Hormones, behavior, reproductive fitness, and endocrine disruption in the fathead minnow

Environmental estrogens (EEs) are compounds found in the environment that mimic the actions of the female hormone estradiol (E2). Although it is well established that most sewage treatment plants (STPs) release EEs, the effects of these compounds on fish populations are poorly understood. My dissertation examined the estrogenic properties of effluent from two STPs and determined their effects on the hormones, behavior and reproductive fitness of male fathead minnows (Pimephales promelas). A study conducted in 2002 estimated the estrogenic activity of effluent released by the STP in St. Paul (Minnesota) to be 40 ng estrogen equivalents (EEQ)/L using enzyme immunoassay (EIA). Male fathead minnows exposed to this effluent for three weeks had elevated levels of vitellogenin (VTG), lower levels of androgenic hormones and impaired reproductive success. Reproductive impairment was particularly pronounced amongst minnows placed with non-exposed (control) males. This result appeared to have been caused by EEs because another group of minnows exposed to 30 ng/L of waterborne E2 suffered nearly identical behavioral and reproductive difficulties, while exposure to the methyltestosterone produced the opposite effect. Notably, the first result was confirmed by a study which exposed minnows to Western Lake Superior Sanitary District (WLSSD) STP effluent. The estrogenic activity of this effluent was later measured using an in vitro receptor binding assay in combination with an in vivo assay which measured both VTG and VTG mRNA in minnows. These assays measured daily variation in the EEQ of the effluent (varying from 10 to 90 ng EEQ/L). A third exposure study found that male fathead minnows exposed to levels of E2 which fluctuated on a daily rate similar to that of the WLLSD plant had similarly elevated VTG levels and reduced behavioral competence as males exposed to a constant dose of E2. Both groups suffered large decreases in competitive ability. Together, my studies demonstrate that exposure to EEs found in STP effluents decrease the reproductive fitness of individual male fish, especially in competitive situations that typify the natural world. This phenomenon could alter gene flow within populations if fish are exposed to EEs in uneven manners.