Artificial enhancement refers to procedures for increasing natural Laminaria stocks either by creating new Laminaria forests or by increasing the density of already existing Laminaria beds. Since Laminaria japonica is not indigenous to China, having been introduced from Japan in the 1930's, therefore all Laminaria stocks presently existing in China have been created through enhancement techniques.
Because of the extensive development of Laminaria raft culture along China's seacoasts, ecological conditions have changed at some seafarming areas. For example, intensive Laminaria seafarming has resulted in higher nutrient content in the seawater, increased numbers and species of organisms able to thrive because of the sheltering provided by Laminaria, and a marked increase in the density of Laminaria spores in coastal seawater.
In China, the historical sequence of events has been the following: introduction of natural stocks of Laminaria→ production of artificially cultured Laminaria stocks from introduced natural stocks ---> production and enhancement of natural stocks from artificially cultured Laminaria stocks. [Introduced wild ---> artificially cultured ---> enhanced wild.] This chapter will look at the third stage in this sequence, namely the various methods used for enhancement of naturally occurring Laminaria stocks.
The artificial creation and enhancement of Laminaria forests has many beneficial ecological effects on the inshore sea environment:
Laminaria forests provide shelter and protection for fish fry, young fish, and small-sized species of fish.
Many species of fish and cephalopods spawn in Laminaria beds, their sticky eggs adhering to Laminaria fronds.
Laminaria forests improve the base of the ecological food chain. Laminaria beds provide nutrients and gather detritus, enhancing plankton growth which sustains species of smaller invertebrates, in turn providing feed for fish and other organisms, such as sea urchins, abalone, sea cucumbers, and other commercially useful organisms.
In raft culture of Laminaria, kelp are harvested and must be recultured annually. Whereas enhancement of wild stocks creates permanent kelp forests that grow naturally and which can be harvested for many years. Investment is not required for building seedling–rearing stations or for constructing kelp rafts. The savings on manpower and materials lower costs enormously.
Only a one–time investment for enhancement operations is required and thereafter yield from many harvests is obtained. Production costs for enhancement methods of growing Laminaria are only 10% of production costs for raft culture. For example, in China the cost of enhancement using the method of sinking 800 parent Laminaria per mu is 200 CNY (Chinese yuan, 3.6 yuan = 1 USD). The cost of enhancement using the method of sinking stones covered with Laminaria spores is 300 CNY, which includes the cost of 400 parent Laminaria as a source of spores, 2,000 stones weighing 15–20 kg each, depreciation or annual rental costs of four wooden boats and manpower costs.
In 1953–54 an experiment on enhancement was done in Qingdao, using the method of sinking stones for collecting natural Laminaria spores. In autumn months stones were sunk on the sea floor. In the following summer Laminaria were harvested. On average, there were 23.5 Laminaria plants attached to each stone, with the mean length of plants being 100 cm. Harvest was 5,000 kg of fresh weight Laminaria per mu, equal to 666 kg of dry weight product. The coefficient relating fresh weight to dry weight was 7.5, i.e. every 7.5 kg of fresh weight produced 1 kg of dry weight Laminaria. With a market price of 0.5 CNY for each 1 kg of dry weight, the first harvest yielded 333 CNY, enough to recover total investment in only 1 year. Thus Laminaria enhancement is economically more profitable than using culture methods.
Mature sporophyte plants can tolerate a maximum water temperature of 26–27o C. If the water temperature exceeds this temperature, sporophyte plants can neither survive nor reproduce. Therefore areas selected for enhancement should be places where the highest summer seawater temperature is below 27o C. This condition is true for coastal regions in northern China from Liaoning Province in the north to Shandong Peninsula in the south. In other words, as we have noted previously, naturally occurring Laminaria stocks cannot grow further south than about 36o N latitude.
Substratum (Sea Bottom) Characteristics
Shallow sea regions where natural reefs exist are best for Laminaria enhancement projects. Sea areas should have rising and falling, rather than flat reef bottoms. Ideally, sea bottoms should be littered with large rocks on which Laminaria spores can settle and attach. If the sea bottom is composed of fragmented stones, pebbles or loose grit, then the enhancement method of sinking stones for attachment of Laminaria spores should be employed.
Distribution of Laminaria
Care must be taken to locate Laminaria beds in the subtidal zone. Laminaria beds should be located in areas where water depth during spring tides does not fall below 1.0–2.5 m. Care should be taken to avoid intertidal areas where plants will suffer frequent exposure. It is important, therefore, to make observations of water depths during spring tides when selecting sites for Laminaria enhancement.
In addition to avoiding frequent intertidal exposure, care must also be taken to avoid locating Laminaria beds in deepwater regions. Laminaria growth depends on adequate illumination. If water is too deep penetrating sunlight will be insufficient for growth requirements. With proper illumination, Laminaria will grow evenly and form a well–spaced forest of healthy plants. If placed at too deep a location, Laminaria plants may survive only in weak stands, clusters or widely spaced patches.
Seawater areas with high transparency and a natural nitrogen/nitrate content of 10 mg/m3 are suitable for kelp cultivation. The higher the dissolved nitrogen/nitrate levels in seawater, the better. Areas with muddy seawater and reef surfaces covered with loose sediments are not good for Laminaria enhancement. Loose deposits on reefs may prevent attachment of Laminaria spores and, if spores do succeed in adhering, may prevent healthy growth by settling on Laminaria fronds.
Competing Seaweeds and Attaching Organisms
Laminaria forests will have difficulty becoming established if hardier competing species, such as calcareous or sac seaweeds, already occupy the region. Similarly, an area infested with barnacles, sea urchins and other attaching organisms will be inhospitable to the introduction of Laminaria.
Few naturally occurring Laminaria beds are found in shallow sea regions where water current exceeds 60 cm/sec. The reason may be that it is difficult for Laminaria spores to adhere to reefs where strong currents prevail. The most suitable areas for Laminaria spore adhesion seem to be near the mouths of bays where coastal currents are moderately but not excessively strong.
Many methods have been used to enhance Laminaria beds in China. Among them are the following, of which (i) and (ii) are the best methods:
sinking stones covered with Laminaria spores;
sinking parent Laminaria to produce spores;
sinking stones to collect natural Laminaria spores;
sinking baskets to collect natural Laminaria spores;
clearing reef surfaces and sinking (transplanting) parent Laminaria to produce spores.
Laminaria spores are artificially induced to adhere to stones, and then the spore–covered stones are sunk to the sea floor in areas suitable for Laminaria growth. Though this method is more complex and its costs higher than other methods, significantly better results are obtained. Permanent Laminaria forests can be created using this method. The stones are sunk in autumn and the first harvest of Laminaria can take place as early as the following summer. Investment costs can be recovered after only one year's growth and harvest.
Size of Stones
Stones are chosen according to varying conditions in different sea regions. Where winds and waves are moderately strong, stones should weigh about 15 kg. Where strong winds and waves prevail, stones should weigh about 15–20 kg. Best stones are ones with rough surfaces and concave depressions. Smooth round stones are not good, since their surfaces prevent good spore attachment.
Time and Method for Collecting Spores
The best time to collect spores in northern China is between the end of October and the middle of November, when the seawater temperature is between 17–19° C.
Wooden boats of 4–8 tons carrying capacity are used for the stone–sinking operation, with four boats in a group, three for carrying the stones that Laminaria spores will adhere to and one for holding a supply of seawater that will be used to gather the spores produced by parent Laminaria.
Each parent Laminaria plant releases numerous spores, enough to adhere to the surface area of six stones. Parent Laminaria are stimulated to release spores by being partially dried in a shaded area. The parent Laminaria are then placed in the clean holds of the boat carrying seawater, where zoospores are released and dispersed. When the density of spores in the seawater reaches 30–50 spores/100x (100x magnification field), then the seawater containing spores can be pumped into the holds of the other boats containing stones. The water containing spores should be diluted by adding seawater until density of spores is reduced to 15 spores/100x. Using this method, density of spores settling and attaching on stones will be about 10–20/100x.
Method of Sinking the Stones
Depth and physical features of the sea bottom should be investigated and marked before the stone–sinking operation begins. Each stone–carrying boat is anchored with two cables fixed to two anchors, one cable across the stern and the other across the bow. The forward cable is gradually winched–in, drawing the boat slowly forward. As the boat moves forward, stones can be sunk to the sea bottom. About 1,600–1,800 evenly distributed stones should be sunk per mu.
Shallow sea regions with natural reefs are good locations for enhancing Laminaria stocks, using the method of sinking parent Laminaria as a source of spores. Generally, shallow sea reefs are exposed during neap tides. However, care should be taken to observe conditions during spring tides, in order to note such factors as greatest water depth, water transparency and strength of tidal currents in different sea regions.
Preparation of Parent Laminaria
There are two ways of collecting parent Laminaria: (i) divers can collect parent Laminaria from natural stocks, or (ii) parent Laminaria can be selected from culture rafts.
If natural stock parent Laminaria are to be used, mature plants should be collected by divers when mature sporangial sori are seen forming on the Laminaria fronds. Collected Laminaria plants are then attached at intervals of 2 cm to a 2 m long rope, with about 20 plants on each rope. The parent plants should then be temporarily cultured on rafts. When the sporangial sori are observed to have reached full maturity, the parent Laminaria plants are ready to be sunk to the sea bottom.
Reef depth and sea bottom features should be investigated and marked carefully before sinking the parent Laminaria. The parent Laminaria should be stimulated to release spores by drying them in a shaded place for a short time. Stones weighing 10 kg are then tied to the ends of each 2 m rope holding 20 plants. The ropes with attached Laminaria plants are sunk to the sea bottom during low tides. Each rope is oriented at cross-currents, i.e. at right angles to prevailing tidal currents. About 800 parent Laminaria per mu should be sunk in this manner.
A unique method for sinking parent Laminaria plants to seed kelp forests has been developed in Japan. It is used in inshore seawater regions about 15 m deep with suitable substratum. In this method, the main floating raft rope is submerged. The ends and middle parts of the raft rope are anchored in place with cement blocks and the rope is held buoyant, about 8 m above the sea floor, with plastic floats (Fig. 10.1). Parent Laminaria are attached to short kelp ropes and these are tied along the submerged raft rope, so that kelp plants are suspended above the bottom reefs. When the parent Laminaria mature they release spores which settle and adhere to the sea bottom.
Fig. 10.1. Hanging Rope Enhancement Method
1: main float 2: auxiliary float 3: raft rope 4: anchor rope 5: kelp rope 6: cement block 7: protecting plastic tube 8: marker buoy
Good results have been reported from this method. In one report parent Laminaria were suspended over bottom reefs in a bay in Japan in 1972. By 1974 average density of Laminaria on the sea bottom had reached 21 mature plants per square metre of reef surface and had produced a large scale Laminaria forest. The permanent forest is now being harvested annually, allowing a quota of mature fronds to remain for parent stock.