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This circa 2009 photo provided by Bent Christensen show perch fish swimming together in Sweden. Researchers around the world have been taking a close look at the effects of pharmaceuticals in extremely low concentrations, measured in parts per billion. Such drugs have turned up in waterways in Europe, the U.S. and elsewhere over the past decade. (AP Photo/Bent Christensen)

Associated Press
This circa 2009 photo provided by Bent Christensen show perch fish swimming together in Sweden. Researchers around the world have been taking a close look at the effects of pharmaceuticals in extremely low concentrations, measured in parts per billion. Such drugs have turned up in waterways in Europe, the U.S. and elsewhere over the past decade. (AP Photo/Bent Christensen)
This circa 2009 photo provided by Bent Christensen show perch fish swimming together in Sweden. Researchers around the world have been taking a close look at the effects of pharmaceuticals in extremely low concentrations, measured in parts per billion. Such drugs have turned up in waterways in Europe, the U.S. and elsewhere over the past decade. (AP Photo/Bent Christensen)
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