EFFECTS OF ARTIFICIAL EUTROPHICATION ON THE METABOLISM OF THE JAPANESE OYSTER CRASSOSTREA-GIGAS

Matsushima Bay, one of the richest oyster-culture areas in Japan, is subject to heavy artificial eutrophication, mainly from the inflow of city and factory sewages. The physiological activity in the oyster Crassostrea gigas in hanging cultures (i.e., suspended in wire-mesh cages) in this embayment declines markedly with progressive development of the gonads; this decline in activity coincides with eutrophication-induced accumulation of fatty material in the epithelia of the digestive organs; the oyster is thereby forced to depend on these accumulated fats for respiratory substrates in order to maintain its increased physiological needs. However, the fats are inefficient energy sources for the oyster and hence, during each spawning season, 50% mortality occurs. In addition, such phenomena as overmaturation of the gonad and disturbance of the lipid and steroid metabolism seem to accelerate this mass mortality. On the basis of the results, the author has compiled a schematic diagram which illustrates the possible causes of mass mortality among C. gigas in an eutrophic environment such as Matsushima Bay. © 1979 Springer-Verlag.