RESTON, Virginia -- The discovery of a high incidence of intersex (male fish exhibiting female characteristics) in smallmouth bass of the Potomac River Basin has prompted an investigation of water quality and wastewater discharge into the upper Potomac, and blood-plasma studies on this fish species. Recently, U.S. Geological Survey (USGS) scientists found pesticides, flame retardants, and personal-care products containing known or suspected endocrine-disrupting chemicals in several tributaries to the Potomac River and in the smallmouth bass that inhabit them.
The endocrine system excretes hormones in an organism that govern many functions, including sexual and reproductive characteristics. Endocrine disrupters in the environment include pharmaceuticals in untreated wastewater and agricultural runoff. Agricultural, industrial, and household products often contain compounds that mimic estrogen when ingested. Endocrine disrupters of this type may contribute to the high percentage of male smallmouth bass found in the Potomac that exhibit female characteristics.
"We analyzed samples of 30 smallmouth bass from six sites, including male and female fish without intersex and male fish with intersex," said Douglas Chambers, USGS scientist and lead investigator. "All samples contained detectable levels of at least one known endocrine-disrupting compound, including samples from fish without intersex."
Known or suspected endocrine-disrupting chemicals from pesticides, flame-retardants, and personal-care products were also present in water samples taken from all eight sites, including those where fish did not exhibit intersex. Wastewater from several sites that discharge municipal effluent or from sites contributing runoff was examined to identify point sources of these compounds. Antibiotics were found in wastewater samples, with municipal effluent having at least seven such compounds, but were not detected in water from other sites.
The reproductive anomalies in the Potomac's smallmouth bass population are not readily apparent on gross examination of an affected fish -- they were discovered by accident. In 2003, scientists investigating massive fish kills and widespread lesions found oocytes, precursors of egg cells that females normally produce, while looking at tissues from the testes of male fish under the microscope.
High intersex occurrence in aquatic species has been documented at other locations in the U.S. and in Europe. It is not unique to the Potomac Basin, and not unique to smallmouth bass. Previous studies have found that known or suspected endocrine disrupters are widespread in the environment.
The USGS continues to examine the occurrence of known or suspected endocrine disrupters in the Potomac Basin to determine the potential for these compounds to interfere with the development and reproduction of aquatic life.
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