Collapse of a fish population after exposure to a synthetic estrogen

TLDR

Municipal wastewater contains estrogenic compounds that feminize male fish by inducing vitellogenin production and early‑stage eggs, yet it remains unclear whether chronic low‑level exposure threatens wild fish population sustainability. The study aimed to determine whether chronic exposure to low concentrations of 17α‑ethynylestradiol would affect fathead minnow populations over a seven‑year whole‑lake experiment. Researchers conducted a seven‑year whole‑lake experiment at the Experimental Lakes Area, exposing fathead minnows to 5–6 ng L⁻¹ EE2 and monitoring vitellogenin mRNA and protein, gonadal development, intersex occurrence, and oogenesis changes. Chronic EE2 exposure induced male feminization, intersex, altered female oogenesis, and ultimately drove the species toward near extinction, showing that estrogen concentrations found in freshwater can jeopardize fish population sustainability.

Abstract

Municipal wastewaters are a complex mixture containing estrogens and estrogen mimics that are known to affect the reproductive health of wild fishes. Male fishes downstream of some wastewater outfalls produce vitellogenin (VTG) (a protein normally synthesized by females during oocyte maturation) and early-stage eggs in their testes, and this feminization has been attributed to the presence of estrogenic substances such as natural estrogens [estrone or 17β-estradiol (E2)], the synthetic estrogen used in birth-control pills [17α-ethynylestradiol (EE2)], or weaker estrogen mimics such as nonylphenol in the water. Despite widespread evidence that male fishes are being feminized, it is not known whether these low-level, chronic exposures adversely impact the sustainability of wild populations. We conducted a 7-year, whole-lake experiment at the Experimental Lakes Area (ELA) in northwestern Ontario, Canada, and showed that chronic exposure of fathead minnow ( Pimephales promelas ) to low concentrations (5–6 ng·L −1 ) of the potent 17α-ethynylestradiol led to feminization of males through the production of vitellogenin mRNA and protein, impacts on gonadal development as evidenced by intersex in males and altered oogenesis in females, and, ultimately, a near extinction of this species from the lake. Our observations demonstrate that the concentrations of estrogens and their mimics observed in freshwaters can impact the sustainability of wild fish populations.