Correlation between Overall Pesticide Effects Monitored by Shrimp Mortality Test and Change in Macrobenthic Fauna in a River

The relationship between change in benthic fauna in the Suna River and shrimp mortality in the river water samples (test was conducted two times per week from May to September) was investigated to assess the insecticide impact on the benthic community. The river was selected because pesticide application had been restricted for a long time in the upper reaches, while aerial insecticide spraying had been done four times on paddy fields in the lower reaches in summer. Species richness and density of benthos were high at the stations in the upper reaches. High shrimp mortality was measured in the water samples collected from a downstream station during the pesticide spraying period, from late July to late August. Species richness and density of benthos markedly decreased at the stations in the lower reaches in that period. However, several species of chironomids,Cheumatopsyche(Trichoptera), which demonstrated tremendous insecticide resistance, andBaetis sahoensis(Ephemeroptera), which has a high recovery potential, were not affected, even in the insecticide spraying period. The benthic fauna in the lower reaches tended to recover from autumn to spring, although not sufficiently. It is considered that the temporal recovery of the benthic community was brought about through recruitment from the benthic community in the 5-km upper reaches. A close correlation was found between the high mortality of shrimp in the water samples and the deterioration in the benthic fauna in this river which was contaminated with insecticides during the summer.