tied to bamboo poles. This effectively doubled production.^[76] A cheaper variant of this method is called thehibi method—ropes stretched between bamboo poles. In the early 1970s, demand for seaweed and seaweed products outstripped supply, and cultivation was viewed as the best means to increase production.^[77]

In the tropics, commercial cultivation ofCaulerpa lentillifera (sea grapes) was pioneered in the 1950s inCebu, Philippines, after accidental introduction ofC. lentillifera to fish ponds on the island ofMactan.^[78][79] This was further developed by local research, particularly through the efforts ofGavino Trono, since recognized as aNational Scientist of the Philippines. Local research and experimental cultures led to the development of the first commercial farming methods for other warm-water algae (since cold-water red and brown edible algae favored inEast Asia do not grow in the tropics), including the first successful commercial cultivation ofcarrageenan-producing algae. These includeEucheuma spp.,Kappaphycus alvarezii,Gracilaria spp., andHalymenia durvillei.^{[80][81][82][83]} In 1997, it was estimated that 40,000 people in thePhilippines made their living through seaweed farming.^[29] The Philippines was the world's largest producer of carrageenan for several decades until it was overtaken byIndonesia in 2008.^{[84][85][86][87]}

Seaweed farming spread beyond Japan and the Philippines to southeast Asia, Canada, Great Britain, Spain, and the United States.^[88]

In the 2000s, seaweed farming has been getting increasing attention due to its potential formitigating both climate change and other environmental issues, such asagricultural runoff.^[89][90] Seaweed farming can be mixed with otheraquaculture, such as shellfish, to improve water bodies, such as in the practices developed by American non-profitGreenWave.^[89] The IPCCSpecial Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" as a mitigation tactic.^[10]

In 2024 a commercial-scale seaweed farm began construction within the Hollandse Kust Zuid (HKZ) 139 turbine wind farm. The project uses 13-metre long "Eco-anchors" that cover the surface with a marine life habitat using materials such as oyster shells, wood, and cork.^[91]

In 2025 theFood Policy Institute recommended a series of policies that would aid with commercialising seaweed growth in the United Kingdom.^[92]

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 This article incorporates text from afree content work. Licensed under CC BY-SA 3.0 IGO (license statement/permission). Text taken fromIn brief, The State of World Fisheries and Aquaculture, 2018​, FAO, FAO.

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• "Seaweed farming: an economic and sustainable opportunity for Europe".YouTube. euronews. June 9, 2020.

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