Sublethal effects of three pesticides on Japanese medaka.

One- to 2-day-old medaka (Oryzias latipes) larvae were exposed for 4 days to the rice field pesticides methyl parathion, molinate, carbofuran and a mixture of all three. Pesticide concentrations were one-half the 96 h LC50 ("high concentration") and levels approximating those measured in receiving waters from rice field runoff ("low concentration"). Maximum swimming speed, spontaneous muscular activity, acetylcholinesterase activity, dry weight, RNA:DNA ratio, and five morphometric variables were determined at the end of the exposures. Larvae were retained for an additional 10 days in non-contaminated water, and the same measurements taken to investigate residual effects. Results are compared to a parallel study on striped bass larvae to evaluate the suitability of this species as a surrogate for the bass in toxicological studies involving sublethal exposures. There was no relationship between mortality rate and pesticide exposure either during the exposures or during the ten day subsequent period. Only the high concentration of carbofuran caused an impairment of swimming performance. Spontaneous activity was stimulated in the high concentration of molinate and the combined pesticides groups. Acetylcholinesterase was severely inhibited in parathion and molinate, and this persisted in some cases after 10 days in non-contaminated water. The pesticides had little effect on growth rate except for molinate which acted as a stimulant. Combining the three pesticides caused a less than additive effect. Except for decreases in acetylcholinesterase, the sublethal effects of the pesticides tested at the very low concentrations used were subtle. Apparently, larvae of this species are less sensitive to these pesticides than are striped bass larvae.