Hisanori Okuda

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.

Toxicity of fenitrothion degradation products to medaka (Oryzias latipes)

Japanese agriculture, currently uses chemicals in large quantities. Many investigators bave reported the toxic effects of these chemicals on organisms. However, when a chemical is actually applied to a field, it breaks down just after application, and various chemicals result from its degradation. It may be more important to determine the toxic effects of degradation products than those of undegraded chemicals to assess the actual toxic effects of chemicals on organisms in nature. The effects of such degradation products on organisms bave been studied very little. Only Miyamoto et al. (1978) have examined in the acute toxicity of each degradation product of MEP and have reported the results. However, they did not examine their collective toxicities. In nature, it is rather important to assess the total effect on organisms. Thus experiments were conducted for clarification of collective toxicity of fenitrothion and its degradation products.

Time Course Study of Occurrence of Anomalies in Medaka's Centrum by Cadmium or Fenitrothion Emulsion

Recently researchers have presented a summary of the skeletal abnormalities of fish, in which a few cases of skeletal abnormalities in medaka are cited. But there is no mention of the time at which skeletal abnormalities become apparent as a result of exposure to chemical substances during egg development, and the frequency of such occurrence with medaka growth. In the present study, developing medaka eggs were exposed to cadmium (Cd) or a fenitrothion emulsion (MEP), and the transition of incidence of centrum-damaged fish was examined.

Toxicity to medaka of solutions of fenitrothion degraded by strong alkali.

Various experiments were conducted to examine the effects of degradation products on organisms. Fenitrothion (MEP) emulsions adjusted to pH 10 or 14 were degraded by exposure to natural sunlight in winter with heat. Medaka eggs were exposed to various concentrations of the degraded solutions and an untreated MEP emulsion for 5 days starting 4-5 h following fertilisation. The eggs were then allowed to develop in dechlorinated tap water until the fish reached 3 months of age. The hatching rate and rate of survival in the degraded solutions tended to be lower than those in the untreated MEP emulsion, and the incidence of abnormal fry in the degraded solutions was higher than that of the untreated MEP emulsion. It thus appears that degradation products formed by MEP have toxic effects on medaka.