Study finds highest rate of feminized fish in Mississippi River in Minn.

The Mississippi River in Minnesota has the highest rate of feminized fish found in a study conducted by the U.S. Geological Survey.

Scientists are concerned when they find male fish with female sex organs. In the Mississippi River, near Lake City Minnesota, 73 percent of the smallmouth bass had characteristics of both sexes.

The feminization is thought to be caused by hormone-disrupting chemicals in the environment. They can include pesticides, PCBs, heavy metals, household compounds such as laundry detergent and shampoo, and many pharmaceuticals.

Large and smallmouth bass seem to be particularly vulnerable to the chemicals.

The USGS study sampled fish in nine rivers around the country. The Yukon River basin in Alaska was the only one where researchers did not find at least one intersex fish.

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