173
Oyster culture techniques
Traditional oyster culturing technique using small lumps of igneous rock as surfaces for oyster spat attachment remained basically unchanged until the beginning of the 1960s, when oyster farmers started experimenting with different shapes of concrete block to replace rock. Bromhall (1958) reports that the oysters were removed from rocks, placed into sampans and the rocks thrown back into the water for further spat collection. The rocks would often be covered in silt or sedentary biota before the next spat fall, so it is not surprising that various forms of artificial substrate have been adopted which could be "planted" at appropriate times. Mok, 1974a reports that concrete tiles 250 mm × 130 mm × 15 mm were the most common form of cultch used. Morton and Wong, 1975 confirmed that concrete tiles and posts were replacing stones and old oyster shells as cultch, and considered that diminishing supply of rocks was one reason for their introduction. The concrete posts currently used are 40 mm × 40 mm × 450 mm. Chinese oystermen also use concrete blocks 150 mm × 150 mm × 150 mm. All concrete items are inserted to about 50% of their volume into bottom sediments.
The Chinese oyster farmers said that in shallow-water beds, tiles tended to be used in very shallow inshore water, and posts in deeper water. Inspection of some of the shallow inshore Hong Kong beds showed however that posts were widely used. Whether this was by design or because of some temporary supply problems could not be ascertained.
The landward extent of the shallow oyster beds is governed by exposure to air, which should be no more than 7 to 8 hours in a day. In view of the usual tidal cycle described earlier such duration of exposure is infrequent. The shallow water oyster beds extend to about 0m PD, at which depth farmers can wade in water and work if necessary.
Concrete tiles are found in rows about 0.6 m apart and spaced at 35 to 75 mm intervals, (see plates 4 and 5). Concrete posts are spaced at similar intervals to the tiles, but in some instances double or treble the density is used particularly when oysters are being
173
Oyster culture techniques
Traditional oyster culturing technique using small lumps of igneous rock as surfaces for oyster spat attachment remained basi- cally unchanged until the beginning of the 1960s, when oyster farmers started experimenting with different shapes of concrete block to replace rock. Bromhall (1958) reports that the oysters were removed from rocks, placed into sampans and the rocks thrown back into the water for further spat collection. The rocks would often be covered in silt or sedentary biota before the next spat fall, so it is not surprising that various forms of artificial substrate have been adopted which could be "planted" at appro- priate times. Mok, 1974a reports that concrete tiles 250 mm × 130 mm x 15 mm were the most common form of cultch used. Mor- ton and Wong, 1975 confirmed that concrete tiles and posts were replacing stones and old oyster shells as cultch, and considered that diminishing supply of rocks was one reason for their intro- duction. The concrete posts currently used are 40 mm × 40 mm × 450 mm. Chinese oystermen also use concrete blocks 150 mm × 150 mm × 150 mm. All concrete items are inserted to about 50% of their volume into bottom sediments.
The Chinese oyster farmers said that in shallow-water beds, tiles tended to be used in very shallow inshore water, and posts in deeper water. Inspection of some of the shallow inshore Hong Kong beds showed however that posts were widely used. Whether this was by design or because of some temporary supply problems could not be ascertained.
The landward extent of the shallow oyster beds is governed by exposure to air, which should be no more than 7 to 8 hours in a day. In view of the usual tidal cycle described earlier such duration of exposure is infrequent. The shallow water oyster beds extend to about Om PD, at which depth farmers can wade in water and work if necessary.
Concrete tiles are found in rows about 0.6 m apart and spaced at 35 to 75 mm intervals, (see plates 4 and 5). Concrete posts are spaced at similar intervals to the tiles, but in some instances dou- ble or treble the density is used particularly when oysters are being
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