SUBLETHAL EFFECTS OF 3 PESTICIDES ON JAPANESE MEDAKA

One- to 2-day-old medaka (Oryzias latipes) larvae were exposed for 4 d ays to the rice field pesticides methyl parathion, molinate, carbofura n and a mixture of all three. Pesticide concentrations were one-half t he 96 h LC50 (''high concentration'') and levels approximating those m easured in receiving waters from rice field runoff (''low concentratio n''). Maximum swimming speed, spontaneous muscular activity, acetylcho linesterase activity, dry weight, RNA:DNA ratio, and five morphometric variables were determined at the end of the exposures. Larvae were re tained for an additional 10 days in non-contaminated water, and the sa me measurements taken to investigate residual effects. Results are com pared to a parallel study on striped bass larvae to evaluate the suita bility of this species as a surrogate for the bass in toxicological st udies 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 c arbofuran caused an impairment of swimming performance. Spontaneous ac tivity was stimulated in the high concentration of molinate and the co mbined pesticides groups. Acetylcholinesterase was severely inhibited in parathion and molinate, and this persisted in some cases after 10 d ays in non-contaminated water. The pesticides had little effect on gro wth rate except for molinate which acted as a stimulant. Combining the three pesticides caused a less than additive effect. Except for decre ases in acetylcholinesterase, the sublethal effects of the pesticides tested at the very low concentrations used were subtle. Apparently, la rvae of this species are less sensitive to these pesticides than are s triped bass larvae.