Aquaculture of giant kelp

At the beginning of the 20th century California kelp beds were harvested for their potash.[1][5][6] Commercial interest increased during the 1970s and the 1980s due to the production of alginates, and also for biomass production for animal feed due to the energy crisis.[5][6][7] However commercial production for M. pyrifera never developed. With the end of the energy crisis and the decline in alginate prices, research into farming Macrocystis declined.[2]

The supply of M. pyrifera for alginate production relied heavily on restoration and management of natural beds during the early 1990s.[2][8] Other functions such as substrate stabilization were explored in California, where the “Kelp bed project” transplanted 3-6m adult specimens to increase the stability of the harbor and promote diversity.[8][3][9]

The most common method of cultivating M. pyrifera was developed in China in the 1950s. It is called the long line cultivation system, where the sporelings are produced in a cooled water greenhouse and then planted in the ocean attached to long lines.[15] The depth at which they are grown varies. This species alternates generations in its life cycle, cycling between a large sporophyte and a microscopic gametophyte. The sporophyte is harvested as seaweed. The mature sporophytes form the reproductive organs called sori. They are found on the underside of the leaves and produce the motile zoospores that germinate into the gametophyte.[16][17] To induce sporalation, plants are dried for up to twelve hours and placed in a seeding container filled with seawater of about 9-10 °C; salinity of 30% and a pH of 7.8-7.9.[12][15][18] Photoperiod is controlled during sporolation and growth phases. A synthetic twine of about 2 – 6mm in diameter is placed on the bottom of the same container after sporalation. The released zoospores attach to the twine and begin to germinate into male and female gametophytes.[12][15][18] Upon maturity these gametophytes release sperm and egg cells that fuse in the water column and attach themselves to the same substrate as the gametophytes (the twine).[12][15][18] These plants are reared into young sporophytes for up to 60 days.[15][18]

These strings are either wrapped around or are cut up into small pieces and attached to a larger diameter cultivation rope. The cultivation ropes vary, but extend approximately 60m with floating buoys attached.[12] The depths vary. In China, M. pyrifera is cultivated on the surface with floating buoys attached every 2-3m and the ends of the rope attached to a wooden peg anchored to the substrate. Individual ropes are usually hung at 50 cm intervals.[15] In Chile M. pyrifera is grown at a depth of 2m using buoys to keep the plants at a constant depth.[18] These are then let alone to grow until harvest.

Problems that afflict this method include management of the transition from spore to gametophyte and embryonic sporophyte which are done on a terrestrial facility with careful control of water flow, temperature, nutrients and light.[15] The Japanese use a forced cultivation method where 2 years of growth is achieved within a single growing season by controlling inputs.[15]

In China a project for offshore/deep water cultivation used various farm structures to facilitate growth, including pumping nutrients from deep water into the beds. The greatest benefit for this approach was that the algae were released from size constraints of shallow waters. Issues with operational and farm designs plagued deep water cultivation and ended further exploration.[15]

In the UK, legislation defines giant kelp as a nuisance. invasive specimens are mechanically removed.[20]

The demand for M. pyrifera centers on fertilizers, bioremediation and feed for abalone and sea urchins.[2][8]

• Edible seaweed
• Kelp forest
• Seaweed farming
• Ocean fertilization
• Blue carbon