Feminizing Male Bass

WASHINGTON — Male smallmouth bass collected from rivers near human population centers and inside extensive agricultural areas were found to have higher rates of "intersex" characteristics than fish captured in more rural areas with fewer farms.

These findings are based on collaborative research conducted by the U.S. Geological Survey (USGS), the West Virginia Department of Natural Resources, the U.S. Army Corps of Engineers, and the Virginia Department of Game and Inland Fisheries. 

"We collected smallmouth bass from the Shenandoah River, the south branch of the Potomac River, and [outside] of the [Potomac River] basin for comparison," said USGS study leader Vicki Blazer. "The fish from the sites with the highest human population and the most farming had the highest incidences of intersex."

Previously linked to endocrine disruption caused by pollution, intersex characteristics are those defined as both male and female. Their characteristics are not typical in fish species like black bass. The USGS-led team discovered immature female eggs in the testes of male smallmouth bass, and they labeled the cells "testicular ootids."

The Shenandoah River had the highest rate of male smallmouth bass with testicular ootids, ranging from 80 to 100 percent. In the more remote, less agricultural areas outside the Potomac basin, the rate dipped as low as 14 percent.

The intersex rate for bass in the south branch of the Potomac River ranged from 47 to 77 percent.