Severe intersex is predictive of poor fertilization success in populations of rainbow darter (Etheostoma caeruleum).

Municipal wastewater effluent (MWWE) contains emerging contaminants such as pharmaceuticals and personal care products (PPCPs) that have estrogenic properties. PPCPs are thought to be responsible for feminization of male fish in heavily urbanized areas around the globe. While many observations of feminized male fish have been made, the impact of feminization on reproductive success is not well understood. To address this lack of knowledge of the impacts of feminization, we examined the reproductive fitness of rainbow darter (Etheostoma caeruleum), a fish that is also known to have been feminized in some reaches of the Grand River, Kitchener-Waterloo, Ontario, Canada. In order to assess their reproductive health, somatic indices, gonadal steroid production, fecundity, and histological severity of intersex were measured in male rainbow darter collected through an urban gradient. Reproductive fitness was assessed by stripping milt and eggs from wild spawning fish, fertilizing eggs manually, and rearing embryos to hatch. The fertilization success and survival of embryos to hatch were compared among sites. In this study, it was found that rainbow darter collected at sites near a large municipal wastewater treatment plant (MWWTP) had decreased gonad size, increased severity of intersex, and decreased androgen production relative to other sites. Fish collected near the largest MWWTP also had lower fertilization success and survival to hatch. In contrast, fish collected near a second MWWTP farther upstream had comparable fertilization success, but lower survival to hatch relative to the upstream rural reference site. Intersex severity was negatively correlated with fertilization success, but not survival to hatch, suggesting that intersex is a good indicator of a population's fertilization success. Further investigation is required in order to determine if feminization will impact the sustainability of wild populations of fish.