Influence of forest decrease and land-use development on fishery resources in coastal lake region

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The dynamics of suspended sediment concentration and the impact on a lake-inhabiting bivalve were studied in the Abashiri River basin, in eastern Hokkaido. The Japanese corbicula (bivalve : Corbicula japonica) is the most important fisheries resource in Abashiri Lake, which has an area of 32.3 km^2 and a mean depth of 6.1 m. Eighteen water-collection stations were established based on different land-use and geological conditions. The water was collected from spring to summer during freshet. The amounts of forest cover and agricultural land, as well as the slope gradient, lithological hardness, and the number of faults in each tributary were also calculated using Geographic Information Systems. Sediment concentrations greater than 1600 mg/l were observed at all stations along the main channel in the summer of 2003, except at the uppermost station and the station downstream from the lake. Suspended sediment (SS) concentrations >1600 mg/l were also measured in the tributaries. The factors influencing SS concentration were analyzed by multiple regression analysis. This led to the identification of soft bedrock and fault as significant factors influencing sediment concentration. Fine-sediment particles less than 10 μm in diameter accounted for more than 50% of the samples at all stations. Since the lake bivalve prefers coarse substrates, a negative relationship was observed between the amount of fine sediment. The substrate around the mouth of the river was also composed of fine sediment, which indicated that it had been transported from an upstream tributary. Together, the data show that the suspended sediment produced from the mountain tributaries has deteriorated the lake environment, especially for corbicula survival and growth. Careful watershed management, such as monitoring the effect of forestry and land-use practices, is required to protect the fisheries resources.