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Culture of seaweeds for industrial use
Descriptions of seaweed culture practices are mainly focused on edible sea-weeds. However, a considerable amount of research and development is underway on establishing viable practices of intensive culture of seaweeds as sources of agar, alginates and carrageenans. Mathieson (1986) has reviewed the work done on a number of species and in this section experience in commercial farming of some of the more important species is summarized.
Methods of enhancing the production of Gelidium amansii and related species (used inagar production) have been practised in Japan for many years. Cuttings of the plants are sown in protected bays, where they generate new fronds. Rope cultivation techniques, very similar to those used for Undaria, are presently employed and the growth is enhanced with fertilizer pellets.
Species of Gracilaria, especially G. confervoides and G. gigas, are cultured in Taiwan inold milkfish ponds. The species can adapt towide variations in environmental conditions; for example, it can withstand salinities ranging from 8 to 25ppt and temperature up to 25°C. Cuttings are planted uniformly on the pond bottom at the rate of about 3000–5000 cuttings per ha of pond, supported by bamboo sticks dug into the bottom. The depth of water in the pond is increased to about 60–80cm, with the rise in temperature in June. Exchange of tidal water is maintained to provide additional nutrients and to adjust salinity levels. Inorganic or organic fertilizers are applied at the rate of about 3kg/ha of urea every week, or 120–180kg/ ha of fermented pig manure every two or three days at times of water exchange. If the water temperature falls below 8°C, either the depth of the pond water is increased or the plants are transferred to protected wintering ponds. Major pests and competitors in the ponds are overgrowths of algae, including Enteromorpha and Chaetomorpha. The common control measures are to lower the water level and reduce the water exchange; plant additional large Gracilaria to utilize nutrients in the pond water; and stock adult milkfish or tilapia (about 150g weight) at the rate of 500–1000 per ha to feed on the green algae.
Harvesting of Gracilaria from the ponds is done by hand or with scoop nets, from June to November. The annual yield is around 10 tons/ ha. The main costs of production are labour (about 53 per cent) and seed (18 per cent) and the rate of return on initial investment is 50 per cent according to Shang (1976). When polyculture with crabs is practised, the production ofGracilaria is reduced to about 9 tons/ha and theoperating costs increased more than four times, but an additional production of 6.3 tons/ha of crabs and shrimps is obtained. The harvested plants are sundried on bamboo screens or plastic sheets for export.
The culture of the red algae (Eucheuma spp.) has become a commercial operation in the Philippines for production of kappa carrageenan. The most successful species so far isE. cottonii, but the related species E. muricatum (= spinosum) has also been tried. For the culture of E. cottonii, protected coral reef areas, with a good water flow and temperatures between 26 and 32°C are selected. Nets and long lines are used for suspended culture. A major factor in the success of commercial culture of E. cottonii in the Philippines is the use of a clone of the species known as ‘tambalang’, produced by natural selection. It grows and multiplies rapidly and can survive in a wide variety of environmental conditions. Vegetative parts of the plant are attached to polyethylene nets and strung parallel to the bottom between poles. The seed is attached with thin plastic strips which allow movement and do not cut the fronds. When the fronds reach a weight of 1200–1500g, harvesting is done by pruning about one-third. Further harvesting is done when the fronds grow to the desired size, and this cycle is repeated. Long lines are preferred to nets for grow-out as they are easy to maintain, although the productivity per unit area may be lower.
Experimental culture of several other species of seaweeds is being carried out by many institutions and private enterprises. Chondruscrispus (Irish moss) has been cultivated ingreenhouse tanks flushed with sea water. Eucheuma isoforme has been cultured in tankswith slanted bottoms and circulating water to keep the plants in suspension. Studies on the growth of Hypnea musciformis in an artificial upwelling system in the US Virgin Islands have been referred to earlier. Mass culture techniques using sporophytes as seed have been developed for enhancing natural stocks of the kelp Macrocystis (North, 1972).
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