Matsushima Bay, one of the richest oyster-culture areas in Japan, was subject to heavy artificial eutrophication, mainly from the inflow of city and factory sewage. The physiological activity in the oyster,
Crassostrea gigas, in hanging cultures in this embayment declined markedly with progressive development of the gonads; this decline in activity coincided with eutrophication-induced accumulation of fatty material in the epithelia of the digestive organs; the oyster was thereby forced to depend on these accumulated fats for respiratory substrates in order to maintain its increased physiological needs. However, the fats were inefficient energy sources for the oyster and hence, during each spawning season, 50% mortality occurred. In addition, such phenomena as overmaturation of the gonad and disturbance of the lipid and steroid metabolism seemed to accelerate this mass mortality.
The progress of eutrophication in recent years is due not only to an increase in the inflow of city and factory sewage, but also to an expansion of aquaculture areas in coastal waters. Dense cultivation can markedly accelerate eutrophication, especially in a small bay or inlet, leading to deterioration or self-pollution of culture areas. In a eutrophic embayment or inlet, the inflow of organic matter and the sedimentation of dead organisms often lead to an increase in organic content of the sea-bottom sediments. In summer, oxidative decomposition of these organic substances results in decrease of dissolved oxygen of both the water layers near the bottom and the intermediate stratum, with liberation of free hydrogen sulphide. The oysters naturally fail to survive in such an environment.
A high density that is artificially created can result in symptoms of intraspecific physiological stress. Dense oyster culture in an area often facilitates the introduction and propagation of new strains of bacteria or parasites that were never present before. This sometimes causes disruption in the structure and function of the ecosystem.
Hence, one must take precautions against artificial eutrophication of coastal waters and dense oyster culture in order to maintain steady production of oysters by means of hanging cultures.
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