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marsh, which has been enclosed by bunds to form kei wais, (iv) a shallow channel running parallel to the outer bund; (v) a zone of mangrove plants some 15-25 m. wide with Kandelia candel, Avicennia marina and Aegiceras corniculatum as the commonest species, (vi) Deep Bay, which forms part of the estuary of the Pearl River and also receives water from the Sham Chun River and smaller streams. At intervals channels run through the mangrove zone to join channel (iv) beside the outer bund to Deep Bay; water moves along these channels during the rise and fall of the tide and thus the kei wais are filled.
Within the kei wais are small, irregularly shaped, muddy islands. Essentially all of our results were obtained from Mai Po kei wai No. 7 (total area ca 9.3 ha, area of islands 5.6 ha, area of open water 3.7 ha). This kei wai is 820 × 120 metres with its shorter sides parallel to the shoreline. It is surrounded by a bund which, on the seaward side, is pierced by a sluicegate with a concrete frame. The frame is about 2 m. wide and is usually closed by a gate consisting of heavy wooden planks, placed horizontally, which can be raised or lowered in slots in the concrete frame (Plates 7-9). The sluicegate is essential to the operation of the kei wai because it enables the operator to control exchange of water between the kei wai and the adjoining estuary. The kei wai, like others in the neighbourhood, was held on lease renewable every seven years.
Source of Future Produce
As already mentioned, an important source of future produce is the adjoining estuary: fry and larvae are carried on the high tide through the open sluicegate into the kei wai where they are "trapped" when the gate is closed. Species entering in this way include marine fish, shrimps and crabs. In addition, the source of produce is supplemented and diversified by the operator, who may add fry of tilapia (Sarotherodon mossambicus, syn. Tilapia mossambica) and the brackish-water striped mullet (or grey mullet, Mugil cephalus). Small oysters (Crassostrea gigas) are bought from local fishermen and used to stock oyster beds set up in the vicinity of the sluicegate; their growth period is 1.5 to 3 years.
The basis of production of the kei wai is the usual two food