After the first summer, male minnows were producing egg proteins. By the second year their sperm cells were undeveloped. Shortly after that they produced eggs as well.
In addition, females were producing more egg protein than normal, and their sexual development was also delayed, Kidd said.
The result was an impaired ability of the minnows to reproduce, which caused the population to collapse in the second year of the study, the researchers found.
The population even failed to recover in the two years after the researchers stopped adding estrogen, indicating the effects were quite persistent, according to Kidd.
"Low concentrations of an estrogen can have very dramatic, very severe effects on fish reproduction and fish population," she said.
Kidd and colleagues report the findings this week in the journal Proceedings of the National Academy of Sciences. The Canadian government and the American Chemical Council funded the research.
Douglas Chambers is a biologist with the U.S. Geological Survey's West Virginia Water Science Center who has studied possible causes of feminized fish in the Potomac River Basin.
(Read related story: "Sex-Changing Chemical Found in Potomac River" [January 22, 2007].)
He said the finding of a sharp drop in the minnow population attributed only to synthetic estrogen was surprising.
He had suspected that feminization in addition to other environmental stresses could lead to population collapse. The new study shows feminization alone causes the decline.
Study leader Kidd and colleagues believe that if they had continued to add estrogen to the experimental lake for more than three years, the estrogen would also have had an impact on the populations of longer-lived fish in the lake—trout, white suckers, and pearl dace.
"It's critical that our municipal wastewaters are treated at least with secondary treatment to reduce the input of these estrogens into surface waters," Kidd said.
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