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A seven-year whole-lake experiment in northwestern Ontario examined how chronic exposure to the synthetic estrogen 17α-ethynylestradiol (EE2) affected fathead minnows. Scientists added small amounts of EE2 to Lake 260 during three open-water seasons beginning in 2001. The exposure increased vitellogenin responses, affected gonadal development, and produced reproductive changes in both male and female fish. After two seasons of additions, the fathead minnow population collapsed because of a loss of young-of-the-year fish. Reproductive failure continued after the additions ended, and the species experienced near extinction in the lake. The study demonstrated potential population-level consequences of chronic estrogen exposure.