CURRENT SITUATION OF SEAWEED AQUACULTURE DEVELOPMENT IN ROK

Jung Jin Tae *

INTRODUCTION

I. Geographic characteristic of Korean Coast

The surrounding waters of the Korean peninsula have special environmental characteristics.

The water in the Eastern coast suddenly deepens, the surf is strong, there are many submerged rocks/reefs, and the coastline is monotonuos without special features. The water current is more influenced by the Liman cold current rather than Tsushima warm current. Seaweed culture in this area is possible with the long-line rope method. Species that can be grown are sea mustard and sea tangle.

The southern coast is characteristized by a highly curved coastline. There are many islands including many bays of all dimensions. The water current is influenced strongly by Tsushima warm current. Due to the suitable tidal range as well as to the moderate water depth this coast is most adequate for installing various cultural structures such as hanging, long-line rope, and bamboo raft type. This coastal area has become the centre of aquaculture in Korea, and almost all of the major seaweeds cultured such as sea mustard and layer, are raised in this area.

The western coast is characteristized by many islets and a rugged coastline, though less pronounce than the southern coast. This coast is also characterized by the large and worldwide known tidal range. As a result laver culture has been practised for a long time and floating net culture system of laver is now well established.

II. The Sea Current and Water Masses

There are interactions between the sea currents of the Korean adjacent seas and those from the Pacific ocean. The currents of Korean waters are mainly influenced by Tsushima warm current, which is a branch of the Kuroshio current which originates from eastern Philippines and flows upwards to the Western Pacific, through the eastern China Sea. Therefore, the characteristics of currents and water masses in relation to the three sides of the Korean peninsula are as follows: In the southern coast, the Kuroshio current forms the West Sea (Yellow Sea) warm current flowing into the Western sea, through the southern part of Jeju island. In the eastern coast, the Tsushima current forms the Korean East Sea warm current, and this is faced with the Liman cold current, the North Korea cold current flowing southwards with temperature lowered by cold weather and thawed ice during winter. In the western coast, the Yellow sea warm current, flowing in through western Jeju island as a branch of the Tsushima current, typically flows upwards to the north western coast of China during April to August. The strength of this lowered by cold weather and thawed ice during winter. In the western coast, the Yellow sea warm current, flowing in through western Jeju island as a branch of the Tsushima current, typically flows upwards to the north western coast of China during April to August. The strength of this current is relatively weak compared to the China coastal current flowing downward. Consequently, the two currents form a reflux in the central part of the Yellow Sea, and as a result the Yellow Sea bottom cold current is generated. (Fig. 1).

* Scientist, National Fisheries Research & Development Agency, Republic of Korea

III. The Distribution of Water Temperature in Surrounding Waters of Korea

The seasonal variation of sea surface temperature in Korean waters ranges from 5 to 27 °C. The sea surface temperature (SST) in Korean waters depends not only on incoming radiation but also on heat advections by wind and ocean current as mentioned above. The heat advection by the Asian monsoon lowers the mean SST and increases the annual range of SST, while the heat advection by warm currents such as the Tsushima and Yellow Sea currents increases the mean SST and decreases the annual range of SST. The higher mean SST and smaller range of SST in the East sea (the Sea of Japan) compared to the West sea (the Yellow Sea) is due to the fact that the heat advection into the East sea by the Tsushima current is more pronounced than that into the West sea by the Yellow sea warm current. The maximum SST in the neighboring seas of Korea occurs between mid-August and early September (Fig. 2 and Fig. 3).

IV. The Distribution of Seaweed

The algae species distribution in the Korean coastal area are typical species from the temperate zone (this flora occupies over 70% of the total amount of algae). With regard to the different coasts the flora distribution is as follows:

East Coast

Although the predominant seaweeds are cold water species resulting from the influence of cold current, the central part of the eastern coast is also inhabited with temperate water species.

West coast

Temperate water species dominate due to the weak north cold current The flora of seaweed in this cost is relatively poor.

South coast

Temperate water species occupy about 70% of the total seaweed flora. The south coast is characterized by numerous large and small islands as well as a highly indented coastline. Many subtropical temperate algae species are found in the coast of Jeju island.

Figure 1. Sea currents of the Korean coastal waters.

Figure 2. Oceanographic chart of the Korean coast line (SST: February and August).

Figure 3. Monthly SST variation in relation to major culture areas.

The major algae found in Korean coastal waters are:

1. The annual plants

   Laver, sea mustard, green laver, and others.

2. The perennial plants

   Hijiki, Irish moss (rock moss), seaweed cava, tangle (kelp), Agar-agar, Tenella, Zostera marina and others.

V. Major Cultured Seaweeds

The major algae being cultured now in Korea are the following:

Laver (Porphyra spp.)

This is a major cultured algae and has the longest history of culture. The culture area stretches from the south coast to the east coast of the country. The centre of culture area is located around Wando, Shinan, Hadong etc. in the southern coast and Seochon, Anmyundo etc. in the western coast.

Sea mustard (Undaria pinatifida)

This is one of the artificially propagated species and it is distributed all along the coastal zone. There is abundant, good quality Undaria in the eastern coast growing naturally. The technique of artificial seeding was introduced in Korea in 1962 and a production of to 300,000 MT was reached in 1986. The main culture area is Wando in the southern coast and Ilgwang, Kijang in Pusan.

Sea tangle (Laminaria)

This is cultured abundantly in the northeast coast up to latitude 38°. It is also cultured in the southeast coast on a smaller scale.

Hijiki (Hijikia)

Although this is found almost all over the Korean coast the culture of this species is practiced only in Jeju island including Wando in the southern coast.

Agar-agar (Gelidium spp.)

Although this species is found all over the Korean coast, the production of this algae is from natural stocks. Recently the high price of raw material for agar-agar products has prompted culture trials of Gelidium amansii in the southern coast.

VI. The Status of Shallow Sea Mariculture in Korea

Korea has shown rapid progress in marine culture fisheries from the late 1960's. Although the total amount of mariculture production reached 19,000 MT, this was only 4.1% of the total Korean fisheries production of 947,000 MT in 1982. This represents a 50% increase over 1962 and occupied 26% of total Korean fisheries production (Table 1 and Fig. 4, 5 & 6). On the other hand, the exploited culture area, which reached about 100,000 ha in 1986, occupied about 55% of total possible mariculture area estimated at about 183,000 ha (Table 2). Mariculture has made such rapid progress due to the influence of decreased fisheries resources in off-shore and curtailment of deep-sea fishing because of the 200-mile economic zone of coastal states, along with increased demand of sea food. Consequently extension service for mariculture, particularly in supplying artificial seed produced by the government as well as the activation of the licencing system in private cultivation areas, have contributed to the rapid progress of mariculture in Korea.

VII. The Status of Seaweed Culture in Korea

The major sea weeds cultivated in Korea are laver, sea mustard, kelp, including Hijiku, and green laver (Table 3). The culture of laver has been steadily increasing, while sea mustard under stagnation. Laver and sea mustard highest production was recorded in 1986. The major problems related to seaweed culture are how to maintain production stable and strengthening the regulations concerning licencing of private culture areas. Figure 7 shows the major seaweed culture grounds in Korea.

CULTURE OF PORPHYRA SPP. IN KOREA

I. Development of Cultural Technique

The original culture method of laver reportedly began in 1623 in Kwangyang Bay (adjacent in the estuary of Sumjin river) in the southern coast of Korea. This was 50 years earlier than Japan. The original culture method, now obsolete and nearly extinct in Korea, involved placing bundles of leafless branches of bamboo, or other trees at or just above the mean water level located well away from brackish water during September to October. This was improved to a fixed semi-floating type made of split bamboo and fixed with poles on either or both sides. It was only in 1962 that artificial production of monospore and culture of conchocellis filament was practiced in Korea (Fig. 8). Owing to this technical development, the technique of nursery nets storage and floating net culture system was introduced from Japan, and free-living conchocellis culture method was dispersed during 1970s. As a result, laver production attained over 23 × 10⁸ sheets of dried products from 1979. Since 1980 the annual production has been kept over 30 × 10⁸ sheets, and over 62 × 10⁸ sheets of dried laver was produced in 1986. Production in 1987 decreased to 34 × 10⁸ sheets resulting from bad condition in marine environment (Fig. 8). The harvest of cultured laver is greatly dependent on the marine environment condition.

Table 1. Annual yield of aquaculture products in Korea from 1962 to 1986. (Total culture = total marine culture products).

Table 2. Seaweed culture area in Korea from 1962 to 1986.

Table 3. Annual yields of major aseaweed species in Korea from 1962 to 1986.

Figure 4. Total yield of fisheries products in 1986.

Figure 5. Total yield of cultured seaweed in 1986.

Figure 6. Total yield of mariculture products in 1986.

Figure 7. Major seaweed culture grounds in Korea.

Figure 8. Annual yield of laver from 1962 to 1987 (dried algae).

Figure 9. Lyfe cycle of laver.

II. Characteristics of Laver

There are about 10 species cultured in Korea.

Among them P. tenera Kjellman and P. yezoensis are the most important species.

1. Phylum : Rhodophyta
   Class    : Bangiophyceae
   Order    : Bangiales
   Family   : Bangiaceae
   

2. Size:

   Length of fully grown frond of P. yezoensis varies from 15 to 36 cm, and that of P. tenera from 17 to 35 cm, sometimes up to 1 m

3. Color:

   Fronds are usually dark purple

4. Life cycle:

   Life cycle of both species are the same (Fig. 9 and Fig. 10). The algae germinate from spores released from conchocelis filaments (sporophyte) from September to November and appear as small germlings 1 mm in length from mid- to late October, when the water temperature drops to 22 °C. The germlings grow rapidly to fronds (gametophyte) 15 to 20 cm long or more by mid- to late November and flourish during winter in waters of 3 to 8 °C. In April, fronds of the algae wither, and disappear by May when the water temperature rises to 14 °C.

   Germlings of 150 micron to 1 mm in length form a large number of neutral spores during the period between late September and early November. These neutral spores also germinate and grow into fronds. When germlings grow into fronds of 3 to 5 cm in length in November, sexual reproductive organs are formed on the fronds. These organs continue to release fertilized carpospores until the fronds disappear in May. Released carpospores attach to shells on the sea bed to germinate and become Conchocelis filaments. Conchocelis filaments bore into the shell and grow in the pearl layer of the shell to form colonies of 1 cm in diameter by August to September when they start to release spores called conchospores.

   Generally speaking, the life cycle and ecology of Laver differ from almost all of the cultivated species, in that it carries out accessory reproduction by asexual spores given off by the young thalli. The growing of laver into fronds is the result of repeated and complicated process of life cycle such as: (1) carphospore → (2) conchocelis rosa → (3) conchospore (monospore) → (4) germlings (young thalli) (5) neutralispore (monospore) → (6) fronds of thalli. The process between (4) and (5) differ from other cultured species, that is, germlings germinated from carposphore form neutral spore; in turn, these neutral spores germinate into germlings. After repeating this process several times, the germlings grows into fronds of thalli.

5. The process of Laver culture

Figure 10. Process of laver culture.

III. Culture technique

Culture of conchocelis filaments

This is carried out by placing oyster shells, either loose or strung on wires, in indoor tanks in the early spring and adding chopped thalli. The conchocelis plants are then cultured in the tank until the artificial seeding of conchospores begins in October.

Culture by oyster shell

This starts with the collection of thalli during December to mid-January. The thalli are then cold-stored until March to April. Prior to the culture of conchocelis the thalli are dried in a dark place until the moisture contents reaches a level of about 20–30%. The conchocelis culture begins with inputting the carpospore solution (*) are either placed into a concrete tank perpendicularly (stringed oyster shells), or by placing the shells loosely into a plastic container filled with sea water (Fig. 11 and Fig. 12).

* Preparation of carpospore solution: The cold stored thalli are placed into a container filled with sea water (10 g dried thalli per litre of sea water). The thalli will release carpospores over 2–6 hours, which causes the water color to change. The solution prepared is placed into the culture tank filled with sea water, and the carpospore will bore into the shells in one week and develop into conchocelis filaments.

Culture by free living conchocelis

This method was introduced in Korea in 1975. Over 30% of the culturists have now adopt this method. The method grows the matured free living conchocelis filaments until monospore collection can be carried out.

Recently it was found out that the free living conchocelis release spore by forming carposporangium without the need of a substrate, and monospore collection by free living conchocelis without substrate has become possible.

Collection of conchospore

Conchospores are monospore released from the conchocelis filaments or neutral spore germinated from germlings (young plant). The monospores can be (1) collected with nets in the sea during the right season, or (2) artificially collected in the sea or in the tank, with oyster shells bearing matured conchocelis (Fig. 13).

Spore collection in the sea

Culture nets are set up on the seashore in fall to collect released conchospore, when water temperature lowers to about 22 °C. A variety of collecting devices are used. The collection of monospores is best when the water temperature is 22 to 23 °C. In the western coast of Korea collection is practiced everyday in the morning.

Artificial seeding of conchospores in the sea

This is carried out by placing the conchocelis-bearing oyster shell under the spore collecting nets which are spread horizontally in the sea with supporting poles.

Figure 11. Artificial seeding in container.

Figure 12. Stringed oyster shells used for the collection of the carpospores.

Artificial seeding of conchospores in concrete tanks

Mature conchocelis-bearing oyster shells are placed on the bottom of concrete tanks, and the conchospores released from the conchocelis filaments attached to the nets dipped into the tank. The release and attachment of spores is induced by stirring the water in the tank. Nets are the same quality as those used in the sea. They are placed one over the other up to 30 to 50 layers in thickness. There are several methods of seeding the spores onto the nets. The seeding devices are classified as follows: (1) rotary type (2) vertical movement type (balance type) (3) running water type (belt conveyor type) and (4) bubbling type (Fig. 14).

Correlation between timely artificial seeding and growing of laver

At the start of September all culturist carefully time spore collection along with the placing of nursery nets in the sea. Generally, spore collection decreases as the sea water temperature drops. However, if falling of sea water temperature is late in occurring and artificial seeding is late along with placing of nursery nets, the annual harvest is usually rich. The amount of harvest is related to the seeding and placing the culture nets in the culture grounds at the correct time. Harvesting of adult laver usually begins in November, and it takes about 40 to 60 days to reach harvest size from the time of seeding. If the growth rate of thalli is fast, large quantities of nutritive salts in sea water are demanded, followed by a high discharge of waste materials. Laver becomes actively photosynthetic at water temperature of 15 to 16 °C, therefore the demand of nutrient salts becomes highest at this time. The convection of current is usually weak at this time. As a result, the thalli suffer from the imbalance in the demand and supply of the nutrient salts which causes the outbreak of diseases and usually terminates the life cycle of the plant.

Germination of conchospores and growth of germlings

The laver nets seeded with conchospores are placed in the sea to induce spore germination. To enhance the water flow over the nets, only up to 5 nets are stacked over each other for germination. The nets are set at a depth which allows them to be exposed to air for about 4 hours a day, in order to get rid of the undesirable algae. About 15 to 20 days after seeding, laver germlings become visible on the nets. The germlings grow to 2–3 cm, 40–50 days after seeding and the nets become ready for use as a nursery nets. The density of germlings is several hundreds per cm of twine. The nets are either placed in the sea for cultivation, or stored in freezer for use in later months.

Figure 13. Oyster shells bearing mature conchocelis fitted on nets.

Figure 14. Porphyra indoor tank seed collector (rotary type).

Growth of laver plants

The growing season of laver fronds last from November to early April. Culture grounds should be located in the sea with as much nutrient salts and sea water movement as possible. Waves and wind are desirable to secure rapid water exchange. The nursery nets are spread horizontally in the top layer of the sea to enhance growth of the laver fronds.

IV. Laver culture structures

There are many types of structures for growing laver, ranging from bamboo bundles to the more recently developed floating net systems fixed with anchors. The most popular structure in use is the movable type with pole, and the floating net culture system (Fig. 15).

Laver culture structures used in Korea

1) According to materials:

2) According to structure types:

a) Fixed type:

Bamboo bundles, oak trees are no longer used except in the estuary of Sumjin river/Kwangyang Bay

Figure 15. Laver culture structures. 1) pine/bamboo bunble method; 2) bamboo movable type fixed with poles; 3) net movable type fixed with poles; and 4) floating net culture system.

b) Movable type fixed with pole:

• semi-floating type, now used only by few culturists.

• floating type:

  1. elevator type fixed with pole; used in western coast in which there is high tidal range.

  2. fixed with pole on either side; used in both western and southern coasts.

  3. fixed with pole on both sides; used along the coast of Shinan area.

c) Floating net culture system fixed with anchors.

V. Other technical problems

Developing floating net culture system for an effective use of the culture grounds

The laver is cultured in shallow waters. The nets should be exposed to air two times a day. This is necessary to check for diseases. The floating net culture system was developed in Japan and was introduced in Korea in 1972. The extensive use of the floating net culture system begun in 1981. It occupied 1.1% of total amount of culture structure in 1982, and 13.7% in 1987. The floating net culture system differs from other structures in water depth. It is designed to be placed under 10 metres of water. Other structures such as the floating but fixed with pole type is designed under 7 meter in water depth. With the spread of floating net culture, we are now faced with the problem not only of shallow culture ground under 10 meter becoming limited, but also with the difficulty of observing the intervals of placing the structure over 500 meters. It will bring up the problem of high intensive culture. Finally, in order to develop the cultivation ground more effectively, it is necessary to correct the Fisheries regulation concerning the depth. The depth to place the floating net cultural system is designated now “under 10 meters”. It should be changed to “under 20 meters”.

Utilizing cold stored nursery nets and extending the culture period

When sea water temperature rises at night in the culture ground, the algae suffer from physiological injury. The thalli fall out from the nets. Rising sea water temperature occurs during early to mid-November making it necessary for the nursery nets to be stored in a freezer. Generally, the nursery net germinated in autumn can be stored in a freezer for 1 to 2 months for cultivation in later months. Attached germling of 1 to 3 cm in length are dehydrated with air drying in a dark place or in a centrifuge to reduce the moisture contents to 30–40% and stored in -20 °C freezer. Usually stored until early December, this is called cold-stored nursery nets. On December the oceanographic conditions become favourable for the nursery nets to be placed in cultivation area. The germlings of 1 to 3 cm in length begin to growth again. Harvest of mature laver begins in early November. But the thalli decline in quality with the curtailment of the culture period. Various diseases also occur. Many culturist in Korea are not equipped with freezer for cold storage of nursery nets except some industrialized culturists. The problem is how to supply cold storage systems for nursery nets to small scale fisheries.

Arrangement of culture grounds and intensive culture

The placement of culture structures in the culture grounds is regulated by the Ministry of Agriculture, Forest and Fisheries, to keep the ratio between the area covered by the nets and the licenced culture ground to 20– 25% in case of floating nets and 20–30% in case of any other structure. The law, however, does not differentiate between the floating net culture system, which could be placed off-shore, and other structures, which could be placed only on sea shore. With the floating net culture system expanding, it is most likely that the off-shore culture ground will be intensively utilized. Consequently, in order to protect laver from diseases resulting from the reduced water circulation, it is necessary to control the number of culture structures per unit area. In the floating net culture system the present ratio of 20–25% (culture nets area/licenced culture area) should be reduced to 10–15%, and in case of other structures the ratio of 20–30% should be 20% on off-shore and 15% on estuary/shore areas respectively.

VI. Harvesting and Processing

Harvesting

Harvesting of mature laver fronds is usually carried out from early November to April. This is done by hand picking or with a vacuum cleaner-like machine fitted with a rotating knife (Plate 1 and Plate 2). However, harvest is done almost all by hand picking. After harvesting, the smaller fronds left on the nets flourish again. Thus, laver is usually harvested 5 times from each net until growth begins to decline. In some parts of southern coast such as Kohung peninsula (Jeonnam province), harvesting is done nearly 10 times from each net. This is possible due to the development of the floating net culture system which exposes the underside of the floating net once in a while.

Processing

The harvested fronds are processed into dried laver sheets directly by the culturists or by the processors. Mechanical drying is expensive: 800–1000 won per bundle of dried product but the basic cost is about 400–500 won per bundle. The entire process is by machine. The manufacturing process begins with washing the laver fronds in sea water, followed by chopping the washed laver into small pieces (10–15 mm²), and stirring in fresh water to make a laver suspension (about 1 kg of laver pieces are placed in 20 liters sea water and stirred). The suspension is poured over a small rectangular wooden frame placed on top of a screen mat made of fine stems of bamboo, reed, or synthetic resin sticks. The wet sheet of laver formed on the screen is dried under the sun or in a hot air chamber (40 +/- 1 °C) and then peeled off the screen. The sheets of dried laver are folded in half and then packaged for marketing. One 1.8 × 40 m² culture net produces an average of 7,500 sheets or 20 kg of dried laver product. If one-fourth of the culture area is covered by laver nets, a total of 100 nets/ha will produce a total of 20,000 kg of dried product in a 6- to 8-months.

Plate 1. Harvesting of laver: hand picking.

Plate 2. Harvesting of laver: mechanical harvest.

The standard size of 1 sheet of dried laver product is 21 cm × 19 cm. One sheet weighs from 2.4 to 3.0 g. One bundle equals 10 piles which consist of 10 sheets, so 1 bundle equals 100 sheets. The yield of dried product is 600–800 g per 10 kg of wet raw material. In accordance with Fisheries Statistics in 1986, the production ratio showed 2.08 kg of laver (wet) per m² of culture net. Japan produces 3.17 kg per m² of culture net.

VII. Marketing

Almost all of the processed laver is now consumed in the country. There are different marketing routes according to the district. The marketing channels, now in Korea, is shown in Fig. 16: either the producer/culturist themselves deal with wholesalers/retailers directly, or through the Local Fisheries Cooperative Association (LFCA). Over 90% of culturists process dried products by machine and 10% by sun-drying.

The consumer's price is about 3 to 5 thousand won per bundle. The moisture content after processing is about 10–12%, therefore it should be dried again to 5% before storage. Long storage of over 2 months causes changes of color, taste etc., if the moisture content is over 10%.

Dried laver is sold as a set of 10 sheets packaged. This weighs approximately 24–30 g.

VIII. Problems of Laver Industry in Korea

Recently, the laver culture industry in Korea has shown a remarkable increase in production, owing to the technical development such as artificial seeding, availability of cultural nets made of synthetic fibers, floating net culture system, and improved cultured species (such as P. tessera Kjellman form, Tamatsuansis mura, and P. yezoensis). Yet, the production is unstable because of variations in the marine environment, along with deficiency in cold storage system for the storage of nursery nets. On the other hand, expansion of culture areas has increased production. Hand harvesting and drying have been replaced rapidly by mechanical harvesting and drying. Therefore, expenditure for purchasing of machines as well as equipment for the culture structure has enhanced inevitably the cost of the final product. The price is being kept at 3,000 to 4,000 won per bundle, or 12,000 won per kg of dried products. The processors are faced with the problem of how to keep the price steady and how to lower the basic cost without affecting the culturist, through the development of various products at low cost.

Figure 16. Marketing channels of laver.

CULTURE OF UNDARIA PINATIFIDA IN KOREA

I. Development of Culture Technique

Sea mustard or brown algae belonging to the genus Undaria is a cold water algae occurring widely in the temperate zone. For the entire life cycle a temperature of 10 to 20 °C is most suitable. It typically occurs in open sea and the growth of the algae is greatly affected by the inflow of freshwater from rivers. This algae grows all along the Korean coast except at higher latitudes over 40° in western coast.

Culture by artificial seeding started in 1963. The culture method before then was rather primitive consisting of placing stones at suitable depths for the attachment of young plants. From 1967, cultivation by hanging rope method was developed, and in 1970 over 100,000 MT (wet) were produced. In 1974 the amount was recorded at 180,000 MT. The decreased consumer's price resulting from over-production caused production to decrease, however in 1975 a market was developed in Japan. At present the annual yield of cultured product is about 200–300 thousand MT (Fig. 17). Natural harvest, was 28,000 MT in 1972, however it is now under 6,000 MT. Cultivation of sea mustard now occupies over 60% of the total yield of cultured seaweed. The major culture centres are Wando in Jeonnam district and Kijang, Ilgwang near Pusan in the southern coast which produce over 80% of all the sea mustard in Korea.

II. Characteristics of Sea Mustard

There are three species, Undaria pinatifida (Harvey) Suringar, U. undarioides (Yendo) Okamura, U. peterseniana (Kjellman) Okamura. Of these U. pinatifida is most widely distributed and favoured for consumption.

1. Phylum : Pheophyta
   Class    : Pheophyceae
   Order    : Laminariales
   Family   : Alariaceae

2. Size and shape:

   Maximum 1–2 mm in thallus length.

3. Color:

   Fresh fronds are dark brown to greenish brown, but when cooked (boiled) the color turns brownish green to green

4. Process of culture

   Spore sowing

   culture of gametophyte germling

   floating culture of sporophyte to market size

   harvest, processing and sale

5. Life cycle and ecology (Fig. 18):

   Undaria pinnatifida is an annual plant which grows on rocks and reefs at a depth of 1 to 15 m in areas facing the open sea. When the water temperature rises above 14 °C in April in southern part of Korea, the discharge of zoospores begins from the sporangium formed on sporophylls at the base of the fronds. The discharge of zoospores continues until the temperature reaches 23 °C, peaking at 17 to 22 °C.

   The discharged zoospores are 9 micron in length and motile. They drift in the sea with the water current, adhering to any substrate they come in contact with. Zoospores germinate on the substrate and grow to gametophytes at water temperatures up to 24 °C. They stop growing at temperatures higher than 24 °C and form resting gametophytes with thicker cell walls to tolerate the high temperature.

   Gametophytes are microscopic structures which perform sexual reproduction when mature, at water temperatures of about 20 °C in October. The fertilized eggs germinate into sporophytes which grow well at temperatures lower than 17 °C. Sporophytes grow fast in winter to form large edible fronds. In the spring, the sporophytes form sporangium on the sporophylls at the base of the fronds which asexually reproduce gametophytes. Following the discharged of zoospores, the sporophytes wither and die.

III. Culture Technique

The life history of Undaria pinatifida, like other brown algae, is complex. The asexual form, or sporophyte, is the plant from which the fronds are used as food.

The sporophyte phase grows during the winter months when the temperature is between 10 and 15 °C. The culture of Undaria begins with the collection of the sporophylls.

During winter months the sporophyte develops asexual zoospore, which are released from the sporophyll in spring and early summer when the water temperature rises above 14 °C. After a brief planktonic life, the zoospores settle and adhere to a solid surface (stone, shells etc.), and produce the macroscopic sexual plant, the gametophyte.

Germination of zoospore and growth of the gametophyte occurs between water temperature 15 °C and 20 °C.

Artificial spore production

In order to collect spore, the mature sporophytes are brought into the laboratory and placed in concrete/plastic tanks. Rectangular plastic/wooden frames (generally 40 × 50 cm²) wrapped with cotton strings are placed in the tanks (Fig. 19).

Figure 17. Annual yield of sea mustard (wet basis) from 1962 to 1987.

Figure 18. Life cycle of Undaria.

The zoospores are released from the plant and attach to the strings in late spring or early summer, and the young sporophytes develop on the strings in summer and early fall. The method for spore collection is as follows: after drying the sporophylls, in a dark place for one to several hours, they are placed into a tank filled with sea water. The sporophylls start discharging zoospores within 5 minutes lasting for 20 to 30 minutes. The sporophylls are removed 30 minutes later, and the spore collectors placed into the tank. After 30 minutes the spore collectors are removed from the tanks and transferred to the sea or tanks for the culture of gametophyte germlings. The suitable amount of sporophylls used in spore collection is about 3 kg (wet basis) per 1,000 m of string, or 10 individuals per 200 m length of string.

The sea water in the tanks should be controlled under 20 °C, and specific gravity over 1.020.

Culture of gametophyte germling

There are two methods for culturing germlings; one is to culture them in the sea by hanging the spore collector from a raft, the other is to culture them in indoor concrete tanks. In Korea, the culture of germlings is carried out in indoor concrete tanks. The spore collectors with attached zoospores are hung vertically in the tank where zoospores germinate on the strings of the spore collectors.

Water temperature, light and water flow are carefully adjusted according to the growth stage. In some cases nutrient salts such as sodium nitrate and sodium phosphate are added to the water to stimulate the growth of germlings. Gametophytes enter a resting stage at temperatures over 25 °C in summer. During this phase the germlings are kept in still water at temperatures below 28 °C because at higher temperatures the germlings are easily killed. In the southern part of Korea the sea water temperature often exceeds 25 °C in August, therefore growth of the gametophyte continues until early August whilst they enter the resting phase around mid-August. In this case the supply of nutrients becomes necessary. When the water temperature drops below 23 °C in autumn the gametophytes resume their growth, and the light condition of the holding tanks is increased to 2000–4000 Lux. Gametophytes mature at 20 °C and sexual reproduction usually takes place. When the fertilized eggs germinate as sporophytes the light is increased to over 5000 Lux and the exchange of water and addition of nutrient salts are controlled to accelerate the growth of the sporophytes.

If the growth of the sporophytes is not efficient the water is cooled to 20 °C, or the spore collectors are hung in the sea.

Conditions of water temperature and light in relation to the various life stages are shown below:

Floating culture of Undaria sporophytes

In September or November the sporophytes grow to 100 to 1000 micron in size attached to the strings of the spore collectors and are ready for further culture in the sea. Thus, floating culture of Undaria begins in autumn, from September to November when the water temperature drops below 20 °C. Culture grounds should be located where the water temperature is below 15 °C, and the specific gravity above 1.025.

Prior to the culture of the sporophytes by floating rope method, the spore collectors are hung to the main rope of the culture structure for about two weeks until the young thalli grow to 0.5–1 cm in length. After that, strings covered with young thalli (sporophytes) of Undaria are wound around the main branch line of the floating rope structure.

Harvesting

Undaria fronds are harvested from a boat with a knife from December to May (Plate 3). However, there are some differences in harvesting time between the east and the central parts of the country's South coast, due to the different oceanographic characteristics of the areas. Generally, harvesting in the central part terminates around April, while in the east part it continues until the end of May. However, due to the recent appearance of Harpacticoida which affects the thalli, considerable problems in the culture areas have been recorded especially after March. The damage resulting from Harpacticoida have in recent year reduced the total annual harvest by 20%.

The floating culture system of Undaria pinatifida

In Korea the culture method widely adopted for Undaria is the floating rope or hanging rope systems (Fig. 20).

The main lines of the culture structure are 1–3 cm in diameter and 70–80 metres long made of synthetic fibers (or strong rubber) kept afloat and fixed to the sea floor at suitable intervals with anchors.

In the floating rope method branch lines are hung from the main lines. The strings covered with young thalli (sporophytes) of Undaria are either wound around the main or branch lines or cut into short pieces, several cm long, and inserted between the strands of the main or branch lines. The method of winding around the main branches is still popular. The main lines should be set 1–5 metres below the water surface. The depth from the sea surface varies from 1–4 metres depending on the growth stage.

Processing

Market size fronds are harvested by cutting the strips near the sporophyll with a knife. At present harvesting is manual. Undaria from natural grounds are collected by diving, which is practised in Korea only in the eastern coast of Kangwon Province.

Undaria is processed as a sun-dried product, blanched-salted product, etc. Processing methods in Korea are as follow:

Sun-dried product

The harvested fronds of sea mustard are washed with fresh water after harvesting and cut into two similar halves by removing the midrib or cut into small pieces by removing the end part of the thalli. They are dried in the sun or in a hot air dryer. The yield from raw material is about 10 %.

Blanched-salted product

The raw fresh fronds are heated immediately after harvest at 90–98 °C for about 40–60 seconds and then cooled with water. The fronds (now vivid green in color) are mechanically mixed with salt in a ratio of 3:10 (w/w). They are preserved in a tank for 1–2 days, and then packed in a bag to remove excess water. The product is then stored in a cold room at -10 °C for sale. This kind of product has a very strong market and most of it is exported to Japan.

Figure 19. Undaria spore collector.

Plate 3. Harvesting of Undaria pinatifida. A) harvesting part and B) growing part.

Figure 20. Undaria floating culture method. a), b), e) and f) single rope, c) double rope, d) rectangular rope structure.

Cut products

The blanched-salted product is desalted by washing with fresh water, centrifuged to remove excess water, cut mechanically into small pieces, and dried in a rotary type of flow-through dryer. In Korea this type of processing is done by fishermen themselves who sun-dry after desalting the blanched-salted product.

Marketing

At present, the price of cultured Undaria is 10,000 won per 100 kg of fresh wet product. A small amount of the harvest is usually sun-dried by the producers themselves and them marketed to the consumer through wholesale/retail dealers in the domestic market (Fig. 21). Only the blanched-salted product derived from high quality fronds is exported to Japan. Generally all the product is exported by March. Because of the good quality of the fronds the price has been kept at about 120,000 won per 100 kg after harvesting during the last few years. The standard of this product is regulated to have a moisture content under 63 %, the midrib removed, and the salt content from 25–40 %. The yield of the product from the raw material is about 40%. The exported product is packed in wooden boxes and the unit of package is 15 kg/box. Other kinds of products are sold to the domestic market.

Constraints in the Undaria industry

Undaria is the most productive cultured species in the shallow coastal areas in Korea. Although the total production reached 350,000 tons in 1986, the income of the culturists has decreased due to the stagnation in price. The disease Harpacticoida, arising from intensive cultivation, is now a serious problem. The damage in terms of decreased harvest was estimated at about 20%. The reason this causes serious damage to Undaria culture is that the disease of the fronds occurs in March, when the harvest of good quality fronds reaches its peak. There is no special counter measure to keep the fronds from this damage. The preventive measure is to (1) control all culture process thoroughly, (2) prevent intensive culture, and (3) keep the marine environment free from pollution, etc.

On the other hand, as the processed products of Undaria are consumed only as soup, the price is limited in the domestic market. Consequently from now on, in order to increase the consumption in the domestic market higher graded products should be developed such as Undaria tea, powdered products, seasoned products, fish pasted product, Undaria noodles and others.

Figure 21. Undaria marketing channels.