BIOMONITORING WITH SHRIMP TO DETECT SEASONAL CHANGE IN RIVER WATER TOXICITY

Seasonal changes in toxicity of Kokai River water to a freshwater shri mp (Paratya compressa improvisa) were monitored from 1989 to 1993. A 4 -d toxicity test was conducted three times a week from April to August from 1989 to 1993 to measure mortality. Seventeen pesticides were ana lyzed during the biological monitoring. Expected shrimp mortalities we re calculated by adding mortalities caused by each insecticide at the measured concentrations. Shrimp mortalities increased in late May to m id-June and could be attributed to additive effects of insecticides su ch as pyridaphenthion, malathion, and fenobucarb. Mortalities decrease d toward the end of June but increased again from July to mid-August a nd could be attributed to insecticides such as diazinon or fenobucarb, which drifted directly into the river after aerial pesticide spraying on paddy fields. High mortalities decreased rapidly after heavy rains in August.