Aquaculture of giant kelp
Giant kelp, Macrocystis pyrifera, has been utilized for many years as a food source; it contains many compounds such as iodine, potassium, other minerals vitamins and carbohydrates and thus has also been used as a dietary supplement. In the beginning of the 20th century California kelp beds were harvested as a source for potash. With commercial interest increasing significantly during the 1970s and the 1980s this was primarily due to the production of alginates, and also for biomass production for animal feed due to the energy crisis during that period. However commercial production for M.pyrifera never became a reality. With the end of the energy crisis and the decline in prices of alginates, the research into farming Macrocystis also declined.
The demand for M.pyrifera is increasing due to the newfound uses of these plants such as fertilizers, cultivation for bioremediation purposes, abalone and sea urchin feed. There is current research going into utilizing M.pyrifera as feed for other aquaculture species such as shrimps. The supply of M.pyrifera for alginate production relied heavily on restoration and management of natural beds during the early 1990s. Other functions such as substrate stabilizing ability of this species was also recognized in California, called the “Kelp bed project” where the adult plants of 3-6m in length were transplanted to increase the stability of the harbor and promote diversity; this was done in efforts to restore the natural environment after extensive harvesting.
With the global demand for aquatic plants increasing there have been great advances with the technology and methods of cultivating them. China and Chile have taken it on a broader scale; these two countries are currently the largest producers of aquatic plants each producing over 300,000 tones each in 2007. Data on how much of this total can be attributed to actual M.pyrifera harvesting is sketchy because as opposed to animals where the details on individual species harvested is kept on record, aquatic plants however are usually lumped in to a single category. Both these countries culture a variety of species, in Chile 50% of the production involves several species of Phaeophytes and the other 50% results from harvesting Rhodophytes. China produces a larger variety of seaweeds which also includes chlorophytes. There are also experiments undergoing in Chile to produce hybrids between this species and M.integrifolia in efforts to produce a super cultivar.
The most common method of cultivating M.pyrifera was developed in China in the 1950s called the long line cultivation system. Where the sporelings are produced in a cooled water greenhouse and then later planted out in the ocean attached to long lines. The depth to which they are grown is varied in different countries. Since this species has an alternation of generations in its life cycle, which involves a large sporophyte and a microscopic gametophyte. The sporophyte is what is harvested as seaweed. The mature sporophyte form the reproductive organs called sori, these are found on the underside of the leaves and produce the motile zoospores that germinate into the gametophyte. To induce sporalation, the selected plants are dried from a couple to up to twelve hours and placed in a seeding container filled with cool seawater of about 9-10°C; salinity of 30% and a pH of 7.8-7.9. Photoperiod is also controlled for during the sporolation and the growth phases. A synthetic twine of about 2 – 6mm in diameter is placed on the bottom of the same container after sporalation and the released zoospores attach themselves to the twine and begin to geminate into male and female gametophytes. 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 (i.e. the synthetic twine). These plants are then reared up into young sporophyte plants for up to 60days.
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 are approximately 60m with floating buoys attached. The depths at which they are grown in the water column vary for some of the countries. 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 to each other. In Chile however M.pyrifera is grown at a depth of 2m using buoys to keep the plants at a constant depths. These are then left alone to grow until ready to harvest. There are several problems with this method of cultivation as there are difficulties that lay in the management form the transition in the juvenile stages; from spore the gametophyte and embryonic sporophyte which are all done on a land based facility with careful control of water flow, temperature, nutrients, and light. The Japanese use a force cultivation method where a 2 year growth rate is achieved within a single growing season by controlling for the above requirements.
In China a project for offshore or deep water cultivation was also looked at where various farm structures were designed to facilitate the growth of M.pyrifera; nutrients from the deep waters were pumped up into the growing kelps. The greatest benefit for this approach was that the algae were released from size constrains that are found in shallow waters. Issues with operational and farm designs plagued the project for deep water cultivation, and prevented further cultivation in this manner.
The duration of the cultivation is varied upon the region and intensity of the farming, this species is usually harvested after two growth seasons (2 years). For M.pyrifera that is artificially cultivated on ropes, they are harvested by a pulley system that is attached on boats that pull the individual lines on the vessels for cleaning. Other countries such as the United States of America (USA) which rely primarily on naturally grown M.pyrifera use boats to harvest the surface canopy, the surface canopy is harvested several times per year this is possible due to the fast growth of this species and the vegetative and reproductive parts are left undamaged.
In the UK, legislation defines giant kelp as a plant which should not be allowed to grow in the wild and these kelp are mechanically removed.
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