"It was really unexpected that they would react so quickly and so dramatically," said Karen Kidd, the lead researcher of the University of New Brunswick study, which was published Monday. "The crash in the population was very evident, very dramatic, very rapid, and related directly to the estrogen addition." Researchers began adding small amounts of estrogen to the freshwater facility in 2001. Shortly after, the number of fathead minnows plummeted to just one percent of the original population. Fortunately, after removing the estrogen from its habitat, the fathead minnow population rebounded fully, and just as quickly.