Seaweed farming or kelp farming is the practice of cultivating and harvesting seaweed. In its simplest form farmers gather from natural beds, while at the other extreme farmers fully control the crop's life cycle.
The seven most cultivated taxa are Eucheuma spp., Kappaphycus alvarezii, Gracilaria spp., Saccharina japonica, Undaria pinnatifida, Pyropia spp., and Sargassum fusiforme. Eucheuma and K. alvarezii are attractive for carrageenan (a gelling agent); Gracilaria is farmed for agar; the rest are eaten after limited processing. Seaweeds are different from mangroves and seagrasses, as they are photosynthetic algal organisms and are non-flowering.
The largest seaweed-producing countries are China, Indonesia, and the Philippines. Other notable producers include South Korea, North Korea, Japan, Malaysia, and Zanzibar (Tanzania). Seaweed farming has frequently been developed to improve economic conditions and to reduce fishing pressure.
The Food 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 marine aquaculture.
Seaweed farming is a carbon negative crop, with a high potential for climate change mitigation. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" as a mitigation tactic. World Wildlife Fund, Oceans 2050, and The Nature Conservancy publicly support expanded seaweed cultivation.
Methods
The earliest seaweed farming guides in the Philippines recommended the cultivation of Laminaria 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.
Long-line cultivation methods can be used in water approximately 7 meters in depth. Floating cultivation lines are anchored to the bottom and are widely used in North Sulawesi, Indonesia. Species cultured by long-line include those of the genera Saccharina, Undaria, Eucheuma, Kappaphycus, and Gracilaria.
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.
Ecological impacts
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 fed aquaculture. Many of the impacts of seaweed farms, both positive and negative, remain understudied and uncertain.
Nonetheless, many environmental problems can result from seaweed farming. For instance, seaweed farmers sometimes cut down mangroves to use as stakes. Removing mangroves negatively affects farming by reducing water quality and mangrove biodiversity. Farmers may remove eelgrass from their farming areas, damaging water quality.
Seaweed farming can pose a biosecurity risk, as farming activities have the potential to introduce or facilitate invasive species. For this reason, regions such as the UK, Maine and British Columbia only allow native varieties.
Farms may also have positive environmental effects. They may support welcome ecosystem services such as nutrient cycling, carbon uptake, and habitat provision.
Seaweed can be used to capture, absorb, and incorporate excess nutrients into living tissue, aka nutrient bioextraction/bioharvesting, is the practice of farming and harvesting shellfish and seaweed to remove nitrogen and other nutrients from natural water bodies.
Similarly, seaweed farms may offer habitat that enhances biodiversity. Seaweed farms have been proposed to protect coral reefs by increasing diversity, providing habitat for local marine species. Farming may increase the production of herbivorous fish and shellfish. Pollinac reported an increase in Siginid population after the start of farming of Eucheuma seaweed in villages in North Sulawesi.
Bacterial infection ice-ice stunts seaweed crops. In the Philippines 15 percent reduction in one species appeared in 2011 to 2013, representing 268,000 tonnes of seaweed.
Economic impacts
In Japan the annual production of nori 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.