The environment of the brackish lakes on the coast of the Sea of Okhotsk, in Hokkaido, has changed dramatically due to increased human activities since the Meiji period. Previous paleoenvironmental studies of these lakes have revealed historical changes in the lake topography and ecosystems over the last 10000 years. However, few studies have focused on recent environmental changes in these lakes, especially owing to the high economic growth in the region from the 1960s to the present. The purpose of this study was to examine change in the lake environment and aquatic organisms in Lake Shibunotsunai and Lake Mokoto, since 1945, by conducting field surveys as well as using existing data. From 1945 to the early 1960s, there was little human impact on the lake environment. However, since the 1960s, the lake environment has changed rapidly, mainly due to the progressive construction work on the inflow river channel and outflow channel from the lakes to the sea. Furthermore, changes in land use have led to eutrophication of the lake water and an influx of large amounts of mud in the lakes, making them shallower and smaller. Such changes in the lake environment have had a strong impact on the abundance and fishery activity of the brackish water clam
Corbicula japonica in both lakes. Further studies of core samples from Lake Shibunotsunai are necessary to corroborate the results of this study.
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