PACIFIC OYSTER INDUSTRY IN HONG KONG
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in order to collect the maximum number of spat, the cultch must be laid at the optimum period which is typically from late May to early June, usually at the time of Extreme Low Water Spring tide (ELWS). Mok (1973) has reported a semi-lunar periodicity in the release of eggs by C. gigas in Deep Bay; a similar breeding pattern is seen in other oysters, e.g., O. edulis (Korringa, 1947; Knight-Jones, 1952). Before the laying of the cultch commences, a site is first selected and marked out by the placing of tall bamboo poles at the four corners of the area during low tide when the oyster bed is exposed. During high tide, the cultch is taken by boat to the site indicated by the bamboo sticks and deposited on the sea bed. On the same day, during low tide, the cultch is laid. The oyster-farmer and his assistants visit the oyster bed at low tide by riding on a wooden sledge (Plate 15; E, F, and G). The cultch is laid in rows some 2 feet apart. Within each row, the arrangement is different according to the cultch type (Plate 14). The shell cultch is placed closely together in groups of three or more. The concrete tiles are half-inserted into the mud and placed approximately two inches apart. The concrete posts are similarly inserted into the mud to half their length but spaced some six inches apart.
Two to three weeks later, the oyster spat collected on the cultch can be seen as tiny, gleaming spots. The cultch, if initially placed inshore, is now taken further offshore and relocated. Because of the high rate of sedimentation within the bay, particularly in the summer, the cultch has to be periodically lifted out of the mud and transferred to the empty spaces between the rows to prevent it from sinking too deeply into the mud, thereby smothering the spat. This is especially important after typhoons. Usually, the oysters are tended until 3 to 4 years of age and are then cropped. The normal marketable size is approximately 10–15 cm. However, the age at which the oysters are cropped varies with demand, so that at times of great demand, even younger individuals can be marketed, and with reduced demand, they are left longer in the sea, and as a consequence, 6-year-old individuals almost 30 cm long have been found.
The oysters are harvested continuously throughout the year; no account being taken of breeding season. During winter, when the water is cold, the clusters of oysters are brought up into the boat by means of a pair of tongs (Plate 15; B) comprising two long (10–12 feet) bamboo poles loosely tied together and each possessing an inwardly directed four-pronged fork at one end. Similar tongs are