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 completely under their control.

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 things firm. Gracilaria is farmed for agar, another gelling material. The other types are eaten after some processing. Seaweeds are different from mangroves and seagrasses because they are photosynthetic algae and do not have flowers.

In 2022, the top seaweed-producing countries were China (58.62%) and Indonesia (28.6%). South Korea (5.09%) and the Philippines (4.19%) followed. 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 used to help improve economies and reduce pressure on fishing industries.

According to the Food and Agriculture Organization (FAO), global seaweed production in 2019 was more than 35 million tonnes. North America produced about 23,000 tonnes of wet seaweed. Alaska, Maine, France, and Norway each increased their seaweed production by more than 100% since 2018. In 2019, seaweed made up 30% of marine aquaculture. In 2023, the global seaweed extract market was worth $16.5 billion, with expected growth.

Seaweed farming is a carbon-negative practice, meaning it absorbs more carbon dioxide than it releases. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate suggests more research on seaweed farming as a way to reduce climate change. Organizations like the World Wildlife Fund, Oceans 2050, and The Nature Conservancy support expanding seaweed farming.

Methods

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

Long-line farming methods work in water about 7 meters (23 feet) deep. These methods use floating lines that are secured to the ocean floor and are common in North Sulawesi, Indonesia. Seaweed species grown using long-line methods include types from the Saccharina, Undaria, Eucheuma, Kappaphycus, and Gracilaria groups.

Seaweed farming in Asia typically uses simple tools and requires a lot of human labor. Efforts to use tanks on land to grow detached seaweed plants and reduce labor have not yet become successful in business.

Diseases

A bacterial infection known as ice-ice slows the growth of seaweed crops. In the Philippines, a 15% decrease in one seaweed species occurred between 2011 and 2013, which equals 268,000 tonnes of seaweed. The spread of ice-ice disease is closely linked to rising seawater temperatures.

Ecological impacts

Seaweed is a type of plant that grows in the ocean and requires little fertilizer or water. Because of this, seaweed farms usually have a smaller effect on the environment compared to other types of farming or fish farming. However, many effects of seaweed farming, both good and bad, are not fully understood.

Some environmental problems can happen because of seaweed farming. For example, farmers sometimes cut down mangrove trees to use as supports for their seaweed. This harms the environment by lowering water quality and reducing the number of plant and animal species in mangrove areas. Farmers may also remove eelgrass from their farming areas, which harms water quality. Seaweed farms are often built on top of seagrass meadows, especially in places like Southeast Asia and the Western Indian Ocean, leading to many negative effects.

Seaweed farming can also create risks for the environment. Farming activities might bring in or spread non-native species that can harm local ecosystems. Because of this, places like the UK, Maine, and British Columbia only allow seaweed farming with species that naturally live in those areas.

Seaweed farming can also have benefits for the environment. It can help with important processes like recycling nutrients, absorbing carbon dioxide, and providing homes for marine life.

Studies show that seaweed farming can help in several ways, such as improving human diets, feeding animals, creating fuel from plants, slowing climate change, and offering homes for ocean animals. However, these benefits can only happen if seaweed farming grows in a way that does not harm the environment. One way to expand seaweed farming is by using remotely operated vehicles (ROVs) to place low-cost anchors that allow seaweed to grow in areas that are not protected.

Seaweed can take in extra nutrients from water and store them in its tissues. This process, called nutrient bioextraction or bioharvesting, involves farming and harvesting seaweed and shellfish to remove nitrogen and other nutrients from water.

Seaweed farms may also help increase the number of different species in the ocean. Some research suggests that seaweed farms could help protect coral reefs by increasing biodiversity and offering homes for local marine animals. Farming might also increase the number of herbivorous fish and shellfish. A study by Pollinac found that the population of Siginid snails increased 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 large amount of work, which provides many job opportunities.

A study in the Philippines found that one hectare of land used for Eucheuma farming can earn farmers 5 to 6 times more than the average wage for agricultural workers. The study also showed that seaweed exports increased from 675 metric tons (MT) in 1967 to 13,191 MT in 1980, and then to 28,000 MT by 1988.

Each year, commercially harvested seaweeds remove about 0.7 million tonnes of carbon from the sea. In Indonesia, seaweed farms make up 40 percent of the country’s total fisheries production and provide jobs for about one million people.

The Safe Seaweed Coalition is a group that works to support seaweed farming through research and industry efforts.

Seaweed farming has had a major impact on the economy and society in Tanzania. It is a key resource for women and is the third largest source of foreign currency for the country. About 90% of seaweed farmers in Tanzania are women, and much of the seaweed is used in skincare and cosmetics products.

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

• Zanzibar’s seaweed growers face challenges from climate change. A farmer tends to her seaweed farm in Paje, on the southeast coast of the island.
• Mwanaisha Makame and Mashavu Rum have farmed seaweed in Zanzibar for 20 years. They walk through the shallow water to reach their farm.
• The seaweed grows underwater for 45 days. When it reaches one kilogram, it is harvested, dried, and packed into bags for export to countries like China, Korea, and Vietnam. There, it is used in medicines and shampoos.
• Farmers face many problems because of climate change. Twenty years ago, 450 seaweed farmers worked in Paje. Now, only about 150 remain.
• Mwanaisha holds up a healthy piece of seaweed. Then she shows seaweed that is unusable because of a hard white substance called ice-ice disease, which is caused by warmer ocean temperatures and strong sunlight.
• Farmers learned to make soap from seaweed at the Zanzibar Seaweed Center, which began as an NGO in 2009. At home, they mix water, ground seaweed powder, coconut oil, caustic soda, and essential oils in a large plastic container.
• Later in the week, the farmers sell their finished soaps in Zanzibar town or to local customers. As seaweed supplies decrease, they have found ways to increase the value of their work.
• The final product is a bar of seaweed soap.

Uses

Farmed seaweed is used in industrial products, as food, in animal feed, and as a material for making biofuels. Seaweeds are used to create chemicals that can be used in industrial, pharmaceutical, or food products. Two important products made from seaweed are carrageenan and agar. These materials are used in industries such as medicine, food, and cosmetics.

Carrageenans are a type of natural substance made from red seaweed. They are used in the food industry because they help thicken, stabilize, and gel food. They are often found in dairy and meat products because they bind well to proteins in food. Carrageenans are also being studied for use in medical treatments, such as helping repair tissues and deliver medicines.

Agar is a jelly-like substance made from red algae. It is made up of two parts: agarose and agaropectin. Agar is found in the cell walls of certain algae and is released when the algae are boiled. Agar is used in food production after the agaropectin is removed, leaving mostly agarose.

Edible seaweeds are types of seaweed that can be eaten. They are rich in fiber and belong to groups of algae such as red, green, and brown algae. Seaweeds are also used to extract substances like alginate, agar, and carrageenan. These substances, called hydrocolloids, are important in food production because they help with gelling, thickening, and holding water.

Algae fuel is a type of energy made from algae. It is an alternative to fuels made from corn or sugarcane. When made from seaweed, it is sometimes called seaweed fuel or seaweed oil. However, these fuels are not yet widely used.

Growing seaweed in the ocean can help reduce carbon dioxide in the atmosphere. Seaweed forests near the shore can store carbon because seaweed waste is carried into deeper ocean areas. Giant kelp, a type of seaweed, stores carbon faster than any other plant. If 9% of the world's oceans were covered with kelp forests, it could produce enough energy to replace all fossil fuels and remove 53 billion tons of carbon dioxide from the air each year.

Seaweed farming can help reduce the effects of climate change. It can protect shorelines by absorbing wave energy, help balance ocean acidity, and provide oxygen to coastal waters. Scientists suggest that growing seaweed in the open ocean, using artificial methods, could store carbon if the seaweed is buried deep in the ocean.

Seaweed contributes about 16–18.7% of the total carbon stored by marine plants. In 2010, there were about 19.2 million tons of aquatic plants worldwide, including 6.8 million tons of brown seaweed, 9.0 million tons of red seaweed, 0.2 million tons of green seaweed, and 3.2 million tons of other plants. Much of this seaweed is carried into the deep ocean, where it stores carbon permanently.

Ocean afforestation is a plan to grow seaweed to remove carbon from the atmosphere. After seaweed is harvested, it can be broken down into biogas, which contains methane and carbon dioxide. Methane can be used as fuel, and carbon dioxide can be stored to prevent it from entering the air.

Since 2008–2010, seaweed has been growing more rapidly across the oceans, increasing by about 13.4% each year. Some organizations suggest that seaweed can be farmed in the ocean using methods inspired by permaculture. This includes using floating platforms to grow seaweed and fish together. Trials of this method have been successful in places like Hawaii, the Philippines, Puerto Rico, and Tasmania. This idea is also featured in the documentary 2040 and 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, records from the 15th century describe the cultivation of gim (laver). Seaweed farming in Japan began as early as 1670 in Tokyo Bay. Each autumn, farmers placed bamboo branches in shallow, muddy water. Seaweed spores attached to the branches. After several weeks, the branches were moved to a river estuary. Nutrients from the river helped the seaweed grow.

In the 1940s, the Japanese 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. It uses ropes stretched between bamboo poles. In the early 1970s, demand for seaweed and its products grew faster than supply. Cultivation became the best way to increase production.

In tropical regions, commercial farming of Caulerpa lentillifera (sea grapes) began in the 1950s in Cebu, Philippines. This happened after sea grapes accidentally entered fish ponds on Mactan Island. Local research, especially by Gavino Trono, a National Scientist of the Philippines, helped develop farming methods for warm-water algae. These methods were used to grow other algae, such as Eucheuma spp., Kappaphycus alvarezii, Gracilaria spp., and Halymenia durvillei. These algae produce carrageenan. In 1997, about 40,000 people in the Philippines earned a living through seaweed farming. The Philippines was the world’s largest producer of carrageenan for many years until Indonesia surpassed it in 2008.

Seaweed farming expanded beyond Japan and the Philippines to countries in Southeast Asia, Canada, Great Britain, Spain, and the United States.

In the 2000s, seaweed farming gained more attention because it can help reduce climate change and environmental problems, such as agricultural runoff. Seaweed farming can be combined with other aquaculture, like shellfish farming, 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 more research on seaweed farming as a way to reduce environmental harm.

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

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