Whole-lake exposure to a widespread estrogenic pollutant leads to unanticipated reproductive behaviors in wild fish.

Reproductive behaviors are critical to the sustainability of wild fish populations and are threatened by the presence of potent endocrine-disrupting chemicals.1,2,3 Laboratory studies have routinely shown that key behaviors (e.g., nesting and aggression) of male fish are reduced following exposure to hormone mimics,4,5,6,7 yet the applicability of these findings to nature and consequences for wild populations are unknown. Here, we quantified, in situ, the reproductive behavior of male fathead minnows (Pimephales promelas) before, during, and after (spanning 12 years in total) a 3-year period in which the synthetic estrogen, ethinylestradiol (EE2, ∼5 ng/L) was added to an experimental lake, while also recording fish in nearby unmanipulated reference lakes.8,9 In contrast to expectations, nesting males showed heightened aggression during the exposure period that was triggered by the unexpected presence of more conspecifics at nest sites. Consequently, paternal care became more fragmented, males spent less time at the nest, and they had fewer eggs in their nests. These altered breeding dynamics, including marked reductions in male secondary sexual characteristics, contributed to a population collapse during EE2 amendments. Our findings underscore that social interactions in natural systems provide a challenging but necessary arena in which to pursue ecotoxicological studies if we are to understand whether risk assessments meaningfully protect wild populations.