Study on the mechanism of the large amount death of Corbicula japonica

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To clarify the mechanisms of the large amount death of Corbicula japonica occurred in Lake Shinji in 1997, experiments were carried out with containers in different population densities of C.japonica in 1999. It revealed that higher density results in (1) increase of the death rate, (2) decrease of the filtration rate, (3) decrease of anoxic tolerance of the bivalve. Before the large amount death of C.japonica was recognized among containers of higher population density (3000 ind./m^2 4000 ind./m^2) in summer, the rapid decrease of the flesh dry weight was observed. To clarify the value of the elements effective to check the condition of C.japonica, some experiments were carried out in 2000. Seasonal changes of Condition Index, filtration rate and tolerance of anoxic condition were examined for C.japonica living in an artificial lagoon built beside the Lake Jinzai, Shimane Prefecture. Although the Condition Index reduced largely from May to September, the filtration rate and the tolerance of anoxic condition did not reduce so apparently. It indicates that the artificial lagoon was kept in a comfortable condition for C.japonica. The Condition Index of C.japonica kept in an anoxic condition artificially reduced rapidly. It appears that the large amount death occurs when the reducing rate or the Condition Index becomes larger than 3%/day. In these cases of large amount death, the filtration rate is reduced to under 0.1 l/ind. hr. The combination of these two values may provide a guide to check the condition of C.japonica and effective to predict the large amount death,