Corbicula japonica is a typical bivalve in blackish water environment in Japan and is an important resource in fishery. It is also importance for nutrient circulation in blackish water ecosystem. C. japonica has a high ability to tolerate the anoxic condition and adapt to wide range of salinity concentration. However, a large amount death of C. japonica occurred in 1997 in Lake Shinji, a representative lake for fishery of C. japonica in Japan. We considered that there are causes for this large amount death in addition to the final deterioration in their health conditions.
For this study, several containers with different set of population densities of C. japonica were submerged in Ohashi River, an outflow of Lake Shinji, from the early August to the end of October. Death rate in each container, tolerance ability and filtration rate of C. japonica were collected periodically from each container. The following results were obtained: 1) The death rate increased in high population density of C. japonica. 2) The filtration rate decreased in high population density of C. japonica. 3) The tolerance ability for an anoxic condition decreased in high population density of C. japonica. 4) These effects occurred in the population of over 2000 ind./m^2 of C. japonica with ca. 2 cm length of shell size. 5) Tolerance ability for anoxic condition increased when water temperature decreased under 20℃, and little dead shells were observed in October. 6) The rapid decrease in dry flesh weight of C. japonica was observed before the decrease of tolerance in anoxic condition with increase in number of death of C. japonica. It is concluded from these results that the health condition of C. japonica, such as the decrease of flesh weight, seemed to be an important factor for decrease in tolerance, and over population of C. japonica in Lake Shinji before the large amount death in 1997 was considered to be one of the cause of this phenomena.