Introduction

Seaweed cultivation takes many forms but there is a kind of evolutionary process through which it develops, the rate of which is market-driven. If demand is low and natural resources adequate, cultivation is unnecessary. As demand increases, natural populations frequently become inadequate and attempts are made to increase production by resource management techniques such as improving harvesting techniques, removing competing species, adding artificial habitats and seeding cleared areas. Such techniques are most highly developed in Japan, China and south-east Asia.

Should such management prove to be inadequate, the use of artificial structures to grow seaweeds becomes inevitable. Fragments of adult plants, juvenile plants, sporelings or spores are seeded onto ropes or other substrata and the plants grown to maturity in the sea. To do this, intimate knowledge of both the biology and life history of the plants is critical. For example, kelps cannot be grown from fragments as there is a high level of specialization and fragments of sporophytes do not regenerate. On the other hand, many red algae do not have this degree of specialization and can easily be grown from portions of the adult plant. Knowledge of the life history is critical in many cases and on-land cultivation of particular life history phases is often necessary for seeding. A considerable amount of technology has gone into the development of reliable methods for the cultivation of seed-stocks and their improvement.

The penultimate development in seaweed cultivation is the growing of plants in artificial impoundments on land. This involves the use of either tanks or ponds into which seawater is pumped and the seaweeds are grown detached and at very high densities. This necessitates the careful study of the growth parameters of the seaweeds involved and the development of special strains, preferably with high growth rates, but more importantly, adapted to the artificial conditions. In some instances, the cultivation of seaweeds in association with animals has been attempted, which is often referred to as "polyculture".

The ultimate development of seaweed cultivation will probably be the growth of genetically-improved strains in fermenter-like reactors, in which all environmental factors will be controlled and artificially-compounded seawater-like growth media used. It is likely that such strains will be developed by means of biotechnological techniques.

Low vs high-technology cultivation

The world's most successful seaweed cultivation industries are in Asia, where low labour costs married to simple and intelligent maricultural techniques have proved very successful. The labour intensiveness of seaweed mariculture and the absence of a ready market have been the main reasons why seaweed mariculture has not developed to any great extent in the west. If seaweed-based cultivation is to develop in Europe and north America, we must look at the market potential of seaweeds and the various ways in which seaweed mariculture can be improved so as to reduce the labour content

Large-scale cultivation of seaweeds

Large-scale seaweed mariculture is carried out only in Asia, where there is a very high demand for seaweed products and burgeoning populations to create market growth. Most cultivated seaweeds are grown for the food market, although the Eucheuma co-operatives in the Philippines and Gracilaria cultivation in association with milkfish production in Taiwan are exceptions to this rule, the seaweeds being used for carrageenan and crude agar production, respectively.

Cultivation of seaweeds in Asia is a relatively low-technology business in that the whole, attached plants are placed in the sea and there is a high labour content in the operation. Several attempts have been made in various western countries to introduce high technology to the cultivation of detached plants grown in tanks on land in order to reduce labour content but none of these has attained commercial viability to date.

Nori cultivation in Japan

"Nori" is the Japanese name and "zicai" (purple vegetable) is the Chinese for a flat blade-like red seaweed belonging to the genus Porphyra. The use of this seaweed was introduced into Japan from China; initially, field-gathered plants were used but when the supply became inadequate, cultivation was started in the 17th century; production was initially confined to Tokyo Bay.

Before World War II, production of nori was at rather low levels and it was a much-prized and expensive food. Various new techniques developed after the War including net cultivation, cultivation with an open-water system of floating nets, artificial seeding of conchospores, low temperature storage of nursery nets, and mechanisation of dried nori manufacturing processes, all of which made a rapid increase in output possible.

The ever-increasing demand for this seaweed has made nori cultivation the largest marine aquaculture industry in Japan. Nori is now cultivated mainly in the Inland Sea of Japan in south eastern Japan, with smaller units being found along the coasts of north eastern Honshu and Hokkaido. Annual production of nori in Japan in the period 1925 - 1957 was about 3-5,000 t wet weight per annum; in the period 1958 - 1980 production jumped from 5,000 t to about 35,000 t per year, largely due to the artificial seeding of nets using the Conchocelis -phase. The Japanese nori industry is now a highly mechanised, efficient operation that employs some 60,000 people on a part-time basis and some 67,000 hectares of Japanese waters are occupied by Porphyra nets. The wholesale price of nori in 1979 was 350 yen per kg wet weight. Today, about 350,000 tonnes of wet nori are produced in Japan with a retail value of in excess of US$1 billion.

In China, 7,200 dry tonnes are produced with a value of about US$30 million, making Porphyra the single most valuable plant or animal crop grown by cultivation in the sea. Nori is a high-value crop and although some science and technology has gone into the development of cultivation, all nori plants are still grown in the sea. In Japan, the industry remains largely in the hands of small co-operatives.

Nori, which is usually sold as a rectangular sheet measuring 19 x 21 cm, is the most commonly eaten alga in Japan. The sheets are broiled lightly over a fire, cut into small pieces and seasoned with soy sauce for eating with rice. It is used in a similar fashion in China and Korea. Consumption levels of nori in Europe are currently at a minuscule level, but it may be that in the future a higher demand for this product will emerge. Some areas of the west coast of Scotland, Ireland, France, Spain and Portugal may be suitable for nori cultivation but viability studies are necessary. It is unlikely that the Mediterranean will be a suitable area for nori cultivation. Currently, cultivation is being attempted in British Columbia on a commercial scale; this is the first time that nori cultivation has been attempted outside of Japan, China and Korea.

Recently, Porphyra yezoensis, the premier species in cultivation in Japan, has been reported growing in the wild at Helgoland (Kornmann, 1986), but it has been shown recently that this report is based on a misidentification.

Nori species used

There are about 20 species of Porphyra growing on the coast of Japan but only two of these, P. yezoensis (shown here on the left) and P. tenera, are cultivated by the Japanese, and the former now forms the bulk of the crop.

The thallus of the erect frond of Porphyra species is in the form of a flat, lanceolate or broadly elliptical blade. The fronds are composed entirely of either small rectangular or rounded cells which are arranged in one or, more rarely, two cell layers. The plants generally grow up to 35 cm long in the commercially-used species, rarely to l m; they are dark purplish to brownish red. In the wild, Porphyra species normally grow attached to rocks or as epiphytes in the intertidal or shallow subtidal and are generally highly seasonal in their appearance and growth.

Life history of nori

In 1949, Dr Kathleen Drew-Baker of the University of Manchester made a discovery that was to revolutionist the Japanese Nori industry by allowing reliable cultivation of Nori seedlings. She found that the shell-boring seaweed known as Conchocelis rose was part of the life cycle of a North Atlantic species, Porphyra umbilicalis. Although Conchocelis is widely assumed to be shell-boring, its most common habitat may be on the calcareous plates of barnacles in the high intertidal.

The most commonly used species of Porphyra in Japan for the cultivation of Nori is Porphyra yezoensis. This species normally grows on open coasts in areas affected by cold ocean currents throughout the year whilst the other species that is commonly cultivated in Japan, P. tenera, prefers warmer waters in sheltered, low salinity areas. The transplantation of P. yezoensis to cultivation areas by Nori farmers has largely displaced P. tenera from its former wild habitats. The life histories of both species are roughly the same: the blades germinate from conchospores liberated by the Conchocelis-phase from September to November (short days, relatively low temperatures) and appear as germlings 1 mm in length from mid- to late October, when the water temperature drops to 22?C. The germlings grow rapidly and form blades 15-20 cm or more long by mid- to late November and flourish during the winter at temperatures of 3-8?C. In April, the fronds start to decay and have disappeared by May, when the water temperature rises to 14?C. After fertilization, zygotospores are produced and these are used to grow new Conchocelis-phase plants for the next season.

Cultivation techniques

At the end of the growing season (late February to early April) high quality, sporulating Porphyra plants are selected from the nets. Zygotospores (diploid spores formed by the female gametangial plant; above) are released into a 20-litre container and a suspension is then sprayed onto clean oyster shells using a watering can. About 1 kg of ripe Porphyra is necessary to seed about 20,000 shells.

The zygotospores (left) germinate best in water of 10-15?C to form unbranched filaments that penetrate into the shells. Microscopic examination is carried out and if enough spores have penetrated the shells, they are then suitable for the next phase.

It has been found that the best development of the sporophyte phase takes place when the shells are suspended from ropes (below) rather than lying on the bottom of the tanks (left). Two holes are usually drilled in each shell to make strings of 15 shells, with the tips touching. This is usually carried out at Prefectorial (Japan is divided into local government areas called Prefectures.) Seedling Centres, which are government-sponsored and run. The shells are grown indoors in tanks 2 x 3 x 0.7 m deep in which 16 bamboo sticks are placed lengthways, just above the water level, and about 35 strings of shells are hung looped over the sticks. The rooms in which the tanks are placed have windows in the roof and walls, which are provided with curtains to control the light intensity.

Prefectorial Seedling Centre (above)usually has 24 tanks, giving a total facility for rearing some 200,000 shells. The tanks are not aerated but water temperature and light intensity are carefully controlled (above). A rise in water temperature in early summer is desirable and should not drop below 23?C too soon as this stimulates premature spore formation

Light intensity should be maintained at about 500 lux (about 10 ?mol photons per meter squared per second). Generally, there is no need to change the seawater in the tanks during the summer season, but if water quality deteriorates then it is renewed. The shells and the water usually contain enough nutrients to feed the Conchocelis-phase but small amounts of N and P may be added to stimulate the growth of the alga. The shells are kept about 5 months in indoor tanks and the Conchocelis- phase plants soon become apparent as greyish-purple spots.

At the Seedling Centres, nets of synthetic fibres 2-3 mm in diameter and 18.3 m long x 1.5 m wide with a mesh size of 15 cm square (unstretched) are used for seeding. The most common seeding mechanism used at present is to place the nets on drums 1.5 m in diameter and 2 m across driven by an electrical motor at 2 revolutions per min. These are dipped into concrete tanks about 7 x 6 x 0.5 m deep to a depth of 0.25 m. About 30 nets are wound around the drums at a time and sporulating Conchocelis- phase shells are placed, still attached in strings, on the bottom of the tanks. The rotation of the drums keeps the water sufficiently turbulent to stop the conchospores from settling on the floor of the tank. It takes about 20-60 min to seed the nets in this way. The fibres of the nets are examined microscopically to ensure that a sufficient number of spores have settled. One Seedling Centre may seed up to 20,000 nets in one season. It takes about 10 shells adequately to seed one net.

Net storage

Until relatively recently, the seeded nets were placed in the sea as quickly as possible, but in recent years various methods of storage have been evolved as this allows the farmers to extend the growing season by using two successive sets of nets on each rack. For short-term storage, nets are kept in concrete basins in seawater but in the longer term, cold storage is needed. Provided the nets are folded and covered with polyethylene sheets to keep them moist they can be stored in cold storage for several weeks.

Young nori plants can also be kept at -20?C if they are first air-dried to 20-30% moisture (top). They can be kept for more than 6 months using this method, which has the added advantage that contaminants such as diatoms are eliminated.

Rack design and location

 Racks are built before bringing the nets to the Seedling Centres. Well-sheltered sites are chosen, preferabley with current speeds of about 30 cm per second at full tide. The racks consist of two rows, each of nine long bamboo poles which are driven into the sea bed using a power pump and a rubber hose, which forces water at pressure through a nozzle. The poles are placed so that a stretched net fits precisely between them. Usually, four or five such rows are placed next to each other so that most of the poles serve two nets. For five nets only 54 poles are thus required.

On ground that is unsuitable for rack building, floating rafts are used, but nori plants need to be exposed to the air from time to time, especially during the first three weeks of being pt into the sea. Floating frames, 18 m long, made of plastic tubing are used for this purpose. Up to 20 nets can be attached to a single frame. Having been brought to the site, the frame is slotted into the raft in such a way that the nets are exposed to the air for at least 2 h every day. Rafts may be constructed of a light bamboo framework which allows the nets to be attached directly to the frame. Styrofoam floats are placed under the framework when it is necessary to lift the plants out of the water. Nylon frames, anchored to the seabed at the corners, are also used and supported by floats, but a plastic subframe has to be used to lift the nets out of the water when necessary.

Growth of nori

In the first month the fronds grow slowly but, once they reach about 1 cm long, growth to 10-15 cm can take place in a tidal cycle of 15 days. About 50 days after seeding the nets the fronds are 15-20 cm long. During this period the plants are particularly susceptible to disease and factors such as water temperature and salinity have to be carefully considered and the nets should be lowered and raised to fit in with the tidal cycle.

Nori harvesting

Most harvesting takes place from December to March, as nori is a winter crop. Formerly, the only way to collect the plants was to pick them one by one from the nets, a cold, tedious, and slow job. Today, petrol-driven rotary cutters are used which resemble an inverted lawnmower in construction. One or two men pull the nets over the cutter while another manages the boat. The harvesters tie and untie the nets as they pass. The process can be adjusted so as to select only the larger fronds and within a fortnight the remaining fronds and frond stumps will have grown sufficiently to allow a further crop to be cut. As a rule, nets are harvested 3-4 times but the crop gets progressively smaller each time. At the end of the harvesting season, the nets are carefully cleaned with freshwater, dried and stored for the next season. Some harvesters use another type of mowing machine with a vacuum pump incorporated to collect the cut plants.

Immediately after harvesting, the nori is thoroughly washed in sea water; this is generally done on board the cutting boats, which may be equipped with a cylindrical washer. The washer consists of a cylindrical fine-mesh cage with an internal agitator. The nori plants are fed into a hopper at the top, and a powered pump with a hose feeds the washer with ample seawater.

Nori processing

Nori is not sold in the fresh state but is immediately dried into sheets. This was formerly carried out by a laborious sun-drying process, but the job is now highly mechanised. The harvesters only take what they can process in one day. The nori is first washed with freshwater then fed into a shredding machine which reduces it to pieces 0.5 x 1 cm in size. The cut nori is then thoroughly mixed with freshwater, 4 kg of nori per 100 litres. This nori/water mixture is then fed into a machine which rather resembles a paper-making machine; it is metered automatically on to wooden frames about 30 cm square on the outside, into which fits mats of split bamboo 20 x 18 cm in size, and placed over a wire netting screen. About 600 ml of the mixture is fed into each frame and the water drains away through the mats and the screen. The frames then move slowly along a production line and finally over a heated surface. The nori and the bamboo mats are then removed and the frames return in a circle for more nori-water mixture. The nori sheets are then piled up and put into an oven to reduce the moisture content to about 18%. In order to obtain a good-quality product, the temperature of the drying process should not exceed 50?C. The bamboo mats are then removed, the nori is put together in 10s and packed in bundles of 100s. They are then shipped to a co-operative shipping point. Here they are carefully packed and sealed in cellophane so that moisture uptake will not reduce their quality, and shipped to all parts of Japan. The total Japanese output is about 7 billion sheets. Korean production is 60-100 million sheets.

Farming risks

The selection of a site for nori farming is of crucial importance. Where there is not enough shelter, the Pacific surf may cause a lot of damage, especially when the fronds are fully-grown. Some rain is an advantage, since it brings nutrients via run-off but low salinities can be deleterious as this reduces the plants' resistance to disease.

Generally, there would not seem to be any serious problem from herbivores but fouling species such as the green algae Enteromorpha and Ulva and certain diatoms can be troublesome. These can be controlled to an extent by careful manipulation of the levels of the nets but care has to be taken to avoid drying out the nori completely.

About 10 different diseases attack the nori plants including bacteria, viruses and fungi. The usual remedy for these diseases is to select a level for the nets that optimises the resistance of the plants but is not favourable to the survival of the pathogen. Since growth is faster at depth, the farmers are inclined to lower the nets, but they are aware that disease will strike more readily if this is done. Cross-breeding of strains to find resistant types may prove successful in the future. If disease is observed in the crop, great care has to be taken in washing and drying of the nets or a reservoir of infection could be built up.

Nori economics

There is considerable government support for the nori industry in Japan. Extensive basic research was funded in the 1960s and 1970s and a network of scientific advisors was set up. The Seedling Centres charge a fee for the seeding of nets on a per-net basis but it appears that this does not cover the entire running costs of the Centres. The local co-operative collects 3% of the annual yield from each nori grower as payment for the use of the farming plots. Equipment such as boats, cutters, washers, sheet-making machine, and dryers all have a limited life of 5-10 years. Nets, ropes, frames, and rafts last 2- 3 years. All this has to be costed in the operation and it must be borne in mind that nori farming is strictly seasonal and most farmers rely on fishing as their main source of income (about 60%).

About 35,000 people are employed in the nori industry in Japan, a figures that respresents a decline from a peak of about 65,000. Most of these are organised in small family co-operatives and nori is an important part of the rural infrastructure in Japan.

Cultivation of nori in China and Korea

Some cultivation of nori is carried out in South Korea and in the People's Republic of China, using more or less the same methods practised in Japan.

The use of nori would appear to have originated in China where the earliest records of its food value appeared in books published in the years 533-544 AD. In the Sung Dynasty (960-1279 AD), Porphyra, called "zicai" in China, was the symbolic item chosen to present to the emperor each year from the Haitan Island of the Fujian Province. Porphyra haitanensis and P. yezoensis are the two species cultivated in China. In 1970-80, China produced some 7,200 t of Porphyra by cultivation from about 3,400 hectares of farms and is cultivated in more or less the same manner as nori is cultivated in Japan. The southern species, P. haitanensis, accounts for about 90% of this production.

In Korea, about 100 million dry sheets were produced in 1994 but normal production is generally 60-80 million sheets. Racks are used in the south and rafts in the northern part of South Korea.

SOURCE : Nationnal University of Ireland, Galway

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