A tentative assessment of water pollution by the medaka egg stationing method: aerial application of fenitrothion emulsion.

To determine how centrum abnormalities in medaka could be used to indicate the degree of water pollution, several rearing experiments were carried out. Medaka eggs were placed at certain points on a stream flowing from an area in which fenitrothion emulsion (organophosphorous pesticide--MEP) was being applied aerially. There was sufficient probability that the eggs were exposed to the MEP. They were brought to the laboratory, and reared in dechlorinated water to adulthood. The external forms and vertebral centra of each adult fish were closely observed. It was found that the incidence of centrum-damaged fish in the exposure experiment groups was significantly higher than in the control and blank experiments, and the influence of the aerial application of MEP was clearly indicated in this manner. Consequently, it was confirmed that centrum abnormalities in medaka may be used as a biological index for indicating the degree of water pollution by agricultural chemicals.