Though aquaculture as practised today is largely based on vertebrate and invertebrate animal species, plants contribute a substantial proportion of world production through aquatic farming. Seaweed agriculture production in 1999 was estimated at 9.5 million tons (wet weight), which was about 22 per cent of that year’s overall aquaculture production (FAO, 2001). The bulk of this production was from China (7284887 tons), followed by the Phillipines (620620 tons), Korean Republic (473175 tons), Korea DPR (413000 tons), Vietnam (18200 tons), Malaysia (11847 tons) and the rest of the countries (119496 tons).
In the major producing countries, namely Japan, China and Korea, seaweeds are grown mainly for human consumption. Seaweeds are also used as fodder and in the manufacture of agar, carrageenan, alginates, mannitol and iodine. China now produces on a commercial scale analogue foods such as ‘shredded jellyfish’ from seaweeds. Many other countries, including the USA, Canada and some Carribbean Islands, are now undertaking experimental and pilotscale culture to produce raw material for industrial uses. Small-scale farming of fresh-water aquatic plants like the water chestnut (Trapa spp.), water cress (Nasturtium spp.) and water spinach (Ipomoea spp.) has been under-taken by Asian farmers for many centuries, but large-scale farming of aquatic plants has only been done in the marine environment. It originated in Japan about three centuries ago with the culture of ‘nori’ or the laver (Porphyra spp.), which continues to be the most important species cultivated for human consumption.
Seaweed culture, particularly of Laminaria, has advanced rapidly in China.
Doubts have been expressed about the value of seaweeds in human nutrition. Their use as a condiment or vegetable is limited to oriental countries and among certain ethnic groups else-where.Though the total per capita consumption is not very high, its protein content is not low (35.6 per cent in dried nori, Bardach et al., 1972). The amino acid composition is reported to be 10–30 per cent of the dry weight, and thecontents of vitamins A, B1, B2, B6, B12, C and niacin are very high. In addition, these edible seaweeds have higher contents of the important minerals calcium and iron than vegetables and fruits (Fujiwara-Arasaki et al., 1984). Irrespective of the direct nutritional value, seaweeds have an important role in the overall food consumption of the people and therefore obviously in their nutrition. An unsatisfied demand for good-quality seaweed products for food, additives for food products and other industrial uses provided the rationale for the increased interest in introducing or expanding seaweed culture in several countries. The main groups of seaweeds cultivated for human food are the following:
Red algae (Rhodophyceae)
Brown algae (Phaeophyceae)
Green algae (Chlorophyceae)
All are typically marine species, but there are differences between species in their salinity and temperature tolerance. Many of them cannot withstand exposure to wide variations in salinity. Many of the edible seaweeds require lower temperatures, between 10 and 20°C, for rapid growth. They are largely intertidal and subtidal species, and the lower limits of vertical distribution are governed by the levels of light intensity. Reproduction can be both sexual and asexual. Some species of red algae exhibit a biphasic (gametophyte, carposporophyte) type of alternation of generation, while others are triphasic (gametophyte, carposporophyte, tetrasporophyte). The discovery of the micro-scopic conchocelis phase in Porphyra spp. has been a landmark in the understanding of the summer phase of the reproductory cycle of these seaweeds. Asexual reproduction by means of asexual (neutral) spores occurs in the rather young stage of the leafy plants and this often accounts for the heavy settlement on collectors in culture operations. Vegetative propagation is also common among most of the cultivated species.
Gracilaria spp., favoured as agarophytes, arebeing widely cultivated in several Asian countries, notably China, the Phillipines and Vietnam (FAO/NACA, 1996). Chile has begun Gracilaria cultivation on a large scale lately andrecent production as high as 120000 tons has been reported (Buschmann et al., 2001). Integrated farming of Gracilaria along with prawns (FAO/NACA, 1996) and with salmonids in cages (Troell et al., 1997) has also been reported. The latter has some ecological advantages because seaweeds make use of the nutrients N and P and organic matter released by the aquatic animals, leading to sustainable farming systems.