Reproductive pattern of oysters found along the coast of Pakistan(Northern Arabian Sea)

摘要

The oyster species of Pakistan are a dwindling resource. At the beginning of the last century the stocks of edible oysters on the coast of Sindh were being depleted rapidly. Hornell thought that the cause of this depletion was over-exploitation and pollution. At the present time, many of the recognized beds of edible oysters on the coast have more or less disappeared. Even the spatfall of edible oysters was not occurring in the backwaters and creeks so new stocks could not build up there. Researchers have pointed to an excess of turbidity on the coast of Sindh, resulting from high erosion that is injurious to filter feeders, such as the edible oysters. This may have been the cause of the demise of the oysters. About 15 years ago there were sizable oyster reefs in the Hub River Delta on the coast of Balochistan, where the edible oysters, especially Crassostrea gryphoides (it was called C. rivularis then) and C. madrasensis were abundant. These oyster reefs have now disappeared altogether, so where live oyster beds were once found, now only oyster boxes and depleting reefs can be seen. This may have been the result of the construction of Hub Dam, which deprived the oyster reefs of freshwater. The reefs may also have disappeared because of the presence of increasing amount of silt and sand in the water. Yet a third reason may be the gradual elevation/uplifting of the oyster habitat out of the water. The coast of Balochistan, in Pakistan, is well-known for subduction and uplifting of the terrain so that 120- 150 m high cliffs stand at the back of the tidal zone, bearing a rich record of fossil marine fauna on top.