Enhancement of rooted algae on the rocky shore is an overarching practice to manage and promote wildstock enhancement and marine ranching for commodity production and ecological sustainability. Recently, natural fisheries resources are decreasing in the coastal waters due to various reasons coming mainly from industrialization near the shores. Construction of artificial seaweed beds is presently looked into as a necessary factor for the recovery of natural resources. Seaweed bed is an important part of the marine food web. There are many kinds of seaweeds growing on the coast, forming a marine forest in coastal waters where they make a good habitat and spawning grounds for a number of fisheries resources. Of them, marine forests of Sargassum and Laminaria are of interest in Korea.
The coastal line of Jeju island is 253km long. The coast has played an important role in the fisheries economy of the area, producing lots of valued marine products. Recently, however, sea walls for the coastal protection of the reclaimed land have been constructed from the sea bottom at depths of more than 17m. As a result, many seaweed beds have been reduced in areas. Coastal pollution accumulated in last decades also caused the reduced natural products in the area.

Table 1. Areas of barren ground in Jeju island of Korea  

Factors influencing seaweed bed

    Factors affecting seaweed beds are climate, water temperature, nutrients, and grazing. The monsoon-generated waves and storm affect to some extent the seaweed bed. On the coastal areas which show barrens in Korea, resulted in the decline of fisheries resources in these areas. Barren grounds are mainly due to the warm water from Kuroshio Extension which is characterized by malnutrition.

Effects of grazing in seaweed beds

    The activities of grazers affect the distribution and abundance of the rooted brown algae. The influence of grazing generally changes abruptly at the boundary between the intertidal and subtidal zones. Even on shores where the abundance of herbivorous gastropods (such as limpets and trochids) is high, their distribution tends to end at the zone of dense algae (normally fucoids) found in the immediate subtidal area. Herbivorous gastropods are normally the major grazing invertebrate. Herbivorous fishes may also affect the subtidal algal assemblage.

1. Invertebrate grazers

   Some herbivorous invertebrate organisms can graze heavily on new fronds and broken-ends of old fronds of brown algae. Abalone and sea-stars may have some small-scale effects upon algal abundance in Laminaria forests. Small crustaceans may be very abundant in algal turfs and may be major grazers of algal spores, thus influencing recruitment in the seaweed bed.
Sea urchins may have dramatic effects on kelp assemblages, on most temperate shores, in both hemispheres. In areas where sea urchins are abundant, their effects have been generally documented in three categories: (1) whole-scale removal of algae; (2) the alteration of species
diversity via feeding preferences and selective removal of algal species; (3) the provision of cleared primary substratum suitable for kelp recruitment. It is commonly observed worldwide that dense aggregations of sea urchins may remove large tracts of algae, creating barren ground.

2. Effects of fish grazers

    A recent study suggested that fishes, particularly the halfmoon (Medialuma californiensis) and opaleye (Girella nigricans) can be important grazers on small algal spores. These fishes seasonally appear in seaweed beds. Opaleye are grazers on the fronds of Laminaria in autumm. Rabbitfishes are notorious herbivores in the Eucheuma farming areas of the Philippines.

Methods for the construction of artificial seaweed beds

    Techniques developed for the construction of artificial seaweed beds can be classified into 2 general types: 1) a spore technique, and 2) a vegetative transplanting technique. The following techniques may fall under either of the above classifications:

 1. Spore dispersal technique as used for Macrocystis (North, 1971)

     Techniques have been developed for raising Macrocystis plants in mass cultures, from liberated zoospores, via the gametophyte, to the embryonic sporophyte. The zoospores are allowed to settle on various substrata which are then placed in flowing seawater under continuous

illumination. The gametophyte phase requires 10-20 days to reach sexual maturity and another 5-20 days are allowed for development of embryonic sporophyte stages. Cultures then are scraped·free from their substrata and dispersed close to the bottom in coastal areas suitable for kelp growth. The embryonic, microscopic plants are able to reattach at this stage, if the sites are fairly calm. Preliminary estimates indicate that about 10s embryos must dispersed to yield a single, attached, identifiable Macrocystis juvenile about 15 cm tall. Mass culture systems, however, can produce very large numbers of embryos (upto 105-106 embryos.cm-2 of culture substratum). Consequently, the low survival rates following dispersal do not preclude the practical application of this system as a means to enhance the development of new stands of kelp in the sea.

2. The spore-bag technique

    Mesh-bags ("spore-bags") packed with fertile, adult plants have proven to be a successful method for the establishment of Sargassum beds The spore-bags are usually suspended over rocky substrata or artificial reefs during the reproductive season. During this time, spores are released naturally and eventually settle on the hard bottom surface. The distance of settlement and establishment, from the site of the spore bag depends on prevailing water currents. From previous experience, this Is usually less than 40 m. Young Sargassum plants become noticeable on the natural or artificial substrata, as the case may be, after a few months.

The spore bag method of establishing a seaweed population.

3. Rope-seeding technique

    Fertile thalli of Laminaria may be allowed to sporulate in indoor tanks. Once ropes are seeded, the seeded ropes are then firmly attached to concrete blocks for growing.

              Transplantation of Laminaria plants.

4. Adult-plant transplantation method

The transplantation of adult plants of Ecklonia and Laminaria has been undertaken by fishermen for many years. This practice was initiated when natural recruitment was poor as a result of the barren-grounds (caused by excessive sea urchin grazing). Recently, this method of algal transplantation was improved by fishery scientists in Korea, making the construction of seaweed beds a little easier. There are various modifications of this technique depending on the species.

Adult-plant transplantation of Laminaria and defence of herbivore.

5. Concrete blocks

    Transplantation of Ecklonia can be carried out in September to December, at the time when seawater temperature is optimal for growth. One or two adult individuals can be attached to concrete blocks with strong rubber bands or ropes. In some cases, to increase the area of the attachment surface, the concrete blocks may be covered with coils of plastic material. Whichever case is used the plants are then covered with plastic sheets or placed in large cages for protection against fish grazing. The holdfasts of Ecklonia were found to attach to the blocks after one month of growth.

    New blades of adult plants of Ecklonia appear in late December and become young plants in early January. In May, new beds of Ecklonia will be formed around the blocks. A transplanted area of 9 m ³ can be expected to form a new Ecklonia bed of about 3000 m² area.

    Although many plants may gradually disappear (due to grazing and wave action during summer), surviving plants begin to form spores until the end of August. Thereafter. a new community of Ecklonia can be expected to become established on the same rocks. within an area of approximately 400 m ², in March of the following year.

    Transplantation, by attachment of adult plants on concrete blocks, has proven to be a most effective method of establishing new algal beds in Korea.

     Various concrete blocks for construction of seaweed beds.

6. The "threading technique" as used for Macrocystis

    This method is used in California, USA. Several Macrocystis plants can be threaded, at intervals by means of a rope passed through their holdfasts. After threading onto the rope, the plants can be moved to the site for transplantation by boat. The Macrocystis plants can then be moored by means of anchors and buoys until they start to gain permanent hold on the substratum.

7 "Gravel-bag" technique used for Macrocystis transplantation

   A more elaborate method of attaching Macrocystis to an artificial substratum was devised by larger and Neushul (1983) . Nylon mesh-bags are filled with coarse gravel (3 cm diameter) and their rims are drawn around the holdfast of the plant. Once transplanted, the bags gradually accumulate sand and sediments thereby increasing stability. Eventually the kelp haptera grow out of the bag and down to the sandy bottom.

   About 50 kg of coarse-gravel can be used for the anchor, which underwater, will weigh approximately 28 kg. Several plant bags can then be anchored to a grid of chain and rope to an optimum stocking density. Maximum stocking density of a Macrocystis artificial bed was determined by larger and Neushul (1983) to produce a higher biomass if fertilized with ammonium sulphate (less than optimum density dissipates the fertilizer before it can be absorbed by the plants) .

8. Transplantation of young plants

    Young plants, particularly of Laminaria and Undaria, can be anchored using concrete blocks or ropes as substrata. The substrata with attached young plants are transplanted on the bottom of rocks or artificial reefs after they are seeded.

      Young plants transplantation of Ecklonia.

Subjects for the future

    The possibility of management-free seaweed bed restoration by reducing local limiting factors, e.g. by rasing bottom and improving substrata for seaweeds at actual sites, has been suggested.

    In the future, it is supposed that the transparency in coastal water will decrease due to the continuous water pollution of coastal areas, and the distribution of new natural and artificial seaweed beds will be limited on many coasts by such factors. Therefore, it is indispensable to improve water quality to ensure the high transparency of coastal waters.

Marine forest enhancement on east coast of Korean Peninsula

1. Whitening area (barren area) on eastern coasts of Korean peninsula

2. Artificial reefs for rooted seaweed attachment

3. Slags and stones for the seaweed habitation

4. Laminaria japonica seeded on the artificial reefs

5. Costaria costata grown on the reefs

6. Undaria pinnatifida enhanced in the whitening area

7. Sargassum horneri forest enhanced in the previously whitened area

8. Eisenia bucyclis forest

9. Management of marine forest enhanced

10. Young seaweeds growing on the artificial reefs

11. Total recovery from the whitening