Intermittent Exposure of Fish to Estradiol

The majority of testing for the effects of chemicals on fish has, historically, been designed to provide continuous exposure to constant concentrations. However, in the environment, fish are typically exposed intermittently or to fluctuating levels of contaminants, due to their mobility between polluted and clean waters and to variations in effluent discharge concentrations. The effects of such intermittent exposure of fish to estrogenic substances are unknown. Therefore, male fathead minnows (Pimephales promelas) were exposed to nominal concentrations of 17-β estradiol (an endogenous and environmental estrogen) at 30, 60, or 120 ng L-1 dosed continuously, or 120 ng estradiol L-1 dosed intermittently, for 21 or 42 days. The estrogenic effects of these dosing regimes were evaluated by determination of plasma vitellogenin concentrations and changes in gonad weight. Intermittent exposure resulted in plasma vitellogenin concentrations approximately equal to those in response to continuous exposure to the same concentration and significantly higher than continuous exposure to the equivalent time-weighted average concentration. Thus, the response to the intermittent regime was greater than would be expected from a simple integration of concentration and time. Changes in gonadosomatic index were less sensitive to the dosing regimes. Exposed fish were also allowed to depurate to determine the rate at which the estrogenic response disappeared after removal of the stimulus. After a 21 day depuration, the elevated plasma vitellogenin concentrations remained high, suggesting that this estrogenic effect would be sustained in the fish for long periods after transient estrogenic exposure.