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.
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
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,
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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%).
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.
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