Levels of fenitrothion in Oryzias eggs and the effects on hatchability

Levels of MEP in Oryzias eggs and the effects of MEP on hatchability were investigated. When Oryzias eggs were exposed to MEP-containing water, the eggs contained more MEP the higher the MEP concentration of the water, the more advanced the stage of development, and the longer the exposure. The MEP contents accumulated in the eggs were reduced to 2-19% after they were transferred to dechlorinated tap water for 96 h. There was a significant correlation between the MEP concentration in the water and the MEP content in the eggs. When Oryzias eggs in an early stage of development were exposed to 4 ppm MEP for 96 h or 64 ppm MEP for 6 h and were then placed in dechlorinated tap water until hatching, their hatching rates became significantly lower than those of the control, and abnormal fry occurred frequently. This experiment clarified that increases of MEP in Oryzias eggs were greatly affected by the MEP concentration in the water, and that the hatching rate was reduced and abnormal fry increased, even though the eggs contained the MEP only temporarily.