Seaweed farming, also called kelp farming, is the process of growing and collecting seaweed. Some farmers collect seaweed from natural areas, while others grow it in controlled environments where they manage every stage of its growth.

The seven most commonly farmed types of seaweed are Eucheuma spp., Kappaphycus alvarezii, Gracilaria spp., Saccharina japonica, Undaria pinnatifida, Pyropia spp., and Sargassum fusiforme. Eucheuma and K. alvarezii are grown for carrageenan, a substance used to make jellies. Gracilaria is farmed for agar, a material used in food and science. The other types are eaten after being processed. Unlike mangroves and seagrasses, seaweed is a type of algae that uses sunlight to make food and does not have flowers.

In 2022, the largest seaweed-producing countries were China (58.62%) and Indonesia (28.6%). South Korea (5.09%) and the Philippines (4.19%) were next. Other countries that produce seaweed include North Korea (1.6%), Japan (1.15%), Malaysia (0.53%), Zanzibar (Tanzania, 0.5%), and Chile (0.3%). Seaweed farming is often developed to help improve the economy and reduce pressure on fish populations.

According to the Food and Agriculture Organization (FAO), global seaweed production in 2019 was over 35 million tonnes. North America produced about 23,000 tonnes of wet seaweed. Alaska, Maine, France, and Norway each more than doubled their seaweed production since 2018. In 2019, seaweed accounted for 30% of all marine aquaculture. In 2023, the global seaweed extract market was valued at $16.5 billion, with expected growth.

Seaweed farming is a carbon-negative crop, meaning it helps reduce carbon in the environment. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate suggests more research on seaweed farming as a way to fight climate change. Organizations such as the World Wildlife Fund, Oceans 2050, and The Nature Conservancy support expanding seaweed cultivation.

Methods

The earliest seaweed farming guides in the Philippines suggested growing Laminaria seaweed and using reef flats at about one meter deep during low tide. Farmers were also advised to cut seagrasses and remove sea urchins before building seaweed farms. Seedlings are tied to thin lines and hung between mangrove stakes in the area. This method, called off-bottom farming, is still widely used today.

Long-line farming can be done in water about 7 meters (23 feet) deep. These methods use floating lines that are tied to the bottom of the ocean and are commonly used in North Sulawesi, Indonesia. Species grown using long-line farming include those from the groups Saccharina, Undaria, Eucheuma, Kappaphycus, and Gracilaria.

Seaweed farming in Asia often uses simple techniques that require a lot of manual labor. Some efforts to use technology for growing seaweed in tanks on land to reduce labor have not yet become successful in commercial farming.

Diseases

A bacterial infection known as ice-ice disease harms seaweed farms. In the Philippines, the number of one seaweed species dropped by 15% between 2011 and 2013, which equals 268,000 tonnes of seaweed lost. The spread of this disease is connected to rising seawater temperatures.

Ecological impacts

Seaweed is a crop that requires little fertilizer or water, meaning seaweed farms usually have a smaller impact on the environment compared to other farming or fish farming methods. However, many effects of seaweed farming, both good and bad, are not fully understood.

Some environmental problems can arise from seaweed farming. For example, farmers sometimes cut down mangroves to use as stakes. Removing mangroves harms water quality and reduces the variety of plants and animals in those areas. Farmers may also remove eelgrass from their farming areas, which harms water quality. Seaweed farms are often placed on seagrass meadows, especially in Southeast Asia and the Western Indian Ocean, leading to several negative effects.

Seaweed farming can create risks for the environment by introducing non-native species. Because of this, places like the UK, Maine, and British Columbia only allow seaweed that naturally grows in their areas.

Seaweed farms can also help the environment. They may support important natural processes, such as moving nutrients through ecosystems, absorbing carbon dioxide, and providing homes for marine life.

Evidence shows seaweed farming can have benefits, such as adding to human diets, feeding animals, creating fuel, reducing climate change effects, and offering habitat for marine life. However, these benefits can only be achieved if farming grows in a sustainable way. One way to expand seaweed farming is by using ROVs, which can place low-cost anchors to grow seaweed in areas not previously used for farming.

Seaweed can capture and store extra nutrients in its tissues, a process called nutrient bioextraction. This involves farming seaweed and other organisms to remove nitrogen and other nutrients from water.

Seaweed farms may also improve biodiversity by creating habitats for marine life. They have been suggested as a way to protect coral reefs by increasing species diversity and providing homes for local marine animals. Farming may also increase the number of herbivorous fish and shellfish. A study found more of a certain species of snail after seaweed farming began in villages in North Sulawesi.

Economic impacts

In Japan, the yearly production of nori is worth US$2 billion. This high level of production requires a lot of work, providing many job opportunities.

A study in the Philippines found that one hectare of land used for farming Eucheuma can produce income that is 5 to 6 times higher than what an average agriculture worker earns. The study also showed that seaweed exports increased from 675 metric tons (MT) in 1967 to 13,191 MT in 1980 and 28,000 MT by 1988.

Each year, about 700,000 tonnes of carbon are taken from the ocean by seaweeds that are harvested for sale. In Indonesia, seaweed farms make up 40% of the country’s fishery production and provide jobs for about one million people.

The Safe Seaweed Coalition is a group that works on seaweed farming research and industry.

Seaweed farming has helped the economy and people’s lives in Tanzania. It is a major source of resources for women and the third biggest way the country earns money from other countries. About 90% of seaweed farmers in Tanzania are women, and much of the seaweed is used in skincare and cosmetics.

In 1982, Adelaida K. Semesi started research on seaweed farming in Zanzibar. This research led to more investment in the industry.

• Zanzibar’s seaweed farmers deal with changing weather patterns. A farmer in Paje tends to her seaweed farm on the southeast coast of the island.
• Mwanaisha Makame and Mashavu Rum have farmed seaweed on Zanzibar for 20 years. They walk through the low tide to reach their farm.
• The seaweed grows underwater for 45 days. When it weighs one kilogram, it is picked, dried, and packed in bags to be sent to countries like China, Korea, and Vietnam. There, it is used in medicines and shampoos.
• Farmers face many challenges because of climate change. Twenty years ago, 450 seaweed farmers worked in Paje. Now, only about 150 remain.
• Mwanaisha shows a healthy seaweed sample and one that is not usable. A hard white substance called ice-ice disease, caused by warmer ocean temperatures and strong sunlight, affects the seaweed.
• At the Zanzibar Seaweed Center, farmers learned to make soap from seaweed. They mix water, seaweed powder, coconut oil, caustic soda, and essential oils in a large container at home.
• Later in the week, farmers sell their seaweed soap in Zanzibar town or to local customers. As seaweed supplies decrease, they have found ways to increase the value of their work.
• The finished product is a bar of seaweed soap.

Uses

Farmed seaweed is used in industrial products, as food, as an ingredient in animal feed, and as a source material for biofuels.

Seaweeds are used to produce chemicals that can be used for various industrial, pharmaceutical, or food products. Two major derivative products are carrageenan and agar. Bioactive ingredients can be used for industries such as pharmaceuticals, industrial food, and cosmetics.

Carrageenans or carrageenins are a family of natural linear sulfated polysaccharides. They are extracted from red edible seaweeds. Carrageenans are widely used in the food industry for their gelling, thickening, and stabilizing properties. Their main application is in dairy and meat products, due to their strong binding to food proteins. Carrageenans have emerged as a promising candidate in tissue engineering and regenerative medicine applications as they resemble animal glycosaminoglycans (GAGs). They are used for tissue engineering, wound coverage, and drug delivery.

Agar is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from the Gracilaria genus (Irish moss, ogonori) and the Gelidiaceae family (tengusa). As found in nature, agar is a mixture of two components: the linear polysaccharide agarose and a heterogeneous mixture of smaller molecules called agaropectin. It forms the supporting structure in the cell walls of certain species of algae and is released on boiling. These algae are known as agarophytes, belonging to the Rhodophyta (red algae) phylum. The processing of food-grade agar removes the agaropectin, and the commercial product is essentially pure agarose.

Edible seaweeds are seaweeds that can be eaten and used for culinary purposes. They typically contain high amounts of fiber. They may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. Seaweeds are also harvested or cultivated for the extraction of polysaccharides such as alginate, agar, and carrageenan, gelatinous substances collectively known as hydrocolloids or phycocolloids. Hydrocolloids have attained commercial significance, especially in food production as food additives. The food industry exploits the gelling, water-retention, emulsifying, and other physical properties of these hydrocolloids.

Algae fuel, algal biofuel, or algal oil is an alternative to liquid fossil fuels that use algae as the source of energy-rich oils. Also, algae fuels are an alternative to commonly known biofuel sources, such as corn and sugarcane. When made from seaweed (macroalgae), it can be known as seaweed fuel or seaweed oil. These fuels have no practical significance but remain an aspirational target in the biofuels research area.

Seaweed cultivation in the open ocean can act as a form of carbon sequestration to mitigate climate change. Studies have reported that nearshore seaweed forests constitute a source of blue carbon, as seaweed detritus is carried into the middle and deep ocean, thereby sequestering carbon. 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. According to one study, covering 9% of the world's oceans with kelp forests could produce "sufficient biomethane 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."

Seaweed farming may be an initial step towards adapting to and mitigating climate change. These include shoreline protection through the dissipation of wave energy, which is especially important to mangrove shorelines. Carbon dioxide intake would raise pH locally, benefitting calcifiers (e.g., crustaceans) or in reducing coral bleaching. Finally, seaweed farming could provide oxygen input to coastal waters, thus countering ocean deoxygenation driven by rising ocean temperature.

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.

Seaweed contributes approximately 16–18.7% of the total marine-vegetation sink. In 2010, there were 19.2 × 10⁶ tons of aquatic plants worldwide, 6.8 × 10⁶ tons for brown seaweeds; 9.0 × 10⁶ tons for red seaweeds; 0.2 × 10⁶ tons of green seaweeds; and 3.2 × 10⁶ 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.

Ocean afforestation is a proposal for farming seaweed for carbon removal. After harvesting seaweed, it is decomposed into biogas (60% methane and 40% carbon dioxide) in an anaerobic digester. The methane can be used as a biofuel, while the carbon dioxide can be stored to keep it from the atmosphere.

Without explicit human support, seaweed has been expanding its footprint across the oceans, accelerating beginning in 2008–2010, increasing 13.4%/yr since then.

Similarly, the NGO Climate Foundation and permaculture experts claimed that offshore seaweed ecosystems can be cultivated according to permaculture principles, constituting marine permaculture. 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. Seaweeds and fish can be sustainably harvested. As of 2020, successful trials had taken place in Hawaii, the Philippines, Puerto Rico, and Tasmania. The idea featured as a solution covered by the documentary 2040 and in the book Drawdown: The Most Comprehensive Plan Ever Proposed to Reverse Global Warming.

History

Human use of seaweed dates back to the Neolithic period. In Korea, people began growing gim (laver) as early as the 15th century, as recorded in books from that time. In Japan, seaweed farming started in Tokyo Bay as early as 1670. Each autumn, farmers would place bamboo branches in shallow, muddy water, where seaweed spores would attach. After several weeks, the branches were moved to a river estuary, where nutrients from the river helped the seaweed grow.

In the 1940s, Japanese farmers improved this method by using synthetic nets tied to bamboo poles. This change doubled seaweed production. A less expensive version of this method is called the hibi method, which uses ropes stretched between bamboo poles. By the early 1970s, demand for seaweed and seaweed products grew faster than the supply, leading to increased focus on seaweed farming to meet needs.

In tropical regions, commercial seaweed farming began in the 1950s in Cebu, Philippines, after C. lentillifera (sea grapes) accidentally entered fish ponds on Mactan Island. Local research, especially by Gavino Trono, who is now recognized as a National Scientist of the Philippines, helped develop the first commercial farming methods for warm-water algae. These methods included successful cultivation of carrageenan-producing algae such as Eucheuma spp., Kappaphycus alvarezii, Gracilaria spp., and Halymenia durvillei. In 1997, about 40,000 people in the Philippines relied on seaweed farming for their livelihood. The Philippines was the world’s largest producer of carrageenan for many years until Indonesia surpassed it in 2008.

Seaweed farming later expanded to other regions, including southeast Asia, Canada, Great Britain, Spain, and the United States.

In the 2000s, seaweed farming gained more attention because of its potential to help reduce climate change and environmental problems, such as pollution from agriculture. Seaweed can be grown together with other aquatic life, like shellfish, to improve water quality. Practices like those developed by the American non-profit GreenWave demonstrate this. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate suggests that further research on seaweed farming could help reduce environmental harm.

In 2024, a commercial seaweed farm began construction near the Hollandse Kust Zuid (HKZ) 139 turbine wind farm. The project uses 13-meter-long "Eco-anchors" that create a habitat for marine life using materials like oyster shells, wood, and cork.

In 2025, the Food Policy Institute proposed policies to support the growth of seaweed farming in the United Kingdom.