In 2001, scientists began adding tiny amounts of synthetic estrogen to an Ontario lake; within the next two years, its fathead minnow population collapsed

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.