Seaweed farming is the practice of cultivating and harvesting seaweed. In its simplest form, it consists of the management of naturally found batches. In its most advanced form, it consists of fully controlling the life cycle of the algae. The main food species grown by aquaculture in Japan, China and Korea include Gelidium, Pterocladia, Porphyra, and Laminaria. Seaweed farming has frequently been developed as an alternative to improve economic conditions and to reduce fishing pressure and overexploited fisheries. Seaweeds have been harvested throughout the world as a food source as well as an export commodity for production of agar and carrageenan products.
Global production of farmed aquatic plants, overwhelmingly dominated by seaweeds, grew in output volume from 13.5 million tonnes in 1995 to just over 30 million tonnes in 2016.
Seaweed farming began in Japan as early as 1670 in Tokyo Bay. In autumn of each year, farmers would throw bamboo branches into shallow, muddy water, where the spores of the seaweed would collect. A few weeks later these branches would be moved to a river estuary. The nutrients from the river would help the seaweed to grow.
In the 1940s, the Japanese improved this method by placing nets of synthetic material tied to bamboo poles. This effectively doubled the production. A cheaper variant of this method is called the hibi method — simple ropes stretched between bamboo poles.
Seaweed farming has been introduced to several different islands in Indonesia, such as Rote island. The Australian government first brought agar seeds to Rote 15 years ago to keep locals out of poverty.
In the early 1970s, there was a recognized demand for seaweed and seaweed products, outstripping supply, and cultivation was viewed as the best means to increase productions.
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 then tied to monofilament lines and strung between mangrove stakes pounded into the substrate. This off-bottom method is still one of the primary methods used today.
There are new long-line cultivation methods that can be used in deeper water approximately 7 meters in depth. They use floating cultivation lines anchored to the bottom and are the primary methods used in the villages of North Sulawesi, Indonesia. Species cultured by long-line include those of the genera Saccharina, Undaria, Eucheuma, Kappaphycus, and Gracilaria.
Cultivation of seaweed in Asia is a relatively low-technology business with a high labor requirement. There have been many attempts in various countries to introduce high technology to cultivate detached plants growth in tanks on land in order to reduce labor, but they have yet to attain commercial viability.
Environmental and ecological impacts
Several environmental problems can result from seaweed farming. Sometimes seaweed farmers cut down mangroves to use as stakes for their ropes. This, however, negatively affects farming since it reduces the water quality and mangrove biodiversity due to depletion. Farmers may also sometimes remove eelgrass from their farming areas. This, however, is also discouraged, as it adversely affects water quality.
Seaweed farming helps to preserve coral reefs by increasing diversity where the algae and seaweed have been introduced, and it also provides an added niche for local species of fish and invertebrates. Farming may be beneficial by increasing the production of herbivorous fishes and shellfish in the area. Pollnac & et al 1997b reported an increase in Siginid population after the start of extensive farming of eucheuma seaweed in villages in North Sulawesi, Indonesia.
Seaweed culture can also be used to capture, absorb, and eventually incorporate excessive nutrients into living tissue. "Nutrient bioextraction" is the preferred term for bioremediation involving cultured plants and animals. Nutrient bioextraction (also called bioharvesting) is the practice of farming and harvesting shellfish and seaweed to remove nitrogen and other nutrients from natural water bodies. (See main article Nutrient pollution.)
Seaweed farming can be an actor in biological carbon sequestration.
The practice of seaweed farming has long since spread beyond Japan. In 1997, it was estimated that 40,000 people in the Philippines made their living through seaweed farming. Cultivation is also common in all of southeast Asia, Canada, Great Britain, Spain, and the United States.
In Japan alone, the annual production value of nori amounts to US$2 billion and is one of the world's most valuable crops produced by aquaculture. The high demand for seaweed production provides plentiful opportunities and work for the local community. A study conducted by the Philippines showed that plots of approximately one hectare could have a net income from eucheuma farming that was 5 to 6 times that of the minimum average wage of an agriculture worker. In the same study, they also saw an increase in seaweed exports from 675 metric tons (MT) in 1967 to 13,191 MT in 1980, which doubled to 28,000 MT by 1988.
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