Fang Jianguang *
There are more than twenty species of oysters along the coast of
China. Among them, Crassostrea rivularis, C. gigas and
Saccostrea cucullata have been regarded as the most suitable
species for culture and have been widely cultivated for many
Crassostrea rivularis is a species that is mainly found in the estuarine area of Pearl River in Guangdong province, South of China (Fig. 1). It is the biggest among the three species. After growing for three years, the average length of the shell can reach 20 cm.
Saccostrea cucullata, which is smaller than C. rivularis, can be found everywhere along the coast of China from the norther Province of Liaoning to the souther Province of Guangxi. The growth of this oyster is relative fast and can reach the commercial size of 6–7 cm in shell length within one year of culture. Due to its highly value meat, it is an important species for culture throughout the nation, but especially in Fujian Province in Southeast China.
The Pacific oyster, Crassostrea gigas, which has been called Ostrea talienensis for many years in China, mainly exists along the coast of the Bohai and Yellow Seas in Northern China. It is slightly smaller than C. rivularis but bigger than Saccostrea cucullata. Because its growth rate is highest among all the species, it is considered to be the most suitable species for culture in the north.
* Researcher at the Yellowsea Fisheries Research Institute, Qingdao
II. Methods of culture
During the oyster breeding period, fishermen place the spat collectors such as oyster and clam shells, stones and roofing tiles on the bottom of the intertidal zone. When a sufficient number of spats has settled on the collectors they are shifted to the on-growing area where the environmental factors are favourable for the oysters to grow up to the commercial size.
Several culture methods are being practised in China:
Raft-Cage Culture. Only in the last few years did Chinese
fishermen began to cultivate C. rivularis in cages. These are
made of net and are hung under floating rafts. During the
spatfall season the fishermen fix small collectors such as oyster
and clam shells into the cage and hang them under the rafts. The
number of cages per raft depends on the buoyancy of the raft
itself. Usually one cage is divided into several layers and on
each of them several collectors are attached. This culture
method is somewhat similar to the scallop culture system
practised in Northern China. The major difference is that
scallop cages are suspended from long lines while the oyster
cages are suspended under rafts.
It has been shown that the major advantages of this culture method are better growth rates and quality. The survival rate is also better when compared to bottom culture methods mainly due to the fact that bottom dwelling predators (e.g. starfish and drills) are unable to reach the cultured oysters. However, with this method, the raft and cages can be easily damaged by a storm and the cages fouled up by a number of benthic organisms and seaweeds. To avoid being blocked up, the cages are periodically cleaned.
Figure 1. Major oyster species cultured in China.
Bottom Culture. This is mainly applied in the culture of
Crassostrea gigas and Saccostrea cucullata. The best sites for
this culture practice are protected bays or inlets with muddy
bottoms and a relatively slow current.
Fishermen spread the collectors on a satisfactory intertidal collecting site during the spatfall period. When the density of attached spats reaches a certain point (80–200/150 cm2) the collectors are either transplanted to the on-growing areas or allowed to remain on the same site until the oysters attain commercial size. Generally the collectors are spread over the surface and kept 5–20 cm apart. In September, in both Guangdong and Fujian Provinces, fishermen separate the clumps of oyster in order to reduce crowding and shell malformation, by striking the base of the oyster clump with a metal object. To promote better growth and recovery the oyster beds are periodically raked or pulled from the mud. Predators are removed from time to time from the oyster beds in order to reduce predation pressure. The number of collectors spread over a certain area is closely dependent on the environmental factors prevailing at the culture site.
Stone Culture. This method is particularly suitable in bays with
muddy substrates where oyster bottom culture can not be practised
mainly due to the fact that the oysters would tend to sink into
the sediments. This culture system can also be applied in areas
affected with strong waves or current action.
Fishermen erect stone barriers on the tidal zone during the oyster breeding period. These stone slats which can be regular or irregular in shape are often 1.2 m long and 20 cm wide. They are driven to a depth of 30–40 cm to keep them in place. Spats which attach to these stone structures are left growing until they reach harvestable size. Oysters growing on stone slats often become heavily fouled with barnacles, mussels and other organisms, and tend to trap drifting seaweed. No effective treatment has yet been developed to prevent barnacles from fouling these structures. However, if the density of mussels attached to the stone slats is acceptable, they are often cultivated along with the oyster and harvested as a secondary crop. Seaweed caught on the stone slats are removed by hand.
Prior to the spatfall season, the undesired organisms attached to the permanent collectors are thoroughly removed. Following spatfall, thinning is usually carried out, due to the often over collection of spats, until a suitable density is reached. If few spats have been collected and the stone slats are heavily fouled, it would be necessary to clean out the stone barriers throughout and new spats collected. During the culture period, fouling organisms and predators are eliminated on a regular basis.
The methods described above are rarely adopted by fishermen in Northern China. It is common practice here to collect the oysters from natural beds which are guarded and protected by the fishermen themselves. Harvesting begins towards the end of year whether the oysters reach commercial size or not.
Harvesting of marketable oysters generally begins in January and
ends towards April. Over this period the oysters are in a better
condition than they would be the rest of the year.
During high tide fishermen, while standing on the boat, clip the oysters and/or collectors out of water onto the boat and transport them ashore for accurate harvesting.
When the density of the oysters has been reduced due to repeated harvesting, divers begin to collect the oysters growing on the sea bed. Fishermen usually harvest by hand the oysters growing on the bottom of the tidal zone at low tide.
When the oysters are transported ashore they are shucked by the fishermen with the aid of a special oyster knife. So far, this procedure is still entirely a manual operation.
A number of methods of processing oyster meat can be found in China. Oysters are however mainly processed into dried oyster meat, canned oyster meat and oyster sauce. With the improvement of the freezing technique, fresh or cooked frozen oysters are now widely marketed in Northern China.
V. Annual Production
The total production of fresh oyster meat in 1983 was about 35,500 tons; 40,688 tons in 1984; and 50,872 tons in 1985. In 1986 the total national production was 54,994 tons. From 1980 to 1982 no precise figures are available on the total national oyster production.
VI. Law on Oyster Culture
There have been no restrictions on oyster culture and growing methods so far. This seafarming practice is strongly encouraged and right of access to tidal zones is protected by the Chinese government. Damaging the oysters being cultivated by fishermen is illegal and punishable by law.