Comparison between the effects of the phytosterol β‐Sitosterol and pulp and paper mill effluents on sexually immature rainbow trout

Abstract Smaller gonads, delayed sexual maturity and depression of sex steroids have been reported in fish exposed to pulp and paper mill effluents. β-Sitosterol, a phytosterol present in pulp and paper mill effluents that causes reductions in sex steroid secretion in fish, may contribute to some of these effects. In this study, the effects of 17β-estradiol (E2), β-sitosterol from two sources, and two Canadian bleached mill effluents (BME) on sexually immature rainbow trout were investigated in a 21-d in vivo experiment. Both β-sitosterol sources and the two BMEs exerted similar effects. Plasma vitellogenin levels were elevated by all treatments establishing the presence of estrogenic compounds in BME and confirming the estrogenicity of β-sitosterol. β-Sitosterol and BME had little or no effect on testosterone levels. Pregnenolone was significantly reduced by β-sitosterol and BMEs but not by E2, suggesting that this was not an estrogenic effect. Plasma cholesterol was reduced by β-sitosterol but not by BME or E2. Feral female white sucker exposed to BME showed significantly reduced plasma cholesterol compared with females from a reference site. Mixed-function oxygenase (MFO) activity was significantly induced in β-sitosterol- and BME-exposed fish. Both BME and β-sitosterol were shown to affect biological responses through more than one mechanism of action. These studies suggest that β-sitosterol could contribute to some of the physiologic effects of pulp and paper mill effluents.