Wang Feijiu *
In China, the seaweed with commercial importance are mainly Laminaria, Undaria, Porphyra, Gelidium, Gracilaria, Eucheuma, and Macrocystis.
Laminaria is the most important economic seaweed in China. Mariculture of Laminaria on artificial floating rafts started in 1952 and the production increased steadily until 1980 when the highest production of 252,907 tons of the dry product was recorded. In recent years, the cultivation area and total yield of Laminaria declined because the culture of shellfish developed so quickly that the farmers began to prefer cultivating shellfish to Laminaria. At present annual yield of Laminaria is over 200,000 tons.
Undaria is cultivated by the same raft method as the Laminaria and is often mix-planted with the latter on the same floating raft in Qingdao and Dalian. The annual production is only a few thousand tons. It is usually used for feeding abalone and some are exported to Japan. Porphyra is mainly cultivated in Jiangsu, Zhejiang and Fujian Province. It is used for food and extraction of agar. Gracilaria and Eucheuma are cultivated in Guangdong Province, also used for extracting phycocolloid.
The culture methods of other brown algae, such as Undaria and Macrosystis are almost similar to Laminaria.
The production procedure of Laminaria includes 4 steps: (1) culture of Laminaria sporelings; (2) culture of sporelings in the sea and transplantation; (3) culture methods of Laminaria; and (4) harvesting and processing.
II. Culture of Laminaria sporelings
The first step consists in the collection of the zoospores onto the seeding cords. To do this Laminaria fronds with mature sporangial sori are subjected to partial drying in the air and then placed in a small container with seawater. The liberated zoospores will readily attach themselves to the substratum, the seeding cords. The gametophytes and early sporophytes are cultured in water of 8–10 °C in glass-house for three months. After that, the juvenile sporophytes can reach 2–3 cm in length.
III. Culture of sporelings in the sea and transplantation
When the water temperature has dropped to about 20 °C, sporeling cords are removed from the glass-house and placed into the sea and hung on the floating raft. In a month or so the sporelings will have grown to juveniles of 10–15 cm or longer. These juvenile sporophytes will be eventually brought to the transplanting room and placed in tanks filled with seawater. Due to their fast growth on the cords they are thinned at some stage. Usually they are removed from the original sporeling cord and inserted in the twists of the kelp ropes, generally, with about 30 juveniles to each rope of 2 m.
* Fisheries Scientist, Ministry of Agriculture, Animal Husbandry and Fisheries, Beijing, People's Republic of China.
IV. Culture methods of Laminaria
There are two basic floating raft kelp cultivation methods. One is hanging kelp rope (also called single-raft) cultivation method. Another is horizontal kelp rope (also called double-raft) cultivation method. The floating line, is about 60 meter long floated at the surface by buoys generally made of glass or plastics and anchored terminally by anchoring lines to wooden pegs driven into the sea bottom. Each kelp rope has about 30 kelps twisted in it and is about 2 meter in length. In the single-raft method, the kelp ropes are hung down from floating line and weighted down by a small piece of stone. In the double-raft method, the two kelp ropes are linked or tied together at one end and the other ends tied to floating lines. The hanging kelp rope method has the advantage of better water movement but has the defect of uneven growth of kelps. The horizontal kelp rope method gives an even growth of kelp. However, it has the defect of being more resistant to water motion. Generally speaking, the “single-raft” method is better adapted to comparatively clearer water regions, and the 'double-raft” method to turbid regions with lower water transparency, such as, the Zhejiang coast.
V. Harvesting and processing
Harvesting takes place when the fronds are mature. The time for harvesting is important to kelp farmers. Since the Laminaria is sold on the market on the basis of dry weight, and since the wet weight to dry weight ratio changes from month to month, the criterion for selecting harvest time should take into consideration the highest per-unit area production rate plus the lowest wet-to-dry ratio.
Table 1 shows the results of an experiment conducted on the quantity and quality of the production on kelp drop at different dates.
From the table it is readily seen that mid to late June is the best time for harvesting. It is therefore recommended that harvesting begin in mid June and continue until late June or early July in North China.
Table 1. Kelp production harvested at different periods.
|Production in wet wt.|
|Production in dry wt.|
|Wet weight Dry weight|
In harvesting the Laminaria, the kelp ropes are detached from the floating line, and collected in small boats, many of which are towed in a long line by a motor boat. When the boats reach the wharf or shore, the Laminaria are transported to land and dried under the sun.
VI. Utilization and marketing of Laminaria
Most of the Laminaria japonica produced in China is used for food. Besides its role as a “health” vegetable, in China Laminaria is also important as raw material for its algin, mannitol and iodine contents in a special program for the comprehensive utilization of Laminaria. Recently the kelp produced in China has been employed in the processing of synthetic feed use in mariculture. Formerly the kelp was sold on the market only in its crude dried form, but recently small package of shredded and seasoned forms with different flavours have appeared on the market and are very well accepted by the people.