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Chapter: Aquaculture Principles and Practices: Seaweeds

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Culture of Undaria - Seaweeds

Undaria, known popularly as ‘wakame’, is animportant group of cultivated edible seaweeds. Its culture is important in Japan, Korea and China.

Culture of Undaria

 

Undaria, known popularly as ‘wakame’, is animportant group of cultivated edible seaweeds. Its culture is important in Japan, Korea and China.

 

The main species of Undaria cultivated is U.pinnatifida, which is in great demand in Japanfor the traditional as well as the salted products made from it. Production by culture has outstripped harvests from natural sources. Two other species, U. undarioides and U. peterseniana, are also presently cultivated on a smallerscale in the southern parts of Japan. These are cold-water, open-sea species and can be grown only in areas where winter temperatures remain below 22°C. Optimal temperatures are different for different stages of the life cycle. Salinities between 30 and 33ppt are reported to be optimum. The life history of Undaria includes an alternation of sexual and asexual forms. The macroscopic plant is the asexual form or sporophyte, which grows during the winter months at temperatures between 10 and 15°C. Asexual zoospores are produced by the sporophyte during this season and are released when the water temperature rises above 14°C. The planktonic zoospores settle within a short time on solid substrates like rocks, shells, etc., and germinate at temperatures between 15 and 20°C, to produce microscopic gametophytes or the sexual plants. During summer the gameto-phytes develop and by the end of the season, around September, the sperms are released and fertilize the ova within the oogonuim to form the zygote, which develops into the sporophyte.

 

The systems of culture are essentially the same as for Porphyra. The spores are collected and reared on socalled ‘seeding twines’, made of synthetic yarns of 2–3mm diameter. These twines, which are usually about 100m in length,are wound round a square (50cm x 50cm) plastic frame, leaving intervals of about 1cm. Seeding is performed in large concrete or plastic tanks kept in cool shaded positions. Mature sporophylls (sporophyte plants) are partially dried and then placed in the tanks filled with fresh sea water. The twine frames are arranged in layers inside the tank to collect the zoospores released from the sporophylls. After about two hours they are transferred to large culture tanks with a depth of about 1m, in which the frames are hung vertically. The gametophytes and the young sporophytes develop on the strings throughout summer. The light intensity in the tanks is regulated to facilitate rapid growth and high survival rates. If the water quality deteriorates it is exchanged; fertilizers may be added if the growth is poor. In areas with short periods of seed rearing and a lower likelihood of fouling, tank culture of gametophytes can be avoided, but most seaweed growers find it beneficial to perform nursery rearing in indoor tanks.

Grow-out of thalli starts in autumn, between September and November, when the water temperature falls below 20°C and there is less likelihood of fouling by epiphytic organisms. Twines containing the seed are set out on cultivation ropes in water depths up to 5m, depending on local conditions. The cultivation ropes are made of synthetic fibre of 10–20mm diameter and the seed twines are attached to them at intervals of about 15cm. Sometimes the twines are cut into 5–6cm pieces and inserted into the braided strands of the cultivation rope.

 

In exposed areas the cultivation rope is set out with heavy anchors and a sufficient number of floats. This system of floating ropes has made it possible to culture Undaria in deeper waters (up to 50m depth) exposed to heavy seas. In protected areas, like bays, the ropes can be stretched out on rafts.

Undaria grows rapidly and can be ready forharvest in about three months. Though heavy seas may damage the floating rope system, they aid rapid growth of the plants. Water temperature is a major factor affecting growth. The optimum is between 15 and 17°C, and below 5°C growth will be greatly retarded. The harvesting length of the thalli is greater than 50cm.

 

Harvesting is done according to the growth pattern of the thalli which is itself very much dependent on the length of the growing period.

When there is dense growth and the growing period is long, the well-developed thalli are harvested first. This helps the remaining ones to grow faster and enables repeated harvesting. In areas with poor growth, the thalli are harvested by cutting the upper portion first and allowing the remaining portion to grow. In areas with short growing periods, all the thalli may be harvested at one time, when they have grown to the maximum size.

Undaria is marketed in the dry state. Dryingmay be done in the sun or in a dryer. Salted Undaria is preferred by many consumersbecause it is convenient to use. Some are sold fresh in certain areas and, if required, can be stored under refrigeration.


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