Sept. 14, 2026, Hamilton, ON – A team of researchers has found that a common wastewater pollutant turned male fathead minnows into worse fathers and fiercer rivals, revealing ecological impacts that laboratory studies missed.
The pollutant, called ethinylestradiol (EE2), is a synthetic estrogen used in oral contraceptives and other hormone therapies. While municipal wastewater treatment removes much of the compound, small but potent amounts still enter rivers and lakes. Scientists have long known it can affect fish reproduction. What remained unclear was whether it could also alter fish behaviours that are critical for population survival.
While laboratory studies suggested that exposure to EE2 would reduce aggression in male fish, researchers studying an entire lake ecosystem found the opposite. Researchers observed more fish around active nests, increasing competition and causing nesting males to spend more time defending territories and eggs.
“This is a great example of how animals can respond differently in the real world than they do in laboratory experiments,” explains Karen Kidd, a professor in the Department of Biology. “When fish are living in a natural environment, their behaviour is influenced by competition and social interactions that are difficult or impossible to recreate in the lab.”
To better understand the hormone’s effects in a natural setting, researchers conducted a whole-lake experiment, monitoring male fathead minnows before, during and after a three-year period where trace amounts of EE2 were added to an experimental lake. They compared the fish with populations living in nearby lakes that were not exposed.
The researchers found that EE2 changed the social dynamics of the fish population.
Male fathead minnows normally establish territories, court females and guard the eggs laid in their nests. During the exposure period, competition around active nests increased. The added rivalry, combined with reduced parental care and fewer eggs in nests, likely contributed to the population collapse observed during the experiment.
These results were published in the journal Current Biology .
“The hormone didn’t just affect individual fish. It changed the social conditions in the lake. Those changes had ripple effects on reproduction across the population,” says Erin McCallum, an associate professor at the Swedish University of Agricultural Sciences who co-led the research with Kidd and Paul Blanchfield from Fisheries and Oceans Canada.
The findings suggest scientists may be underestimating the ecological risks posed by hormone pollution and other contaminants.
Many environmental studies are done under controlled laboratory conditions, where researchers can isolate the direct effects of a pollutant. Whole-lake studies like this provide a unique opportunity to understand how pollution influences wildlife populations over time.
“In the lab, the environment and animal interactions are controlled and simplified; in these whole-ecosystem experiments we capture the complex and often unpredictable responses happening in response to chemical pollution. That’s why large-scale experiments like this are so valuable,” says Kidd. “They allow us to see consequences that would otherwise remain hidden.”
The researchers hope the findings will help improve environmental risk assessments and strengthen efforts to protect freshwater ecosystems from hormone pollution.
Current Biology
Whole-lake exposure to a widespread estrogenic pollutant leads to unanticipated reproductive behaviors in wild fish
7-Sep-2026