Seaweed and Kelp Farming: Marine Macroalgae Aquaculture

Seaweed farming grows marine macroalgae with no feed or fertiliser input. Learn the species, seeded-line methods, uses from food to hydrocolloids, and benefits.

Seaweed farming is the cultivation of marine macroalgae and, by volume, one of the largest aquaculture sectors in the world. In 2019 world production was over 35 million tonnes, and seaweed made up about 30% of marine aquaculture. As of 2022, China dominated with about 58.6% of global output, followed by Indonesia at about 28.6%, with South Korea and the Philippines also major producers. Like bivalves, seaweed is extractive: it needs no feed and little fertiliser, taking carbon and nutrients straight from the water.

Major cultivated species

A handful of taxa account for most farmed seaweed, grown for food and for industrial gels (hydrocolloids).

Cultivation methods

Most farming begins in a nursery. For kelp, spores are settled onto thin twine or string in a land-based hatchery; the young plants spend a few weeks indoors under controlled conditions before going to sea. For sugar kelp, spores settle on the substrate within about 24 to 36 hours and the juveniles are reared indoors for roughly 4 to 6 weeks.

• Nursery: seed spores or sporelings onto lines, strings or nets in a hatchery
• Out-planting: deploy the seeded line at sea, commonly on longlines suspended a few feet below the surface between buoys
• Other systems: nets and rafts, with intertidal stake-and-line methods used in some regions
• Grow-out: blades grow over the cold season and are harvested when they reach the size needed for their intended use

Uses

• Food: nori, wakame, kelp and other edible seaweeds
• Hydrocolloids: agar (from Gracilaria), carrageenan (from Eucheuma and Kappaphycus) and alginate (from kelps)
• Animal feed
• Biofuel
• Fertiliser

Harvest and drying

Seaweed is harvested when blades reach the right size for their intended purpose; cold-water kelps such as sugar kelp grow through winter and are typically harvested in spring, with fresh product available roughly February to May in temperate regions. Much of the crop is dried after harvest, both to stabilise it for storage and transport and to prepare it for food or hydrocolloid processing.

Environmental benefits

Because seaweed photosynthesises and needs no feed or chemical fertiliser, its environmental footprint is low. Cultivation removes excess nutrients such as nitrogen and phosphorus along with carbon dioxide from the water while releasing oxygen, improving local water quality. Commercially harvested seaweeds remove on the order of 0.7 million tonnes of carbon from the sea each year, and seaweed is a natural partner crop in integrated multi-trophic aquaculture (IMTA), where it absorbs nutrients released by fed species.