WU CHAO YUAN
Institute of Oceanology, Academia Sinica
7 Nanhai Road, Qingdao, China
In 1980 the total world production of seaweeds was estimated at about 65,000 tons (ADB/FAO Market Studies, 1983). More recent figures indicate that the total production of cultured seaweeds in the Asia-Pacific region represents more than 90% of the world's (Csavas, 1985). In this region, four countries, China, Japan, Republic of Korea and the Philippines, are principally engaged in the cultivation of six economic species. It was also estimated that the world seaweed production contributed 20% to the total volume of the global aquaculture production (Csavas, 1985).
Farming of sea plants, specifically algae, has a relatively short history, perhaps three hundred years. Scientific farming of algae started only some 30 years ago. Porphyra was one of the algae which has long attracted man's interest. The Orientals appreciate the Porphyra so much that natural production was unable to meet demand. Undoubtedly the Orientals many years ago must have tried hard to produce the much appreciated Porphyra. However, it was not until the early sixties that Porphyra cultivation began to be made on a truly scientific basis. Another alga greatly appreciated in the Orient is the Laminaria, especially L. japonica. This seaweed has been appreciated by the Chinese people for at least fifteen hundred years, but it was not until the early fifties that the Laminaria was successfully produced by artificial cultivation on floating rafts. These two cases show that, even with present-day scientific knowledge, it is still not easy matter to domesticate a wild alga into a real crop plant that can grow better than their wild relatives. So far only a few algae can be regarded as truly marine crop plants. These would include Eucheuma, Undaria, Gracilaria, Macrocystis and Caulerpa besides the above mentioned Porphyra and Laminaria.
Cultivation of Laminaria
Laminaria Japonica is one of the several species successfully cultivated in commercial scale. Starting from the mid-fifties, the following seven crucial problems concerning the artificial cultivation of Laminaria had been solved:
The floating raft method of artificial cultivation. This method is characterized by three important processes. The first is the spore collection and sporeling cultivation process. The second is the setting-up of the floating raft for cultivation. The superiority of the raft method lies in maintaining the plants at the desired water level. This is one of the keys to successful cultivation. The third is the sporeling transplantation. This is a crucial process ensuring appropriate density for the growth of the plants.
Method of low temperature culture of summer sporelings. Spore collection takes place in early summer instead of autumn. Gametophytes and young sporelings are cultured under controlled low temperature conditions. In late autumn, the sporelings are taken off to the open sea. This method enhances the production by 50%, since the summer sporelings have an advantage of two to three months' growth (Tseng, 1955).
It has been found that Laminaria cannot grow well in places where the total nitrogen content is lower than 5mg/m3, but in fertile places, for example, the total nitrogen content is as high as 20 mg/m3, the plants can grow fast and reach commercial standard within several months. Fertilizing the sea is, however, a difficult activity. In the mid-fifties, the clay bottle method of fertilizer application was devised to take advantage of the porous nature of earthenware. Clay bottles containing fertilizer solution were hung under the raft. The porosity of the bottles effectively controlled the outflow of the fertilizer which became available to the plant in the immediate vicinity. Loss into the open sea was also minimized. In the early sixties, the clay bottle method was replaced by plastic bags punched with minute pores, allowing the nutrient solution to leach out slowly. Later, experiments showed that with large farms hundreds of hectares in area, application of the fertilizer solution by spraying to the open sea can be generally adopted (Tseng, 1955).
Southward transplantation. L. japonica is native to the cold temperate coastal regions along northern Japan and Siberia. Experimental results showed that, although the optimal temperature for Laminaria growth was 5–10 °C, growth was still good enough at 13 °C; even at 20 °C, a frond of 1–2 m was still able to attain some growth. Based on the above experiments, Chinese phycologists conducted an experiment in 1956 on the cultivation of the Laminaria at Gouqi Island, Zhejiang Province. The results of the experiment fully confirmed their postulation that Laminaria of commercial standard can be cultivated along the Eastern China sea coast. Production in the area now accounts for one third of China's entire production (Tseng, 1957).
Genetical studies and breeding of new strains. Genetic studies showed that natural populations of the Laminaria now under cultivation in China are genetically mixed in nature and are considered to possess a high level of hybridity. In the sixties, three varieties were bred, one with broad frond, one with long frond and one with thick frond. In the seventies, two new varieties with high iodine content and high yield were bred, which answered better the demands of the iodine industry of China (IOAS and QIMF, 1976).
Pathogenic diseases. In recent years three kinds of pathogenic diseases in cultured Laminaria have been recognized. One of these is the malformation disease of summer sporelings in green house. The disease is characterized by the death of oogonia and the malformation of the young sporophyte. The cause of this disease is the presence of hydrogen sulfide which is produced by sulfate-reducing and hydrogen sulfide producing saprophytic bacteria (Wu et al., 1976). The other well-documented disease is the frond twist disease of cultivated Laminaria on floating rafts. The diseased plants have abnormally twisted fronds with swollen stipes and shortened holdfasts. The disease is caused by a kind of mycoplasma-like organism. Another kind of disease found in the greenhouse is the rot disease of summer sporelings. This is characterized by the detachment of the sporelings from the sporeling-rope and the rot of the blade and the stipe. It has been found that the alginic acid decomposing bacteria Pseudomonas is the cause of the decay of the holdfast by enzymatic action of arginase, finally resulting in the detachment and loss of the sporelings (Wu et al., 1982).
Frond tip-cutting method. Based on the study of the transportation and accumulation of assimilate devised in the late fifties. By this method the distal part of the frond, sometimes as much as one third of the entire frond, are cut off at certain time to improve the lighting condition of the frond. Such measure improves the conditions for the growth of the frond and enhances the quality of the product. The cutoff distal part of the frond will be cast off anyway under the natural condition; this part is not good for food but is good raw material for the algin industry. By taking such measures, the increase of production generally amounts to about 15% (Wu et al., 1981).
Since the mid-fifties, the above achievements for enhancing the production and improving the quality of Laminaria have been successfully devised one after the other. Now, commercial cultivation of Laminaria is being practiced on the China coast from Dalian of the north to Fujian Province of the south. In the 1978–1979, more than 18,000 hectares of farms were engaged in the commercial cultivation of Laminaria, and about 275,500 tons in dry weight of this alga were produced.
Being the 'home' of Laminaria japonica, Japan had been the principal producer of this much desired seaweed, producing mainly from the nature 120,000 tons in wet weight annually in the late seventies. Natural production is not steady and fluctuates greatly. Artificial production was initiated in the early fifties, two methods attempted being the stone-planting technique and the blasting of reefs (Hasegawa, 1976). Improvement in production was, however, not promising, and production was unable to meet the demands. Since 1968, the so-called forced cultivation method has been employed, and the output has increased from 30 tonnes in 1969 to over 7,500 tonnes in wet weight in 1974, amounting to 16% of the natural production on reefs (Hasegawa, 1976).
Cultivation of Porphyra
Both Japan and China have a long history of the cultivation and utilization of Porphyra. Commercial cultivation of Porphyra in Japan was initiated more than 300 years ago by the primitive method of inserting bundles of bamboo twigs, called hibi, for collecting spores. In China, more than 200 years ago the simple 'rock clearing' method of cultivation was devised by mechanically clearing seaweeds from the rocks in early autumn. This was done just before the mass liberation of the spores. The surface of the rock would then provide the substratum for the spores to attach and grow. The whole process is simple, but people have to depend upon nature's mercy to give them spores. This condition is similar to the Japanese 'bamboo-hibi planting' method. Up to the early fifties the source of the spores had been a mystery to the phycologists. In the early fifties, Drew discovered the conchocelis as a stage in the life history of Porphyra. Later on, both the Japanese and the Chinese phycologists (Tseng & Chang, 1954), who independently discovered the missing link, the conchospore. The shell of Meretrix sp., was found to be an excellent substrate for conchospore. It was not until the '60's that, with the introduction of the artificial collection of conchospores, the commercial cultivation methods became truly modernized. In China, the intertidal semifloating raft method for growing the leafy Porphyra from conchospores is preferred over the fixed pillar method. Conchospore-seeded nets are first allowed to stay in the intertidal zone until the leafy Porphyra appear and then are transferred to deeper areas. In Japan, with the innovation of the cold-storage net and the use of floating nets, Porphyra production increased steadily. P. tenera and P. yezoensis are the two principal species cultivated, although six species are grown commercially. In 1981, the Japanese farmers produced 34,000 t (dry weight) of Porphyra. In China 9,987 t of Porphyra was produced in 1981, while the Republic of Korea has been produced 8,000 t annually these past few years.
Cultivation of Undaria
The other seaweed now under commercial cultivation and qualified to be called a marine crop is Undaria. Today Undaria pinnatifida is the main species under cultivation. Undaria undariodoides and Undaria peterseniana are cultivated to a minor extent. In Japan, the cultivation of Undaria has been developed only in the sixties when the natural resources were not sufficient to cover the ever-increasing demand for this alga. At present, production of Undaria through culture is estimated to be 91,000 t (wet weight) in Japan (1981), 100,000 t (wet weight) in the Republic of Korea these few years, and several thousand tons in China in the eighties each year.
The cultivation of Undaria consists of the following three stages:
Collection of zoospores and growing of sporelings. Collection of zoospores begins at about April to June when the plants become fertile. The matured sporophylls are kept in a dark moist container for several hours to induce the mass discharge of the spores. These spores attach themselves to substrates and develop into male and female gametophytes. The fusion of the gametes results in the formation of zygotes which give rise to young sporophytes. The favourable temperature for the growth of gametophytes and the formation of oogonia and antheridia is at 15–25 °C (Li et al., 1982). In China and northern Japan the seeded ropes were directly cultured in the open sea under a raft where the young sporelings are allowed to grow to about 2–3 cm long.
Outgrowing of the plant. The outgrowing of sporelings starts in the autumn when the water temperature is about 20 °C. The sporeling ropes are cut into 5–6 cm long pieces, which are inserted and tied in the twists of the cultivation ropes. The cultivation ropes with the attached sporelings are set into the sea. The depth of the water where outgrowing is done ranges from 0.5–5 m depending on the transparency of the water. The range of optimum temperature is at 5–10 °C (Zhang, 1984). The plants are harvested when they reach a length of 0.5–1 meter. In areas where the growing season is long, several harvests can be made from the same ropes. Since Undaria has an early short-growing season, maturing much earlier than Laminaria, it is often mixed-planted with Laminaria in China. In that case, Undaria does not interfere with the growth and maturation of Laminaria, which is harvested in June (Tseng, 1981).
Productivity of Laminaria, Porphyra and Undaria
Seaweeds are potentially very productive. Porphyra yezoensis, Laminaria japonica and Undaria pinnatifida are believed to be the potential food products of the tidal zone. The commercial production of these benthic macroscopic marine algae reaches 110 gm-2 for Porphyra, 1,500 gm-2 for Laminaria and 150 gm-2 for Undaria (Wu, 1984). As these seaweeds are cultivated under more or less artificially controlled conditions, it is likely that the production will be raised even higher under certain favourable conditions. The spectacular rates of primary production are found in all these three algae. Under the cultivated condition in North China, the annual primary productivity of P. vezoensis is calculated to be 270 g m-2, that of L. japonica 2,200 g m-2 and U. pinnatifida 160 g m-2 (Wu, 1984). The raft method of cultivation maintains the seaweeds at the desired depths, which generally ensures a light-saturated rate of photosynthesis on clear days to enhance productivity. On the other hand, the low respiratory rates as well as low light compensation points especially at low temperatures are considered as another reason to save heat energy, resulting in a high productivity, especially in case of Laminaria (Wu et al., 1984).
References
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